Reconfigurable optical add/drop multiplexer having an array of micro-mirrors

BACKGROUND OF THE INVENTION

[0003] 1. Technical Field

[0004] The present invention relates to a tunable optical device, and more particularly to a reconfigurable optical add/drop multiplexer (ROADM) including an array of micro-mirrors to selectively add and/or drop at least one optical channel to and/or from a wavelength division multiplexing (WDM) optical signal.

[0005] 2. Description of the Related Art

[0006] In general, micro-electro-mechanical system (MEMS) micro-mirrors have been widely explored and used for optical switching applications. The most commonly used application is for optical cross-connect switching. In most cases, individual micro-mirror elements are used to ‘steer’ a beam (i.e., an optical channel) to a switched port or to deflect the beam to provide attenuation on a channel-by-channel basis. Each system is designed for a particular ‘wavelength plan’ —e.g. “X” number of channels at a spacing “Y”, and therefore each system is not ‘scalable’ to other wavelength plans.

[0007] In networking systems, it is often necessary to route different channels (i.e., wavelengths) between one fiber and another using a reconfigurable optical add/drop multiplexer (ROADM) and/or an optical cross-connect device. Many technologies can be used to accomplish this purpose, such as Bragg gratings or other wavelength selective filters.

[0008] One disadvantage of Bragg grating technology is that it requires many discrete gratings and/or switches, which makes a 40 or 80 channel device quite expensive. A better alternative would be to use techniques well-known in spectroscopy to spatially separate different wavelengths or channels using bulk diffraction grating technology. For example, each channel of an ROADM is provided to a different location on a generic MEMS device. The MEMs device is composed of a series of tilting mirrors, where each discrete channel hits near the center of a respective mirror and does not hit the edges. In other words, one optical channel reflects off a single respective mirror.

[0009] One issue with the above optical MEMS device is that it is not “channel plan independent”. In other words, each MEMS device is limited to the channel spacing (or channel plan) originally provided. Another concern is that if the absolute value of a channel wavelength changes, a respective optical signal may begin to hit an edge of a corresponding mirror leading to large diffraction losses. Further, since each channel is aligned to an individual mirror, the device must be carefully adjusted during manufacturing and kept in alignment when operated through its full temperature range in the field.

[0010] It would be advantageous to provide an add/drop multiplexer that mitigates the above problems.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide a reconfigurable optical add/drop multiplexer (ROADM) having a spatial light modulator that includes a micro-mirror device having an array of micro-mirrors, wherein a respective plurality of micro-mirrors direct separate optical channels of an optical WDM input signal to selectively switch at least one optical channel to add to and/or drop from the optical WDM input signal, which advantageously permits the ROADM to be reconfigurable by changing a switching algorithm that drives the micro-mirrors, without having to change the overall hardware configuration.

[0012] In accordance with an embodiment of the present invention, the optical add/drop multiplexer includes an optical arrangement for receiving one or more optical signals, each optical signal having one or more optical bands or channels, and includes a spatial light modulator having a micro-mirror device with an array of micro-mirrors for reflecting the one or more optical signals provided thereon. The optical arrangement features a free optics configuration having one or more light dispersion elements for separating the one or more optical input signals so that each optical band or channel is reflected by a respective plurality of micro-mirrors to selectively add or drop the one or more optical bands or channels to and/or from an optical input signal.

[0013] The one or more light dispersion elements may include either a diffraction grating, an optical splitter, a holographic device, a prism, or a combination thereof. The one or more diffraction gratings may include a blank of polished fused silica or glass with a reflective coating having a plurality of grooves either etched, ruled or suitably formed thereon. The diffraction grating may also be tilted and rotated approximately 90° in relation to the spatial axis of the spatial light modulator.

[0014] The spatial light modulator may be programmable for reconfiguring the optical add/drop multiplexer by changing a switching algorithm that drives the array of micro-mirrors.

[0015] In one embodiment, the optical add/drop multiplexer may include a first collimator that collimates an optical input signal. The optical input signal comprises a plurality of optical input channels, each of which are centered at a central wavelength. A first light dispersion element substantially separates the optical input channels of the collimated optical input signal. A second collimator collimates an optical add signal. The optical add signal comprises at least one optical add channel, which is centered at a central wavelength. A second light dispersion element substantially separates the optical add channels of the collimated optical add signal. A spatial light modulator reflects each separated optical input channel along a respective first optical path or second optical path, and reflects at least one optical add channel along the respective first optical path in response to a control signal. The spatial light modulator includes a micro-mirror device that has an array of micro-mirrors selectively disposable between a first and a second position in response to the control signal. Each separated optical input channel is incident on a respective group of micro-mirrors. Each separated optical add channel is also incident on the respective group of micro-mirrors. Each respective separated optical input channel reflects along the respective first optical path when the micro-mirrors are disposed in the first position, or along the respective second optical path when the micro-mirrors are disposed in the second position. At least one optical add channel reflects along the respective first optical path when the micro-mirrors are disposed in the first position. A controller generates the control signal in accordance with a switching algorithm.

[0016] Many other embodiments are shown and described below.

BRIEF DESCRIPTION OF DRAWING

[0017] The drawing, which are not drawn to scale, include the following:

[0018]
FIG. 1A is a plan view of a block diagram of one embodiment of a reconfigurable optical add/drop multiplexer (ROADM) in accordance with the present invention;

[0019]
FIG. 1B is a side elevational view of a block diagram of the ROADM of FIG. 1;

[0020]
FIG. 2 is a plan view of a block diagram of another embodiment of a reconfigurable optical add/drop multiplexer (ROADM) in accordance with the present invention;

[0021]
FIG. 3 is a block diagram of a spatial light modulator of the ROADM of FIG. 1A having a micro-mirror device having optical channels of a WDM input signal distinctly projected thereon in accordance with the present invention;

[0022]
FIG. 3A is a block diagram of an alternative spatial light modulator having a micro-mirror device with mirrors tilting on a spectral axis that is perpendicular to the spectral axis of WDM input signal distinctly projected thereon in accordance with the present invention;

[0023]

FIG. 4

a
is a pictorial cross-sectional view of the micro-mirror device of FIG. 3 showing a partial row of micro-mirrors disposed in a first position perpendicular to the light beam of the WDM input signal in accordance with the present invention;

[0024]

FIG. 4

b
is a pictorial cross-sectional view of the micro-mirror device of FIG. 3 showing a partial row of micro-mirrors disposed in a second position non-orthogonal to the light beam of the WDM input signal in accordance with the present invention;

[0025]
FIG. 5 is a plan view of a micro-mirror of the micro-mirror device of FIG. 3 in accordance with the present invention;

[0026]
FIG. 6 is a block diagram of a spatial light modulator of the ROADM of FIG. 3, showing four groups of micro-mirrors tilted to drop and/or add an optical channel from and/or to the WDM input signal in accordance with the present invention;

[0027]
FIG. 7A is a block diagram of another embodiment of an ROADM in accordance with the present invention;

[0028]
FIG. 7B is a plan view of another embodiment of an ROADM in accordance with the present invention;

[0029]
FIG. 7C is a side elevational view of the ROADM of FIG. 7B;

[0030]
FIG. 8 is a block diagram of another embodiment of an ROADM in accordance with the present invention;

[0031]
FIG. 9 is a block diagram of a spatial light modulator of the ROADM of FIG. 8 having a micro-mirror device, wherein the optical channels of a WDM input signal are distinctly projected onto the micro-mirror device, in accordance with the present invention;

[0032]
FIG. 10 is a block diagram of a spatial light modulator of the ROADM of FIG. 8, wherein four groups of micro-mirrors are tilted to drop and/or add four optical channels from and/or to the WDM input signal, in accordance with the present invention;

[0033]
FIG. 11 is a perspective view of a portion of a known micro-mirror device;

[0034]
FIG. 12 is a plan view of a micro-mirror of the micro-mirror device of FIG. 11;

[0035]

FIG. 13

a
is a pictorial cross-sectional view of the micro-mirror device of FIG. 11 showing a partial row of micro-mirrors, when the micro-mirrors are disposed in a second position non-orthogonal to the light beam of the input signal in accordance with the present invention;

[0036]

FIG. 13

b
is a pictorial cross-sectional view of the micro-mirror device of FIG. 11 showing a partial row of micro-mirrors, when the micro-mirrors are disposed in a first position perpendicular to the light beam of the input signal in accordance with the present invention;

[0037]
FIG. 14 is a pictorial cross-sectional view of the micro-mirror device of FIG. 11 disposed at a predetermined angle in accordance with the present invention;

[0038]
FIG. 15 is a graphical representation of the micro-mirror device of FIG. 14 showing the reflection of the incident light;

[0039]

FIG. 16

a
is a graphical representation of a portion of the optical filter wherein the grating order causes the shorter wavelengths of light to image onto the micro-mirror device that is closer than the section illuminated by the longer wavelengths, in accordance with the present invention;

[0040]

FIG. 16

b
is a graphical representation of a portion of the optical filter wherein the grating order causes the longer wavelengths of light to image onto the micro-mirror device that is closer than the section illuminated by the shorter wavelengths, in accordance with the present invention;

[0041]
FIG. 17A is a plan view of a block diagram of another embodiment of an ROADM in accordance with the present invention;

[0042]
FIG. 17B is a plan view of a block diagram of another embodiment of an ROADM in accordance with the present invention;

[0043]
FIG. 18 is an expanded view of the micro-mirror device of the spatial light modulator of FIG. 17A, wherein optical channels of a WDM input signal are distinctly projected onto the micro-mirror device in accordance with the present invention;

[0044]
FIG. 19 is a plot showing four filter functions of the ROADM similar to the ROADM of FIG. 1A having a micro-mirror device of FIG. 11 at the drop output/port 74 in accordance with the present invention;

[0045]
FIG. 20 is a plot showing four filter functions of the ROADM similar to the ROADM of FIG. 1A having a micro-mirror device of FIG. 11 in accordance with the present invention;

[0046]
FIG. 21 is a graphical representation of the light of an optical channel reflecting off a spatial light modulator, wherein the light is focused relatively tight, in accordance with the present invention;

[0047]
FIG. 22 is a graphical representation of the light of an optical channel reflecting off a spatial light modulator, wherein the light is focused relatively loosely compared to that shown in FIG. 16, in accordance with the present invention;

[0048]
FIG. 23 is a plan view of a block diagram of another ROADM in accordance with the present invention;

[0049]
FIG. 24 is a side elevational view of a block diagram of the ROADM of FIG. 23;

[0050]
FIG. 25 is a block diagram of a spatial light modulator of the ROADM of FIG. 23 having a micro-mirror device, wherein optical channels of a WDM input signal are distinctly projected onto the micro-mirror device, in accordance with the present invention;

[0051]

FIG. 26

a
is a pictorial cross-sectional view of the micro-mirror device of FIG. 11 showing a partial row of micro-mirrors, when the micro-mirrors are disposed in a first position, in accordance with the present invention;

[0052]

FIG. 26

b
is a pictorial cross-sectional view of the micro-mirror device of FIG. 11 showing a partial row of micro-mirrors, when the micro-mirrors are disposed in a second position, in accordance with the present invention;

[0053]
FIG. 27 is a plan view of a block diagram of another embodiment of a ROADM in accordance with the present invention;

[0054]
FIG. 28 is a plan view of a block diagram of another embodiment of a ROADM in accordance with the present invention;

[0055]
FIG. 29 is a plan view of a block diagram of another embodiment of a ROADM in accordance with the present invention;

[0056]
FIG. 30 is a plan view of a block diagram of another embodiment of a ROADM in accordance with the present invention;

[0057]
FIG. 31 is a plan view of a block diagram of an optical drop device in accordance with the present invention;

[0058]
FIG. 32 is a block diagram of an optical system including a pair of optical drop devices and an optical add device in accordance with the present invention;

[0059]
FIG. 33 is a block diagram of another embodiment of an ROADM, which includes a plurality of ROADMs in accordance with the present invention;

[0060]
FIG. 34 is a block diagram of the spatial light modulator of the ROADM of FIG. 27, wherein the optical channels of a plurality of WDM input signals are distinctly projected onto the micro-mirror device, in accordance with the present invention;

[0061]
FIG. 35 is a block diagram of a spatial light modulator of the ROADM of FIG. 27, wherein groups of micro-mirrors are tilted to drop and/or add optical channels from and/or to the plurality of WDM input signals, in accordance with the present invention;

[0062]
FIG. 36A is an exploded view of a collimator assembly according to the present invention;

[0063]
FIG. 36B is an exploded view of a fiber array holder subassembly that forms part of the collimator assembly shown in FIG. 36A;

[0064]
FIGS. 36C and 36D are exploded views of a fiber V-groove subassembly shown in FIG. 36B;

[0065]
FIG. 36E is a view of a constructed collimator assembly shown in FIG. 36A;

[0066]
FIG. 37 shows an alternative embodiment of an ROADM having one or more optic devices for minimizing polarization dispersion loss (PDL);

[0067]
FIG. 38 shows an embodiment of an ROADM having a chisel prism in accordance with the present invention;

[0068]
FIG. 39 shows an alternative embodiment of an ROADM having a chisel prism in accordance with the present invention;

[0069]
FIG. 40 shows an alternative embodiment of an ROADM having a chisel prism in accordance with the present invention;

[0070]
FIG. 41 is side elevational view of a portion of the optical channel filter of FIG. 40;

[0071]
FIG. 42 is a block diagram of an optical cross-connect;

[0072]
FIG. 43 is a plan view of a block diagram of a reconfigurable optical cross-connect including a spatial light modulator in accordance with the present invention;

[0073]
FIG. 44 is a block diagram of a spatial light modulator similar to that in FIG. 4, wherein four groups of micro-mirrors are tilted to selectively switch optical channels between a pair of WDM input signals, in accordance with the present invention;

[0074]
FIG. 45 is a block diagram of an optical interleaver device that is known in the art;

[0075]
FIG. 46 is a block diagram of an optical de-interleaver device that is known in the art;

[0076]
FIG. 47 is a plan view of a block diagram of a reconfigurable optical interleaver/de-interleaver device including a spatial light modulator in accordance with the present invention; and

[0077]
FIG. 48 is a block diagram of a spatial light modulator of the interleaver/de-interleaver device, wherein six groups of micro-mirrors are tilted to redirect a respective optical channel of the WDM input signal, in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0078] FIGS. 1-6 show an embodiment of the basic invention which features a reconfigurable optical add/drop multiplexer (ROADM) generally indicated as 10 having an optical arrangement generally indicated as 15, 16 in combination with a spatial light modulator 30. The optical arrangement 15, 16 receives an optical input signal 12 and an optical add signal 21 having one or more optical bands or channels. The spatial light modulator 30 has a micro-mirror device 82 (FIGS. 3-6) with an array of micro-mirrors 84. The optical arrangement 15, 16 features a free optics configuration having one or more light dispersion elements 24, 54 for separating the optical input signal and optical add signal so that each optical band or channel is reflected by a respective plurality of micro-mirrors 100, 101, 102, 103 (FIG. 6) to selectively add or drop the one or more optical bands or channels to and/or from the optical input signal 12.

[0079] The optical arrangement 15, 16 includes a first optical portion 15 and a second optical portion 16 that provide the optical input signal 12 and the optical add signal 21 to the spatial light modulator 30, and also provide the optical output signal 48 having the remaining optical bands or channels after bands or channels have been added and/or dropped and the one or more optical signals 76 dropped from the optical input signal 12 from the spatial light modulator 30. The scope of the invention is not intended to be limited to any particular type of optical portion. Embodiments are shown and described by way of example below having many different types of optical portions. The scope of the invention is not intended to be limited to only those types of optical portions shown and described herein.

[0080] The one or more light dispersion elements 24, 54 may include either a diffraction grating, an optical splitter, a holographic device, a prism, or a combination thereof. The one or more diffraction gratings 24, 54 may include a blank of polished fused silica or glass with a reflective coating having a plurality of grooves either etched, ruled or suitably formed thereon. The diffraction grating 24, 54 may also be tilted and rotated approximately 90° in relation to the spatial axis of the spatial light modulator 30.

[0081] The spatial light modulator 30 is programmable for reconfiguring the optical add/drop multiplexer 10 by changing a switching algorithm that drives the array of micro-mirrors 84 (FIG. 3) to accommodate different WDM input signal structures ( i.e. channel spacing, beam shape). For example the ROADM may be modified to accommodate WDM signals having a 50 GHz or 100 GHz spacing.

[0082] In FIG. 1A, the reconfigurable optical add/drop multiplexer (ROADM) 10 selectively adds and/or drops one or more desired wavelength band(s) of light (or optical channel(s)) from and/or to an optical WDM input signal 12. FIG. 3 shows each of the optical channels 14 of the input signal 12 centered at a respective channel wavelength (λ1, λ2, λ3, . . . , λN). One optical portion 15 receives the optical input signal 12, and the other optical portion 16 receives the optical signal 21 to be added thereto, as known herein as the optical add signal 21. FIG. 1A is a plan view of the ROADM 10 in the horizontal plane. Each optical portion 15, 16 includes substantially the same components disposed in substantially the same configuration. To better understand the ROADM 10 of FIG. 1A, a side elevational view of one of the optical portions 15 is illustrated in FIG. 1B and will be described with the understanding that the other complementary optical portion 16 functions in a similar manner.

[0083] In FIG. 1B, the optics of the optical portion 15 are disposed in two tiers or horizontal planes. Specifically, the optical portion 15 includes a three port circulator 18 and an optical fiber or pigtail 20. The free optics configuration includes a collimator 22, a light dispersive element 24, a mirror 26, and a bulk lens 28 for directing light to and from the spatial light modulator 30. As shown, the pigtail 20, the collimator 22 and the light dispersive element 24 are disposed in a first tier or plane parallel to the horizontal plane. The mirror 26, bulk lens 28 and the spatial light modulator 30 are disposed in the second tier also parallel to the horizontal plane.

[0084] In FIGS. 1A and 2, the first three-port circulator 18 directs light from a first port 32 to a second port 33 and from the second port 33 to a third port 34. The first optical fiber or pigtail 20 is optically connected to the second port 33 of the circulator 18. A capillary tube 36, which may be formed of glass, is attached to one end of the first pigtail 20 such as by epoxying or collapsing the capillary tube onto the first pigtail. The circulator 18 at the first port 32 receives the WDM input signal 12 from an optical network (not shown) via an optical fiber 38, and directs the input light to the first pigtail 20. The input signal 12 exits the first pigtail (into free space) and passes through the first collimator 22, which collimates the input signal. The collimator 22 may be an aspherical lens, an achromatic lens, a doublet, a GRIN lens, a laser diode doublet or similar collimating lens. The collimated optical input signal 40 is incident on the first light dispersion element 24 (e.g., a diffraction grating or a prism), which separates spatially the optical channels of the collimated input signal 40 by diffracting or dispersing the light from (or through) the first light dispersion element 24.

[0085] In one embodiment, the first diffraction grating 24 is comprised of a blank of polished fused silica or glass with a reflective coating (such as evaporated gold or aluminum), wherein a plurality of grooves generally indicated as 42 (or lines) are etched, ruled or otherwise formed in the coating. The first light dispersion element 24 has a predetermined number of lines, such as 600 lines/millimeter (mm), 850 lines/mm and 1200 lines/mm. The resolution of the ROADM 10 improves as the number of lines/mm in the grating increases. The grating 24 may be similar to those manufactured by Thermo RGL, part number 3325FS-660 and by Optometrics, part number 3-9601. Alternatively, the first diffraction grating 24 may be formed using holographic techniques, as is well known in the art. Further, the first light dispersion element 24 may include a prism or optical splitter to disperse the light as the light passes therethrough, or a prism having a reflective surface or coating on its backside to reflect the dispersed light.

[0086] As best shown in FIG. 1B, the diffraction grating 24 directs the separated light 44 to the first mirror 26 disposed in the second tier. The first mirror 26 reflects the separated light 44 to the first bulk lens 28 (e.g., a Fourier lens), which focuses the separated light 44 onto the spatial light modulator 30, as shown in FIG. 3. In response to an input command signal 46 from a controller performing the switching algorithm, the spatial light modulator 30 reflects selected optical input channel(s) away from the first bulk lens 28 (i.e., the dropped channels) to the other optical portion 16 and reflects the remaining optical input channel(s) (i.e., returned or express optical channel(s)) back through the same optical path to the first pigtail 20, as best shown in FIG. 1A and described hereinbefore. The returned optical input channel(s) propagates from the second port 33 to the third port 34 of the optical circulator 18 to provide an express output signal 48 from an optical fiber 50.

[0087] The dropped channel(s) passes through the other optical portion 16 of the ROADM 10. Specifically, the dropped channel(s) passes through a second bulk lens 52 (e.g., a Fourier lens), and then reflects off a second mirror 58 onto a second light dispersion element 54, which is similar to the first light dispersion element 24. The second diffraction grating 54 converges the dropped channel(s) into a collimated beam. A second collimator 60, which is similar to collimator 28, focuses the dispersed light 62 onto a second pigtail 64, which is optically connected to a second 3-port circulator 66. The second circulator 66 directs light from a first port 68 to a second port 69 and from the second port to a third port 70. A capillary tube 72, which may be formed of glass, is attached to one end of the second pigtail 64 such as by epoxying or collapsing the tube onto the second pigtail. The dropped channel(s) propagates from the second pigtail 64 to the output optical fiber 74, which is optically connected to the third port 70 of the second circulator 66, to provide an optical drop signal 76.

[0088] One or more optical channels 19 of an optical WDM add signal 21 may be added to the express/output signal 48 by providing to the optical fiber 78 the optical channels to be added. The added channel(s) 19 propagates from the optical fiber 78 to the second pigtail 64 through the second circulator 66.

[0089] The added channel(s) 19 (FIG. 3) of the optical signal 21 exits the pigtail 64 and passes through the second collimator 60 to the second diffraction grating 54, which separates spectrally the add channels of the collimated add signals 21 by dispersing or diffracting from (or through) the second diffraction grating 54. The diffraction grating 54 directs the separated light 80 to the second mirror 58 disposed in the second tier, similar to that described above in FIG. 1B for the optical portion 15. The mirror 58 reflects the separated light 80 to the second bulk lens 52, which focuses the separated light 80 onto the spatial light modulator 30. The spatial light modulator 30 reflects selected add channel(s) of the separated light 80 to the first bulk lens 28 and reflects the remaining add channel(s) away from the spatial light modulator 30, as shown by arrows 81 in FIG. 1A.

[0090] The selected add channel(s) 19 passes through the first bulk lens 28, which are then reflected off the first mirror 26 onto the first diffraction grating 24. The first diffraction grating 24 converges the selected add channel(s) onto the first collimator 22 which focuses the selected add channels to the first pigtail 22. The selected add channel(s) propagates from the first pigtail 20 to optical fiber 50, to thereby add the selected added channel(s) to the express/output signal 48. As will be described hereinafter, the add channels 19 and input channels 14 (FIG. 3) of the optical signal 12 at the same wavelengths reflect off the same portion of spatial light modulator 30, and therefore when an add channel is added to the express signal 48, the corresponding input channel 14 is dropped simultaneously.

[0091] As shown in FIG. 3, the spatial light modulator 30 comprises a micro-mirror device 82 having a two-dimensional array of micro-mirrors 84, which cover a surface of the micro-mirror device. The micro-mirrors 84 are generally square and typically 14-20 microns (μm) wide with 1 μm spaces between them. FIG. 4a illustrates a partial row of micro-mirrors 84 of the micro-mirror device 82, when the micro-mirrors are disposed in a first position to reflect the light back along the return path and provide the input channel 14 to the express output 50. FIG. 4b illustrates a partial row of micro-mirrors 84 when the micro-mirrors are disposed in a second position, and therefore drop the corresponding input channel 14 to the drop output 74, and add a selected add channel 19 to the express output 50, as will be described in greater detail hereinafter. The micro-mirrors may operate in a “digital” fashion. In other words, as the micro-mirrors either lie flat in a first position, as shown in FIG. 4a, or be tilted, flipped or rotated to a second position, as shown in FIG. 4b.

[0092] As described herein before, the positions of the mirrors, either flat or tilted, are described relative to the optical path wherein “flat” refers to the mirror surface positioned orthogonal to the light path, either coplanar in the first position or parallel as will be more fully described hereinafter. The micro-mirrors flip about an axis 85 parallel to the spectral axis 86, as shown in FIG. 5, wherein the spectral axis is defined by the direction the channels (λn) of the optical input signal 12 is spread by the diffraction grating 24. One will appreciate, however, that the micro-mirrors may flip about any axis, such as parallel to the spatial axis 88, at a 45 degrees angle to the spatial axis, or any desired angle.

[0093] Referring to FIG. 3, the micro-mirrors 84 are individually flipped between the first position and the second position in response to the control signal 87 provided by the controller 90 in accordance with the switching algorithm and the input command signal 46. The switching algorithm may provide a bit (or pixel) map indicative of the state (flat or tilted) of each of the micro-mirrors 84 of the array to return, drop and/or add the desired optical channel(s) 14 to provide the express/output signal 48 at optical fiber 50 (see FIG. 1), and thus requiring a bit map for each configuration of channels to be dropped and added. Alternatively, each group of mirrors 84, which reflect a respective optical channel 14, may be individually controlled by flipping the group of micro-mirrors to direct the channel along a desired optical path (i.e., return, drop or add).

[0094] One will appreciate that the ROADM 10 may be selectively configured or modified for any wavelength plan by simply modifying the software. For example, an ROADM for filtering a 50 GHz WDM optical signal may be modified to filter a 100 GHz or 25 GHz WDM optical signal by simply modifying or downloading a different switching algorithm, without modifying the hardware. In other words, any changes to the WDM signal structure (such as varying the spacing of the channels, the shapes of the light beams, and center wavelength of the light beams) may be accommodated within the ROADM by simply modifying statically or dynamically the switching algorithm (e.g., modifying the bit map).

[0095] As shown in FIGS. 1A and 4a, the micro-mirror device 82 is oriented to reflect the focused light 92 of the input signal 12 back through the first bulk lens 28 to the first pigtail 20, as indicated by arrows 94, to provide the express signal 48, and to reflect the focused light 98 away from the first optical portion, as indicated by arrows 81, when the micro-mirrors 84 are disposed in the first position. As shown in FIGS. 1A and 4b, the focused light 92 reflects away from the first bulk lens 28 to the second bulk lens 52, as indicated by arrows 96, and the focused light 98 when the micro-mirrors 84 are disposed in the second position. Further, when the micro-mirrors 84 are disposed in the second position, the same micro-mirrors may also reflect an add channel(s), as indicated by arrows 98, back through the first bulk lens 28 to the first pigtail 20, as indicated by arrows 94, which is added to the express/output signal 48. This “digital” mode of operation of the micro-mirrors advantageously eliminates the need for any type of feedback control for each of the micro-mirrors. The micro-mirrors are either “on” or “off” (i.e., first position or second position), respectively, and therefore, can be controlled by simple binary digital logic circuits.

[0096]
FIG. 3 further illustrates the outline of the optical channels 14 of the optical input signal 12 and add channels 19, which are dispersed off respective diffraction gratings 24, 54 and focused by bulk lens 28, 52 respectively, onto the array of micro-mirrors 84 of the micro-mirror device 82. Each optical channel 14, 19 is distinctly separated from other channels across the spectrum and have a generally circular cross-section, such that the optical channels do not substantially overlap spatially when focused onto the spatial light modulator 30. The optical channels have a circular cross-section to project as much of the beam as possible over a multitude of micro-mirrors 84, while keeping the optical channels separated by a predetermined spacing.

[0097] One will appreciate though that the diffraction gratings 24, 54 and bulk lens 28, 52 may be designed to reflect and focus any optical channel or group of optical channels with any desired cross-sectional geometry, such as elliptical, rectangular, square, polygonal, etc. Regardless of the cross-sectional geometry selected, the cross-sectional area of the channels 14 should illuminate a plurality of micro-mirrors 84, which effectively pixelates the optical channels. In an exemplary embodiment, the cross sectional area of the optical channels 14, 19 is generally circular in shape, whereby the width of the optical channel beam spans over approximately 11 micro-mirrors.

[0098] One will appreciate that while the spacing between the channels are predetermined, the spacing between may be non-uniform. For example, one grouping of channels may be spaced to correspond to a 100 GHz spacing, and another group of channels may be spaced to correspond to a 50 GHz spacing.

[0099]
FIG. 6 is illustrative of the position of the micro-mirrors 84 of the micro-mirror device 82 for dropping and/or adding the optical channels 14, 19 at λ3, λ5, λ6, λ10, for example. The outline of each channel 14, 19 is shown to provide a reference to visually locate the groups of tilted mirrors 100-103. As shown, the group of mirrors 100-103 associated with each respective optical channel at λ3, λ5, λ6, λ10 are tilted away from the return path to the second position, as indicated by the blackening of the micro-mirrors 84. Each group of tilted mirrors 100-103 provides a generally rectangular shape, but one will appreciate that any pattern or shape may be tilted to redirect an optical channel. In an exemplary embodiment, the group of micro-mirrors 100-103 reflects substantially all the light of each respective optical channel 14, 19 and does not reflect substantially light of any adjacent channels. The micro-mirrors 84 of the other optical channels 14, 19 at wavelengths of λ1, λ2, λ4, λ7, λ8, λ9, λ11-λN are flat (i.e., first position), as indicated by the white micro-mirrors, to reflect the light 92 back along the return path to the first pigtail 20, as described hereinbefore.

[0100] As shown, the optical input channel 14 of the input signal 12 and the add channel 19 of the add signal 21, which are centered at the same wavelength, are focused onto the same group of micro-mirrors. For example, both the optical input channel 14 at λ3 and optical add channel 19 at λ3 reflect off the same group of mirrors 100. Consequently, when the micro-mirrors are disposed in the tilted (or second position), the respective input channel 14 is dropped and the corresponding add channel 19 is added to the express/output signal 48, such that an add channel cannot overlap on an existing input channel on the express/output signal 48.

[0101]
FIG. 2 shows an alternative embodiment generally indicated as 10a to the ROADM 10 shown in FIGS. 1A and 1B, wherein the DMD device 30 is oriented so that the mirrors 84 pivot or tilt an axis 85 that is perpendicular to the spectral axis 86, rather than parallel to the spectral axis 86 as shown in FIG. 6. This embodiment is particularly important when implementing the chisel prism arrangement discussed below in relation to FIGS. 38-40. Similar elements in FIGS. 1A and 2 are labelled with similar reference numerals.

[0102]
FIG. 7A shows another exemplary embodiment of an ROADM generally indicated as 110 that is substantially similar to the ROADM 10 of FIG. 1A, and therefore, common components have the same reference numeral. In the ROADM 110, the circulators 18, 66 of FIG. 1 are replaced with a pair of pigtails 112, 114. Each pigtail 112, 114 has a glass capillary tube 116, 118 respectively attached to one end of the pigtails 112, 114. Each of the pigtails 112, 114 receives the optical channels reflected from the micro-mirror device of the spatial light modulator 30 back along a respective optical path. Specifically, the pigtail 112 receives the returned optical input channels 14 and the add channels 19 reflected back along the return optical path, and the pigtail 114 receives the dropped channels reflected back from the micro-mirror device of the spatial light modulator 30.

[0103] To accomplish these expected return paths, the spatial light modulator 30 cannot lie in the image plane of the first pigtail 20 along the spatial axis 88. These conditions can be established by ensuring that the lens systems 22 and 28 are astigmatic. In particular, the lens 28 may be a cylindricalized lens with its cylindrical axis parallel to the spatial axis 88. By tilting the spatial light modulator 30, the return path can be displaced to focus at the pigtail 112. Using similar component in the second optical portion 16, the drop channel can be focused onto pigtail 114 and the add channel 19 will be focused onto the express pigtail 112.

[0104]
FIGS. 7B and 7C show alternative embodiments generally indicated as 110′ and 110″ to the ROADM 110 shown in FIG. 7A, wherein the DMD device 30 is oriented so that the micro-mirrors 84 tilt on the axis 85 that is perpendicular to the spectral axis 86, rather than parallel to the spectral axis as shown in FIG. 6. (As shown, the spatial axis 85 runs into and out of the FIGS. 7B. These embodiments are particularly important when implementing the chisel prism arrangement discussed below in relation to FIGS. 38-40. Similar elements in FIGS. 7A, 7B and 7C are labelled with similar reference numerals, and in FIGS. 7B, 7C all the elements are shown for ease of understanding.

[0105]
FIG. 8 illustrates another embodiment of an ROADM generally indicated as 170 in accordance with the present invention, which is similar to the ROADM 10 of FIG. 1A, and therefore similar components have the same reference numerals. The ROADM 170 is substantially the same as the ROADM depicted in FIG. 1A, except the optical components of the ROADM 170 are disposed in one horizontal plane, rather than two tiers or planes, as shown in FIG. 1B. Rather than using a mirror 26, 58 (in FIGS. 1A and 2) to direct the dispersed light 44, 80 to the bulk lens 28, 52 and the spatial light modulator 30, the diffraction grating is tilted and rotated 90 degrees to directly disperse the light onto the bulk lens 28, 52 which focuses the light onto the spatial light modulator 30.

[0106] Functionally, the ROADM 170 of FIG. 8 and ROADM 10 of FIG. 1A are substantially similar. For illustrative purposes, however, the diffraction gratings 24, 54 and the bulk lens 28, 52 of the ROADM 170 are different so as to provide dispersed optical channels 14, 19 incident on the micro-mirror device 82 having a substantially elliptical cross-section, as shown in FIG. 9. As described, the diffraction gratings 24, 54 are rotated approximately 90 degrees such that the spectral axis 86 of the optical channels 16, 19 is parallel to the horizontal plane, and the micro-mirror device 82 in FIG. 9 is similarly rotated approximately 90 degrees such that the spectral axis 86 of the optical channels 16, 19 is perpendicular to the tilt axis 85 (FIG. 5) of the plurality of micro-mirrors 84.

[0107]
FIG. 10 is illustrative of the position of the micro-mirrors 84 of the micro-mirror device 82 for dropping and/or adding the optical channels 14 at λ3, λ5, λ6, λ10, for example. The outline of each channel 14, 19 is shown to provide a reference to visually locate the groups of tilted mirrors generally indicated as 100-103. As shown, the groups of micro-mirrors 100-103 associated with each respective optical channel at λ3, λ5, λ6, λ10, are tilted away from the return path to the second position, as indicated by the blackening of the micro-mirrors 84. Each group of tilted mirrors 100-103 provides a generally rectangular shape. In an exemplary embodiment, the group of micro-mirrors 100-103 reflects substantially all the light of each respective optical channel 14, 16 and does not reflect the light of any adjacent channels. The micro-mirrors 84 of the other optical channels 14, 16 at wavelengths of λ1, λ2, λ4, λ7, λ8, λ9, λ11-λN are flat (i.e., first position), as indicated by the white micro-mirrors, to reflect the light back along the return path to the first pigtail 22, as described hereinbefore.

[0108]
FIG. 11 illustrates a pair of micro-mirrors 84 of a typical micro-mirror device generally indicated as 200 manufactured by Texas Instruments, namely a digital micro-mirror device (DMD™). As a person skilled in the art would appreciate the micro-mirror device 200 is monolithically fabricated by CMOS-like processes over a CMOS memory 202. Each micro-mirror 84 includes an aluminum mirror 204, 16 μm square that can reflect light in one of two directions, depending on the state of the underlying memory cell 202. Rotation, flipping or tilting of the mirror 204 is accomplished through electrostatic attraction produced by voltage differences between the mirror and the underlying memory cell. With the memory cell 202 in the on (1) state, the mirror 204 rotates or tilts approximately +10 degrees. With the memory cell in the off (0) state, the mirror tilts approximately −10 degrees. As shown in FIG. 12, the micro-mirrors 84 flip about an axis 205. The micro-mirror device 82 shown in detail in FIGS. 3, 6, 9 and 10 is similar to the Digital Micromirror Device™ (DMD™) manufactured by Texas Instruments and described in the white paper entitled “Digital Light Processing™ for High-Brightness, High-Resolution Applications”, white paper entitled “Lifetime Estimates and Unique Failure Mechanisms of the Digital Micromirror Device (DMD)”, and news release dated September 1994 entitled “Digital Micromirror Display Delivering On Promises of ‘Brighter’ Future for Imaging Applications”, which are incorporated herein by reference.

[0109]

FIGS. 13

a
and 13b illustrate the orientation of the micro-mirror device 200 similar to that shown in FIG. 12, as used in the embodiment shown in FIG. 8. As shown, neither the first or second position (i.e., “on” or “off” state) of the micro-mirrors 84 is parallel to the base or substrate 210 of the micro-mirror device 200. (Compare the orientation to that shown in FIGS. 4a and 4b.) Consequently, as shown in FIG. 13a, the base 210 of the micro-mirror device 200 is mounted at a non-orthogonal angle a relative to the collimated light 80 shown in FIG. 8 to position the micro-mirrors 84, which are disposed at the first position, perpendicular to the collimated light 44, so that the light reflected off the micro-mirrors in the first position reflect substantially back through the return path, as indicated by arrows 94, to provide the express signal 48 at optical fiber 50. Consequently, the tilt angle of the mirror between the horizontal position and the first position (e.g., 10 degrees) is approximately equal to the angle α of the micro-mirror device. FIG. 13b is illustrative of the micro-mirror device 200 when the micro-mirrors 84 are disposed in the second position to drop an input channel 14 and/or add an add channel 19 to the express signal 48 at optical fiber 50.

[0110]
FIG. 14 illustrates the phase condition of the micro-mirrors in both states (i.e., State 1, State 2) for efficient reflection in either condition. In using the micro-mirror array device 200, it is important that the reflection from each micro-mirror 84 adds coherently in the far-field, so the angle a to which the micro-mirror device 200 is tilted has a very strong influence on the overall efficiency of the device.

[0111] In the micro-mirror device 200 shown in FIG. 14, the effective pixel pitch ρ is about 19.4 μm (see also FIG. 18), so for a mirror tilt angle β of 9.2 degrees, the array is effectively blazed for Littrow operation in the n=+2 order for the position indicated as Mirror State 1 in FIG. 14 (i.e., first position). For Mirror State 2, the incident angle γ on the micro-mirror device 200 is now 9.2 degrees and the exit angle ε from the array is 27.6 degrees. Using these numbers, the micro-mirror device is nearly blazed for fourth-order for mirrors in Mirror State 2 in FIG. 14.

[0112]
FIG. 15 graphically illustrates the micro-mirror device 200 wherein the micro-mirrors 84 are disposed in the retro-reflective operation (i.e., first position), such that the incident light reflects back along the return path, as indicated by arrows 202. For retro-reflective operation, the micro-mirror device 200 acts as a blazed grating held in a “Littrow” configuration, as shown in FIG. 1, with the blaze angle equal to the mirror tilt a (e.g., 10 degrees). The grating equation provides a relationship between the light beam angle of incidence, θ1; angle of reflection, θm; the pitch of the micro-mirror array; the mirror tilt; and the wavelength of the incident light. Because the wavelength varies across the micro-mirror array for parallel input beams, the angle of reflection of the beams varies across the apparatus. Introducing the micro-mirror device 200 at the focal plane 215 implements the critical device feature of providing separately addressable groups of mirrors to reflect different wavelength components of the beam. Because of the above reflection characteristics of the micro-mirror device 200, the beam is reflected as from a curved concave mirror surface, effectively with the micro-mirror device 200 in the focal plane 215. Consequently, when the micro-mirror device is oriented to retro-reflect at a wavelength hitting near the mirror center, wavelengths disposed away from the center are reflected toward the beam center as if the beam were reflected from a curved concave mirror. In other words, the micro-mirror device 200 reflects the incident light 212 reflecting off the central portion of the array of micro-mirrors directly back along the incident angle of the light, while the incident light 212 reflecting off the micro-mirrors disposed further away from the central portion of the array progressively direct the light inward at increasing angles of reflection, as indicated by 214.

[0113]
FIGS. 16A and 16B illustrate a technique to compensate for this diffraction effect introduced by the micro-mirror array, described hereinbefore.

[0114]
FIG. 16A illustrates the case where a grating order causes the shorter wavelength light to hit a part of the micro-mirror array 100 that is closer than the section illuminated by the longer wavelengths. In this case the Fourier lens 34 is placed at a distance “d” from the grating 30 that is shorter than focal length “f” of the Fourier lens. For example, the distance “d” may be approximately 71 mm and the focal length may be approximately 82 mm. It may be advantageous to use this configuration if package size is limited, as this configuration minimizes the overall length of the optical train.

[0115]
FIG. 16B illustrates the case where the grating order causes the longer wavelengths to hit a part of the micro-mirror array 100 that is closer than the section illuminated by the shorter wavelengths. In this case the Fourier lens is placed a distance “d” from the grating 30 that is longer than focal length “f” of the Fourier lens 34. This configuration may be advantageous to minimize the overall area illuminated by the dispersed spectrum on the micro-mirror array.

[0116]
FIG. 17A shows an embodiment of an ROADM generally indicated as 250, where the effective curvature of the micro-mirror device 200 may be compensated for using a “field correction” lens 222. The ROADM 250 is similar to the ROADM 10 of FIG. 1A, and therefore similar components have the same reference numeral. The ROADM 250 includes a field correction lens 222 disposed optically between respective bulk lens 28, 52 and the spatial light modulator 252, which includes the micro-mirror device 200. The “field correction” lens 222 respectively compensate for the channels reflecting off the spatial light modulator 252.

[0117]
FIG. 17B shows an alternative embodiment to that shown in FIG. 17A, wherein the DMD device 30 is oriented so that the micro-mirrors 84 tilt on the axis 85 that is perpendicular to the spectral axis 86. (As shown, the tilt axis 85 runs into and out of FIG. 17B.) This embodiment is particularly important when implementing the chisel prism arrangement discussed below in relation to FIGS. 38-40. Similar elements in FIGS. 17A and 17B are labelled with similar reference numerals.

[0118]
FIG. 18 shows the micro-mirror device 200 having the optical input channels 14 and/or the add channels 19 focused thereon such that the spectral axis 86 of the optical channels 14, 19 is parallel to the tilt axis 205 of the micro-mirrors. As shown, the micro-mirrors 84 flip about a diagonal axis 205, similar to that shown in FIGS. 12 and 18. This configuration is achieved by rotating the micro-mirror device 200 by about 45 degrees when compared to the configuration shown in FIG. 3.

[0119] Alternatively, the optical channels 14, 19 may be focused such that the spectral axis 86 of the channels are perpendicular to tilt axis 205 of the micro-mirrors similar to that shown in FIGS. 8 and 9. Further, one will appreciate that the orientation of the tilt axis 205 and the spectral axis 86 may be at any angle.

[0120]
FIGS. 19 and 20 show graphs of data of an ROADM similar to that shown in FIG. 1 having the micro-mirror device 200, in which the flipping of the micro-mirrors 84 is controlled by the above described switching algorithm.

[0121]
FIG. 19 shows a plurality of filter functions 260-263 at the drop port (at optical fiber 50) of a single dropped channel 261 and bands of dropped channels 260-263, wherein a “band” of channels is defined as a predetermined number of adjacent optical channels. Specifically, the filter function 260 corresponds to a single channel drop, the filter function 261 corresponds to a two channel drop, the filter function 262 corresponds to a three channel drop, and the filter function 263 corresponds to a four channel drop. While the widest band shown in FIG. 19 is four drop channels, one will recognize that any plurality of adjacent optical channels may define a band.

[0122]
FIG. 20 shows a graph of a plurality of filter functions 265-268 at the express/output port (at optical fiber 74) of a single dropped channel 265 and bands of dropped channels 265-268, wherein a “band” of channels is defined as a predetermined number of adjacent optical channels. Specifically, the filter function 265 corresponds to a single channel drop, the filter function 266 corresponds to a two channel drop, the filter function 267 corresponds to a three channel drop, and the filter function 268 corresponds to a four channel drop. While the widest express band shown in FIG. 20 is four channels, one will recognize that any plurality of adjacent optical channels may define a band.

[0123]
FIGS. 21 and 22 illustrate the effect of the ringing of micro-mirrors during their transition.

[0124] In the operation of the micro-mirror device 200 manufactured by Texas Instruments described above, all the micro-mirrors 84 of the device 200 release when any of the micro-mirrors are flipped from one position to the other. In other words, each of the mirrors will momentarily tilt towards the horizontal position upon a position change of any of the micro-mirrors. Consequently, this momentary tilt of the micro-mirrors 84 creates a ringing or flicker in the light reflecting off the micro-mirrors. To reduce or eliminate the effect of the ringing of the light during the transition of the micro-mirrors 84, the light is focused tightly on the micro-mirror device 200.

[0125] Both FIGS. 21 and 22 show an incident light beam 310, 312, respectively, reflecting off a mirror surface at different focal lengths. The light beam 310 of FIG. 22 has a relatively short focal length, and therefore has a relatively wide beam width. When the micro-mirror surface 314 momentarily tilts or rings a predetermined angle τ, the reflected beam 316, shown in dashed lines, reflects off the mirror surface at the angle τ. The shaded portion 318 is illustrative of the lost light due to the momentary ringing, which represents a relatively small portion of the incident light 310. In contrast, the light beam 312 of FIG. 22 has a relatively long focal length, and therefore has a relatively narrow beam width. When the micro-mirror surface 314 momentarily tilts or rings the predetermined angle τ, the reflected beam 320, shown in dashed lines, reflects off the mirror surface at the angle τ. The shaded portion 322 is illustrative of the lost light due to the momentary ringing, which represents a greater portion of the incident light 312, than the lost light of the incident light of FIG. 21. Consequently, the sensitivity of the momentary tilt of the micro-mirrors is minimized by tightly focusing the optical channels on the micro-mirror device 200. Advantageously, tightly focusing of the optical channels also reduces the tilt sensitivity of the micro-mirror device due to other factors, such as thermal changes, shock and vibration.

[0126] FIGS. 23-26b show an embodiment of an ROADM generally indicated as 350 that is similar to the ROADM 10 of FIG. 1A having a micro-mirror device 200 of the spatial light modulator 300, and therefore, similar components have the same reference numerals. The ROADM 350 directs both the optical input signal 12 and the add signal 21 through a set of common optical components. FIG. 24 shows a side elevational view of the input optical components 18, 20 and the common optical components 22, 24, 26, 28, 300 to better understand the ROADM 350.

[0127] In FIG. 24, the optical components are disposed in two tiers or horizontal planes. Specifically, the first three-port circulator 18, the first pigtail 20, the collimator 22 and the diffraction grating 24 are disposed in a first tier or horizontal plane. As would be appreciated by a person skilled in the art, the second circulator 66 and the second pigtail 64 are disposed in the first tier. The mirror 26, the bulk lens 28 and the spatial light modulator 300 are disposed in the second tier or horizontal plane. Further, mirrors 352, 354 and lens 356, 358 of FIG. 23 are disposed in the second tier.

[0128] In FIGS. 23 and 24, the first circulator 18 directs the input signal 12 from the optical fiber 38 to the first pigtail 20. The input signal 12 exits the first pigtail (into free space) and passes through the collimator 22, which collimates the input signal. The collimated input signal 40 is incident on the diffraction grating 24, which separates spatially the optical input channels 19 of the collimated input signal 40 by diffracting or dispersing the light from the diffraction grating. As best shown in FIG. 24, the diffraction grating 24 directs the separated light 44 to the mirror 26 disposed in the second tier. The mirror 26 reflects the separated light 44 to the bulk lens 28 (e.g., a Fourier lens), which focuses the separated light onto the micro-mirror device 200 of the spatial light modulator 300, as shown in FIG. 25. In response to a switching algorithm and the input command 46, the micro-mirror device 200 of the spatial light modulator 300 selectively reflects each optical input channel 14 in one of two optical paths 360, 362 away from the bulk lens 28 through a pair of respective focusing lens 356, 358 to corresponding mirrors 352, 354.

[0129] As will be described in greater detail hereinafter, the input channels directed along the optical path 360 reflect back to the first pigtail 20 to provide the express/output signal 48 at optical fiber 50, while the input channels directed along the optical path 362 are redirected to the second optical pigtail 64 to provide the drop signal 76 at optical fiber 74.

[0130] Similarly, the optical add channels 19 of the add signal 21 propagates through the common optical components to the micro-mirror device 200 of the spatial light modulator 300, which selectively reflects each add channel 19 in one of the two optical paths, as described hereinbefore. The add channels directed along the optical path 360 reflect back to the first pigtail 20 to be added to the express/output signal 48 at optical fiber 50, while the add channels directed along the optical path 362 are redirected to the second optical pigtail 64 to be added to the drop signal 76 at optical fiber 74.

[0131]
FIG. 25 shows the micro-mirror device 200 having the outline of the optical input channels 14 of the optical input signal 12 and add channels 19 of the optical add signal 21, which are dispersed off the diffraction grating 24 and focused by the bulk lens 28 onto the array of micro-mirrors 84 of the micro-mirror device 200. The input and add channels 14, 19 are spectrally separated and have a generally circular cross-section, such that the optical channels 14, 19 of each optical signal 12, 21 do not substantially overlap spatially when focused onto the micro-mirror device 200. Further, ends or edges of the input channels 14 and the add channels 19 are positioned (e.g., spatially spaced) such that the input channels 14 and the add channels 19 are initially focused onto different groups of mirrors. In other words, the spectrum of the input channels and the spectrum of the add channels are spaced spatially along the spatial axis 88.

[0132] Further, FIG. 25 also shows the position of the micro-mirrors 84 of the micro-mirror device 200 for dropping and/or adding the optical channels 14, 19 at λ2 and λ5, for example. The outline of each channel 14, 19 is shown to provide a reference to visually locate the groups of tilted mirrors 370 and 372. As shown, the group of mirrors 370 and 372 associated with each respective optical channel at λ2 and λ5 are tilted away from the incident light 92 to the second position (see FIG. 26), as indicated by the blackening of the micro-mirrors 84 to the mirror 354. Each group of tilted mirrors 370, 372 provides a generally rectangular shape. In an exemplary embodiment, the group of micro-mirrors 370 and 372 reflects substantially all the light of each respective optical channel 14, 19 and reflects substantially no light of any adjacent channels. The distance between the micro-mirror device and the mirror 354 is approximately two times the focal length (i.e., 2f), which causes the input channel 14 and add channel 19 to switch spatially such that the input channel 14 reflects off the micro-mirror device 200 to the second pigtail 64 to drop the input channel, while the add channel 19 reflects off the micro-mirror device to the first pigtail 20 to be added to the express signal 48.

[0133] Conversely, the micro-mirrors 84 of the other optical channels 14, 19 at wavelengths of λ1, λ3, λ4, λ6-λN are disposed in the first position, as indicated by the white micro-mirrors, to reflect the light 92 along the optical path 360 to the mirror 352. The distance between the micro-mirror device and the mirror 352 is approximately four times the focal length (i.e., 4f), which causes the input channel 14 and add channel 19 to return to the same group of micro-mirrors 84 such that the input channel 14 reflects off the micro-mirror device 200 back to the first pigtail 20 to provide the express signal 48 at optical fiber 50, while the add channel 19 reflects off the micro-mirror device back to the second pigtail 64 to drop the add channel 19.

[0134] As shown in FIG. 26a, the micro-mirror device 200 is oriented to reflect the focused light 92 of selected input channels 14 and/or add channels 19 to mirror 354, as indicated by arrows 362, which are then reflected back along corresponding optical paths 376, as described hereinbefore, when the micro-mirrors 84 are disposed in the second position.

[0135] As shown in FIG. 26b, the focused light 92 of selected input channels 14 and/or add channels 19 reflects off the micro-mirror device 200 to mirror 352, as indicated by arrows 360, which are then reflected back along the same optical paths, as described hereinbefore, when the micro-mirrors 84 are disposed in the first position. It should be realized that with astigmatic optics, mirrors 352, 354 could be tilted such that the return beams are displaced from the input pigtails 20, 64 and can be received by a second set of output pigtails eliminating the need for circulators 18, 66.

[0136]
FIG. 27 shows an exemplary embodiment of an ROADM 400 that is similar to the ROADM 10 of FIG. 1A, and therefore, similar components have the same reference numerals. The ROADM 400 directs both the optical input signal 12 and the add signal 21 through a common set of optical components. The optical components are disposed in two tiers or horizontal planes similar to the embodiments discussed hereinbefore. Specifically, the three-port circulators 18, 66, the pigtails 20, 64, the collimator 22 and the diffraction grating 24 are disposed in a first tier or horizontal plane. The mirror 26, the bulk lens 28 and the spatial light modulator 30 are disposed in the second tier or horizontal plane, which is parallel to the first horizontal plane. Further, the ROADM 400 has a mirror 402 and a lens 404 disposed in the second tier.

[0137] In operation, the first circulator 18 directs the input signal 12 from the optical fiber 38 to the first pigtail 20. The input signal 12 exits the first pigtail (into free space) and passes through the collimator 22, which collimates the input signal. The collimated input signal 40 is incident on the diffraction grating 24, which separates spatially the optical input channels 14 of the collimated input signal 40 by diffracting or dispersing the light from the diffraction grating. The diffraction grating 24 directs the separated light 44 to the mirror 26 disposed in the second tier. The mirror 26 reflects the separated light 44 to the bulk lens 28 (e.g., a Fourier lens), which focuses the separated light onto the micro-mirror device 82 of the spatial light modulator 30, as shown in FIG. 2. In response to a switching algorithm and the input command 46, the spatial light modulator 300 selectively reflects each input channel through the lens 404 to the mirror 402, or back through the common set of optical components to the pigtail 20.

[0138] The micro-mirrors 84 of the spatial light modulator 30 are tilted to a first position to reflect selected input channels 14 of the input signal 12 back along the return path 94 to provide the output/express signal 48 at optical fiber 50. The micro-mirrors 84 of the spatial light modulator 30 are tilted to a second position to reflect the remaining input channels (i.e., dropped input channels) through the lens 404 to the mirror 402. The mirror 402 is tilted such that the dropped input channels are reflected along a slightly different path, as indicated by arrows 406 than the return path 94. The dropped input channels propagate to the second pigtail 72, as indicated by arrows 406, to provide the drop signal 76 at the optical fiber 74.

[0139] Similarly, the optical add channels 19 of the add signal 21 propagate through the common set of optical components to the micro-mirror device 82 of the spatial light modulator 30, which selectively reflects each add channel 19 in one of the two optical paths, as described above. The add channels directed along the optical return path 94 reflect back to the first pigtail 20 to be added to the express/output signal 48 at optical fiber 50, while the add channels directed along the optical path 410 are redirected to the mirror 402 and reflected back to the second optical pigtail 64 along the optical path 406 to be added to the drop signal 76 at optical fiber 74.

[0140]
FIG. 28 shows an exemplary embodiment of an ROADM generally indicated as 500 that is similar to the ROADM 10 of FIG. 1A, and therefore, similar components have the same reference numerals. The ROADM 500 operates similarly to the ROADM 10 except the drop signal 76 is provided at the optical fiber 50 and the express/output signal 48 is provided at the optical fiber 74. To accomplish this, the ROADM 500 has a mirror 502 and a focusing lens 504 disposed in the second tier, as described hereinbefore, to reflect selected add channels back to the second pigtail 64, which is then added to the express signal 48.

[0141] In operation, the optical input channels 14 of the input signal 12 propagate through the first optical portion 15 to the micro-mirror device 82 of the spatial light modulator 30, which selectively reflects each input channel 14 in one of the two optical paths. When the micro-mirrors 84 of the spatial light modulator 30 are tilted to a first position, selected input channels 14 of the input signal 12 reflect back along the return path 94 to provide the drop signal 76 at optical fiber 50. When the micro-mirrors 84 of the spatial light modulator 30 are tilted to a second position, the remaining input channels (i.e., express channels) reflect along the optical path indicated by arrows 96 to the second pigtail 64 to provide the express signal 48 at the optical fiber 74.

[0142] Similarly, the optical add channels 19 of the add signal 21 propagate through the second optical portion 16 to the micro-mirror device 82 of the spatial light modulator 30, which selectively reflects each add channel 19 in one of the two optical paths. When the micro-mirrors 84 of the spatial light modulator 30 are tilted to the first position, selected input channels 14 of the input signal 12 reflect through the lens 504 to the mirror 502, as indicated by arrow 506. The mirror 502 then reflects the selected add channels 19 along the optical path 505 to the second optical portion 16 along the optical path 96 to the second pigtail 64. The add channels 19 then propagate to the optical fiber 74 to add the add channels to the express signal 48. When the micro-mirrors 84 of the spatial light modulator 30 are tilted to the second position, the remaining input channels 14 of the input signal 12 reflect along the optical path 94 to provide the drop signal 76 at optical fiber 50.

[0143]
FIG. 29 shows an embodiment of an ROADM generally indicated as 600 that is similar to the ROADM 10 of FIG. 1A, and therefore, similar components have the same reference numerals. The ROADM 600 operates similarly to the ROADM 10 except the drop signal 76 is provided at the optical fiber 50 and the express/output signal 48 is provided at the optical fiber 74. Further, the add signal 21 is provided through a third optical portion 615, which is substantially similar to the first and second optical portions 15, 16.

[0144] In operation, the optical input channels 14 of the add signal 21 at the third pigtail 620 propagate through the first optical portion 15 to the micro-mirror device 82 of the spatial light modulator 30, which selectively reflects each input channel 14 in one of the two optical paths. When the micro-mirrors 84 of the spatial light modulator 30 are tilted to a first position, selected input channels 14 of the input signal 12 reflect back along the return path 94 to provide the drop signal 76 at optical fiber 50. When the micro-mirrors 84 of the spatial light modulator 30 are tilted to a second position, the remaining input channels (i.e., express channels) are reflected along the optical path indicated by arrows 96 to the second pigtail 64 to provide the express signal 48 at the optical fiber 74.

[0145] Similarly, the optical add channels 19 of the add signal 21 propagate through the third optical portion 616 to the micro-mirror device 82 of the spatial light modulator 30, which selectively reflects each add channel 19 in one of the two optical paths. When the micro-mirrors 84 of the spatial light modulator 30 are tilted to the first position, selected input channels 14 of the input signal 12 propagating along the optical path 692 reflect along the optical path 96 through the second optical portion 16 to the second optical pigtail 64. The add channels 19 then propagate to the optical fiber 74 to add the add channels to the express signal 48. When the micro-mirrors 84 of the spatial light modulator 30 are tilted to the second position, the remaining input channels 14 of the input signal 12 reflect along the optical path 94 to provide the drop signal 76 at optical fiber 50.

[0146]
FIG. 30 shows an embodiment of an ROADM generally indicated as 700 that is similar to the ROADM 170 of FIG. 8, and therefore, similar components have the same reference numerals. The ROADM 700 operates similarly to the ROADM 170 except the first diffraction gratings 24, 54 are rotated 90 degrees so that the input channels 14 of input signal 12 and add channels 19 of add signals 21 are dispersed on micro-mirror device 82 of the spatial light modulator 30 such that the spectral axis 86 of optical channels 14, 19 are perpendicular to the horizontal plane that the optical components of the ROADM 700 are disposed. Further, the diffraction grating 54 is tilted at a predetermined angle to reflect the optical channels 14, 19 in an optical path 62 (upward as shown in FIG. 30) to equalize the path length of each of the optical channels through the ROADM 700.

[0147] While the present invention has shown and described embodiments of the present invention as having a combined add function and drop function, the present invention also contemplates optical devices that function separately as an optical dropping device or an optical add device.

[0148] For example, FIG. 31 illustrates an optical drop device 800, which is substantially the same as the ROADM 10 of FIG. 1 except the second circulator 66 (see FIG. 1) is not included. One will recognize that the drop device 800 may also function as an optical add device by simply providing an add signal 21 to the second pigtail 64, rather than the drop signal 76.

[0149] Discrete optical drop devices 800a, 800b, an optical processing device 802 and an optical add device 801 may be used in combination to provide distinct advantageous. For example, FIG. 32 shows a configuration generally indicated as 805 having a pair of concatenated drop devices 800a, 800b for dropping the optical input channels Drop1, Drop2 and may be necessary to provide the desired extinction of the selected drop channel. Further, the add device 801 and drop device 800b may be optically separated to enable the optical processing device 802 (e.g., conditioning and filtering) to process one particular channel or group of channels, and not another. In FIG. 32, the optical processing device 802, such as a dynamic gain equalization filter (DGEF), may be optically disposed between the drop device 800b and the add device 801.

[0150] While the embodiments of the present invention described hereinabove illustrate a single ROADM using a set of optical components, it is also envisioned to provide an embodiment including a plurality of ROADMs that uses a substantial number of common optical components, including the spatial light modulator.

[0151] For example, FIG. 33 shows an embodiment of an ROADM generally indicated as 900, which is substantially the same as the ROADM 10 in FIG. 1A having a spatial light modulator 300 in FIG. 11. Common components between the embodiments have the same reference numerals. The ROADM 900 provides a pair of ROADMs (i.e., OADM1, OADM2), each of which use substantially all the same optical components, namely the collimating lens 22, 60, the mirrors 26, 58, the diffraction gratings 24, 54, the bulk lens 28, 52 and the spatial light modulator 300. The first ROADM (OADM1) is substantially the same as the ROADM 10 of FIG. 10. The second ROADM (OADM2) is provided by adding a complementary set of input optical components 981, 983. The input optical components 81, 83 of OADM1 and the input optical components 981, 983 of OADM2 are the same, and therefore have the last two numerals of the input optical components 981, 983 of OADM2 are the same as those of the similar components 81, 83 of the OADM1.

[0152] To provide a plurality of ROADMs (ROADM1, ROADM2) using similar components, each ROADM uses a different portion of the micro-mirror device 200, as shown in FIG. 34, which is accomplished by displacing spatially the ends 36, 72, 936, 972 of the pigtails 20, 64, 920, 964 of the ROADMs. As shown, the input channels and output channels of each ROADM are displaced a predetermined distance in the spatial axis 88. Similar to that described hereinabove, the groups 370, 372 of shaded micro-mirrors 84 drop and/or add optical channels at λ2 and λ1 of both ROADMs (OADM1, OADM2). One will recognize that while the same optical channels are dropped and/or added in the embodiment shown in FIG. 34, the micro-mirrors 84 may be tilted to individually drop and/or add different optical channels 14, 19, 914, 919 as shown in FIG. 35.

[0153]
FIG. 35 shows an embodiment of the present invention similar to that shown in FIG. 34, wherein the embodiment has N number of ROADMs (OADM1-OADMN) using substantially the same optical components, as described hereinabove.

[0154] By configuring such a plurality of ROADMs in a sequence such that the dropped channel of ROAOM1 are fed to a second ROADM2, the various wavelength channels can be routed to multiple optical fibers.

[0155] While the micro-mirrors 84 may switch discretely from the first position to the second position, as described hereinabove, the micro-mirrors may move continuously (in an “analog” mode) or in discrete steps between the first position and second position. In the “analog” mode of operation the micro-mirrors can be tilted in a continuous range of angles. The ability to control the angle of each individual mirror has the added benefit of much more attenuation resolution than in the digital control case. In the “digital” mode, the attenuation step resolution is determined by the number of micro-mirrors 84 illuminated by each channel. In the “analog” mode, each mirror can be tilted slightly allowing fully continuous attenuation of the return beam. Alternatively, some combination of micro-mirrors may be switched at a predetermined or selected pulse width modulation to attenuate the optical channel or band.

[0156]
FIG. 36A shows a collimator assembly generally indicated as 2000. The collimator assembly 2000 may be used in place of the arrangement of either the capillary tube 36 and the collimator lens 22, the capillary tube 72 and the collimator lens 60, the capillary tube 636 and the collimator lens 622, the capillary tube 936 and the collimator lens 22, the capillary tube 972 and the collimator lens 60, or any combination thereof, in any one or more of the embodiments described above.

[0157] The collimator assembly has a lens subassembly 2002 and a fiber array holder subassembly 2003. The lens subassembly 2002 includes a lens housing 2004 for containing a floating lens cup 2006, a lens 2008, a polymer washer 2010, a spring 2012, a washer 2014 and a C-ring clip 2016. The lens housing 2004 also has two adjustment wedge slots 2018, 2020. The fiber array holder subassembly 2003 includes a fiber V-groove array holder 2022, a subassembly cap 2024 and a clocking pin 2026. The fiber 2028 is arranged in the fiber array holder subassembly 2003. The V-groove array holder 2022 is designed to place the one or more fibers/pigtails 2028 on the nominal origin of an optical/mechanical access. The clocking pin 2026 sets the angle of a semi-kinematic mount, and therefore the angle of the one or more fibers 2028 relative to the nominal optical and/or mechanical access.

[0158]
FIG. 36B shows the fiber array holder subassembly 2003 having a fiber V-groove subassembly cavity generally indicated as 2030 for mounting a fiber V-groove subassembly generally indicated as 2032. The fiber V-groove subassembly 2032 is semi-kinematically mounted and maintained in the fiber V-groove subassembly cavity 2030 by three retention springs 2034, 2036, 2038 and the subassembly cap 2024. For example, the mounting of the fiber V-groove subassembly 2032 is characterized as follows: (1) the precision substrate of fiber V-groove array is arranged in the fiber V-groove subassembly cavity 2030; (2) The retention spring 2036 restrains the fiber V-groove subassembly 2032 in the X direction; (3) the two retention springs 2034, 2038 constrain the fiber V-groove subassembly 2032 in the Y and Z directions; and (4) the subassembly cap 2024 is welded to the fiber V-groove array holder 2022 to complete retention of the fiber V-groove subassembly 2032 in a semi-kinematic mount.

[0159]
FIGS. 36C and D show, by way of example, the fiber array holder subassembly 2003 having a fiber V-groove subassembly body 2040 having a V-groove 2042 arranged therein for receiving the one or more fibers 2028a, 2028b. The fiber V-groove subassembly 2032 also has a fiber V-groove subassembly cap 2048 for enclosing and holding the fibers 2028a, 2028b in the V-groove 2042, as best shown in FIG. 36D.

[0160]
FIG. 36E shows a cut away view of a complete collimator assembly generally indicated as 2000. In the complete collimator assembly 2000, the lens subassembly 2002 is welded to the fiber array holder subassembly 2003. The fully welded collimator assembly 2000 is mounted on a mounting or focusing tool or configuration (not shown) for providing coarse optical/mechanical alignment. Control of the basic mechanics of the mounting configuration is typically in the range of about +/−25 microns and about 0.1°. However, initial and final positioning of other optical components on the mounting configuration require a coarse adjustment of the actual access of the collimator assembly 2000 to match with the optical access of the other components. The coarse adjustment of the collimator optical access is achieved by moving the lens 2008 in the X and Y directions while maintaining a fixed position of the fiber array holder subassembly 2003. Tuning wedges 2050, 2052 are used to move the lens floating cap 2006 in the X and Y directions to provide coarse lens adjustment to about +/−500 microns, as discussed below. However, with use of a piezoelectric impact tool fine displacement with a resolution that is a small fraction of about a micron may be achievable.

[0161] The collimator assembly is assembled as follows:

[0162] First, the lens subassembly 2002 is assembled. The lens 2008 sits in the floating lens cup 2006. The interfaces between the floating lens cup 2006 and the precision tube of the lens housing 2004 are precision ground. The polymer washer 2014 restrains the lens 2008 in the floating lens cup 2006 under force from the compression spring 2012. The washer 2014 and the C-ring clip 2016 are used to provide a reaction surface so that the compression spring 2012 can hold the floating lens cup 2006 against the interface with the inner surface of the subassembly tube of the lens housing 2004. The lens housing has notches 2018, 2020 to accommodate use of the tuning wedges 2050, 2052. As discussed below, the tuning wedge 2050, 2052 may be inserted into the notches 2018, 2020 so as to react against the surface in order to push the floating lens cup 2006 in adjustment relative to the mechanical access of the tube of the lens housing 2004.

[0163] Next, the array holder 2022 is fit into the precision tube of the lens housing 2004 for a focus adjustment and weld. To accomplish the collimation adjustment, the array holder 2022 and the tube of the lens housing 2004 are installed into the focusing tool (not shown) along with the lens subassembly 2002. The lens subassembly 2002 is aligned and adjusted for optimum collimation. The array holder 2022 is welded to the precision tube of the lens housing 2004. At this point, the lens subassembly 2004 and the fiber array holder subassembly 2003 are a matched pair.

[0164] In operation, the collimator assembly 2000 will interface optical signals on an optical fiber with the optics of another optical device by creating a parameter-matched, free space beam; collect a returning beam from the other optical device and re-introduce it into the optical fiber with minimal loss; interface the collimator on the other optical device chassis with accuracy of about +/−25 microns and about +/−1 mR; point the free space beam into the optical access of the other optical device with a coarse adjustment of about +/−2 mR and a fine adjustment of about +/−0.002 mR. Moreover, adhesives are not in the optical path and are not desired for connecting any of the precisely aligned optical/mechanical components.

[0165]
FIG. 37 shows an embodiment of an ROADM generally indicated as 1000 having optical portions 15, 16 with one or more optical PDL devices 1002, 1004, 1006, 1008 for minimizing polarization dependence loss (PDL). The one or more optical PDL devices 1002, 1008 are arranged between the capillary tube 36 and the grating 24, while the one or more optical PDL devices 1004, 1006 are arranged between the grating 24 and the spatial light modulator 30.

[0166] The optical PDL device 1002 may include a polarization splitter for splitting each channel into its pair of polarized light beams and a rotator for rotating one of the polarized light beams of each optical channel. The optical PDL device 1008 may include a rotator for rotating one of the previously rotated and polarized light beams of each optical channel and a polarization splitter for combining the pair of polarized light beams of each channel.

[0167] The one or more optical devices 1002, 1004, 1006, 1008 may be incorporated in any of the embodiments shown and described above, including but not limited to the embodiments shown in FIGS. 1, 1A, 7A, 7B, 7C, 8, 17, 17A, 23, 27-31 and 33.

[0168] In effect, as a person skilled in the art will appreciate, a diffraction grating such as the optical elements 42, 54 has a predetermined polarization dependence loss (PDL) associated therewith. The PDL of the diffraction grating 24 is dependent on the geometry of the etched grooves 42 of the grating. Consequently, means to mitigate PDL may be desired. The λ/4 plate between the spatial light modulator 30 and the diffraction grating(s) 24, 54 (before or after the bulk lens 28, 52) mitigates the PDL for any of the embodiments described hereinbefore. The fast axis of the λ/4 plate is aligned to be approximately 45 degrees to the direction or axis of the lines 42 of the diffraction grating 24. The mirror is angled to reflect the separated channels back through the λ/4 plate to the diffraction grating. In the first pass through the λ/4 plate, the λ/4 plate circularly polarizes the separated light. When the light passes through the λ/4 plate again, the light is linearly polarized to effectively rotate the polarization of the separated channels by 90 degrees. Effectively, the λ/4 plate averages the polarization of the light to reduce or eliminate the PDL. One will appreciate that the λ/4 plate may not be necessary if the diffraction grating has low polarization dependencies, or other PDL compensating techniques are used that are known now or developed in the future.

[0169] As shown and described herein, the polarized light beams may have a generally circular cross-section and are imaged at separate and distinct locations on the spatial light modulator 30, such that the polarized light beams of the optical channels do not substantially overlap spatially when focused onto the spatial light modulator, as shown, for example, in FIGS. 6, 18, 25, 34 and 35.

[0170]
FIG. 38 shows an ROADM generally indicated as 1600 similar to that shown above, except that the micro-mirror device is oriented such that the tilt axis 85 is perpendicular to the spectral axis 86. The ROADM 1600 has a chisel prism 1602 arranged in relation to the spatial light modulator 30, a set of optical components 1604, a retromirror 1605 and a complimentary set of optical components 1606. The underlying configuration of the ROADM 1600 may be implemented in any of the embodiments show and described in relation to FIGS. 2, 7B, 7C and 17A described above in which the pivot or tilt axis of the mirrors of the DVD device is perpendicular to the spectral axis of the channels projected on the DVD device.

[0171] The set of optical components 1604 and the complimentary set of optical components 1606 are similar to the optical portions 15, 16 shown and described herein. For example, see FIG. 1A. The spatial light modulator 30 is shown and described herein as the well known DMD device. The chisel prism 1602 has multiple faces, including a front face 1602a, first and second beveled front faces 1602b, 1602c, a rear face 1602d and a bottom face generally indicated by 1602e. (It is noted that in embodiments having no retroflector or third optical path only two front faces are used, and in embodiments having a retroflector all three front faces are used.) Light from the set of optical components 1604 and the complimentary set of optical components 1606 passes through the chisel prism 1602, reflects off the spatial light modulator, and passes back through the chisel prism 1602.

[0172] The chisel prism design described herein addresses a problem in the optical art when using micro-mirror devices. The problem is the ability to send a collimated beam out to a reflective object and return it in manner that is insensitive to the exact angular placement of the reflective object. Because a light beam is typically collimated and spread out over a relatively large number of micro-mirrors, any overall tilt of the array causes the returned beam to “miss” the optical component, such as a pigtail, intended to receive the same.

[0173] The present invention provides a way to reduce the tilt sensitivity by using a classical optical design that certain combinations of reflective surfaces stabilize the reflected beam angle with respect to angular placement of the reflector. Examples of the classical optical design include a corner-cube (which stabilize both pitch and yaw angular errors) or a dihedral prism (which stabilize only one angular axis.).

[0174] One advantage of the configuration of the present invention is that it removes the tilt sensitivity of the optical system (which may comprise many elements besides a simple collimating lens such as element 26 shown and described above) leading up to the retro-reflective spatial light modulator 30. This configuration allows large beam sizes on the spatial light modulator without the severe angular alignment sensitivities that would normally be seen.

[0175] Patent application Ser. No. 10/115,647 (CC-0461), which is hereby incorporated by reference, shows and describes the basic principal of these highly stable reflective elements in which all the surfaces of the objects being stable relative to one another, while the overall assembly of the surfaces may be tilted without causing a deviation in reflected angle of the beam that is large compared to the divergence angle of the input beam.

[0176]
FIG. 39 illustrates a schematic diagram of an ROADM generally indicated as 1700 that provides improved sensitivity to tilt, alignment, shock, temperature variations and packaging profile, which incorporates such a tilt insensitive reflective assembly. The scope of the invention is intended to include using the chisum prism technology described herein in any one or more of the embodiments described herein.

[0177] Similar to the embodiments described hereinbefore, and by way of example, the ROADM 1700 includes a first set of optical components having a dual fiber pigtail 1702 (circulator free operation), the collimating lens 26, a bulk diffraction grating 42, a Fourier lens 34, a 1/4λ plate 35, a reflector 26 and a spatial light modulator 1730 (similar to that shown above). The dual fiber pigtail 1702 includes a transmit fiber 1702a and a receive fiber 1702b. The first set of optical components typically provide a first optical input signal having one or more optical bands or channels on the receive fiber 1702b, as well as providing an optical output signal on the transmit fiber 1702b.

[0178] Similar to the embodiments described hereinbefore, the ROADM 1700 also includes a complimentary set of optical components 1703 for providing a second optical input signal, which is typically an optical signal to be added to the first optical input signal.

[0179] The ROADM 1700 also includes a chisel prism 1704 having multiple internally reflective surfaces, including a top surface, a back surface, as well as transmissive surfaces including two front surfaces and a bottom surface, similar to that shown in FIG. 38. The micro-mirror device 1730 is placed normal to the bottom surface, as shown. In operation, the chisel prism 1704 reflects the first optical input signal from the first set of optical components and the second optical input signal from the complimentary set of optical components 1703 both to the spatial light modulator 1730, and reflects the optical output signal back to the first set of optical components.

[0180] The chisel prism 1704 decreases the sensitivity of the optical filter to angular tilts of the optics. The insensitivity to tilt provides a more rugged and robust device to shock vibration and temperature changes. Further, the chisel prism 1704 provides greater tolerance in the alignment and assembly of the optical filter 1700, as well as reduces the packaging profile of the filter. To compensate for phase delay associated with each of the total internal reflection of the reflective surfaces of the prism (which will be described in greater detail hereinafter), a λ/9 wave plate 1708 is optically disposed between the prism 1704 and the λ/4 wave plate 35. An optical wedge or lens 1710 is optically disposed between the λ/4 wave plate 35 and the diffraction grating 30 for directing the output beam from the micro-mirror device 1730 to the receive pigtail 1702a of the dual fiber pigtail 1702b. The optical wedge or lens 1710 compensates for pigtail and prism tolerances. The scope of the invention is intended to cover rmbodiments in which the optical wegde 1710 is arranged parallel or oblique to the front surface of the wedge 1704. Moreover, as shown, these components are only arranged in relation to one front surface; however, as a person skilled in the art would appreciate, these optical components would typically be arranged in relation to any one or more front surfaces shown in FIG. 39, as well as the front surfaces in the other chisel prism embodiments shown ad described herein.

[0181] The optical device 1700 further includes a telescope 1712 having a pair of cylindrical lens that are spaced a desired focal length. The telescope 1712 functions as a spatial beam expander that expands the input beam (approximately two times) in the spectral plane to spread the collimated beam onto a greater number of lines of the diffraction grating. The telescope 1712 may be calibrated to provide the desired degree of beam expansion. The telescope advantageously provides the proper optical resolution, permits the package thickness to be relatively small, and adds design flexibility.

[0182] A folding mirror 1714 is disposed optically between the Fourier lens 34 and the λ/4 wave plate 35 to reduce the packaging size of the optical filter 1700.

[0183]
FIG. 40 shows a practical embodiment of a tilt-insensitive reflective assembly 1800 comprising a specially shaped prism 1804 (referred as the “chisel prism”) arranged in relation to the micro-mirror device 1830, a set of optical components as shown, a compliment set of optical components generally indicated as 1805, as well as a retroreflector 1803 consistent with that discussed above.

[0184] Unlike an ordinary 45 degree total internal reflection (TIR) prism, in this embodiment the back surface 1821 of the prism 1804 is cut at approximately a 48 degree angle indicated as 1804a relative to the bottom surface 1820 of the prism 1804. The top surface 1822 of the prism 1804 is cut at a 4 degree angle indicated as 1804b relative to the bottom surface 1820 to cause the light to reflect off the top surface 1822 via total internal reflection. The front surface 1823 of the prism 1804 is cut at a 90 degree angle relative to the bottom surface 1820. The prism 1804 therefore provides a total of 4 surface reflections in the optical assembly (two TIRs off the back surface 1821, one TIR off the micro-mirror device 1830, and one TIR off the top surface 1822.)

[0185] In order to remove the manufacturing tolerances of the prism angles, a second smaller compensating prism or wedge 1810 (or wedge), having a front surface cut at a shallow angle (e.g., as 10 degrees) with respect to a back surface, may also be used. Slight tilting or pivoting about a pivot point of the compensation wedge 1810 causes the light beam to be pointed in the correct direction for focusing on the receive pigtail 1802.

[0186] The combination of the chisel prism 1804 and the compensation wedge 1810 allows for practical fabrication of optical devices that spread a beam out over a significant area and therefore onto a plurality of micro-mirrors, while keeping the optical system robust to tilt errors introduced by vibration or thermal variations.

[0187] In FIG. 41, the input light rays 1826a first pass through the λ/4 wave plate 35 and the λ/9 wave plate 1840. The input rays 1826a reflect off the back surface 1821 of the prism 1804 the micro-mirror device 1830. The rays 1826b then reflect off the micro-mirror device 1830 back to the back surface 1821 of the prism 1804. The rays 1826b then reflect off the top surface 1822 for a total of 4 surfaces (an even number) and passes through the front surface 1823 of the prism 1804. The rays 1826b then pass back through the λ/4 wave plate 35 and the λ/9 wave plate 1840 to the wedge 1810. The wedge 1810 redirects the output rays 1826c to the receive pigtail 1802 (FIG. 39 of the dual fiber pigtails 1802. As shown by arrows 1851, the wedge 1810 may be pivoted about its long axis 1850 during assembly to slightly steer the output beam 1826c to the receive pigtail 1802 with minimal optical loss by removing manufacturing tolerances of the chisel prism.

[0188] In FIG. 40, the prism 1804 (with wave plates 35, 1840 mounted thereto) and the micro-mirror device 1830 are mounted or secured in fixed relations to each other. The prism 1804 and micro-mirror device 1830 are tilted a predetermined angle off the axis of the input beam 614 (e.g., approximately 9.2 degrees) to properly direct the input beam onto the micro-mirrors of the micro-mirror device, as described hereinbefore. The wedge 1810 however is perpendicular to the axis of the input beam 1826a. Consequently, the receive pigtail of the dual fiber pigtail 1802 is rotated a predetermined angle (approximately 3 degrees) from a vertically aligned position with the transmit pigtail. Alternatively, the wedge 1810 may be rotated by the same predetermined angle as the prism and the micro-mirror device (e.g., approximately 9.2 degrees) from the axis of the input beam. As a result, the receive pigtail of the dual pigtail assembly 1802 may remain vertically aligned with transmit pigtail.

[0189] FIGS. 42-44 show an embodiment of the basic invention which features the optical cross-connect generally indicated as 3000 having an optical arrangement 15, 16 for receiving two or more optical signals 12, 13, each optical signal having one or more optical bands or channels, and including a spatial light modulator 30 having a micro-mirror device 82 (FIGS. 44) with an array of micro-mirrors 84 for reflecting the two or more optical input signals provided thereon. The optical arrangement 15, 16 features a free optic configuration having one or more light dispersion elements 24, 54 for separating the two or more optical signals 12, 13 so that each optical band or channel is reflected by a respective plurality of micro-mirrors 100, 101, 102, 103 (FIG. 44) to selectively switch the one or more optical bands or channels between the optical signals 12, 13 in order to provide output signals 48, 76.

[0190] The optical arrangement 15, 16 includes a first optical portion 15 and a second optical portion 16 that provide the more optical input signals 12, 13 to the spatial light modulator 30, and also provide the spatial light modulator 30 the optical output signal 48, 76 having the cross-connected optical bands or channels after bands or channels have been switched between the one or more optical signals. The scope of the invention is not intended to be limited to any particular type of optical portion. Embodiments are shown and described by way of example below having may many different types of optical portions. The scope of the invention is not intended to be limited to only those types of optical portions shown and described herein.

[0191] The spatial light modulator 30 may be programmable for reconfiguring the cross-connect 3000 by changing a switching algorithm that drives the array of micro-mirrors 84 to accommodate different WDM input signal structures ( i.e. channel spacing, beam shape). For example the ROADM may be modified to accommodate WDM signals having a 50 GHz or 100 GHz spacing.

[0192] In FIG. 43, the cross-connect 3000 receives a pair of WDM input signals 12, 13 and selectively switches at least signals to provide a pair of modified output signals 48, 76. Each optical channel 14, 14′ (or wavelength band of light) is centered at a respective channel wavelength (λ1, λ2, λ3 . . . , λN). In one embodiment, as shown, one input signal 12 includes optical channels 14 (e.g., at λ1-λ4), and the other input signal 13 includes optical channels 14′ (e.g., at λ1-λ4). The cross-connect 3000 in response to an input signal and switching algorithm switches the second and third channels at λ2, λ3 between the input signals 12, 13 to provide the output signals 48, 76.

[0193] In FIGS. 43, the optical cross-connect 3000 comprises a pair of optical portions 15, 16 wherein one portion receives the first input signal 12 and the other portion 16 receives the second input signal 13. FIG. 43 is a plan view of the cross-connect 3000 in the horizontal plane. Each optical portion 15, 16 includes substantially the same components disposed in substantially the same configuration. To better understand the cross-connect 3000 of FIG. 43, one may refer to FIG. 1A above which shows a side elevational view of one of the optical portions 15 that is similar to that shown in FIG. 43 and will be described with the understanding that the other complementary optical portion 16 functions in a similar manner.

[0194] The optics of the optical portion 15 is disposed in two tiers or horizontal planes. Specifically, the optical portion 15 includes a three port circulator 18, an optical fiber or pigtail 20, a collimator 22, a light dispersive element 24, a mirror 26, and a bulk lens 28 for directing light to and from a spatial light modulator 30. As shown, the pigtail 20, the collimator 22 and the light dispersive element 24 are disposed in a first tier or plane parallel to the horizontal plane. The mirror 26, bulk lens 28 and the spatial light modulator 30 are disposed in the second tier also parallel to the horizontal plane.

[0195] The first three-port circulator 18 directs light from a first port 32 to a second port 33 and from the second port to a third port 34. The first optical fiber or pigtail 20 is optically connected to the second port of the circulator 18. A capillary tube 36, which may be formed of glass, is attached to one end of the first pigtail 20 such as by epoxying or collapsing the tube onto the first pigtail. The circulator 18 at the first port 32 receives the first WDM input signal 12 from an optical network (not shown) via optical fiber 38, and directs the input light to the first pigtail 20. The first input signal 12 exits the first pigtail (into free space) and passes through the first collimator 22, which collimates the input signal. The collimator 22 may be an aspherical lens, an achromatic lens, a doublet, a GRIN lens, a laser diode doublet or similar collimating lens. The collimated input signal 40 is incident on the first light dispersion element 24 (e.g., a diffraction grating or a prism), which separates spatially the optical channels of the collimated input signal 40 by diffracting or dispersing the light from (or through) the first light dispersion element.

[0196] In one embodiment, the first diffraction grating 24 is comprised of a blank of polished fused silica or glass with a reflective coating (such as evaporated gold or aluminum), wherein a plurality of grooves 42 (or lines) are etched, ruled or otherwise formed in the coating. The first diffractive grating 24 has a predetermined number of lines, such as 600 lines/mm, 850 lines/mm and 1200 lines/mm. The resolution of the cross-connect improves as the number of lines/mm in the grating increases. The grating 24 may be similar to those manufactured by Thermo RGL, part number 3325FS-660 and by Optometrics, part number 3-9601. Alternatively, the first diffraction grating may be formed using holographic techniques, as is well known in the art. Further, the first light dispersion element may include a prism or optical splitter to disperse the light as the light passes therethrough, or a prism having a reflective surface or coating on its backside to reflect the dispersed light.

[0197] The diffraction grating 24 directs the separated light 44 to the first mirror 26 disposed in the second tier. The first mirror 26 reflects the separated light 44 to the first bulk lens 28 (e.g., a Fourier lens), which focuses the separated light onto the spatial light modulator 30. In response to a switching algorithm and input command 46, the spatial light modulator 30 reflects selected optical input channel(s) away from the first bulk lens 28 (i.e., the switched channels) to the other optical portion 16 and reflects the remaining optical input channel(s) (i.e., returned optical channel(s)) back through the same optical path to the first pigtail 20, as shown in FIG. 43. The returned optical input channel(s) propagates from the second port 33 to the third port 34 of the optical circulator 18 to provide a first output signal 48 from optical fiber 50.

[0198] The switched channel(s) passes through the other optical portion 16 of the cross-connect 10. Specifically, the switched channel(s) passes through a second bulk lens 52 (e.g., a Fourier lens), and then reflects off a second mirror 58 onto a second light dispersion element 54, which is similar to the first light dispersion element 24. The second diffraction grating 54 further disperses the switched channel(s). A second collimator 60, which is similar to collimator 28, focuses the dispersed light 62 onto a second pigtail 64, which is optically connected to a second 3-port circulator 66. The second circulator 66 directs light from a first port 68 to a second port 69 and from the second port to a third port 70. A capillary tube 72, which may be formed of glass, is attached to one end of the second pigtail 64 such as by epoxying or collapsing the tube onto the second pigtail. The switched channel(s) propagates from the second pigtail 64 to the output optical fiber 74, which is optically connected to the third port 70 of the second circulator 66, to provide a second output signal 76.

[0199] One or more optical channels 14′ of the second optical WDM input signal 13 may be switched to the first output signal 48. The second input channel(s) 14′ propagates from the optical fiber 78 to the second pigtail 64 through the second circulator 66. The cross-connet 3000 may also be selectively configured to switch no channels therebetween.

[0200] The second input channel(s) 14′ exits the pigtail 64 and passes through the second collimator 60 to the second diffraction grating 54, which separates spectrally the second input channels of the collimated input signals 13 by dispersing or diffracting from (or through) the second diffraction grating 54. The diffraction grating 54 directs the separated light 80 to the second mirror 58 disposed in the second tier, similar to that described above in FIG. 3 for the optical portion 15. The mirror 58 reflects the separated light 80 to the second bulk lens 52, which focuses the separated light 80 onto the spatial light modulator 30. The spatial light modulator 30 reflects the complementary switched channel(s) 14′ of the separated light 80 to the first bulk lens 28 and reflects the remaining second input channel(s) away from the spatial light modulator 30, as shown by arrows 81, to a mirror 83. The remaining second input channel(s) 14′ (i.e., returned optical channel(s)) reflect back off the mirror 83 and through the second optical portion 16 to the second pigtail 64, as best shown in FIG. 2. The returned optical input channel(s) 14′ propagates from the second port 69 to the third port 70 of the second optical circulator 66 to provide a second output signal 76 from optical fiber 74.

[0201] The complementary switched channel(s) 14′ passes through the first bulk lens 28, which are then reflected off the first mirror 26 onto the first diffraction grating 24. The first diffraction grating further disperses the complementary switched channel(s) 14′ onto the first collimator 22 which focuses the complementary switched channels to the first pigtail 22. The complementary switched channel(s) propagates from the first pigtail 20 to optical fiber 50, to thereby switch the complementary switched channel(s) to the first output signal 48. As will be described hereinafter, the second input channels 14′ and first input channels 14 at the same wavelengths reflect off the same portion of spatial light modulator 20, and therefore when a first input channel 14 of the first input signal 12 is switched to the second output signal 76, the complementary input channel 14′ of the second input signal 13 is switched simultaneously.

[0202] The spatial light modulator 30 comprises a micro-mirror device 82 having a two-dimensional array of micro-mirrors 84, which cover a surface of the micro-mirror device. The micro-mirrors 84 are generally square and typically 14-20 microns (μm) wide with 1 μm spaces between them, and operate in a manner consistent with that shown and described in relation to FIGS. 3-6 above.

[0203] One will appreciate that the cross-connect 3000 may be configured for any wavelength plan by simply modifying the software. For example, a cross-connect for filtering a 50 GHz WDM optical signal may be modified to filter a 100 GHz or 25 GHz WDM optical signal by simply modifying or downloading a different switching algorithm, without modifying the hardware. In other words, any changes, upgrades or adjustments to the cross-connect (such as varying the spacing of the channels, the shapes of the light beams, and center wavelength of the light beams) may be accomplishment by simply modifying statically or dynamically the switching algorithm (e.g., modifying the bit map).

[0204] As shown in FIGS. 43, the micro-mirror device 82 is oriented to reflect the focused light 92 of the first input signal 12 back through the first bulk lens 28 to the first pigtail 20, as indicated by arrows 94, to the first output signal 48, and to reflect the focused light 98 of the second input signal 13 off the mirror 83, as indicated by arrows 81, and back (see arrows 85) to the second output 76, when the micro-mirrors 84 are disposed in the first position. As shown in FIG. 43, the focused light 92 of the first input signal 12 reflects away from the first bulk lens 28 to the second output 74, as indicated by arrows 96, and the focused light 98 of second input signal 13 reflects away from the second bulk lens 52 to the first output 50, when the micro-mirrors 84 are disposed in the second position. This “digital” mode of operation of the micro-mirrors advantageously eliminates the need for any type of feedback control for each of the micro-mirrors. The micro-mirrors 84 are either “on” or “off” (i.e., first position or second position), respectively, and therefore, can be controlled by simple binary digital logic circuits.

[0205] Consistent with that described above, the outline of the optical input channels 14, 14′ of the first and second input signals 12, 13, which are dispersed off respective diffraction gratings 24, 54 and focused by bulk lens 28, 52 respectively, onto the array of micro-mirrors 84 of the micro-mirror device 82. The input channels 14, 14′ at each corresponding wavelength illuminate the same area of the micro-mirror device 82 as shown. Each optical channel 14, 14′ is distinctly separated from other channels across the spectrum and have a generally circular cross-section, such that the optical channels do not substantially overlap spatially when focused onto the spatial light modulator 30. The input channels 14, 14′ have a circular cross-section to project as much of the beam as possible over a multitude of micro-mirrors 84, while keeping the optical channels separated by a predetermined spacing. One will appreciate though that the diffraction gratings 24, 54 and bulk lens 28, 52 may be designed to reflect and focus any input channel or group of input channels with any desired cross-sectional geometry, such as elliptical, rectangular, square, polygonal, etc. Regardless of the cross-sectional geometry selected, the cross-sectional area of the channels 14 should illuminate a plurality of micro-mirrors 84, which effectively pixelates the optical channels. In an exemplary embodiment, the cross sectional area of the input channels 14, 14′ is generally circular in shape, whereby the width of the optical channel beam spans over approximately 11 micro-mirrors.

[0206] One will appreciate that while the spacing between each spectrum of input channels 14, 14′ are predetermined, the spacing between may be non-uniform. For example, one grouping of channels 14, 14′ may be spaced to correspond to a 100 GHz spacing, and another group of channels 14, 14′ may be spaced to correspond to a 50 GHz spacing.

[0207]
FIG. 44 is illustrative of the position of the micro-mirrors 84 of the micro-mirror device 82 for switching adding the optical channels 14, 14′ at λ3, λ5, λ6, λ10, for example. The outline of each channel 14,14′ is shown to provide a reference to visually locate the groups of tilted mirrors 100-103. As shown, the groups of mirrors 100-103 associated with each respective optical channel at λ3, λ5, λ6, λ10, are tilted away from the return path to the second position, as indicated by the blackening of the micro-mirrors 84. Each group of tilted mirrors 100-103 provides a generally rectangular shape, but one will appreciate that any pattern or shape may be tilted to redirect an optical channel. In an exemplary embodiment, the groups of micro-mirrors 100-103 reflect substantially all the light of each respective input channel 14, 14′ and do reflect substantially no light of any adjacent channels. The micro-mirrors 84 of the other input channels 14, 14′ at wavelengths of λ1, λ2, λ4, λ7, λ8, λ9, λ11-λN are flat (i.e., first position), as indicated by the white micro-mirrors, to reflect the light 92 back along the return path to the first pigtail 20, as described hereinbefore.

[0208] As described hereinbefore, the input channel 14 of the first input signal 12 and the complementary input channel 14′ of the second input signal 13, which are centered at the same wavelength, are focused onto the same group of micro-mirrors. For example, both the first input channel 14 at λ3 and complementary input channel 14′ at λ3 reflect off the same group of mirrors 100. Consequently, when the micro-mirrors are disposed in the tilted (or second position), the first input channel 14 is switched with the complementary input channel 14′ at the output 50, 74.

[0209]
FIG. 45 shows a known interleaver device that combines at least two optical WDM input signals 2, 3 into a single optical output signal 4. The WDM input signals include a plurality of wavelength bands of light (or optical channels) that are centered at a respective channel wavelength (λ1, λ2, λ3, . . . λN). In one embodiment, as shown, one input signal 2 includes each even input channel 14 (e.g., λ2, λ4, λ6), and the other input signal 3 includes each odd input channel (e.g., λ1, λ3, λ5). The combined input signals 2,3 provide a WDM output signal having each input channels 14, 14′ (e.g., λ1-λ6).

[0210]
FIG. 46 shows another known optical de-interleaver device generally indicated as 5 that separates an optical WDM input signal 6 into at least two optical output signals 7, 8. The WDM input signal includes a plurality of optical channels that are centered at a respective channel wavelength (λ1, λ2, λ3, . . . λN). In one embodiment, as shown, the input signal 6 includes a WDM output signal having input channels at λ1-λ6. The input signal 6 is separated such that one output signal 7 includes each even input channel (i.e., λ2, λ4, λ6), and the other output signal 8 includes each odd input channel (i.e., λ1, λ3, λ5).

[0211] FIGS. 47-48 show an embodiment of the basic invention which features an optical interleaver/de-interleaver device generally indicated as 10 including an optical arrangement 15, 16 for receiving two or more optical signals, each optical signal having a respective set of at least one optical band or channel, and including a spatial light modulator 30 having a micro-mirror device (FIGS. 48) with an array of micro-mirrors 84 for reflecting the two or more optical signals provided thereon. The optical arrangement 15, 16 comprises a free optic configuration having one or more light dispersion elements for separating the two or more optical input signals so that each optical band or channel is reflected by a respective plurality of micro-mirrors 100, 101, 102, 103 (FIG. 8) to selectively either combine two respective sets of the at least one optical band or channel into one optical output signal, or de-combine one set of the at least one optical band or channel into two optical output signals each having a different set of the at least one optical band or channel.

[0212] The optical arrangement 15, 16 includes a first optical portion 15 and a second optical portion 16 that provide the two or more optical signals 2, 3 to the spatial light modulator 30, and also provide the optical output signal 48, 76 having the cross-connected optical bands or channels after bands or channels have been switched between the one or more optical signals. The scope of the invention is not intended to be limited to any particular type of optical portion. Embodiments are shown and described by way of example below having may many different types of optical portions. The scope of the invention is not intended to be limited to only those types of optical portions shown and described herein.

[0213] The spatial light modulator 30 may be programmable for reconfiguring the interleaver/de-interleaver 4000 by changing a switching algorithm that drives the array of micro-mirrors 84.

[0214] In FIG. 47, the reconfigurable optical interleaver/de-interleaver device 4000 may function as an interleaver device of FIG. 45 or a de-interleaver device of FIG. 46. The input signals 2, 3 and output signal 4 of the interleaver device are shown as solid arrows, while the input signal 6 and the output signals 7, 8 of the de-interleaver device are shown as dashed arrows. To simplify the description of the present invention, each of the embodiments are described hereinafter as an interleaver, however, one should appreciate that each of the embodiments may function as a de-interleaver by configuring one of the input ports to an output port, as illustrated by the dashed arrows 6-8.

[0215] Accordingly, the interleaver device 4000 of FIG. 47 comprises a pair of optical portions 15, 16 that focuses and receives light to and from a spatial light modulator 30. FIG. 3 is a plan view of the interleaver device 4000 in the horizontal plane. Each optical portion 15, 16 includes substantially the same components disposed in substantially the same configuration. To better understand the interleaver device 4000 of FIG. 47, a side elevational view of one of the optical portions 15 is illustrated in FIG. 1A above and will be described with the understanding that the other complementary optical portion 16 functions in a similar manner.

[0216] As shown in FIG. 3, the optics of the optical portion 15 is disposed in two tiers or horizontal planes. Specifically, the optical portion 15 includes an optical fiber or pigtail 20, a collimator 22, a light dispersive element 24, a mirror 26, and a bulk lens 28 for directing light to and from the spatial light modulator 30. A three-port circulator 18 is optically connected to the pigtail 20 to provide input signals 2, 3 to and receive an output signal 4 from the optical portion 15. As shown, the pigtail 20, the collimator 22 and the light dispersive element 24 are disposed in a first tier or plane parallel to the horizontal plane. The mirror 26, bulk lens 28 and the spatial light modulator 30 are disposed in the second tier also parallel to the horizontal plane.

[0217] The circulator 18 directs light from a first port 32 to a second port 33 and from the second port to a third port 34. The first pigtail 20 is optically connected to the second port of the circulator 18. A capillary tube 36, which may be formed of glass, is attached to one end of the first pigtail 20 such as by epoxying or collapsing the tube onto the first pigtail. The first port 32 of the circulator 18 receives the first input signal 2 from an optical network (not shown) via optical fiber 38, and directs the input light to the first pigtail 20. The first input signal 2 exits the first pigtail (into free space) and passes through the first collimator 22, which collimates the input signal. The collimator 22 may be an aspherical lens, an achromatic lens, a doublet, a GRIN lens, a laser diode doublet or similar collimating lens. The collimated input signal 40 is incident on the first light dispersion element 24 (e.g., a diffraction grating or a prism), which separates spatially the optical channels of the collimated input signal 40 by diffracting or dispersing the light from (or through) the first light dispersion element.

[0218] In one embodiment, the first diffraction grating 24 is comprised of a blank of polished fused silica or glass with a reflective coating (such as evaporated gold or aluminum), wherein a plurality of grooves 42 (or lines) are etched, ruled or otherwise formed in the coating. The first diffractive grating 24 has a predetermined number of lines, such as 600 lines/mm, 850 lines/mm and 1200 lines/mm. The resolution of the interleaver device improves as the number of lines/mm in the grating increases. The grating 24 may be similar to those manufactured by Thermo RGL, part number 3325FS-660 and by Optometrics, part number 3-9601. Alternatively, the first diffraction grating may be formed using holographic techniques, as is well known in the art. Further, the first light dispersion element may include a prism or optical splitter to disperse the light as the light passes therethrough, or a prism having a reflective surface or coating on its backside to reflect the dispersed light.

[0219] The diffraction grating 24 directs the separated light 44 to the first mirror 26 disposed in the second tier. The first mirror 26 reflects the separated light 44 to the first bulk lens 28 (e.g., a Fourier lens), which focuses the separated light onto the spatial light modulator 30.

[0220] In response to a switching algorithm and input command 46, the spatial light modulator 30 reflects the optical input channel(s) 14 of first input signal back through the same optical path to the first pigtail 20. The returned optical input channel(s) propagates from the second port 33 to the third port 34 of the optical circulator 18 to provide an output signal 4 from optical fiber 50.

[0221] The optical channels 14′ of the second input signal 3 are combined with or added to the output signal 4. The channel 14′ of the second input signal 3 exit the second pigtail 64 and passes through the second collimator 60 to the second diffraction grating 54, which separates spectrally the channels 14′ of the collimated second input signal 3 by dispersing or diffracting from (or through) the second diffraction grating 54. The diffraction grating 54 directs the separated light 80 to the second mirror. 58 disposed in the second tier, similar to that described above in FIG. 3A for the optical portion 15. The mirror 58 reflects the separated light 80 to the second bulk lens 52, which focuses the separated light 80 onto the spatial light modulator 30. The separated light 44 of the first input signal 2 and the separate light 80 of the second input signal 3 occupy different, alternating portion (or sections) of the spatial light modulator 30. The spatial light modulator 30 reflects the channel 14′ of the separated light 80 to the first bulk lens 28.

[0222] The channel 14′ of the second input signal 3 passes through the first bulk lens 28, which are then reflected off the first mirror 26 onto the first diffraction grating 24. The first diffraction grating further disperses the channel 14′ onto the first collimator 22 which focuses the channels 14′ to the first pigtail 22. The channels 14′ propagate from the first pigtail 20 to optical fiber 50, to thereby combine the channels 14′ to the output signal 4.

[0223] The spatial light modulator 30 comprises a micro-mirror device 82 having a two-dimensional array of micro-mirrors 84, which cover a surface of the micro-mirror device. The micro-mirrors 84 are generally square and typically 14-20 μm wide with 1 μm spaces between them. The reader is referred to FIGS. 4a, 4b, which illustrate a partial row of micro-mirrors 84 of the micro-mirror device 82 when the micro-mirrors are disposed in a first position to reflect the light back along the return path and provide the channels 14 of the first input signal 2 to the output fiber 50, as well as a partial row of micro-mirrors 84 when the micro-mirrors are disposed in a second position, and therefore combine/add the channels 14′ of the second input signal 3 to the output fiber 50, as will be described in greater detail hereinafter. The micro-mirrors may operate in a “digital” fashion. In other words, as the micro-mirrors either lie flat in a first position, as shown in FIG. 6a, or be tilted, flipped or rotated to a second position, as shown in FIG. 6b.

[0224] As described herein before, the positions of the mirrors, either flat or tilted, are described relative to the optical path wherein “flat” refers to the mirror surface positioned orthogonal to the light path, either coplanar in the first position or parallel as will be more fully described hereinafter. The micro-mirrors flip about an axis 85 parallel to the spectral axis 86, as shown in FIG. 48. One will appreciate, however, that the micro-mirrors may flip about any axis, such as parallel to the spatial axis 88, at a 45 degrees angle to the spatial axis, or any desired angle.

[0225] The micro-mirrors 84 are individually flipped between the first position and the second position in response to a control signal 87 provided by a controller 90 in accordance with a switching algorithm and an input command 46. The switching algorithm may provide a bit (or pixel) map indicative of the state (flat or tilted) of each of the micro-mirrors 84 of the array to return, drop and/or add the desired optical channel(s) 14 to provide the express/output signal 48 at optical fiber 50 (see FIG. 47), and thus requiring a bit map for each configuration of channels to be dropped and added. Alternatively, each group of mirrors 84, which reflect a respective optical channel 14, may be individually controlled by flipping the group of micro-mirrors to direct the channel along a desired optical path (i.e., return, drop or add).

[0226] One will appreciate that the interleaver device 4000 may be configured for any wavelength plan by simply modifying the software. For example, an interleaver device for filtering a 50 GHz WDM optical signal may be modified to filter a 100 GHz or 25 GHz WDM optical signal by simply modifying or downloading a different switching algorithm, without modifying the hardware. In other words, any changes, upgrades or adjustments to the interleaver device (such as varying the spacing of the channels, the shapes of the light beams, and center wavelength of the light beams) may be accomplishment by simply modifying statically or dynamically the switching algorithm (e.g., modifying the bit map).

[0227] As shown in FIGS. 47 and 4a, the micro-mirror device 82 is oriented to reflect the focused light 92 of the first input signal 2 back through the first bulk lens 28 to the first pigtail 20, as indicated by arrows 94, to provide the output signal 4. As shown in FIGS. 3 and 4b, the channels 14′ of the second input signal 3 reflects, as indicated by arrows 98, back through the first bulk lens 28 to the first pigtail 20, as indicated by arrows 94, which is added to the output signal 4. This “digital” mode of operation of the micro-mirrors advantageously eliminates the need for any type of feedback control for each of the micro-mirrors. The micro-mirrors are either “on” or “off” (i.e., first position or second position), respectively, and therefore, can be controlled by simple binary digital logic circuits.

[0228] The outline of the optical channels 14, 14′ of the first and second input signals 2,3, respectively, which are dispersed off respective diffraction gratings 24,54 and focused by bulk lens 28,52 respectively, onto the array of micro-mirrors 84 of the micro-mirror device 82. Each channel 14,14′ is distinctly separated from other channels across the spectrum and have a generally circular cross-section, such that the optical channels do not substantially overlap spatially when focused onto the spatial light modulator 30. The optical channels have a circular cross-section to project as much of the beam as possible over a multitude of micro-mirrors 84, while keeping the optical channels separated by a predetermined spacing. One will appreciate though that the diffraction gratings 24, 54 and bulk lens 28, 52 may be designed to reflect and focus any optical channel or group of optical channels with any desired cross-sectional geometry, such as elliptical, rectangular, square, polygonal, etc. Regardless of the cross-sectional geometry selected, the cross-sectional area of the channels 14 should illuminate a plurality of micro-mirrors 84, which effectively pixelates the optical channels. In an exemplary embodiment, the cross sectional area of the optical channels 14, 14′ is generally circular in shape, whereby the width of the optical channel beam spans over approximately 11 micro-mirrors.

[0229]
FIG. 48 is illustrative of the position of the micro-mirrors 84 of the micro-mirror device 82 for combining the optical channels 14, 14′ of the input signals 2, 3. The outline of each channel 14, 14′ is shown to provide a reference to visually locate the groups of tilted mirrors 100. As shown, the groups of mirrors 100 associated with each respective optical channel 14′ at λ1, λ3, λ5, λ7, λ9, λ11, of the second input signal 3 are tilted away from the return path to the second position, as indicated by the blackening of the micro-mirrors 84. Each group of tilted mirrors 100 provides a generally rectangular shape, but one will appreciate that any pattern or shape may be tilted to redirect an optical channel. In an exemplary embodiment, each group of micro-mirrors 100 reflects substantially all the light of each respective optical channel 14′ and reflects substantially no light of any adjacent channels. The remaining micro-mirrors 84 reflects substantially all the light of each channel 14 at λ2, λ4, λ6, λ8, λ12 are flat (i.e., first position), as indicated by the white micro-mirrors, to reflect the light 92 back along the return path to the first pigtail 20, as described hereinbefore.

[0230] While the interleaver/de-interleaver device has been described as combining/separating every other channel of a WDM input signal(s), the present invention contemplates selectively combining/separating any group of channels. For example, every third, fourth, fifth or sixth channel may be combined/separated, every other group of channels of a WDM signal(s) may be combined/separated, or any other periodic or aperiodic pattern desired.

The Scope of the Invention

[0231] The dimensions and geometries for any of the embodiments described herein are merely for illustrative purposes and, as much, any other dimensions may be used if desired, depending on the application, size, performance, manufacturing requirements, or other factors, in view of the teachings herein.

[0232] It should be understood that, unless stated otherwise herein, any of the features, characteristics, alternatives or modifications described regarding a particular embodiment herein may also be applied, used, or incorporated with any other embodiment described herein.

[0233] Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein without departing from the spirit and scope of the present invention.

	Number	Date	Country
Parent	10115647	Apr 2002	US
Child	10255141	Sep 2002	US
Parent	10120617	Apr 2002	US
Child	10255141	Sep 2002	US

Reconfigurable optical add/drop multiplexer having an array of micro-mirrors

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