The present application is based on Japanese patent application No. 2012-090182 filed on Apr. 11, 2012, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The invention relates to a wavelength cross connect device.
2. Description of the Related Art
A wavelength cross connect device shown in
A wavelength cross connect device 141 shown in
In the wavelength cross connect device 141, light input from the input optical fiber 142 passes through the microlens array 144 and a macro-lens 145a as one of macro-lenses constituting the macro-lens pair 145, and is demultiplexed into each wavelength by the grating 146. The lights of respective wavelength demultiplexed by the grating 146 pass through a macro-lens 145b as another macro-lens constituting the macro-lens pair 145 and the λ/4 plates 147 and are then incident on the optical switch matrix 148.
The optical switch matrix 148 is formed by oppositely arranging plural MEMS minors 149 and is configured so that each wavelength can be switched by changing a reflection angle of the MEMS mirrors 149. The lights output from the optical switch matrix 148 pass through the λ/4 plate 147 and the macro-lens 145b and are then multiplexed by the grating 146, and the multiplexed light passes through the macro-lens 145a and the microlens array 144 and is then output from the output optical fiber 143.
Meanwhile, as an optical cross connect device for switching single-wavelength light, there is a device shown in
In an optical cross connect device 151, MEMS mirror arrays 152 are oppositely arranged, and a lens 153 lens with a focal length equal to the Rayleigh length is arranged between the two MEMS mirror arrays 152. Distances between the both MEMS mirror arrays 152 and the lens 153 are adjusted to be respectively equal to a focal length (i.e., the Rayleigh length) of the lens 153.
In the optical cross connect device 151, light input from an input-side fiber array 154 is input to one MEMS mirror array 152 through a lens array 155, is reflected by the one MEMS mirror array 152, passes through the lens 153, is then further reflected by another MEMS mirror array 152 and is output from an output-side fiber array 156 via another lens array 155. Here, since the lens 153 converts an angle into a position (offset), change of reflection angle by the one MEMS mirror array 152 is reflected as change of a position on the other MEMS mirror array 152 and switching is thereby carried out.
Meanwhile, another conventionally known wavelength cross connect device is shown in
Patent Literature 1: U.S. Pat. No. 6,289,145, Specification
Non-Patent Literature 1: David T. Neilson and eleven others, “256×256 Port Optical Cross-Connect Subsystem”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 22, No. 6, p. 1499-1509, April 2004
Non-Patent Literature 2: Nicolas K. Fontaine and two others, “N×M WAVELENGTH SELECTIVE CROSSCONNECT WITH FLEXIBLE PASSBANDS” OFC/NFOEC POSTDEADLINE PAPERS, PDP5B.2, March 2012
However, in the conventional wavelength cross connect device 141 shown in
In addition, in the wavelength cross connect device 141, the image on the MEMS mirror 149 does not become a beam waist (a focal point in which a size of image is minimized) but becomes a large image since a space between the oppositely arranged MEMS mirrors 149 is a free space, and accordingly, the MEMS mirror 149 having a larger area is required. Therefore, it is very difficult to provide multiple ports in the wavelength cross connect device 141.
In the optical cross connect device 151 of
However, it is difficult to apply the optical cross connect device 151 to a wavelength cross connect device since switching per wavelength is not taken into consideration at all and a normal two-dimensional lens is used as the lens 153. In detail, in order to realize a wavelength cross connection in the optical cross connect device 151, the number of spectrograph-demultiplexers 161 to be connected needs to be the same as the number of input/output ports to multiplex and demultiplex wavelengths as shown in
The wavelength cross connect device 171 adopts an optical system which passes through the grating 176 four times in total, as described above. However, reflectance of each time is poor since the grating 176 has inherent loss such as unnecessary order of diffractive excitation, and passing through the grating 176 four times makes insertion loss of the wavelength cross connect device 171 significantly worse.
Accordingly, it is an object of the invention to provide a wavelength cross connect device that allows low loss, flat-top response, low crosstalk and multiport to be realized and has a simple and low-cost structure.
an input demultiplexing optical system that demultiplexes light input from a plurality of input ports into respective wavelengths and outputs the demultiplexed lights;
a wavelength switching optical system for switching and outputting the lights of respective wavelengths input from the input demultiplexing optical system to respective desired ports; and
an output multiplexing optical system that multiplexes the lights of respective wavelengths input from the wavelength switching optical system per port and outputs the multiplexed lights through corresponding output ports,
wherein the input demultiplexing optical system and the output multiplexing optical system are configured such that optical paths of the lights from the respective ports are aligned in parallel to each other in a widthwise direction and light from each port is demultiplexed or multiplexed in a vertical direction, and comprise a lens system that has a function of focusing lights independently in vertical and widthwise directions so as to focus the light of each wavelength output to the wavelength switching optical system into a horizontal oval shape or so as to convert the horizontal oval-shaped focal point of the light of each wavelength input from the wavelength switching optical system back into a focal point having the same shape as an image of the output port, and
wherein the wavelength switching optical system comprises:
two light deflector arrays being oppositely arranged at respective focal positions of the lens systems of the input demultiplexing optical system and the output multiplexing optical system and comprising two-dimensional light deflection elements vertically and horizontally arranged so as to correspond to the light of each wavelength of each port to output incoming light of each wavelength after adjusting a horizontal reflection angle of the light; and
a switching lens comprising a lens that has a focal length equal to the Rayleigh length and acts only in a widthwise direction, being arranged between the two light deflector arrays so that respective distances from the two light deflector arrays are both equal to the focal length to perform switching by converting the horizontal angle of the light of each wavelength adjusted by one of the light deflector arrays into a horizontal position on another light deflector array.
In the above embodiment (1) of the invention, the following modifications and changes can be made.
(i) The wavelength switching optical system comprises multi-stage Fourier optical lenses acting only in a vertical direction and is configured to convert a vertical angle into a vertical position and subsequently convert the vertical position back into the vertical angle by the multi-stage lenses.
(ii) The input demultiplexing optical system and the output multiplexing optical system comprise:
waveguide arrays being formed by monolithically integrating a plurality of channel waveguides formed on a flat substrate so as to have a structure having a high refractive index core covered with a low refractive index cladding such that input/output ports on one side of the channel waveguides are used as the input ports or the output ports and input/output ports on another side are aligned in a straight line in a widthwise direction; and
a demultiplexing element vertically demultiplexing the light of each port emitted from the input/output port on the other side of the waveguide array into each wavelength and then outputting the demultiplexed lights to the wavelength switching optical system or multiplexing the light of each wavelength input from the wavelength switching optical system and then making the multiplexed light incident on the input/output port on the other side of the waveguide array, and
wherein the lens system comprises:
a first lens comprising a lens acting only in a vertical direction to collimate the light emitted from the input/output port on the other side of the waveguide array and then output the collimated light to the demultiplexing element or to focus the light input from the demultiplexing element and then make the focused light incident on the input/output port on the other side of the waveguide array;
a second lens comprising a lens acting only in a vertical direction to focus the light of each wavelength demultiplexed by the demultiplexing element and then output the focused light to the wavelength switching optical system or to focus the light of each wavelength input from the wavelength switching optical system and output the focused light to the demultiplexing element; and
a third lens comprising a lens acting only in a widthwise direction and being separately provided in each of the input/output ports on the other side of the waveguide arrays.
(iii) An enlarged-waveguide portion is formed on each of the channel waveguides of the waveguide array, the enlarged-waveguide portion being formed by enlarging the core toward the input/output port on the other side as viewed from the top by using a tapered waveguide or a slab waveguide, and the third lens comprises a bulk cylindrical lens array provided in the vicinity of an output port of the enlarged-waveguide portion.
(iv) An enlarged-waveguide portion is formed on each of the channel waveguides of the waveguide array, the enlarged-waveguide portion being formed by enlarging the core toward the input/output port on the other side as viewed from the top by using a tapered waveguide or a slab waveguide, and the third lens comprises a waveguide lens formed on the core enlarged by the enlarged-waveguide portion of each of the channel waveguides or on a cladding in the vicinity of the enlarged core.
(v) The waveguide lens is formed by filling a cladding material or a resin having a lower refractive index than the core into a plurality of trenches formed by vertically trenching the core of each of the channel waveguides so that the total width of trenches forms a lens shape or Fresnel lens shape that is concave with respect to a light propagation direction as viewed from the top.
(vi) A resin having a lower refractive index than the cladding is used as the resin having a lower refractive index than the core.
(vii) The waveguide lens is formed by filling a resin having a higher refractive index than the core into a plurality of trenches formed by vertically trenching the core of each of the channel waveguides so that the total width of trenches forms a lens shape or Fresnel lens shape that is convex with respect to a light propagation direction as viewed from the top.
(viii) The plurality of trenches are formed so as to be unequally spaced in a light propagation direction.
(ix) The channel waveguides of the waveguide array each comprise a bent portion formed by bending the core.
(x) An optical fiber array comprising a plurality of optical fibers arranged in an array manner is connected to the input/output ports on the one side of the waveguide array.
(xi) The demultiplexing element comprises a grating having ruled line formed in a widthwise direction.
(xii) The grating comprises a reflective blazed grating or a reflective echelle grating or a grism comprising a grating and a prism coating a surface thereof
(xiii) The light deflector array is formed by arranging a plurality of strip-shaped one-dimensional MEMS mirror groups in a widthwise direction in an array manner so as to correspond to each port, the plurality of one-dimensional MEMS mirror groups each comprising a plurality of MEMS mirrors one-dimensionally arranged in a vertical direction.
(xiv) The MEMS mirrors are each configured such that an interval in an array direction thereof corresponds to a signal frequency interval of not more than 12.5 GHz and a gap between the adjacent MEMS mirrors is set to not more than a spot-size of the incoming light.
(xv) The one-dimensional MEMS mirror group is formed by grouping a plurality of the MEMS mirrors so that the grouped MEMS mirrors are controlled to be inclined at the same angle.
(xvi) The light deflector array is formed by arranging a plurality of LCOS chips in a widthwise direction in an array manner so as to correspond to each port.
(xvii) The light deflector array comprises an LCOS chip in one-piece and is configured such that the oval-shaped focal point group corresponding to all operating wavelengths output from each port falls within an effective diameter of the LCOS chip.
(xviii) The LCOS chip comprises a ¼ wavelength layer formed between a liquid crystal layer and a reflective film.
According to one embodiment of the invention, a wavelength cross connect device can be provided that allows low loss, flat-top response, low crosstalk and multiport to be realized and has a simple and low-cost structure.
Next, the present invention will be explained in more detail in conjunction with appended drawings, wherein:
An embodiment of the invention will be described below in conjunction with the appended drawings.
As shown in
The wavelength switching optical system 3 optically couples two optical systems, the input demultiplexing optical system 2 and the output multiplexing optical system 4, and serves to switch the lights of respective wavelengths from the respective ports to the desired ports.
Although
The input demultiplexing optical system 2, the output multiplexing optical system 4 and the wavelength switching optical system 3 will be described in detail below in this order.
Input Demultiplexing Optical System
Firstly, the input demultiplexing optical system 2 will be described.
As shown in
In more detail, the input demultiplexing optical system 2 is composed of a waveguide array 8, a grating 10 as a demultiplexing element and the lens system 7.
The waveguide array 8 is formed by monolithically integrating plural channel waveguides 9 formed on a non-illustrated flat substrate and has a structure in which high refractive index cores 8a are covered with a low refractive index cladding 8b. An input/output port 9a on one side of each channel waveguide 9 is used as the input port 5, and the input/output ports 9a on the one side and input/output ports 9b on another side are respectively aligned in a straight line in a widthwise direction (X-axis direction).
An enlarged-waveguide portion 11, which is formed by enlarging the core 8a toward the input/output port 9b as viewed from the top by using a tapered waveguide or a slab waveguide, is formed on each channel waveguide 9 of the waveguide array 8. In addition, a bent portion 12 formed by bending the core 8a as viewed from the top to eliminate cladding mode is formed on each channel waveguide 9 on the input/output port 9a side (the input port 5 side) of the enlarged-waveguide portion 11. In addition, on the input/output port 9a side (the input port 5 side) of the bent portion 12, an input portion 13 for optically coupling the bent portion 12 to the input port 5 is formed.
Each channel waveguide 9 is formed so as to be aligned in a widthwise direction (X-axis direction). An input optical fiber array 14 formed by arranging plural optical fibers (optical fiber ports) 14a in an array manner is connected to the input ports 5 of the waveguide array 8, i.e., to the input/output port 9a of each channel waveguide 9. The bent portion 12 is to suppress crosstalk by eliminating cladding mode which occurs at the time of coupling the optical fiber array 14 to the input port 5.
The grating 10 on which ruled lines (concave or convex lines) are formed in a widthwise direction (X-axis direction) is used to demultiplex light in a vertical direction (Y-axis direction). The direction of the ruled line of the grating 10 coincides with the array direction of the channel waveguides 9. The grating 10 to be used desirably has large diffraction efficiency and a large difference in a reflection angle of each wavelength (large angular dispersion) and it is desirable to use a blazed grating or a grism. In this regard, the blazed grating is formed by blazing a normal holographic grating so that sawtooth-shaped protrusions are formed on a surface thereof, and the grism is a grating of which optical path is adjusted by covering a grating surface with a high refractive index prism. It is possible to further improve diffraction efficiency by arranging the blazed grating so as to be inclined with respect to the optical path. In addition, it is possible to increase angular dispersion by using a blazed grating having a smaller ruled line pitch or a grating having a larger diffraction order (a reflective echelle grating). In the present embodiment, a transmissive blazed grating used as the grating 10 is arranged so as to be inclined with respect to an X-Y axis plane. Note that, although the optical path after passing through the grating 10 may curve depending on an arrangement angle of the grating 10 or a design center wavelength, light beam with a center wavelength which travels along the Z-axis without curving before and after passing through the grating 10 is shown in the drawing here in order to simplify the explanation. In addition, the grating 10 is depicted as a thin ideal demultiplexing element in the subsequent drawings and the direction and arrangement angle of the blaze are not exact.
The lens system 7 is composed of a first lens 15 and a second lens 16 which act only in a vertical direction (Y-axis direction) and a third lend which acts only in a widthwise direction (X-axis direction).
The first lens 15 is a semi-cylindrical lens acting only in a vertical direction (Y-axis direction), is arranged between the waveguide array 8 and the grating 10 and is configured to collimate light emitted from the input/output port 9b of the waveguide array 8 and to output the collimated light to the grating 10. A distance between the input/output port 9b of the waveguide array 8 and the first lens 15 is equal to a focal length Fy of the first lens 15.
The second lens 16 is a semi-cylindrical lens acting only in a vertical direction (Y-axis direction) in the same manner as the first lens 15, is arranged between the grating 10 and the wavelength switching optical system 3 (a light deflector array 20) and is configured to focus the light of each wavelength demultiplexed by the grating 10 and to output the focused light to the wavelength switching optical system 3 (the light deflector array 20). A distance between the grating 10 and the second lens 16 and that between the second lens 16 and the light deflector array 20 are equal to a focal length fy of the second lens 16.
The first lens 15 and the second lens 16 are shown as a single lens (a semi-cylindrical lens) here in order to simplify the explanation but may be a compound lens to reduced influence of aberration, etc. The same applies to the third lens, a switching lens 22, a fourth lens 24 and a fifth lens 25 which are described below.
The third lens is a lens acting only in a widthwise direction and is separately provided in each input/output port 9b of the waveguide array 8. Here, a lens of which focal length is represented by 1/(n1/L1+1/L2) is used as the third lens, where the effective propagation length of the enlarged-waveguide portion 11 is L1, the effective refractive index of the enlarged-waveguide portion 11 is n1 and a distance between the third lens and the light deflector array 20 is L2. Since L2 is generally much longer than L1, the focal length of the third lens is approximately L1/n1. By such a configuration, the light passing through the third lens is focused on the light deflector array 20. The third lens may be a bulk cylindrical lens array 17′ shown in
The waveguide lens 17 is formed in the core 8a enlarged by the enlarged-waveguide portion 11 near the input/output port 9b of each channel waveguide 9. The enlarged-waveguide portion 11 may be formed by a tapered waveguide 11a as shown in
As shown in
A phase velocity Vp of light is approximately given by the following formula (1):
Vp=c/n (1)
where light speed in vacuum is c and a refractive index is n. Since the refractive index of the core 8a is larger than that of the cladding 8b, the phase velocity of light is smaller in the core 8a than in the cladding 8b. Therefore, the phase velocity of light is larger in the outer side (a peripheral edge) of the core 8a in which the number of the trenches 18 and the proportion of the cladding 8b are large, and is smaller as closer to the center of the core 8a. Accordingly, the light passing through the waveguide lens 17 has a concave wavefront distribution as viewed from the top (electrical field distribution is convex as viewed from the top). Since the light travels in a direction perpendicular to this level surface, i.e., to the wavefront, the light emitted from the waveguide array 8 propagates while being focused.
The reason why the waveguide lens 17 is formed of the divided trenches 18 as shown in
The cladding 8b is used as a medium for filling the trenches 18 in the present embodiment. However, in this case, a large number (division number) of trenches 18 is required in order to function as a lens since the difference in a refractive index between the core 8a and the cladding 8b is small, which results in large optical loss due to influence of the trenches 18. In this regard, however, it is possible to reduce the number of trenches 18 and thus to further reduce optical loss by filling the trenches 18 with a material having a lower refractive index. In other words, as a medium for filling the trenches 18, it is desirable to use a resin having a lower refractive index than the cladding 8b.
Although the case where the trenches 18 formed in the core 8a are filled with a cladding or a resin having a lower refractive index than the core 8a to increase the phase velocity of light at the peripheral edge has been described in the present embodiment, it is obvious that it is possible to configure the waveguide lens 17 in an opposite manner such that the trenches 18 formed at the middle portion of the core 8a are filled with a resin having a higher refractive index than the core 8a to decrease the phase velocity of light at the middle portion.
In this case, as shown in
The shape of the plural trenches 18 constituting the waveguide lens 17 is depicted as a bar-like shape in
Although an example of the waveguide lens 17 in which the total width of trenches forms a lens shape which is concave or convex with respect to a light propagation direction has been described thus far, the shape of the trench 18 may be a concave or convex Fresnel lens shape as shown in
As shown in
The plural trenches 18 formed in a multistage manner in a light propagation direction (Z-axis direction) as shown in
Next, the operation of the input demultiplexing optical system 2 will be described in reference to
As shown in
Here, it is possible to realize Fourier optics configuration by adjusting the distance between the second lens 16 and the light deflector array 20 to be equal to the focal length fy of the second lens 16, and the lights of respective wavelengths passing through the grating 10 pass through the second lens 16 and are then focused at different positions in the Y-axis direction while travelling in parallel, and are form images on the light deflector array 20.
On the other hand, in the switching plane, the light incident from the input optical fiber array 14 is spread in a widthwise direction (X-axis direction) by the enlarged-waveguide portion 11 of the channel waveguide 9, is focused by the waveguide lens 17, then propagates while being focused and is output to the wavelength switching optical system 3 (the light deflector array 20) as shown in
Since the distance between the waveguide lens 17 and the light deflector array 20 is set so that the light passing through the waveguide lens 17 is focused on the light deflector array 20, light beam forming an image on the light deflector array 20 is a beam waist of which spot diameter is the smallest. In addition, on one hand, the spot diameter of the waveguide lens 17 in a widthwise direction (X-axis direction) becomes large to some extent since a distance from the waveguide lens 17 to the focal point thereof is long, and on the other hand, the spot diameter in a vertical direction (Y-axis direction) of the second lens 16 having the focal length fy is small since a distance from the second lens 16 to the focal point thereof is shorter than that of the waveguide lens 17 and also the spot diameter of the light incident on the second lens 16 is large, which results in that the spot shape of the light beam forming an image on the light deflector array 20 becomes a horizontal oval shape.
Output Multiplexing Optical System
Next, the output multiplexing optical system 4 will be described.
As shown in
That is, the output multiplexing optical system 4 is formed by sequentially arranging the second lens 16, the grating 10, the first lens 15 and the waveguide array 8 from the wavelength switching optical system 3 side.
In the output multiplexing optical system 4, the grating 10 serves to re-multiplex the lights of respective wavelengths input from the wavelength switching optical system 3 (a light deflector array 21) and to make the multiplexed lights incident on the input/output ports 9b of the waveguide array 8. Since lights are output from the wavelength switching optical system 3 (the light deflector array 21) so that the order of the wavelengths is vertically revered in the dispersion plane, the grating 10 in the output multiplexing optical system 4 is arranged to be upside down as compared to the input demultiplexing optical system 2. The detail thereof will be described later.
In addition, in the output multiplexing optical system 4, the lens system 7 serves to convert the horizontal oval-shaped focal point of the light of each wavelength input from the wavelength switching optical system 3 (the light deflector array 21) back into a focal point having the same shape as an image of the output port. The second lens 16 serves to focus the light of each wavelength input from the wavelength switching optical system 3 (the light deflector array 21) and to output the focused light to the grating 10, and the first lens 15 serves to focus the light input from the grating 10 and to make the focused light incident on the input/output port 9b of the waveguide array 8.
The input/output port 9a of the waveguide array 8 of the output multiplexing optical system 4 is used as the output port 6 and is connected to an output optical fiber array 14.
Wavelength Switching Optical System
Next, the wavelength switching optical system 3 will be described.
As shown in
The two light deflector arrays 20 and 21 are oppositely arranged at respective focal positions of the lens systems 7 of the input demultiplexing optical system 2 and the output multiplexing optical system 4 (the focal position of the second lens 16 as well as the focal position of the waveguide lens 17). The light deflector arrays 20 and 21 have two-dimensional light deflection elements vertically and horizontally arranged so as to correspond to the light of each wavelength of each port and are configured to output incoming light of each wavelength after adjusting a horizontal reflection angle of the light.
In the present embodiment, a MEMS mirror array 30 composed of two-dimensionally arranged MEMS mirrors 31 which are light deflection elements is used as the light deflector arrays 20 and 21.
As shown in
Each one-dimensional MEMS mirror group 32 has substantially the same structure in which basic structures each composed of the MEMS mirror 31 and an actuator 33 for driving the MEMS mirror 31 are arranged in a vertical direction (Y-axis direction). Each MEMS mirror 31 can be rotated by changing voltage applied to the actuator 33 and it is thereby possible to freely deflect light beam.
Since a signal frequency interval (interval at the reciprocal value of the wavelength) in conventionally and typically used wavelength multiplexing communications is fixed at 100 GHz or 50 GHz, the MEMS mirror 31 is formed so that a pitch (interval in the array direction) W thereof is a width corresponding to such a signal frequency interval in case of applying to general wavelength multiplexing communications.
However, in recent years, a technique to transmit a large amount of information even at the same spectrum by respectively controlling optical phase and amplitude has been being developed and, in such a case, a spectrum width momentarily varies in accordance with data to be transmitted and it is thus not possible to handle by the MEMS mirror array 30 in which the pitch W of the MEMS mirrors 31 is a constant frequency interval of 50 GHz or 100 GHz as described above.
In order to address this problem, in the present embodiment, each one-dimensional MEMS mirror group 32 is configured such that the grouped plural MEMS mirrors 31 can be controlled to be inclined at the same angle so that the frequency interval to be switched can be adaptively changed. Desirably, the pitch W of each MEMS mirror 31 is set to correspond to a frequency interval of not more than 12.5 GHz and a gap between the adjacent MEMS mirrors 31 is set to not more than a spot-size of incoming light.
When the pitch W of each MEMS mirror 31 is set to correspond to a frequency interval of, e.g., 12.5 GHz, a spread signal spectrum of 37.5 GHz can be covered by grouping three MEMS mirrors 31 and a spread signal spectrum of 25 GHz can be covered by grouping two MEMS mirrors 31, as shown in
As such, the grouped plural MEMS mirrors 31 configured to reflect light at the same angle in a parallel manner can be used as one mirror. Very accurate parallelism is required in this case but the parallelism can be controlled by voltage applied to the actuator 33 and is finely adjustable, hence, no problem arises. In addition, since each gap between MEMS mirrors 31 is not more than the spot-size of incoming light, influence of the gap is ignorable.
A lens system of the wavelength switching optical system 3 will be described in reference to
The wavelength switching optical system 3 couples the input demultiplexing optical system 2 to the output multiplexing optical system 4 by using a lens system. The lens system is composed of the switching lens 22 acting only in a widthwise direction (X-axis direction) and the plural Fourier optical lenses 23 acting only in a vertical direction (Y-axis direction) and has a function of independently focusing lights in vertical and widthwise directions.
The switching lens 22 is a columnar lens (a convex lens as viewed from the top) having a focal length fx equal to the Rayleigh length and acting only in a widthwise direction, and is arranged between the light deflector arrays 20 and 21 so that distances from the two light deflector arrays 20 and 21 are both equal to the focal length (i.e., the Rayleigh length) fx. The switching lens 22 converts a horizontal angle of the light of each wavelength adjusted by the light deflector array 20 into a horizontal position (offset) on the light deflector array 21, thereby performing switching.
The focal length (i.e., the Rayleigh length) fx of the switching lens 22 is represented by the following formula (2):
fx=πω
0
2/λ (2)
where ω9 is a spot radius in the X-axis direction on the light deflector, fx is the focal length (the Rayleigh length) and λ is a wavelength of light.
It is generally known that light beam incident from the same distance as a focal length of a lens is Fourier-transformed after passing through the lens and propagating in the focal length such that a positional shift is converted into an angular shift and vice versa and an output spot diameter is converted into a size inversely proportional to an input spot diameter. However, in the lens of which focal length fx satisfies the above formula (2), diameters of input and output beam spots located at a distance fx before and after the lens are the same ω0.
The Fourier optical lens 23 acts only in a vertical direction and is provided in multiple stages so as to convert a vertical angle into a vertical position and subsequently the vertical position back into the vertical angle.
In the present embodiment, two semi-cylindrical lenses, the fourth lens 24 and the fifth lens 25, are used as the Fourier optical lenses 23 such that the fourth lens 24 is arranged between the light deflector array 20 and the switching lens 22 and the fifth lens 25 is arranged between the witching lens 22 and the light deflector array 21. The fourth lens 24 serves to convert the vertical angle into the vertical position and the fifth lens 25 serves to convert the vertical position back into the vertical angle.
Both of the fourth lens 24 and the fifth lens 25 are a lens with a focal length fy which is equal to each of a distance between the fourth lens 24 and the light deflector array 20, that between the fourth lens 24 and the switching lens 22, that between the witching lens 22 and the fifth lens 25 and that between the fifth lens 25 and the light deflector array 21. In this configuration example, in order to form an image of a light spot on the two facing light deflector arrays 20 and 21 in both of the dispersion plane and the switching plane, it is necessary to satisfy the condition of the following formula (3):
2·fy=fx (3)
It should be noted that the condition of (3) may not be satisfied when it is designed such that a compound lens formed by combining plural lenses is used as a lens group (the fourth lens 24, the fifth lens 25 and the witching lens 22) to relate the light deflector array 20 to the light deflector array 21 and equivalent focal lengths in X- and Y-axis directions are respectively fx and fy.
Although the lens having the same focal length as the second lens 16 is used as the fourth lens 24 and the fifth lens 25 here, a lens having a different focal length from the second lens 16 may be used. In this regard, however, the focal length of the fourth lens 24 needs to be the same as that of the fifth lens 25.
The wavelength switching optical system 3 is arranged so as to be inclined with respect to the input demultiplexing optical system 2 and the output multiplexing optical system 4 as viewed from a side (i.e., inclined with respect to the X-Z axis plane). Accordingly, the light deflector array 20 is arranged so that the light of each wavelength input from the Z-axis direction (from the input demultiplexing optical system 2) is reflected obliquely downward, and the light deflector array 21 is arranged so that the light input from obliquely above is reflected in the Z-axis direction (toward the output multiplexing optical system 4). The both light deflector arrays 20 and 21 are arranged so as to be 180-degree rotationally-symmetric about the X-axis and to have tilt angles which are reverse to each other. An inclination angle of the wavelength switching optical system 3 with respect to the input demultiplexing optical system 2 and the output multiplexing optical system 4 only needs to be appropriately determined to the extent that interference does not occur between the wavelength switching optical system 3 and the input demultiplexing optical system 2 or the output multiplexing optical system 4.
Next, the operation of the wavelength switching optical system 3 will be described in reference to
As shown in
The light of each wavelength input from the input demultiplexing optical system 2 is reflected by the light deflector array 20 and is Fourier-transformed by the fourth lens 24. Subsequently, the light is Fourier-transformed again by the fifth lens 25, is reflected by the light deflector array 21 and is output to the output multiplexing optical system 4.
In the dispersion plane, although the order of the focal positions corresponding to the wavelengths are vertically reversed as compared to those of the input lights due to effect of the fourth lens 24 and the fifth lens 25, the same spot diameter as that on the input-side light deflector array 20 can be reproduced on the output-side light deflector array 21. In the dispersion plane, the switching lens 22 basically does not exert any influence. In addition, the both light deflector arrays 20 and 21 adjust only a reflection direction in a widthwise direction (X-axis direction) (only operate one-dimensionally) and thus basically does not exert any influence in the dispersion plane.
On the other hand, as shown in
The reflection angle of the light of each wavelength input from the input demultiplexing optical system 2 is appropriately adjusted in a widthwise direction (X-axis direction) by the light deflector array 20 so as to correspond to a desired switching destination port and the light is then reflected. The reflection angle is controlled by voltage applied to the actuator 33 of the corresponding MEMS mirror 31. The light of each wavelength reflected by the light deflector array 20 passes through the switching lens 22 and is output to the light deflector array 21. Since the angular shift is converted into the positional shift by the switching lens 22 at this time, the light of each wavelength after passing through the switching lens 22 is converted into a parallel beam group of which position is different depending on the reflection angle, and the horizontal angle of the light of each wavelength adjusted by the light deflector array 20 is converted into a horizontal position on the light deflector array 21.
In other words, changing the applied voltage to the light deflector array 20 allows switching of the light of each wavelength to be performed onto the light deflector array 21 as indicated by a dashed line in
ω0−(fx·λ/π)1/2 (4)
In addition, the light of each wavelength can be reflected in a horizontal direction (Z-axis direction) and then output to the output multiplexing optical system 4 by appropriately inclining the deflection angle of the output-side light deflector array 21. In the output multiplexing optical system 4, the light of each wavelength is multiplexed per port and the multiplexed light is output from the output port 6 to each optical fiber 14a of the optical fiber array 14.
As described above, in the wavelength switching optical system 3, the light input from the input port 5 can be switched for each wavelength and output from a given output port 6 by changing voltages applied to the both light deflector arrays 20 and 21,
Although
In the wavelength switching optical system 3, the light of each port is independently switchable without exerting influence on each other and is independently switchable for each wavelength. Therefore, it is possible to independently switch an optical signal having a given wavelength, among optical signals input to a given input port 5, to a given output port 6 and it is thus possible to realize an M×N wavelength cross connect device 1 having an extremely high degree of freedom.
The effects of the present embodiment will be described.
In the wavelength cross connect device 1 of the present embodiment, the input demultiplexing optical system 2 and the output multiplexing optical system 4 are provided with the lens system 7 having a function of independently focusing lights in vertical and widthwise directions such that the spot shape on the light deflector arrays 20 and 21 is a horizontal oval shape.
Providing the lens system 7 having a function of independently focusing lights in vertical and widthwise directions allows ovality of light distribution on the light deflector arrays 20 and 21 to be controlled and it is thus possible to obtain a horizontal oval shape in which a spot diameter in a vertical direction (a demultiplexing direction) is small and a spot diameter in a widthwise direction (a switching direction) is slightly large.
The focal point in the demultiplexing direction (vertical direction) needs to be as small as possible in order to obtain good flat-top response but focal point in a direction of deflecting light beam at the time of switching (widthwise direction) needs to be large to some extent. The present embodiment satisfies these requirements by providing a horizontal oval-shaped focal point using the lens system 7 having a function of independently focusing lights in vertical and widthwise directions. As a result, it is possible to realize flat-top response and low crosstalk even by using the MEMS mirror 31 having a small area, and it is thus easy to provide multiple ports.
In addition, in the wavelength cross connect device 1, since the switching lens 22 having a focal length equal to the Rayleigh length and acting only in a widthwise direction is provided between the two oppositely arranged light deflector arrays 20 and 21 so that switching is performed by converting the angular shift into the positional shift at the switching lens 22, an image on the MEMS mirror 31 is a beam waist and a spot size is small. Therefore, the mirror area in the switching direction can be small and it is easy achieve integration and multiport.
Furthermore, in the wavelength cross connect device 1, the switching lens 22 acting only in a widthwise direction is used to allow light of each wavelength to be independently switched. Therefore, it is possible to realize the wavelength cross connect device 1 having an extremely high degree of freedom and the structure is simple and cheap since the number of spectrograph-demultiplexers to be connected to multiplex and demultiplex wavelengths does not need to be the same as the number of input/output ports unlike the conventional technique.
Still further, in the wavelength cross connect device 1, the waveguide lens 17 is used as the third lens. As the third lens, for example, a cylindrical lens array acting only in a widthwise direction may be used so as to correspond to the input/output port 9b of each channel waveguide 9 but such a lens array is difficult to manufacture and is expensive. The waveguide lens 17 is cheaper than such a lens array and can be easily manufactured.
In addition, in the present embodiment, the plural MEMS mirrors 31 are grouped and are controlled so as to be inclined at the same angle.
A conventional wavelength cross connect device does not have a problem when a wavelength in wavelength-multiplexing communications used for optical system is fixed since one MEMS mirror simply corresponds to one wavelength but it cannot be used when wavelength assignment changes over time. In recent optical communication, it becomes important to flexibly change wavelength by an optical signal modulation method, and in the present embodiment it is possible to accommodate change in the wavelength assignment over time by grouping the plural MEMS mirrors 31.
Since the optical system used in the present embodiment passes only twice through the grating 10 which is an element with relatively large loss, it is not necessary to repeatedly multiplex/demultiplex for several times by the grating unlike the conventional art and it is possible to realize the wavelength cross connect device 1 with low loss.
Next, other embodiments of the invention will be described.
A wavelength cross connect device 81 shown in
The semi-cylindrical lenses 82 are arranged in the vicinity of the light deflector arrays 20 and 21 so as to act on both of light propagating between the gratings 10 and the light deflector arrays 20, 21 and light propagating between the two light deflector arrays 20 and 21. The columnar lens 83 is arranged in the middle between the two light deflector arrays 20 and 21, and the two semi-cylindrical lenses 84 to be the switching lens 22 are arranged so as to sandwich the columnar lens 83 from both sides. Meanwhile, the light focusing function served by the second lens 16 in the wavelength cross connect device 1 is realized by a configuration in which a distance between the input/output port 9b and the first lens 15 is a distance Fy+ΔF which is slightly larger than the focal length Fy of the first lens 15.
The number of the lenses used in the wavelength cross connect device 81 is the same as that in the wavelength cross connect device 1 of
A wavelength cross connect device 91 of
A wavelength cross connect device 101 of
In addition, although the MEMS mirror array 30 is used as the light deflector arrays 20 and 21 in the present embodiment, it is not limited thereto and an LCOS (Liquid Crystal on Silicon) chip array 111 shown in
As shown in
Also in the case of using the LCOS chip array 111 as the light deflector arrays 20 and 21, an optical signal group having the same wavelength forms an image on the same vertical position (Y-axis direction) on each LCOS chip 112 and lights with the same wavelength are aligned in a widthwise direction (X-axis direction) in the same manner as the case of using the MEMS mirror array 30.
As shown in
That is, the LCOS chip 112 used here is different from atypical LCOS chip and has the ¼ wavelength layer 117 formed between the liquid crystal layer 118 and the reflective film 116. The liquid crystal layer 118 of the LCOS chip 112 can change a refractive index of only a polarized component which vibrates in one axis direction.
However, by forming the ¼ wavelength layer 117, incident S-polarized light is reflected by the reflective film 116 and is subsequently converted into P-polarized light or the P-polarized light is converted into the S-polarized light after being reflected in the same manner. This allows the liquid crystal layer 118 to act on both polarized lights to change a refractive index and it is thus possible to realize polarization independence.
In the LCOS chip 112, it is possible to change a refractive index of the liquid crystal layer 118 of the pixel 113 by applying voltage to each pixel 113 which constitutes the LCOS chip 112. For example, voltage applied to each pixel 113 is adjusted so that the liquid crystal layer 118 has sawtooth-shaped refractive-index distribution with a periodic repetition of 0 to 2π as shown in
Alternatively, the light deflector arrays 20 and 21 may be the LCOS chip 112 in one-piece as shown in
In addition, multicast (branch output) which cannot be realized by the MEMS mirror array 30 can be realized when the LCOS chip is used as the light deflector arrays 20 and 21. For example, two orders of diffracted light is mainly exited when rectangular binary refractive-index distribution as shown in
As shown in
The node device 132 is provided with three network interfaces (NW interfaces) 134 corresponding to the three nodes, and the pair optical fibers 133a are connected to the respective corresponding network interfaces 134. In addition, the node device 132 is provided with a TX/RX bank 137 which includes plural wavelength-tunable optical receivers (λ-RX) 135a and plural wavelength-tunable optical transmitters (λ-TX) 135b.
This node device 132 is configured such that the three network interfaces 134 are connected to a wavelength cross connect device 130 of the invention via respective pair optical fibers 133b, and Drop ports 136a and Add ports 136b of the TX/RX bank 137 are connected to the wavelength cross connect device 130 of the invention. Even in the case of further including a backup TX/RX bank, it is only necessary to connect Drop and Add ports of the backup TX/RX bank to the wavelength cross connect device 130.
Meanwhile, a system configuration in a case of using a conventionally-used 1×N wavelength-selective switch (WSS) is shown in
As understood by comparing
The invention is not intended to be limited to the embodiments and it is obvious that the various kinds of changes can be added without departing from the gist of the invention.
Number | Date | Country | Kind |
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2012-090182 | Apr 2012 | JP | national |