Patent Application
20050036193
This invention relates to the domain of optics, and more precisely to the selection of optical beam spectral components, enabling treatment of the optical beam as a function of the wavelengths or particular spectral bands.
Selection of the wavelengths or spectral bands enables a wide variety of applications, particularly in the field of optical telecommunications (particularly at high speeds), for example gain or channel equalisation (or attenuation) of spectrum or spectral bands, extraction and/or insertion of wavelengths (OADM), switching and optical routing. It also can be used for applications in the field of spectroscopy.
Different optical techniques using free space have been proposed to make a selection of spectral components of an optical beam. According to the state of the art, a dynamic spectrum equaliser uses an optical demultiplexer that transforms the spectral multiplex into a spatial multiplex. Each wavelength is thus separated and then imaged on a programmable element (for example a spatial light modulator (SLM) placed in an imagery plane of the input plane.
According to the state of the art, this programmable element may have different functions, and particularly:
This deviation technique involves different approaches:
The purpose of all these techniques is to deteriorate the coupling balance and thus to selectively act on the spectrum of wavelengths, the optical fibre in this case acting as a spatial filter.
Another approach to the state of the art uses the interferometric principle (for example a Mach-Zehnder interferometer), the spectral treatment being obtained by cascading interferometers tuned on a fixed wavelength, for example using a Bragg grating, and acting as the corresponding number of spectral filters (as mentioned in patent documents US 2003/0035616 (“add-drop multiplexer with a signal amplification capacity), U.S. Pat. No. 6,424,763 (“add-drop filter using a resonating tunnel with coupling on the side”) by MIT® and JP2001109022 (“add-drop optical multiplexer with a switching function”) by the NTT® Company.
Wavelength selection or attenuation is obtained by different techniques such as the introduction of a variable delay on one of the two arms of the interferometer (for example as shown in patent document EP0783127 entitled “Mach-Zehnder interferometric coupler with monomode optical fibre).
These different techniques have the disadvantage that they use Bragg gratings centred on fixed wavelengths. Therefore they do not enable continuous selection of bands or wavelengths.
Furthermore, these techniques have the disadvantage of cascading losses, which causes severe losses, particularly when the guide is long.
Furthermore these techniques make it possible to process a relatively small number of channels.
The invention and its various aspects are intended particularly to overcome these disadvantages according to prior art.
More precisely, one purpose of the invention is to supply a compact and robust system for selection of spectral components of an optical beam.
Another purpose of the invention is to provide an optical selection system that is relatively simple to implement and is easily reconfigurable.
Another purpose of the invention is to enable optical selection of spectral components that is easy to control reliably.
Another purpose of the invention is to enable a selection that enables fast variation of the selected wavelength(s) and is particularly well adapted to high-speed telecommunication applications (in particular routing, equalisation, and switching).
These objectives and others which will be referred to later are achieved by the invention using an interferometric system for selection of spectral components of an incident optical beam as a function of their wavelength, the system comprising:
Thus, as a function of phase shifts specific to each of the wavelengths (or spectral bands) of the phase shifted beams, the first and second output beam each comprise all or part of the spectral components of the input signal, in a complementary manner. Therefore, the system can select one or several spectral components, some of these components possibly being equalised (for example the case in which one of the output beams is considered, the attenuation being a function of the phase shift applied to the phase shifted beams), blocked (the attenuation being maximum for the wavelength considered), switched towards one of the output fibres, etc.
According to one particular characteristic, the system is remarkable in that the first phase shift is equal to approximately zero modulo 2π radians, the components of the phase shifted beams being approximately in phase, and in that the second phase shift is approximately equal to π modulo 2π radians, the components of the phase shifted beams being approximately in phase opposition.
Thus, it is relatively simple to block a wavelength or a spectral band, or to route it with minimized optical losses.
According to one particular characteristic, the system is remarkable in that the means of transforming the incident optical beam comprise means for imagery and spatial demultiplexing of at least one optical beam and splitting means.
According to the invention, the splitting means act either on the incident beam or on spatially demultiplexed beams.
According to one particular characteristic, the system is remarkable in that the imagery and spatial demultiplexing means are placed on the optical path between the splitting means and the phase shifting means.
According to one particular characteristic, the system is remarkable in that the splitting means are placed on the optical path between the imagery and spatial demultiplexing means and phase shifting means.
According to one particular characteristic, the system is remarkable in that the imagery means are adapted to propagation of an optical signal in free space.
According to one particular characteristic, the system is remarkable in that the imagery means comprise at least one first lens with a first focal length and a second lens with a second focal length, and in that:
This embodiment is compatible with many applications, and in particular enables introduction of any optical element (for example demultiplexer, filter, etc.) on the optical path particularly in the Fourier plane of the first and second lenses (plane at a distance equal to a multiple of the corresponding focal lengths of the lenses).
According to one particular characteristic, the system is remarkable in that the spatial demultiplexing means are located in a Fourier plane of the first and second lenses.
According to one particular characteristic, the system is remarkable in that the means of transforming the incident optical beam comprise a demultiplexer guided in planar optics, adapted to demultiplexing of at least one optical beam, and splitting means.
Thus, the system is particularly robust.
According to one particular characteristic, the system is remarkable in that the phase shifting means are placed in an imagery plane of the imagery means so as to enable a phase shift as a function of each spatially multiplexed component.
According to one particular characteristic, the system is remarkable in that the phase shifting means include programmable phase shifting means.
In this way, it is possible to control the phase shift as a function of determined wavelengths or spectral bands.
According to one particular characteristic, the system is remarkable in that the programmable phase shifting means are adapted to delay an optical beam in a variable manner and belong to the group comprising:
In particular, the phase shifting means can be made easily in the form of a strip. The optical materials may or may not be isotropic, and may for example be nano-PDLC associated with controllable electrodes.
According to one particular characteristic, the system is remarkable in that the phase shifting means include non-programmable phase shifting means.
In particular, these phases shifting means may be added to compensate an optical path. For example, they may be delay lines made in free space or from a fibre.
According to one particular characteristic, the system is remarkable in that the phase shifting means are associated with at least one mirror.
Thus, the system is particularly easy to implement, at least part of the transformation means being common with the combination means, the splitting means being used to recombine two beams by reverse return.
According to one particular characteristic, the system is remarkable in that the spatial demultiplexing means belong to the group comprising:
According to one particular characteristic, the system is remarkable in that the splitting means comprise a splitter.
Thus, the splitting means are used in the form of a separating blade or a separating cube.
According to one particular characteristic, the system is remarkable in that the splitting means comprise a coupler.
According to one particular characteristic, the system is remarkable in that the recombination means and the transformation means have at least one part in common.
In particular, this simplifies the system and reduces the manufacturing cost.
According to one particular characteristic, the system is remarkable in that it includes means within the group comprising:
Thus, the invention is particularly well adapted to communication links or networks, particularly at high speeds.
According to one particular characteristic, the system is remarkable in that it includes means of measuring the optical spectrum of the incident optical beam.
The system thus enables a very wide variety of applications, particularly in the field of spectroscopy or even telecommunications (for “monitoring” optical beams).
According to one particular characteristic, the system is remarkable in that it forms a monolithic component.
In particular, this makes it compact and very simple to use.
According to one particular characteristic, the system is remarkable in that the transformation means and the phase shifting means are separate.
Thus, the system can easily be configured as a function of the different applications.
Other characteristics and advantages of the invention will become more obvious after reading the following description of a preferred embodiment given as an illustrative and non-limitative example, and the attached drawings among which:
The invention is based on a solution using free space or being extended to planar optics. The invention enables attenuation and/or selection of wavelengths or spectral bands based on the interferometric division—recombination (DRI) principle, applied independently and in parallel on wavelengths output from the WDM signal, unlike the state of the art in which the same result is obtained by cascading the interferometers which is more complex to implement.
More precisely, according to the invention, the interferometric part is materialised by a block for splitting—recombination of two beams (DRI) (the fibre coupler or the beam splitter). The intensity can be modulated almost continuously by varying constructive or destructive interferences (free space) or by coupling modes (case of the guide), and in particular, very good extinctions on the selected wavelength or spectral band can be obtained during phase opposition. Therefore, this system is compatible with existing specifications for making dynamic selectors of spectral bands or wavelength blockers requiring rejections exceeding 35 dB (corresponding to optical amplifiers in terms of noise; part of the optical signal, attenuated or blocked by a DSE (Dynamic Spectral Equaliser) should not be amplified again by passing through optical amplifiers.
The invention proposes an equalisation system comprising three independent optical blocks:
More precisely,
According to
According to
More precisely, the phase shifted beams are recombined as a function of the phase shift value:
The imager block 42 shown is an imagery system in free space of the 4f type that comprises the following elements in sequence along its optical axis that is parallel to the direction of propagation of the incident beam:
According to one variant, the lens 421 is integrated into an input/output fibre.
The imager block 42 shown is an imagery system in free space of the 4f type, where f represents the focal length of the lenses 421 and 422 with the dispersive element 422 placed in the filter plane of the assembly. The lens 421 is placed at a distance f from the input 420 (to which the fibre 1 is attached) and from the dispersive element 422. Therefore, the input signal converges on the imager block 42.
The lens 423 is also placed at a distance f from the dispersive element 422. Moreover, the equalisation systems 3 and 4 are such that there is an optical distance f between the lens 423 and the mirror 44 so as to produce a real image of the signal before it passes through the dispersive element 422, on the mirror 44. As shown symbolically in
According to one variant of the invention, the lenses 421 and 423 have different focal lengths, equal to f1 and f2 respectively. The lens 421 is then placed at a distance f1 from the input 420 and the dispersive element 422. The lens 423 is placed at a distance f2 from the dispersive element 422. The equalisation systems 3 and 4 are also such that there is an optical distance f2 separating the lens 423 and the mirror 44.
According to another variant of the invention, the imager block is of the AWG (Array Wave Guide) type. An AWG type imager block corresponds to a combination of phasars (delay lines in the form of wave guides) with distinct lengths. Due to the different lengths of the phasars, the block includes an intermediate region that is the Fourier plane between inputs/outputs with an intermediate matrix of active modulators that equalises the channels. It is a planar optical solution with a higher cost per channel than a solution in free space. Insertion losses, PDLs (Polarisation Dispersion Losses) and PDM (Polarisation Dispersion Mode) losses are also higher.
The interferometric block DRI 41 comprises at least one element separating the incident optical beam into two independent beams, and recombining them either to one input or to an output by inverse return as a function of the delay generated between the two beams in the spatial modulator block through the demultiplexer imager block. The spatial modulator optionally includes a variable element that can be controlled either to delay or shift the phase of the optical paths between the two arms, and a mirror.
a to 3c show different variant embodiments of the DRI 41 according to the invention used in a configuration of the optical equaliser as shown with reference to
According to one embodiment shown with reference to
According to one variant of the invention, these beams are collimated, for example using a collimation optics coupled to fibre 1.
The two beams pass through the imager/demultiplexer block 42. The spectral components of one of the beams (according to a Michelson type configuration) or of the two beams (according to a Mach-Zehnder type configuration) are imaged laterally on the phase shifters 43 and therefore phase shifted by the phase shifters 43. The two beams are then reflected by the mirror 44, pass through the imager/multiplexer system 42 and then along an inverse path, and recombine on the splitter 411 of the DRI 41. Depending on the nature of the phase shift between the two beams, a recombination is obtained on fibre 1 if the phase shift is zero and on fibre 3 if the phase shift is equal to π (which is used particularly for wavelength blocker, spectral band selector or optical routing type applications). If the phase shift is equal to other modulo 2π values, the recombination is made on the two fibres 1 and 3 at a proportion that depends on the phase shift (so that the incident beam can be equalised).
b shows a DRI 416 comprising a splitter 4165 and two prisms 4166 and 4167 with total reflection. An incident beam 4162 output from the fibre 1 is firstly separated into two beams by the splitter 4165. Each beam is then reflected several times onto one of the prisms 4166 or 4167. Thus, in this set up, a fibre emulates two secondary virtual sources 4161 and 4162 that produce two parallel output beams 4163 and 4164. After processing by the imager 42, the phase shifter 43 and the mirror 44, similar to that described with reference to
According to another variant embodiment of the DRI shown with reference to
Taking account of the demultiplexing device 420 used, each wavelength λi is imaged laterally on an independent phase shifting element and is reflected as indicated in
A programmable element is placed on at least one of the two interferometer arms and can be used either to delay (shift the phase) or to extend optical paths as required on the corresponding arm(s) so as to modify interference formation conditions between two beams output from the interferometric block and to select outputs.
According to the invention, the two outputs from the DRI block 41 imaged in the plane of the phase shifter (SLM (Spatial Light Modulator) corresponding to a sequence of independent phase shifters forming a strip comprising several pixels), are modulated independently by means of a variable delay. This variable delay is obtained according to the invention particularly by extending the optical path (for example using an SLM comprising MEMS type deformable micro-mirrors to cause a delay in the optical path) or by a modulation of the index (for example using liquid crystal or nano-PDLC in the SLM) on one or both of the two arms.
a shows the case in which the index is modulated on a single arm. According to this embodiment, a first incident beam 430 is imaged on the phase shifter 43 comprising a strip SLM enabling variable delays. The beam 430 comprises different components associated with distinct spatially separated wavelengths λi. Each of these components passes through a strip element (pixel) of the phase shifter 43, assigning a particular value to this component specific to the pixel concerned on the strip. Thus, the delay affecting components of the beam 430 depends on the state of the pixel in the strip through which this component passes, and therefore on the spatially multiplexed wavelength. The phase shifted incident beam is reflected by the mirror 44 and once again passes through the phase shifter to produce a reflected signal 431 comprising spatially multiplexed components for which each of the wavelengths is assigned a phase shift that depends on this wavelength.
A second incident beam 432 is simply reflected by the mirror 44 and no phase shift dependent on this wavelength is applied to it.
b shows the case in which an index modulation is made on two arms. In this case, two incident beams 435 and 437 are imaged on phase shifters 433 and 435 respectively, similar to the phase shifter 43 before being reflected by a mirror 439 and passing once again through the phase shifters 433 and 435. Thus, each of the wavelengths of the output beams 436 and 438 is assigned a phase shift that depends on this wavelength.
According to one embodiment with two phase shifted beams, the difference compared with a conventional interferometer lies in the materialisation of an intermediate interference plane (Fourier plane) made possible by a 4f imagery system in free space, enabling the introduction of a dispersive element acting in common and independently on the two beams (arms) and the result of which is to make the interferometer spectrally parallel, this operation being difficult to obtain with a conventional interferometer. The relative delay between the two beams (each associated with one arm) is introduced in the imagery plane of the 4f assembly, in which these two beams are separated and dispersed as a function of the wavelength (according to the embodiments shown with reference to
a shows the system 41 presented with reference to
According to one variant shown with reference to
If it is only a mirror and provided that the optical paths are equivalent, the two beams reformed in the output plane are in phase and the entire energy is recombined in the input fibre (except for system insertion losses).
On the other hand, if the phase of one of the two beams is shifted with respect to the other in the imagery plane, a delay is generated at the recombination and a different energy distribution is observed on the two output arms of the interferometer, that depends on this phase shift.
According to one variant of the invention that is relatively easy to use, the equalisation system uses a conventional demultiplexer (particularly for the first arm) provided with an element that can independently delay each of the spectrum wavelengths and recombine them. The second arm of the interferometer is composed of a delay line that emulates an equivalent path on the non-demultiplexed spectrum (shown with reference to
a shows this type of embodiment and more precisely an equalizer comprising a DRI associated with an input fibre 1, an output fibre 3 and with two inputs/outputs separating an input signal on the two arms 50 and 60 and recombining the signal returned by these two arms 50 and 60. Elements similar to elements in the previous figures are marked with the same references and therefore are not described further. The arm 60 is a 4f imagery system comprising two lenses 601 and 603 with focal length f and a mirror 605. The first lens 601 is placed at a distance f from the corresponding output of the DRI 5; the two lenses 601 and 603 are separated by a distance 2f, and the mirror 605 is placed at a distance f from the lens 603. Thus, the optical signal passes through a distance equal to 1f in the two arms 50 and 60.
According to one variant, the two lenses 601 and 603 have different focal lengths f1 and f2 respectively. The first lens 601 is placed at a distance f1 from the corresponding output of the DRI 5; the two lenses 601 and 603 are separated by a distance equal to the sum of the focal lengths f1 and f2 and the mirror 605 is placed at a distance f2 from the lens 603. Thus, the optical signal passes through a distance 2×(f1+f2) in the arms 50 and 60.
According to one embodiment shown with reference to
In other words, the arms 60 and 61 create optical paths with length equivalent to the length of arm 50.
According to one variant, the equaliser arm, comprising the delay line (particularly arm 60 or 61) is provided with an additional variable attenuator, thus enabling balancing of power at the coupler to maximise the rejection rate.
According to another variant, the equaliser arm, comprising the delay line (particularly arm 60 or 61) is provided with a compensating splitter to correct the unbalance in the arm lengths that could degrade transmission of very high-speed signals.
a to 7c illustrate a system 4 in which the interferometric block 41 is inserted between the phase shifting element 43 and the imager/demultiplexer system 42 as shown with reference to
Obviously, the DRI 41 is used in a similar manner:
Only the nature of the beams processed by the DRI is associated with the architecture of the optical system.
According to this variant, the interferometric block 41 acts directly on the demultiplexed signals, just before passing through the strip(s) of the variable phase shifter 43. This block is easy to make. For example it may be a splitter (semi-transparent) and in practice for practical reasons a separating cube placed in contact with the spatial modulator(s) according to the principle shown in
As shown with reference to
According to variants shown with reference to
According to the variant in
On the other hand, according to the variant in
Depending on the phase shift Δφ between the arms 81 and 82, an output corresponding to one of the inputs/outputs is activated and the intensity function I1 and I2 is given on each channel 85 and 86 respectively by the relations:
where I0 represents the intensity of the incident signal (optical losses being neglected).
Thus, if one of the channels has to be identified as the output channel and the second as the input channel of the device used as is the case here in a set up in reflection, an isolator must be added on the input channel to eliminate the power that could be recoupled in it. This type of solution has the advantage of materialising a DCE input and output (compared with more frequently used embodiments in which the input fibre acts as the output fibre), or to replace a circulator by an isolator.
Embodiments based on a coupler have the advantage of being compact, robust and easy to use. On the other hand, resources at phase shifting must be sufficient to balance optical paths on the output side (for example imagery plane) or a compensation element needs to be introduced on one of the two arms to balance the coupler arms.
Obviously, the invention is not limited to the example embodiments given above.
In particular, those skilled in the art could add any variant to the geometry and embodiments of the DRIs, phase shifters and demultiplexer imager blocks.
In particular, according to the invention, DRIs are used in the form of any system that makes splitting and recombination by inverse return (the DRI splitter is used by the retropropagated beam as a recombiner), and is associated with a multiplexer/demultiplexer.
The optical system according to the invention may be used in a monolithic or modular form. The modular structure, for example comprising three blocks (DRI, imager and phase shifter) enables the user to use several technologies. It also enables a simple adaptation to the required characteristics (particularly an adaptation to the number of channels, the resolution, the pass-band). The cost of the monolithic structure is usually lower and it is more robust.
The invention has many applications, in particular corresponding to applications using any technique enabling variable rejection of spectral bands along the width of the band and in attenuation, or a selection of spectral bands (particularly spectral band routing). Thus, some of the main applications of the invention are:
| Number | Date | Country | Kind |
|---|---|---|---|
| FR 03 09412 | Jul 2003 | FR | national |
