Liquid crystal modulator and polarization diversity optics for optical communications

• US 6,801,310 B2
• Filed: 08/02/2002
• Issued: 10/05/2004
• Est. Priority Date: 11/07/2001
• Status: Expired due to Fees

• Current Assignee: Arroyo Optics, Inc. (NVIDIA Corporation)
• Original Assignee: Arroyo Optics, Inc. (NVIDIA Corporation)
• Inventors: Rakuljic, George, Kewitsch, Anthony S., Leyva, Victor
• Primary Examiner(s): Evans, F. L.
• Assistant Examiner(s): GEISEL, KARA E
• Application Number:
  US10/209,879
• Publication Number:
  US 20030095305A1
• Time in Patent Office: 795 Days
• Field of Search: 356/328, 356/334, 356/323, 398/79, 398/82-84
• US Class Current: 356/328
• CPC Class Codes: G02B 6/2713 cascade of polarisation selective or adjusting operations
• CPC Class Codes: G02B 6/272 comprising polarisation means for beam splitting and combining
• CPC Class Codes: G02B 6/2773 Polarisation splitting or combining
• CPC Class Codes: G02B 6/2793 Controlling polarisation dependent loss, e.g. polarisation insensitivity, reducing the change in polarisation degree of the output light even if the input polarisation state fluctuates
• CPC Class Codes: G02B 6/2931 Diffractive element operating in reflection
• CPC Class Codes: G02B 6/29311 Diffractive element operating in transmission
• CPC Class Codes: G02B 6/29313 characterised by means for controlling the position or direction of light incident to or leaving the diffractive element, e.g. for varying the wavelength response switching G02B6/35
• CPC Class Codes: G02B 6/29391 Power equalisation of different channels, e.g. power flattening
• CPC Class Codes: G02B 6/29395 configurable, e.g. tunable or reconfigurable switching G02B6/35
• CPC Class Codes: G02B 6/29397 Polarisation insensitivity
• CPC Class Codes: G02B 6/29398 Temperature insensitivity
• CPC Class Codes: G02B 6/32 having lens focusing means positioned between opposed fibre ends with lens being an integral part of the single fibre end G02B6/262
• CPC Class Codes: H04J 14/02 Wavelength-division multiplex systems
• CPC Class Codes: H04J 14/0221 Power control, e.g. to keep the total optical power constant

Abstract

For systems which disperse individual wavelength components of a DWDM beam into an array of converging beams, the individual wavelength signals are modified for blocking, equalization or other purposes by reflective liquid crystal cells. Thus modulated or modified components are then recombined by the system into an output beam, as by reverse passage through the system. Controlled full extinction or linear attenuation may be introduced by converging asymmetrical beams of separate polarization components for each wavelength into superposed relation on zero twist nematic crystal cells which are voltage controlled so as to retard for extinction of greater than 40 dB or to transform the state of polarization to a selected angle for attenuation. Polarization sensitive elements in the return paths of the reflected beams then filter the rejected components.

Claims

1. A system for individually modifying wavelength components of a DWDM beam of an arbitrary state of polarization comprising a combination of:

a diffractive beam separation system including spaced apart reflector surfaces for separating the wavelength components of the DWDM beam;

an optical system converging the separate components from the beam separation system into an array of beams of different wavelength forming beam waists serially dispersed in a sagittal direction at a focal plane;

a polarization splitter device disposed in the optical path for separating the polarization components of the wavelength components into adjacent beams;

a plurality of liquid crystal cells dispersed at the focal plane in an array along the sagittal direction and each responsive to the polarization components of a different wavelength component to transform the polarization direction of the components individually to selectable orientations, the cell array being configured to direct the transformed wavelength components back through the optical system for refraction and combination into a modified DWDM beam, and

polarization diversity optics including at least one polarization sensitive element proximate each of the liquid crystal cells for rejecting, in the transformed wavelength components, polarization components of other than a selected orientation.

2. A system as set forth in claim 1 above, wherein the cell is a nematic element having a selected angle of alignment and the at least one polarization sensitive element has an optical axis in predetermined relation to the alignment angle to reject a proportion of the transformed polarization components.

3. A system as set forth in claim 2 above, wherein the beam waists at the focal plane are diffraction limited and wherein the polarization sensitive element is of birefringent material.

4. A system as set forth in claim 2 above, where beams at different wavelengths are propagated toward the cells of the array, the cells of the array are reflective, and the at least one polarization sensitive element is disposed in the path of beams reflected from the cells, the optical axis of the at least one polarization sensitive element being disposed to transmit beams toward the cells in the array without attenuation and to reject selectively transformed polarization components in beams reflected from the cells.

5. A system as set forth in claim 4 above, wherein the at least one polarization sensitive element is a polarizer plate angled to reject components by blocking the light energy thereof.

6. A system as set forth in claim 4 above, wherein the polarization sensitive element is at least one birefringent polarization beam displacer positioned with its optical axis to divert rejected components from the wavelength components.

7. A system as set forth in claim 4 above, wherein each cell comprises a zero twist nematic liquid crystal cell having a selected alignment axis, wherein the at least one polarization sensitive element has an optical axis in a selected relation to the alignment axis, wherein the cells are voltage controllable and the system includes voltage control circuits coupled to the individual cells.

8. A system as set forth in claim 7 above, wherein the optical system converges separate polarization components at each wavelength and with like polarization directions in superposed relation on the liquid crystal, and wherein the liquid crystal is disposed to return diverging transformed beams along reciprocal paths to the incoming beams.

9. A system as set forth in claim 8 above, wherein the system includes, proximate each liquid crystal cell, a polarizer plate and a quarter waveplate disposed adjacent the focal plane and introducing π

/2 round trip phase retardation and circular polarization in the reflected beam at an angle determined by the control voltage.

10. A system as set forth in claim 2 above, wherein the optical system provides the array of beams of different wavelengths as single beams of arbitrary states of polarization, and wherein the at least one polarization sensitive element comprises at least two polarization beam displacers for (1) splitting the single beam for that wavelength into two spaced apart beams of orthogonal polarization and (2) recombining transformed beams from the liquid crystal cell into a single beam while rejecting components in accordance with the beam transformation by the cells.

11. A system as set forth in claim 10 above, wherein the liquid crystal cell is reflective, and wherein two polarization beam displacers at different optical axis alignments are disposed serially proximate the cell, and configured to direct lossy components after beam transformation out of the optical path of useful signals.

12. A system as set forth in claim 10 above, wherein the polarization beam displacers have optical axes in orthogonal relation and at 45°

to the alignment angle of the liquid crystal cell.

13. A system as set forth in claim 2 above, wherein the optical system further includes an input/output structure having different elements disposed at different elevations transverse to the sagittal direction, and the cells and polarization diversity optics are configured to direct transformed wavelength components to either of selected ones of the different elements of the input/output structure.

14. A system as set forth in claim 13 above, wherein said at least one polarization sensitive element comprises at least two polarization beam displacers responsive to the beam transformation at the liquid crystal cells and directing beams at separate elevations corresponding to the elevation differential at the input/output structure back through the optical system to the input/output structure.

15. A system as set forth in claim 14 above, wherein the system includes polarization diversity optics including the said polarization beam displacers, and wherein said polarization beam displacers comprise a series of polarization beam displacers and interspersed polarization rotators for diverting drop signals to a first elevation and express signals to a second elevation, wherein the differential between the first and second elevations corresponds to the difference in elevation at the input/output structure.

16. A system as set forth in claim 13 above wherein the input/output structure includes polarization splitting elements between the input/output structure and the diffractive beam separation system, and the at least one polarization sensitive element comprises two polarization beam displacers disposed between the diffractive beam separation system and the liquid crystal cells for converging polarization components directed toward the liquid crystal cells onto a single beam surface, and returning transformed reflected beams of selected ellipticity and azimuth back to the beam diffraction system at a different elevation for coupling to the input/output structure at that elevation.

17. A system as set forth in claim 2 above, wherein the plurality of liquid crystal cells comprise cells having sagittal dimensions and pitch such that each incident beam at the focal plane impinges on a substantial number of cells, and wherein the system further includes control electronics coupled to the cells for providing control voltages which determine the local transformation introduced at each cell, such that the transformed components of an individual beam form a composite transformed beam having a number of differently transformed elements.

18. A system as set forth in claim 17 above, wherein the cells are configured with a pitch of less than 2 microns per cell, and the cells in the transverse dimension are about two orders of magnitude or more greater than the sagittal dimension, and wherein each beam is incident on at least about 5 cells, and wherein the cells are zero twist nematic liquid crystal cells, and wherein the at least one polarization sensitive element comprises a single element spanning the entire array of liquid crystals.

19. A system as set forth in claim 2 above, wherein the polarization splitter device is disposed in the path of the DWDM beam before the diffractive beam separation system, for providing diverging beams of different polarization at a selected angle of divergence and the polarization splitter device includes elements for placing the divergent beams in parallel polarization relation while retaining equal path lengths prior to the beam separation system.

20. A system as set forth in claim 19 above, wherein the polarization splitter device comprises a Wollaston prism device, a polarization rotator arranged to rotate one polarization component into parallelism with respect to the other and an optical element for equalizing optical path lengths.

21. The system as set forth in claim 20 above, wherein the angle of divergence between the polarization components is less than about 2°

, wherein the optical system provides separated wavelength components that are substantially longer in the transverse direction than in the sagittal direction and convergent to superimpose the beam waists at the focal plane, and the liquid crystal cells are reflective and each have a substantially greater area than the area beam waists of the impinging beams.

22. A system as set forth in claim 2 above, wherein the cells are reflective and the polarization splitter comprises at least two birefringent elements each disposed as serial pairs adjacent the liquid crystal cells subsequent to beam separation, and wherein the at least two polarization sensitive elements have optical axes disposed at different angles to direct rejected components in diverging paths out of the field of view of signal components to be returned to the diffractive beam separation system.

23. A system as set forth in claim 22 above, wherein the system includes polarization diversity optics comprising a single polarized plate before the reflective cells of the array, an input half waveplate before the polarized plate and with optical axis aligned to rotate the polarization direction in a beam propagated toward the cell, the polarized plate and input half waveplate each being configured to span the length of the array, and wherein each cell also includes, immediately prior to the liquid crystal cell, a quarter waveplate providing a fixed phase retardation such that for zero control voltage, the cell is in full transformation state.

24. A system for blocking or modulating the intensity of individual wavelength components in a DWDM optical beam of arbitrary polarization, comprising:

a beam refolding system having facing and spaced apart grating and concave reflector devices, the grating comprising a Littrow grating and the reflector device comprising a Mangin minor providing a convergence factor in the sagittal direction and a collimating factor in the transverse direction, with the grating and the mirror surfaces spanning substantially the same elevations in the transverse direction;

a beam polarization splitter dispersed in the path of the input beam before the beam refolding system;

an input optical structure disposed adjacent the Littrow grating and directing a DWDM beam through the polarization beam splitter and toward the reflector device at a given angle of inclination in the transverse direction, the input optical structure providing an anamorphic beam having its major dimension in the sagittal direction, the Littrow grating and reflector device being configured to serially refold the anamorphic beam while dispersing the wavelength components sagittally to converge to beam waists at a focal plane;

polarization diversity optics comprising at least one polarization sensitive element in the path of the converging diffracted beam components between the beam refolding system and the focal plane, and

an array of reflective liquid crystal cells at the focal plane, the liquid crystal cells being individually controllable to transform the polarization of the beam components to selectable orientations, the at least one polarization sensitive element being oriented to reject polarization components of other than the selected orientation, and to redirect the reflected beam components back through the beam refolding system and the beam polarization splitter to the input optical structure.

25. A system as set forth in claim 24 above, wherein the beam polarization splitter divides the DWDM beam into orthogonally polarized components, and wherein the system further includes an optical device, including a waveplate for aligning the polarization components in the same direction while maintaining equal optical path lengths.

26. A system as set forth in claim 25 above, wherein the liquid crystal cells are zero twist nematic cells having a predetermined optical axis, and wherein the beam refolding system is configured to superimpose the polarization components of the dispersed wavelength components at the focal plane in a selected orientation to the predetermined axis, and wherein the local intensity of an incident beam on a cell does not exceed about 200 W/mm².

27. A system as set forth in claim 26 above, wherein the polarization components are aligned in the parallel and the sagittal directions, wherein the polarization components are converged onto the focal plane at different angles, wherein the dispersed wavelength components are sagittally separated and asymmetric with a long dimension in the transverse direction, and wherein the sagittal pitch of the cells are varied to receive incident wavelength components centrally.

28. A system as set forth in claim 27 above, wherein the dispersed wavelength components are about 200-250 microns in the transverse dimension, and have, including sagittally dispersed sidebands, sagittal dimensions that are substantially less than the transverse dimension, and wherein the cells have a sagittal dimension more than the optical intensity span of the sagittal dimension of the beam.

29. A system as set forth in claim 28 above, wherein the converging polarization components follow different paths of about 3°

and 2°

relative to a sagittal plane and about ±

0.5°

relative to the cell surface and the cell reflects each of the components along the incident beam path of the other component and wherein the at least one polarization sensitive element is a polarizer having a selected optical axis blocking polarization components reflected from a cell at other than a predetermined orientation.

30. A system as set forth in claim 27 above, wherein the at least one polarization sensitive element comprises at least one polarization beam displacer having an optical axis disposed to divert polarization components reflected from the cell at other than a predetermined orientation from the beam path of oriented components.

31. An array for controllably modulating individual optical beams of different wavelengths with different ones of a plurality of voltage controllable cells, wherein each of the cells receives a different incident optical beam disposed along a sagittal direction and comprises:

a liquid crystal element having a light transforming area responsive to a control voltage for transforming incident beam components in accordance with an applied voltage level to elliptically polarized output wavefronts of controlled azimuth, and

polarization diversity optics comprising at least two serially disposed polarization beam displacers in the path of the optical beams adjacent the liquid crystal element, the polarization beam displacers being of like optical thickness and optically aligned at angles differing by 90°

, to displace an input beam into orthogonally polarized beam components by a separation in one direction that is within the dimensions of the light transforming area and to combine separated output beam components in a second direction after transformation while attenuating the output beam components in accordance with the controlled azimuth.

32. An array as set forth in claim 31 above, wherein the polarization diversity optics separate the beam components by a spacing on the order of 100-200 microns, wherein the liquid crystal elements are zero twist nematic cells disposed in an array of sagittally separated positions and longer in the transverse direction than they are wide in the sagittal direction, and wherein the cells receive separate wavelength and orthogonally polarized beam components.

33. An array as set forth in claim 32 above, wherein the beams of different wavelengths are distributed in the sagittal direction in accordance with the beams'"'"' wavelengths to produce a distribution of individual wavelength signals and the widths of the cells and cell pitches along the array are varied in accordance with the distribution of individual wavelength signals, and wherein the optical axes of the polarization beam displacers are aligned to better than 0.5°

of 90°

with respect to each other, wherein the liquid crystal element is aligned to better than 0.1°

of 45°

with respect to the closest polarization beam displacer and wherein the liquid crystal element has an entrance face lying in a plane that is tilted by greater than about 0.5°

to a perpendicular relative to the incident beam axis, and wherein the polarization beam displacers are birefringent crystals having like optical thicknesses and providing matched path lengths that minimize polarization mode dispersion.

34. An array as set forth in claim 33 above, wherein the orthogonally polarized beam components are each about 70 microns in height by about 10 microns in width, and wherein the birefringent crystals are of YVO₄ and about 2.0 mm in length, and wherein the polarization beam displacers that receive the transformed beam components are configured to divert lossy beam components in transverse position from the output beam components.

35. An array as set forth in claim 31 above, wherein the liquid crystal element is a zero twist nematic reflective element and wherein the at least two serially disposed polarization beam displacers propagate both the input beam components in the one direction and the output beam components in the other direction.

36. A system for modifying signals throughout a given spectral band of DWDM optical signals wherein a spectral function is known and serves as the basis for a plurality of control signals, comprising:

a diffractive optical system receiving the DWDM signals and generating wavelength dependent spatially separated beams in a sagittal plane, the diffractive optical system including an optical structure for converging the separated beams toward an image plane, with predetermined beam spot size for each beam;

a beam transformation system positioned to intercept the converging beams adjacent the image plane, the beam transformation system including a linear array of liquid crystal cells, each individually controllable, and aligned in series along the sagittal plane of the beams to intercept the beams, and also including adjacent polarization diversity optics, wherein the cells are sized relative to the beams such that each beam impinges on a number of adjacent cells concurrently, the beam transformation system and polarization diversity optics attenuating the beams in accordance with control signals applied to the cells; and

voltage control circuits responsive to the known spectral function for providing control signals driving the cells to introduce a spectral correction function, in the band.

37. A system as set forth in claim 36 above, wherein the spectral function is a gain versus wavelength function, wherein the control signals provide an equalizing function to linearize gain versus bandwidth over the spectral band, and wherein the system further comprises output optics coupled to the diffractive optical system receiving attenuated beams from the cells for providing a gain equalized DWDM signal.

38. A system as set forth in claim 37 above, wherein the beam transformation system includes a plurality of zero twist nematic reflective cells, the cells having center-to-center spacings in the sagittal plane such that the individual beams are incident on the order of ten cells or more within the 1/e² power distribution of the individual beam, and the beam transformation system further includes polarization diversity optics for attenuating reflected components in accordance with the transformations introduced by the beam transformation system.

39. A system as set forth in claim 38 above, wherein the beam transformation system comprises an array of greater than 10³ pixels having less than 5 μ

m center-to-center spacings, wherein the cells have an interior pixel gap of less than 0.50 μ

m and a transverse dimension of greater than about 300 μ

m, and wherein the cells further comprise reflective back planes and zero twist nematic layers.

40. A system for equalizing gain across a given optical spectrum within which individual wavelength channels have different known gain characteristics varying non-abruptly from channel to channel, signals for the channels being distributed spatially as individual beams of a first cross-sectional size across a given sagittal width at an object plane, the system comprising:

a plurality of controllable light attenuating elements disposed along the object, there being a number of light attenuating elements within the cross-sectional area of each beam;

a plurality of control drivers, each coupled to at least a different one of the elements for controlling the local attenuation thereat in relation to the known gain characteristics of the channel;

an optical system responsive to the controllably attenuated beams from the elements for providing a wavelength division multiplexed output, and wherein the responses minimize transmission ripple from gaps between the attenuating elements, and the system provides a smooth gain function across the given optical spectrum.

41. An optical beam modulator for modifying a beam of at least one given wavelength and arbitrary polarization, comprising:

an optical system providing separated but aligned asymmetric polarization components of the beam in separate paths converging at a focal plane;

a controllable polarization transformer positioned at the focal plane and reflecting both beam components, after transformation, into principal beam paths that diverge reversely relative to the converging paths, and

a polarizer element having a selected optical axis orientation relative to the alignment of the polarization components, the polarizer element being positioned in the path of the reflected beam components and rejecting a portion of the components to modulate the energy in the principal beam paths in accordance with the transformation of the polarization components.

42. A modulator as set forth in claim 41 above, wherein the angle of convergence of the polarization component of the beam is in the range of about 1°

, wherein the asymmetry of the components is greater than five to one, and wherein the beam components pass through the polarizer element in both the converging and diverging directions.

43. A modulator as set forth in claim 42 above, wherein the polarization transformer comprises a zero twist nematic liquid crystal cell having an optical axis at 45°

and the polarizer element has an optical axis at 90°

.

44. An optical modulator for attenuating an individual input optical beam derived from a wavelength division multiplexed optical communications beam, the modulator providing attenuation in the range of 0 to 20 dB with 0.05 dB resolution comprising:

a reflective zero twist nematic liquid crystal cell having an optical axis and including a compensator plate aligned at 180°

to the optical axis, the cell having an active area with a width bounded by a nonconductive element , the input optical beam being diffraction limited and having a 1/e² spot distribution incident on the active area;

a voltage controller coupled to the cell for applying a voltage in a range to induce up to a quarter wave phase retardation of the incident optical beam; and

a polarized element having an optical axis at 45°

to the optical axis of the cell, and in the path of the incident and reflected optical beams to and from the cell to reject a proportion of the reflected beam dependent on the amount of phase retardation.

45. An optical modulator as set forth in claim 44 above, wherein the cell includes substantially parallel and spaced apart front and back plane surfaces confining a liquid crystal material, and a polyimide layer on at least one of the surfaces in contact with the liquid crystal material and having a surface alignment pattern defining the director axis of the cell, the polarized element being proximate the front surface and tilted by greater than 0.5°

relative to the plane surfaces of the cell.

46. A diffractive Fourier optics system which spatially distributes from a DWDM optical input beam a plurality of individual wavelength optical beams along a sagittal plane, a modulator system for individually modifying the wavelength components and returning them for recombination into a modified DWDM optical output beam, with low adjacent channel crosstalk comprising:

an optical system disposed and configured with the diffractive Fourier optics system to direct the sagittally distributed separate wavelength component beams with a predetermined minimum center-to-center spacing of adjacent wavelength beam spots to a focal plane;

an array of reflective zero twist nematic liquid crystal cells sagittally distributed at the focal plane, the cells representing pixels having pixel to pixel spacings chirped to match the distribution of wavelength component beams and including interpixel gap barriers which separate the pixels, wherein the pixel to pixel spacings in relation to the interpixel gap barriers have a dimensional ratio of at least about 15;

1 in the sagittal direction, and

polarization sensitive elements adjacent the cells and having optical axes in selected relation to the alignment direction.

47. A modulator system as set forth in claim 46 above, wherein the center optical wavelength of the spectral band of the DWDM beam is about 1550 nm, wherein the beams are data modulated, wherein the interpixel gap barriers have widths in the range of 2 to 6 microns and the pixel cell widths are in the range of 75-95 microns, and wherein the beams exclusive of modulation components have 1/e² distributions with sagittal widths in the range of about 8 to about 11 microns and with modulation components have 1/e² distributions with sagittal widths in the range of about 30 to about 40 microns.

48. A modulator system as set forth in claim 47 above, wherein fringing fields sagittally extending from the beam spots are incident in overlapping relation on the interpixel gap barriers and the interpixel gap barriers are approximately 2.5 to 3 microns in sagittal width.

54. The method of modifying individual wavelength signals as set forth in claim 48 above, comprising the steps of:

initially separating the DWDM beam into s and p components, transforming p components to parallel alignment to s component;

splitting the two beam components into separate wavelength signals;

transforming the polarization state of each individual p component wavelength signal;

polarization filtering each separate wavelength signal after transformation, and

recombining the polarization components after diffractive recombination.

55. The method as set forth in claim 54 above wherein the positions of individual areas for polarization transformation are varied in accordance with beam position in the sagittal direction, to provide uniform channel spacing between separate wavelength components, and wherein the transverse to sagittal dimension ratio of the separate wavelength components is in the range of 10-100:

1.

56. The method as set forth in claim 55 above, wherein the step of controllably transforming comprises retarding the beam components during transformation, and wherein the beam angles and path provide an extinction ratio in excess of 40 dB for signal blocking or alternatively substantially linear attenuation in the range of 0 to 20 dB for signal equalization.

49. A system for independently controlling different wavelength signals in a DWDM beam, comprising the combination of:

a diffractive Fourier optics system configured to provide compact three dimensional beam refolding at low angles while distributing wavelength components of the DWDM beam in a sagittal plane;

an input/output structure having separate input/output elements at different elevations transverse to the sagittal plane, one of said input/output elements receiving an input DWDM beam;

a liquid crystal spatial light modulator array disposed parallel to the sagittal plane and having a plurality of voltage controllable reflective elements that are sagittally dispersed to receive individual wavelength components of the DWDM beam from the diffractive Fourier optics system, and

polarization diversity optics disposed between the diffractive Fourier optics system and the spatial light modulator array, for applying different wavelength components from the diffractive Fourier optics systems to the different cells of the array and transferring reflected wavelength components from the cells of the array back to the diffractive Fourier optics system at one of two levels corresponding to the transverse spacing between the elements in the input/output array.

50. A system as set forth in claim 49 above, wherein the diffractive Fourier optics system is configured to provide individual wavelength components as single beams, and wherein the polarization diversity optics includes a succession of polarization beam displacers with lengths and optical axes configured (1) to split the individual wavelength component into a pair of beams both incident on the cell of an array and (2) to return reflected beam components from the cell, dependent upon the control imposed thereby, to a different selected level, dependent upon whether the reflected signal is to be sent to one input/output element or the other, the elevation differential between the beams corresponding to that between the input/output elements.

51. A system as set forth in claim 49 above, wherein the input/output structure includes beam polarization splitters in the optical path of beams from each separate input/output element, and wherein the polarization diversity optics comprises a pair of polarization beam displacers interposed in serial fashion between the diffractive Fourier optics system and the elements of the array, the beam polarization displacers having lengths and optical axes configured to converge input signals to superposed beam spots on the associated element, and in response to controlled individual control of the beams at the elements and return other beams to the same input/output device and returning transformed beams to the other of the input/output elements.

52. The method of modulating individual wavelength signals in an arbitrarily polarized DWDM optical beam comprising the steps of:

diffractively separating the wavelengths in a sagittal direction by forming individual arbitrarily polarized wavelength beams;

directing the individual beams to form a sagittally distributed series of converging beams having beam waists at a focal plane;

separately delivering the polarization components of the individually separated beams within a given individual area at the focal plane, the average beam intensity at the focal plane being below a selected level;

controllably transforming the separate polarization components within the given individual area for each beam at the focal plane to a selected orientation corresponding to a desired degree of modulation of the individual beam;

directing the transformed components in a second direction;

rejecting portions of the reflected individual beam components determined by the amount of transformation while transmitting the remaining portions;

recombining the polarization components of the different beams, and

diffractively recombining the individual beams after modulation into a DWDM beam.

53. A method as set forth in claim 52 above, wherein the separate polarization components for each signal are variably transformed to variable ellipticity and azimuth and reflected back for recombination.

57. The method of modifying individual wavelength signals in a DWDM optical beam as set forth in claim 52 above, further comprising the steps of:

diffractively separating the arbitrarily polarized beams while retaining arbitrary polarization;

splitting each separated beam into separate polarization components proximate to the focal plane;

directing the separate polarization components to the focal plane in orthogonal polarization relation and linearly separated in the transverse direction, relative to the given individual area;

reflectively transforming the polarization components at the focal plane to a degree corresponding to the desired amount of modulation;

recombining the polarization components of the individual beams adjacent to the focal plane, while rejecting controlled portions thereof depending upon the rotation angle, and

diffractively recombining the individual beams into a modulated DWDM signal.

58. The method of modulating individual wavelength signals as set forth in claim 57 above, comprising the steps of displacing the beam components in accordance with polarization proximate to the focal plane, and wherein the reflective rotation comprises generating reflected beams of elliptical polarization and the rejected portions of the beams are directed in optical paths separated from the chosen components of the beams.

59. The method of controllably modifying individual wavelength signals in a DWDM optical beam having an arbitrary state of polarization, comprising the steps of:

diffractively separating the individual wavelength components in a sagittal direction into individual wavelength beams which are asymmetric in a direction transverse to the sagittal direction while converging the beams to form beam waists at a focal plane;

also separating the wavelength components into aligned polarization components converging separately in the transverse direction to be superposed at the focal plane,the beam intensity at the focal plane being locally below a selected level;

controllably transforming both polarization components at the focal plane by a selected variable retardation;

thereafter rejecting a part of the individual polarization components dependent upon the selected variable retardation, and

diffractively recombining the polarization components and individual beams into a modified DWDM beam.

60. The method of modifying individual wavelength signals as set forth in claim 59 above, comprising the steps of:

separating the DWDM beam before diffraction into orthogonal polarization components on initially diverging paths;

converging the separate polarization components at the focal plane, and

polarization filtering the retarded beam components after reflection.

61. The method as set forth in claim 60 above, wherein the initial separation of the DWDM beam comprises splitting into s and p components and wherein both components are aligned parallel to the direction of diffraction separation prior to reaching the focal plane, and wherein the polarization components converge at different small angles toward incidence at the focal plane.

62. The method of claim 61 above, wherein the polarization components follow reciprocal paths after reflection which correspond to the incident path of the opposite polarization component, and wherein the areas of the wavelength component incident on the focal plane are approximately 20 times taller than wide.

63. The method as set forth in claim 62 above, wherein the polarization components for each individual wavelength beam are recombined in orthogonal relationship after being rediffracted towards convergence into a modified DWDM beam.

64. The method of modifying individual wavelength signals in a DWDM beam of arbitrary state of polarization comprising the steps of:

diffractively separating the DWDM beam while still in an arbitrary state of polarization;

dividing the separated beams into separate polarization components proximate to the focal plane;

directing the separated polarization components for each wavelength signal toward the focal plane in linear alignment;

separately varying the polarization direction of the polarization components for each wavelength signal;

rejecting a proportion of the polarization components while recombining the separate polarization components after reflection from the focal plane, and

forming a modified DWDM beam by rediffraction of the individual wavelength signals.

65. The method as set forth in claim 64 above further comprising the step of reflectively retarding the polarization components at the focal plane to a degree corresponding to the desired amount of modification to provide elliptically polarized signals, and rejecting components of the elliptically polarized signals dependent upon the amount of retardation, and diffractively recombining the retarded signals.

66. The method as set forth in claim 65 above, wherein the rejection of signals comprises the steps of directing rejected components in an optical path that is out of the field of view of the modified components.

67. The method of equalizing signal gain within the spectral band of a wavelength multiplexed optical beam using a plurality of controllable attenuation elements disposed in a serial array, comprising the steps of:

demultiplexing the wavelength components into angularly dispersed individual beams spread along and impinging on the serial array, each of the beams having a cross-sectional area spanning more than one of the elements,

controlling the attenuation introduced by the individual elements to selectively attenuate impinging portions of the individual beams such that the gain across the spectral band is equalized, and

multiplexing the wavelength components after attenuation into a gain equalized output beam.

68. A method as set forth in claim 67 above, wherein the individual elements are reflective and arrayed in a one-dimensional series with a given pixel pitch such that all elements are within the 1/e² optical impulse response of the individual wavelength component beams.

69. A beam control unit for receiving a nominally single wavelength optical beam from a sagittally dispersive diffractive source and controllably attenuating in the 0 to 20 dB range or extinguishing to greater than 40 dB comprising:

a reflective liquid crystal cell having a front window and a spaced apart back window and nematic liquid crystal material in a cell gap therebetween, the facing surfaces of the windows each having a rubbed polyimide alignment coating aligned along a selected axis but the coatings being rubbed in anti-parallel senses, the cell having a sagittal active surface dimension and a substantially greater transverse active surface dimension, and including a reflecting surface and a distributed control electrode and an interpixel insulative barrier defining at least the sagittal boundary of the active surface area of the cell, the cell gap and interpixel barrier dimensions both being of like widths, and

polarization diversity optics disposed proximate the front window of the cell in the path of the incident beam and comprising at least one birefringent optical element in the path of the single wavelength beam and the beam reflected from the active surface of the cell for rejecting beam components to an extent determined by the voltage on the control electrode.

70. A beam control unit as set forth in claim 69 above, wherein the beam is a single wavelength beam from a sagittally dispersed spectrum in the range of about 1300 nm to about 1700 nm and the beam includes sagittally dispersed sidebands, wherein the cell operates as a zero twist nematic liquid crystal, to phase retard the incident beam to an extent determined by the control voltage and wherein the local beam intensity on the active surface is nowhere greater than about 200 watts/mm²;

and wherein the cell gap and the interpixel barrier are each of the order of 4 microns.

71. A beam control unit as set forth in claim 70 above, wherein the data sidebands increase the 1/e² optical intensity of the sagittal dimension of the beam by about 30 microns and wherein the active surface area of the cell has a sagittal dimension of about 85 microns and a transverse dimension of about 3 mm.

72. A beam control unit as set forth in claim 71 above, wherein the beam comprises converging incident beam components defining orthogonal beam polarization components having a like direction of polarization selected relative to the alignment direction of the cell, and the polarization diversity optics comprises a polarized optical element having a selected polarization vector to propagate the incident beams to the cell and to reject portions of reflected beams dependent on the phase retardation thereof.

73. A beam control unit as set forth in claim 71 above, wherein the beam comprises a single beam of arbitrary state of polarization and the polarization diversity optics comprises at least a serial pair of birefringent elements with optical axes aligned to split the incident beam into spatially displaced components of orthogonal polarization orientation and to recombine reflected components into a single beam while diverting a proportion of the beams to different paths, the proportion dependent on the phase retardation thereof.

74. A system for controlling the power of amplitude modulated individual wavelength beams that have been dispersed diffractively from a wavelength multiplexed optical beam having a spectral band of about 40 nm into a sagittal span of less than about 1 cm, and the beams being widened by amplitude modulation sidebands such that light is distributed with varying intensity with center wavelength peaks across the sagittal span, the system comprising:

an LC-SLM array having a plurality of cells distributed along the sagittal span and spaced to receive centrally the individual wavelength beam peaks, each cell being individually voltage controllable, the cells being zero twist nematic liquid crystal elements which reflect incident beams with selectable phase retardation, and having sagittal dimensions with reflective surface area sized to receive beams including sidebands down to 40 dB below the beam peak intensity, the transverse dimensions of the reflective surface areas being substantially greater than the sagittal such that the local power intensity of the incident beam is nowhere greater than the 200 W/mm² the array including a front window and a parallel back window spaced apart by a cell gap confining liquid crystal material, the back window having areally separated control electrodes for each cell to determine insulative sagittal boundaries between the adjacent cells, the boundaries being of the same order in dimension as the cell gap, to diminish effects of fringing fields between adjacent electrodes such that passbands for the individual beams are well separated by stop bands from adjacent beams, and

at least one polarized optical element proximate to the array in the path of the incident beams in and reflected beams from the cells to reject such proportions of the individual beams as are determined by the phase retardation introduced thereto.

75. A system as set forth in claim 74 above, wherein the spectral band is in the range from 1300 nm to 1700 nm and the multiplexed beam defines about 80-100 channels each including data modulation at a rate of the order of 10 Gbps, wherein the cells have a selected alignment axis, and wherein the at least one polarized optical element has an optical axis in chosen relation to the alignment axis of the cells.