Tunable holographic filter

Abstract

An optical add/drop filter is formed using a hologram which is tuned to a wavelength in an optical signal. The diffraction condition of the hologram may be varied to vary the in add or drop content.

Description

BACKGROUND

[0002] Many different applications operate using optical techniques. Optical techniques allow many different wavelengths to be multiplexed on the same fiber. Each wavelength can represent a separate data stream. In some applications, it may be desirable to obtain parts of said data stream from the waveguide without obtaining other parts of the data stream from the waveguide. It may also be desirable to add new multiplexed data to the optical stream already present on the waveguide.

[0003] An add/drop multiplexer is often used to carry out the addition or removal of channels from an optical signal in a waveguide such as a fiber. Different kinds of add/drop multiplexers are known.

SUMMARY

[0004] The present application teaches a tunable technique which allows adding and/or dropping a wavelength channel in a waveguide such as a fiber.

[0005] A technique disclosed according to the present application operates by use of a hologram which preferentially changes some characteristic of one or more wavelengths of the optical signal without correspondingly changing other wavelengths of the same optical signal. Some aspect of the hologram is tunable in order to select a different wavelength/channel for changing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] These and other aspects of the invention will be described in detail with reference to the accompanying drawings, wherein:

[0007] FIG. 1 shows an embodiment of a tunable add/drop multiplexer where the hologram is rotatable;

[0008] FIG. 2 shows a different embodiment of a tunable add/drop multiplexer where the hologram is also rotatable;

[0009] FIG. 3 shows an electrically tunable holographic add/drop multiplexer;

[0010] FIG. 4 shows an embodiment of a tunable add/drop multiplexer using a moving mirror;

[0011] FIG. 5 shows an intensity chart.

DETAILED DESCRIPTION

[0012] FIG. 1 shows a first embodiment of a technique which uses a reflection hologram, which allows tunable selection of channels to be removed from a waveguide. The waveguide can be a fiber or other optical-signal-containing element.

[0013] The input signal 100 initially travels in the input waveguide 99. The input signal 100 is applied via an optical element 105 which may be a GRIN lens, and to a material 110 which includes a hologram 115 recorded in a holographic recording material, which may be, for example, a photorefractive crystal. The hologram that is formed may be a grating in that crystal.

[0014] Light is diffracted only if the Bragg matching condition is satisfied. The light wavelength λ should fulfill the relation

λ ∝ 2 n Λ,

[0015] where n is the averaged refractive index of the holographic storage material and Λ is the effective period length of the holographic grating, for reflection holograms. For transmission holograms, the relation λ ∝ 2 Λ is followed. The effective period length Λ is the distance between two fringes of the grating along a line that is parallel to the incoming light beam.

[0016] The recorded hologram is rotated around an axis that is shown by the dotted line in FIG. 1. Tunability of the add/drop multiplexer is achieved by changing the effective period length Λ by changing the rotational position of the grating. As this rotational position changes, the light travels in a different direction with regard to the holographic structure stored in the recording material. For incidence perpendicular to the fringes of the holographic grating, a small period length is present and small wavelengths are diffracted. In contrast, for non-perpendicular incidence, the effective period length of the gratings is increased and hence light of larger wavelengths is diffracted.

[0017] The wavelength which matches the Bragg condition of the hologram 115 is reflected through an optical system 120. The hologram 115 is rotated to select the desired selected wavelengths, i.e., to tune the reflected signal wavelength. The wavelengths which do not match the hologram are transmitted through the crystal 110 substantially unattenuated, and coupled into the output waveguide 125. The output waveguide is coupled to a “y junction” 126 which receives signals/wavelengths to be added from an “add” port 127. The final output waveguide 150 includes the filtered signal from 125, and the add signal from 127.

[0018] As an alternative to using an “Y” junction to add light of the desired wavelength, it may also be possible to use the hologram itself for the “add” function, i.e., to add to the element 125 a second beam that is Bragg matched to the grating for the wavelengths that should be added to the data stream.

[0019] The reflected channel, after passing through optical system 120, may be directed to a detector 130 which converts the optical signal into an electronic signal 135 to be used by an electronic device 140. This device may also include a DFB laser to re-modulate the signal. This signal 145 corresponds to the information in the “dropped” output channel.

[0020] Hence, the same device can be used for either an add function, or for a drop function. By doing both operations, the device can be used for both adding and drop.

[0021] For the add function, the detection system 120, 130, 135, 140, and 145 is replaced by an incoming light beam. The channel “Out” becomes “In” and vice versa. Such an “add” coupler would have the advantage that light is coupled in only if the selected wavelength is Bragg matched to the tunable holographic filter.

[0022] Replacing the electronic detection system 120, 130, 135, 135, 140, and 145 by an optical system as described above and using this port as an drop device and utilizing the optical add function as described above in the section describing alternatives to the “y” junction yields an all-optical tunable add-drop multiplexer.

[0023] The device of this embodiment is called an “add-drop” multiplexer. However, any other application where frequencies are added or dropped, including filters, and the like, may also be used. Further modifications of the setup shown in FIG. 1 are possible. However, these modifications may still allow tunability by a mechanical change of the effective period length of the holographic grating.

[0024] Another embodiment is shown in FIG. 2. The wavelength which is dropped in this embodiment is selectable as in the above embodiments. In the embodiment of FIG. 2, the storage material 200 includes a hologram 210, e.g., in the form of a grating. A rotation element 205 is provided to rotate the crystal in a way such that the effective period length Λ of the hologram is changed and by these means the Bragg condition is changed, as is explained above.

[0025] In this embodiment, the optical signal 199 is input from the fiber 220 to an optical system 225 which may include a GRIN lens. The output of the GRIN lens is directed at the hologram 210. As in the embodiment of FIG. 1, the wavelength which matches the characteristic of the hologram 210 is diffracted to optical system 230, detected by detector 235, and coupled as signal 240 to electronic amplifier 245. The output signal 246 becomes a signal including the information from the dropped wavelength.

[0026] The direction of movement of the crystal need not be circular as explained above, but rather could be of any shape. The motor which moves the crystal can then be a linear stepper motor or a DC servo motor, or other device that can move the position of the holographic recording material.

[0027] All other wavelengths, which do not correspond to the information in the hologram 210 are passed through the hologram. The output signal-passed elements may be sent to a double prism 250 which retro-reflects the passed light 249 as light 251, which travels in the same direction as light 249, but in the opposite direction. The light 251 may pass another GRIN lens 252 to a y junction 253 which combines with new light from an add port 254. The composite output 255 then corresponds to light with the selected wavelength dropped and light from the add port 254.

[0028] As in the above, the “y” junction may be replaced by a coupler that couples the light out of the “in” fiber and that directs the light onto the hologram in such a way that it is Bragg matched and collected by the element 252. Also as described above, the electronic detection system may be replaced by an optical system that couples the light, to name one example, back into a fiber. Applying both changes yields again an all-optical tunable add-drop multiplexer.

[0029] Further modifications of the setup shown in FIG. 2 are possible. All modifications, however, may achieve tunability of frequency by a mechanical change of the effective period length of the holographic grating. The major difference to FIG. 1 is the orientation of the axis of the mechanical rotation of the hologram. In FIG. 1, the axis is in the paper plane while in FIG. 2 it is perpendicular to the paper plane. However, other rotational axes are possible as well.

[0030] An alternative embodiment shown in FIG. 3 allows tuning using an electrically-alterable hologram. Any of a number of different various materials may be used, including electro-optic crystals and electro-optic polymers. The material has a hologram formed thereon as a fixed reflection grating, with a length and orientation. The holographic storage material may be formed of any material where the refractive index can be changed by application of an electric field. For example, this may include electro-optic polymers, liquid-crystal dispersed polymers, and electro-optic oxide crystals such as lithium niobate, barium titanate, potassium niobate, strontium-barium niobate mixed, and potassium-tanatalate niobate mixed crystals. In general, any material which can be used to make a hologram, and which has a variable refractive index can be used.

[0031] The refractive index of the crystal may be changed by an external electric field. The refractive index may vary, for example, between 1.35 and 1.45. By varying the refractive index, the effective length, and hence the Bragg wavelength, can be changed. The same formula as it was given above applies: λ ∝ 2 n Λ. By tuning the refractive index n, the wavelength of the reflected light λ is correspondingly tuned. In this way, certain information is reflected based on its wavelength, while other optical information is allowed to pass.

[0032] In operation, the signal 300 passes through the optical system 301 into the material 310. The material 310 is formed with a hologram 315 in the shape of a diffraction grating. Wavelengths within the signal 300 that match certain conditions in the grating are reflected as 304, and output through optical system 319 on to fiber 320. This forms the drop channel 321. Information which does not correspond to the frequency in the grating is passed to the output fiber 325 as signal 326. A “y” junction 327 allows another signal 328 to be added from an add port to form the composite signal 329.

[0033] In operation, the Bragg wavelength can be changed to render the device tunable, to change the dropped signal 321. As explained above, the “y” junction might be replaced by an additional waveguide with a coupler that directs the light onto the hologram that it is diffracted into the “out” channel. This yields an all-optical tunable add-drop multiplexer. Further modifications of the setup are possible. However, the key feature is always the same: The effective optical period length of the holographic grating is changed by a change of the refractive index n while the mechanical effective period length Λ of the diffraction grating is kept constant.

[0034] An embodiment shown in FIG. 4 uses a moving mirror to change the angular relation between the incoming light beam, and the hologram. In contrast to the FIGS. 1 and 2, however, this embodiment changes the direction of the incoming light beam while the holographic grating stays mechanically in a fixed position. An advantage is that the dropped light will always travel in the same direction based on the direction of the hologram grating. Also, since the mirror may have less mass than the hologram, selection may be carried out faster.

[0035] In this embodiment, the input signal 400 is coupled to a lens 402 and to a movable mirror 404 which directs the input light to the hologram element 415. Light which does not correspond to the resonant frequency of the hologram is passed as 420. The passed light is then reflected by reflection element 422, and returned on path 424, via the mirror 404, and to the output fiber 450. Note that the output fiber 450 is in substantially the same orientation and direction as the input fiber 400. The movable first mirror 404 is used as parts of both the optical system for the output (first side of the mirror) and for the dropped output (second side of the mirror).

[0036] The light which does fulfill the Bragg condition is diffracted as 430, to a retro-reflector array 432, 434, and is reflected by the mirror 404, and output as an output signal 440. Signal 440 includes the information from the dropped channel. In this way, the input and output signals may travel in different directions with the output traveling in a direction substantially perpendicular to the input. The mirror 404 may be rotated as shown to change the area 416. An optical add port can the introduced simply by placing nearby the fiber for the drop channel a fiber for the add channel. The “add” light will travel in the reciprocal direction to the beam 440, will be reflected from the mirrors 404, 434, and 432, will be diffracted from the hologram, reflected from mirror 404 and coupled into the output channel 450.

[0037] Combinations of the tuning mechanisms that are disclosed in the FIGS. 1 to 4 are also possible. One example is rough-tuning by mechanical rotation and fine-tuning by external electrical fields.

[0038] FIG. 5 shows a plot with the diffraction efficiency of the hologram as a function of wavelength. 99.8 percent of the transmitted light may be diffracted over the chosen range of wavelengths. Of course, these results are merely exemplary, and in other embodiments it may be acceptable to have less efficiency.

[0039] For example, the above has described the waveguides being optical fibers. Of course, any kind of waveguide which is capable of containing and passing light may be used, including, but not limited to, a waveguide formed on a semiconductor chip.

[0040] In the description the terms “Bragg condition” and diffracted light wavelengths” and “diffraction grating” and related expressions are used. The invention covers all different kinds of gratings which are known in the art, including gratings with spatially varying amplitude (“apodized gratings”) and gratings with spatially varying fringe spacing (“chirped gratings”).

[0041] Other embodiments are within the disclosed invention.

Claims

1. An tunable optical device, comprising: a holographic element, having a hologram therein which has a predetermined relationship to a plurality of wavelengths; a wavelength varying element, coupled to said holographic element, and varying said predetermined relationship; and a first optical system, handling first wavelengths of an optical signal which pass through said holographic element without being changed by said hologram as an output signal; and a second optical system, separate from said first optical system, and handling a second optical signal including said wavelength having said predetermined relationship as varied by said wavelength varying element.

2. A device as in claim 1, wherein said wavelength varying element includes an element which physically moves said hologram.

3. A device as in claim 1, wherein said hologram includes an electro-optic storage medium, and wavelength varying element includes a voltage system that changes an index of refraction of said electro optic storage medium.

4. A device as in claim 1, wherein said second optical system handles wavelengths to be dropped.

5. A device as in claim 4, wherein said holographic element includes a hologram form therein which includes a plurality of different Bragg matching conditions depending on an angle of incidence with respect to an orientation of the hologram, and said wavelength varying element includes an element which physically moves said hologram to apply said optical signal through a different orientation of said hologram which has different Bragg matching characteristics.

6. A device as in claim 5 wherein said physically moving comprises rotating said hologram.

7. A device as in claim 6 wherein said rotating comprises rotating said hologram to form a section of a cone.

8. A device as in claim 5, further comprising a mirror, and wherein said element that moves comprises an element moving said mirror to change an incidence of said optical signal on said hologram.

9. A device as in claim 1 wherein said output signal extends in substantially a same direction as an input signal.

10. A device as in claim 1 wherein said output signal travels in substantially an opposite but parallel direction to an input signal.

11. A device as in claim 4 wherein said second optical signal is a drop output signal which travels in a different direction than either an input signal or said output signal.

12. A device as in claim 11, wherein said different direction is substantially perpendicular to said input signal.

13. A device as in claim 9, further comprising a double prism forming a retroreflecting operation to reflect the output signal in said opposite direction.

14. A device as in claim 1, further comprising an optical detector, receiving said drop signal, and converting said drop signal to an electrical signal indicative thereof.

15. A device as in claim 14, further comprising a laser element, receiving said electrical signal and converting said electrical signal to an optical signal.

16. An apparatus as in claim 15 wherein said laser is a DFB laser.

17. A device as in claim 1, wherein said hologram comprises a light diffracting structure.

18. A device as in claim 4 further comprising an add port, allowing additional wavelengths to be added to the output signal.

19. An apparatus as in claim 18, wherein said add port comprises a Y junction.

20. A device as in claim 1, wherein said holographic element includes a hologram form therein which includes a plurality of different Bragg matching conditions depending on an angle of incidence with respect to an orientation of the hologram, and said wavelength varying element includes an element which physically moves a direction of an input light beam relative to said hologram to apply said optical signal through a different orientation of said hologram which has different Bragg matching characteristics.

21. A device as in claim 1, wherein said second optical system handles wavelengths to be added.

22. A device as in claim 1, wherein said holographic element includes said hologram forming a grating as part of said holographic element, said grating interacting with a wavelength based on a characteristic of a material forming said holographic element.

23. A device as in claim 22, further comprising electrodes which apply an electric field to said holographic element to change a characteristic of the material and thereby change the reflection wavelength of said grating.

24. A device as in claim 22, further comprising a plurality of mirrors, reflecting said optical signal to exit along an axis parallel to an axis of its entry.

25. A device as in claim 18, wherein said second optical signal travels in substantially a same direction as said first optical signal.

26. A device as in claim 18, wherein said second optical signal travels in a direction which is substantially 180 degrees opposite from said first direction.

27. A device as in claim 1, wherein said first optical signal travels in a different direction than said second optical signal.

28. A device as in claim 1, wherein said first optical system includes a first side of a first mirror, and said second optical system includes a second side of the first mirror.

29. A device as in claim 28, wherein said first mirror is movable to change a direction of reflection of the signal.

30. A device as in claim 1, wherein said first optical system includes a lens.

31. A device as in claim 30, wherein said lens is a GRIN lens.

32. A device as in claim 30, wherein said first optical system further comprises a mirror, at an output of said lens, reflecting output light towards said holographic element.

33. A device as in claim 32, further comprising a second mirror, located to reflect light which has passed through said holographic element back towards said holographic element, and to said first mirror, which reflects said light in a first direction, and further comprising an output waveguide, located adjacent said light which is reflected in said first direction to receive said output light.

34. An apparatus as in claim 33, further comprising a third mirror, positioned to receive light which has been changed by said hologram to reflect said light in a specified direction.

35. A device as in claim 34, wherein said third mirror is a retroreflector.

36. A method as in claim 35 wherein said retroreflector reflects light back towards said first mirror, to be reflected by said first mirror.

37. A device as in claim 36, wherein said light reflected by said first mirror after said retroreflector is reflected in a second direction, and further comprising a waveguide for the drop channel, receiving said light reflected in said second direction.

38. A method, comprising: applying an input optical signal having a plurality of wavelengths therein to an area of a hologram; tuning said hologram to one of a plurality of wavelengths; and using said hologram to separately optically process said one of said wavelengths differently from others of said wavelengths.

39. A method as in claim 38, wherein an output signal includes all wavelengths except said one of said wavelengths and producing a dropped signal includes only said one of said wavelengths.

40. A method as in claim 39, further comprising changing a direction of said dropped signal using said hologram.

41. A method as in claim 38, wherein said adjusting comprises physically moving said hologram.

42. A method as in claim 38, wherein said adjusting comprises applying an electric field to said hologram.

43. A method as in claim 38, further comprising using said hologram to merge said one of said wavelengths as added signal with others of said wavelengths as an output signal.

44. A method as in claim 38, wherein said adjusting comprises moving an angle of incidence of light into said hologram.

45. A method as in claim 42, wherein said hologram includes a grating formed therein.

46. A method as in claim 45, wherein a resonant frequency of said grating depends on characteristics of the material forming said hologram.

47. A method as in claim 46, wherein said applying an electric field comprises using said electric field to change characteristics of the material forming said hologram.

48. A method as in claim 47, wherein said characteristics that are changed include refractive index.

49. A method as in claim 48, wherein said first optical output signal travels in substantially a same direction as said input optical signal.

50. A method as in claim 38, wherein a first optical output signal including said one of said wavelengths travels in substantially an opposite direction from an input optical signal.

51. A method as in claim 38, wherein a first optical output signal including said others of said wavelengths and a second optical output signal including said one of said wavelengths travel in different directions.

52. A method as in claim 51, wherein said first and second output signals have a constant angle therebetween.

53. A method as in claim 38 wherein said using comprises changing a direction of said one of said wavelengths to a different direction than another of said wavelengths.

54. A method as in claim 38, wherein said tuning comprises moving said hologram.

55. A method as in claim 38, wherein said tuning comprises moving said incident light to a different angle.

56. A method as in claim 38, wherein said tuning comprises changing a characteristic of the hologram.

57. A method as in claim 56, wherein the changing the characteristic of the hologram comprises electrically changing the characteristic of the hologram.

58. A method as in claim 57, wherein said electrically changing the characteristic of the hologram comprises applying an electric field to a material containing the hologram.

59. A method as in claim 38, further comprising adding an additional optical wavelength to said output signal.

60. An apparatus, comprising: an optical filter element comprising a hologram material with a hologram form thereon; an optical system, positioned to apply an optical signal to said hologram; a tuning element, changing a way that said optical signal is applied to said hologram to change a Bragg matching condition between said optical signal and said hologram and thereby Bragg match to a different resonant wavelength in said optical signal; a first output path for light that is not Bragg matched to said hologram extending in a first direction, and a second output path for light that is Bragg matched to said hologram of extending along a second optical path, wherein said second optical path is in a different direction than said first optical path.

61. A system as in claim 60, wherein said moving comprises moving said hologram.

62. An apparatus as in claim 60, wherein said moving comprises moving an incident angle of the light.

63. A system as in claim 61, wherein said moving said hologram moves said hologram in a way which forms a substantially cone shape.

64. An apparatus as in claim 60, further comprising an add port, which allows adding additional wavelengths to the output signal.

65. An apparatus, comprising: an optical filter element comprising a hologram material with a hologram form thereon, said optical filter element comprising a holographic material, first and second electrodes formed on said holographic material; and an electrical tuning element, which applies electric signals across said first and second electrodes to change a Bragg matching condition of said hologram and thereby Bragg match to a different resonant wavelength in said optical signal; an optical system, positioned to apply an optical signal to said hologram; a first output path for light that is not Bragg matched to said hologram extending in a first direction; and a second output path for light that is Bragg matched to said hologram of extending along a second optical path, wherein said second optical path is in a different direction than said first optical path.

66. An apparatus as in claim 65, further comprising an add port, enabling adding signals to said output signal.

67. An apparatus as in claim 65, wherein said material is a polymer.

68. An apparatus as in claim 65, wherein said material is a liquid crystal.

69. A apparatus as in claim 65, wherein said material is an electro-optic storage medium.

70. A apparatus as in claim 69, wherein said electro-optic storage materials is one of an electro-optic polymers or an electro optic crystal.

71. An apparatus as in claim 69, wherein said electro optic storage medium is one of lithium niobate, barium titanate, potassium niobate, strontium-barium niobate mixed, and potassium-tanatalate niobate mixed crystals.

72. An tunable optical device, comprising: a holographic element, having a holographic element therein which has a predetermined Bragg matching relationship to a plurality of wavelengths depending on an orientation parameter; a wavelength varying element, changing said orientation parameter; an optical system, receiving first wavelengths of an optical signal which has passed through said holographic element without being changed by said hologram as an output signal in a first direction, and receiving a second optical signal including said wavelength having said predetermined relationship in a second direction.

73. A device as in claim 72, wherein said optical system further includes an element which applies light to said hologram to merge said one of said wavelengths as added signal with others of said wavelengths as an output signal.

74. A device as in claim 72, wherein said hologram includes a diffraction structure therein.

75. A device as in claim 72, wherein said hologram includes a grating therein.

76. A device, comprising: a holographic material, including a holographic grating formed therein; an optical system, providing an input optical beam to said holographic material, and obtaining an output optical beam from said holographic material, said input and output optical beams being different; and a tuning system, which tunes the way in which said input and output beam are different by varying an effective period length of a holographic grating.

77. A device as in claim 76, wherein said tuning system comprises an element which mechanically rotates said holographic material.

78. A device as in claim 76, wherein said tuning system comprises an element which changes a refractive index of said holographic material.

79. A device as in claim 78 wherein said element which changes a refractive index comprises a voltage source, and said holographic material is formed of a material whose refractive index is altered by application of an electric field.

80. A device as in claim 79, wherein said holographic storage material is one of an electro-optic polymer, liquid-crystal dispersed polymers, and electro-optic oxide crystals such as lithium niobate, barium titanate, potassium niobate, strontium-barium niobate mixed, and potassium-tanatalate niobate mixed crystals.

81. A device as in claim 76, wherein said tuning system comprises an element which changes an angular orientation of an input optical beam.

82. A device as in claim 81, wherein said tuning system comprises a movable mirror.

83. A device as in claim 76, wherein said output optical beam includes a first and output optical beam and a dropped optical beam, extending in different directions, said first output optical beam having at least one frequency band removed relative to said input optical beam.

84. A device as in claim 76, wherein said input optical beam includes a first input optical beam, and a second input optical beam with at least one wavelength range to be added to contents of said first input optical beam, said first and second input optical beams coming from different directions.

85. A device as in claim 76 wherein said input optical beam and said output optical beam have parts which extend in substantially the same directions.

86. A device as in claim 85, wherein said output optical beam includes a first output optical beam, and a dropped portion, and wherein said first output optical beam extends in substantially the same direction as said input optical beam, and wherein said dropped portion extend sin a substantially different direction than said input optical beam.

87. A device, comprising: a holographic storage element, formed with a hologram therein in the shape of a grating; and an optical tuning element, tuning an operation of said holographic storage element to react to different optical frequencies.

88. A device as in claim 87, further comprising an optical system, coupling an input optical beam to said hologram.

89. A device as in claim 87, wherein said optical tuning element operates to change an optical angle of incidence of an input optical signal.

90. A device as in claim 87, wherein said optical tuning element operates to change a mechanical orientation of said holographic storage element.

91. A device as in claim 87, wherein said optical tuning element operates to change an index of refraction of a medium of said holographic storage element.

92. A device as in claim 91, wherein said holographic storage element is formed of an Electro optic material, and said optical tuning element changes a voltage across said Electro optical material.

93. A device as in claim 88, wherein said optical system includes an input optical fiber, an output optical fiber, and a dropped output optical fiber, wherein said hologram operates to diffract said different optical frequencies selected by said optical tuning element, to said dropped output optical fiber.

94. A device as in claim 88, further comprising a repeater element, receiving an output optical signal, converting said output optical signal to an electrical signal, and reconverting said electrical signal to an optical signal.

95. A device as in claim 93, wherein said optical system includes a double prism, which reflects an output optical signal back in the direction of its incidence, said double prism located in a direction where it will not contact a dropped optical signal for said dropped output optical fiber.

96. A device as in claim 87, wherein said optical tuning element changes an effective period length of the holographic grating.

97. A device as in claim 92, wherein said changing a voltage comprises changing the voltage to change a refractive index of the crystal between 1.35 and 1.45.

98. A device as in claim 89, wherein said changing element is a movable mirror.