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Claims  |
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What is claimed is:
1. An interferometric spatial switch for selectively directing at least one
externally derived beam of substantially coherent light, said switch
comprising:
means for dividing said externally derived light beam into a substantially
identical constituent beam pair;
an optical phase modulating device optically coupled to said beam-dividing
means to receive said constituent beam pair, said phase modulating device
having liquid crystal pixel means situated to shift phase of at least one
constituent beam of said constituent beam pair, said pixel means being
electrically controllable sufficiently fast for selectively shifting the
phase differential between said constituent beam pair, said pixel means
comprising at least one stage having at least one associated pair of
substantially coplanar liquid crystal (LC) pixels each cooperating in
response to predetermined control signals respectively applied thereto to
respectively contribute a selected phase shift to each respective
constituent beam of said constituent beam pair respectively passing
through said associated pixel pair;
whereby each pixel of said one pixel stage respectively contributes to the
overall selected phase differential between said constituent beam pair
passing therethrough;
said pixel means further comprising another pixel stage being positioned in
parallel relationship with said one pixel stage along a common axis, each
of said pixel stages comprising cascading pixel stages having mutually
corresponding associated pixel pairs, each respective corresponding pixel
pair in said cascading pixel stages cooperating in response to
predetermined control signals respectively applied thereto to respectively
impart a selected incremental phase shift to each respective constituent
beam of said constituent beam pair passing through said cascading pixel
stages;
whereby each corresponding pixel of said cascading stages respectively
contributes to the overall selected phase differential between said
constituent beam pair passing therethrough; and
an output unit optically coupled to receive said constituent beam pair
passing from said phase-modulating device, said output unit having
polarization-independent means for combining along coincident collinear
paths respective components of said received constituent beam pair, said
collinear paths being situated such that said combined components of said
constituent beam pair mutually interfere to form a respective output light
beam directed along at least a selected one of first and second output
axes, according to the phase differential imparted by said
phase-modulating device.
2. The switch according to claim 1 wherein said combining means of said
output unit comprises a cube beam splitter having substantially orthogonal
first and second output axes, and a prism situated to deflect incident
light into said cube beam splitter.
3. The switch according to claim 2 wherein said output unit is operatively
capable of spatially multiplexing said output beam along both of said
first and second output axes in accordance with the phase differential
between said received constituent beam pair.
4. The switch according to claim 2 wherein said beam dividing means
comprises an input collimating lens.
5. The switch according to claim 2 wherein said beam dividing means
comprises an input unit optically coupled to pass at least said
constituent beam pair to said phase-modulating device, said input unit
comprising a cube beam splitter having mutually orthogonal first and
second input axes, and a prism situated to deflect incident light into
said phase modulating device.
6. The switch according to claim 5 wherein said associated LC pixel pairs
of said one pixel stage has a predetermined reference optical axis, said
pixel pair being adapted to phase shift linearly polarized light having a
predetermined polarization orientation substantially parallel to said
reference optical axis.
7. The switch according to claim 6 wherein said pixel means in said one
pixel stage comprises a polarization dependent SLM.
8. The switch according to claim 5 wherein said LC pixel pair of said
another pixel stage has a respective reference optical axis aligned
substantially parallel with respect to said predetermined reference
optical axis of said one stage.
9. The switch according to claim 8 wherein said pixel means in said
cascading pixel stages is adapted to phase shift linearly polarized light
having a predetermined polarization orientation substantially parallel to
each of said reference optical axes.
10. The switch according to claim 9 wherein said pixel means in said
cascading pixel stages comprises a polarization dependent SLM.
11. The switch according to claim 5 wherein said associated LC pixel pair
of said another pixel stage has a respective reference optical axis being
aligned mutually orthogonal with respect to said predetermined reference
optical axis of said one stage and said common axis.
12. The switch according to claim 11 wherein said pixel means in said
cascading stages is adapted to phase shift unpolarized light.
13. The switch according to claim 12 wherein said pixel means in said
cascading stages comprises a polarization independent SLM.
14. The switch according to claim 5 further including first and second
input arrays of collimating lenses optically coupled to said input unit to
focus thereinto externally derived light bees passing along said first and
second input axes respectively, said input unit thereby forming a
plurality of constituent beam pairs.
15. The switch according to claim 14 wherein the pixel means of said phase
modulating device comprises a spatial light modulator (SLM), said SLM
comprising an array of individually controllable liquid crystal pixels
patterned to receive respective ones of said plurality of constituent beam
pairs received from said input BS unit;
whereby predetermined LC pixels of said pixel array can individually and
simultaneously provide a selected phase differential between each
respective one of said plurality of constituent beam pairs.
16. The switch according to claim 15 wherein said output unit is
operatively capable of spatially multiplexing respective output beams
along both of said first and second output axes, in accordance with the
phase differential between respective ones of said received plurality of
constituent beam pairs.
17. The switch according to claim 2 wherein said beam dividing means
comprises an array of input collimating lenses situated to receive said
externally derived light beam and additional externally derived light
beams, the lenses of said lens array cooperating to form a plurality of
constituent beam pairs.
18. The switch according to claim 17 wherein the pixel means of said phase
modulating device comprises a spatial light modulator (SLM), said SLM
having an array of individually controllable LC pixels patterned to
receive respective ones of said plurality of constituent beam pairs from
said input lens array;
whereby predetermined LC pixels of said pixel array can individually and
simultaneously provide a selected phase differential between each
respective one of said plurality of constituent beam pairs.
19. The switch according to claim 18 further including first and second
output arrays of collimating lenses optically coupled to said output unit
to receive and focus output light beams emerging therefrom along said
first and second output axes respectively.
20. The switch according to claim 19 further including first and second
photodiode arrays optically coupled to said first and second output arrays
of collimating lenses, respectively, to convert received output light
beams into corresponding electrical signals.
21. The switch according to claim 18 wherein said output unit is
operatively capable of spatially multiplexing respective output beams
along both of said first and second output axes, in accordance with the
phase differential between respective ones of said received plurality of
constituent beam pairs.
22. The switch according to claim 21 further including first and second
output arrays of collimating lenses optically coupled to said output unit
to receive and focus multiplexed output light beams emerging therefrom
along said first and second output axes respectively.
23. The switch according to claim 22 further including first and second
photodiode arrays optically coupled to said first and second output arrays
of collimating lenses, respectively, to convert said multiplexed output
light beams into corresponding electrical signals.
24. An interferometric process for spatially switching at least one
externally derived beam of substantially coherent light comprising the
steps of:
dividing said externally derived light beam into a substantially identical
constituent beam pair;
providing liquid crystal pixel means to phase modulate at least one
constituent beam of said constituent beam pair, said pixel means
comprising at least one stage having at least one associated pair of
substantially coplanar liquid crystal (LC) pixels each cooperating in
response to predetermined control signals respectively applied thereto to
respectively contribute a selected phase shift to each respective
constituent beam of said constituent beam pair respectively passing
through said associated pixel pair;
whereby each pixel of said one pixel stage respectively contributes to the
overall selected phase differential between said constituent beam pair
passing therethrough;
said pixel means further comprising another pixel stage being positioned in
parallel relationship with said one pixel stage along a common axis, each
of said pixel stages comprising cascading pixel stages having mutually
corresponding associated pixel pairs, each respective corresponding pixel
pair in said cascading pixel stages cooperating in response to
predetermined control signals respectively applied thereto to respectively
impart a selected incremental phase shift to each respective constituent
beam of said constituent beam pair passing through said cascading pixel
stages;
whereby each corresponding pixel of said cascading stages respectively
contributes to the overall selected phase differential between said
constituent beam pair passing therethrough;
controlling the pixel means sufficiently fast for selectively shifting the
phase differential between said constituent beam pair; and
combining along coincident collinear paths respective components of said
constituent beam pair, said collinear paths being situated such that said
combined components of said constituent beam pair mutually interfere to
form a respective output beam directed along at least a selected one of
first and second output axes according to the phase differential between
said constituent beam pair.
25. The process according to claim 24 wherein said output beam is spatially
multiplexed along both of said first and second output axes according to
the phase differential between said constituent beam pair.
26. The process according to claim 24 wherein the step of phase-modulating
comprises incrementally phase-shifting said constituent beam pair.
27. The process according to claim 24 wherein the step of dividing further
comprises dividing additional externally derived light beams passing along
either of first and second input axes so as to form a plurality of
respective constituent beam pairs.
28. The process according to claim 27 wherein the step of phase-modulating
further comprises phase-shifting respective ones of said plurality of
constituent beam pairs such that each respective one of said plurality of
constituent beam pairs has an individual predetermined phase differential
shift therebetween.
29. The process according to claim 28 wherein the step of phase-modulating
comprises incrementally phase-shifting respective ones of said plurality
of constituent beam pairs.
30. The process according to claim 24 wherein the step of phase-modulating
is polarization-dependent.
31. The process according to claim 24 wherein the step of phase-modulating
is polarization-independent.
32. An interferometric spatial switch for selectively directing a plurality
of externally derived beams of substantially coherent light, said switch
comprising:
means for dividing said plurality of externally derived light beams into a
corresponding plurality of substantially identical constituent beam pairs,
said beam dividing means comprising an array of input collimating lenses
situated to receive said plurality of externally derived light beams, the
lenses of said lens array cooperating to form said plurality of
constituent beam pairs;
an optical phase modulating device optically coupled to said beam-dividing
means to receive said plurality of constituent beam pairs, said phase
modulating device having liquid crystal (LC) pixel means situated to shift
phase of at least one constituent beam of respective ones of said
plurality of constituent beam pairs, said pixel means being electrically
controllable for selectively shifting the phase differential between
respective ones of said constituent beam pairs, and wherein said pixel
means comprises a spatial light modulator (SLM) having an array of
individually controllable LC pixels patterned to receive respective ones
of said plurality of constituent beam pairs from said input lens array;
whereby predetermined LC pixels of said pixel array can individually and
simultaneously provide a selected phase differential between each
respective one of said plurality of constituent beam pairs; and
an output unit optically coupled to receive constituent beam pairs passing
from said phase-modulating device, said output unit having
polarization-independent means for combining along coincident collinear
paths respective components of said received constituent beam pairs, said
collinear paths being situated such that said combined components of
respective ones of said constituent beam pairs mutually interfere to form
a plurality of output light beams directed along at least a selected one
of first and second output axes, according to the respective phase
differential imparted by said phase-modulating device. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to optical switches and, more particularly, to an
electrically controlled interferometric spatial switch.
An essential component in most optical processing systems is an efficient
light switch. Key characteristics of a light switch used in an optical
processing system, in which the processed light must commonly pass through
many switches, are the amount of optical loss or attenuation that the
light beam experiences in passing through the switch and the on-off
response time of the switch. A commonly used light switch is a lithium
niobate, integrated-optics switch that provides relatively fast on-off
response time but typically also has about 3 dB light loss per switch.
Thus, for example, if a light beam is passed through only seven such
switches in an optical system, it will be attenuated by 21 dB, i.e., the
light intensity of the output beam drops to 1/128th of the intensity of
the input beam.
One efficient, low loss light switch may typically include a polarizing
beam splitter (PBS) and a liquid crystal pixel array to selectively
control the polarization of light beams entering the PBS. One example of
optical switching using a PBS and a liquid crystal array is disclosed in
U.S. Pat. No. 5,117,239 of N. Riza, issued 26 May 1992 and which is
assigned to the assignee of the present invention and incorporated herein
by reference. As efficient as this switch is, its use is limited to
polarized light. Further, the operation of this switch is not based on the
principle of optical interference.
Polarization-independent types of beam splitter switches have been
suggested that enable an unpolarized light beam to be selectively directed
along a selected path. One example of such a polarization-independent
switch is described by Wagner and Cheng in "Electrically Controlled
Optical Switch for Multimode Fiber Applications," Applied Optics, Vol. 19,
No. 17, September 1980, pp 2921-2925. In optical systems, use of
polarization-independent switches can be advantageous as there is no
reduction of light beam intensity (as may occur if a polarizer is used to
polarize light to be used in a polarization-dependent system) and
connections between blocks of the optical system can be made with optical
fibers that do not require polarization-maintaining fibers. Although the
switch suggested by Wagner et al is polarization-independent, it requires
high quality polarization-based optical components, such as PBSs, which
add at least twice to reduction of light beam intensity as well as to the
complexity and cost of the switch, being that high quality
polarization-based optical components are, in general, more elaborate than
non-polarization based optical components. Here again, Wagner et al do not
suggest either an interferometric switch, i.e., a switch based on the
principle of optical interference between substantially coherent light
beams, or the use of non-polarization based optical components.
A high-speed nematic liquid crystal modulator using the so called high
voltage transient nematic effect is described by S. T. Wu in "Nematic
Liquid Crystal Modulator with Response Time Less than 100 .mu.s at Room
Temperature", Applied Physics Letters, Vol. 57, No. 10, September 1990, pp
2921-2925. However, Wu does not suggest how to use the high voltage
transient nematic effect to make an interferometric optical switch based
on a high-speed nematic liquid crystal cell. Additionally, Wu suggests
neither an interferometric optical switch based on a
polarization-independent nematic liquid crystal cell nor a cascading
arrangement of nematic liquid crystal cells to further reduce the on-off
response time of the interferometric optical switch. A need thus exists
for a compact low-loss light switch which can operate either on polarized
or unpolarized light using non-polarizing optical components. Further, it
is desirable to provide a light switch whose on-off response time is
comparable or superior to that of currently available fast-response
switches (e.g., 100 .mu.sec).
Accordingly, one object of the invention is to provide an interferometric
light switch than can operate either on polarized or unpolarized light.
Another object of the invention is to provide an interferometric switch
having a relatively fast on-off response time.
It is yet another object of the invention to provide a compact, low-loss
interferometric light switch which uses non-polarizing optical components.
SUMMARY OF THE INVENTION
In accordance with this invention an interferometric spatial switch is
provided for selectively directing at least one externally derived beam of
substantially coherent light. The switch comprises means, such as, for
example, an input collimating lens or an input unit, for dividing the
externally derived light beam into a substantially identical constituent
beam pair. An optical phase modulating device is optically coupled to the
beam-dividing means to receive the constituent beam pair. The phase
modulating device includes at least one electrically controllable pixel
for selectively shifting the phase differential between the constituent
beam pair. An output unit is optically coupled to receive the constituent
beam pair passing from the phase-modulating device. The output unit
includes a cube beam splitter, for example, and a prism which cooperate to
combine respective components of the received constituent beam pair along
coincident collinear paths. The collinear paths are situated such that the
combined components of the constituent beam pair mutually interfere to
form a respective output light beam directed along at least a selected one
of first and second output axes in accordance with the phase differential
imparted by the phase modulating device. In one aspect of the invention,
the output unit is operatively capable of spatially multiplexing the
output beam along both the first and the second output axes in accordance
with the phase differential between the received constituent beam pair.
In one embodiment of the invention, the phase-modulating device may
comprise one pixel stage made up of an associated pixel pair which
cooperates in response to predetermined control signals respectively
applied thereto to respectively contribute a selected phase shift to each
respective constituent beam passing therethrough. At least another pixel
stage can be positioned in parallel relationship with the one pixel stage
along a common axis so as to form cascading pixel stages having mutually
corresponding associated pixel pairs. Each associated pixel pair in the
respective cascading pixel stages cooperates in response to predetermined
control signals respectively applied to each associated pixel pair to
respectively contribute a selected incremental phase shift to each
respective constituent beam passing through the cascading pixel stages.
The pixels in each of the cascading pixel stages comprise, for example, a
nematic liquid crystal cell whose reference optical axis, also commonly
referred to as the nematic molecular director, can be predeterminedly
aligned to impart a selected phase shift either to polarized light or to
the respective mutually orthogonal polarization components of unpolarized
light.
In another aspect of this invention, the phase modulating device may
comprise a spatial light modulator (SLM) having an array of individually
controllable pixels patterned to receive respective ones of a plurality of
constituent beam pairs corresponding to additional externally derived
light beams received by the beam-dividing means. In this case,
predetermined LC (liquid crystal) pixels of the pixel array can
individually provide a selected phase differential between respective ones
of the plurality of constituent beam pairs; thereby, the interferometric
switch can individually and simultaneously direct each of the additional
externally derived light beams along at least a selected one of the output
axes. The output unit may be optically coupled to associated photodiode
arrays for converting received output light beams into corresponding
electrical signals. Input and output arrays of collimating lenses may be
respectively used for focusing the externally derived light beams passing
into the interferometric switch and the light beams emerging therefrom.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth with
particularity in the appended claims. The invention itself, however, both
as to organization and method of operation, together with further objects
and advantages thereof, may best be understood by reference to the
following detailed description in conjunction with the accompanying
drawings in which like numerals represent like parts throughout the
drawings, and in which:
FIG. 1 is a schematic diagram showing a plan view of a
polarization-dependent interferometric switch in accordance with one
embodiment of the present invention;
FIGS. 2A and 2B are schematic diagrams showing respective plan views of a
polarization-dependent interferometric switch, in accordance with the
present invention.
FIGS. 3A and 3B are schematic diagrams showing respective plan views of a
polarization-independent interferometric switch, in accordance with the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a polarization-dependent interferometric switch 10 capable of
selectively directing at least one externally derived beam B.sub.i of
substantially coherent, linearly polarized light, being represented in
FIG. 1 by a cluster of light rays 12 diverging from an input port 14. For
the purpose of explanation of operation, it is assumed that the externally
derived, linearly polarized light beam (e.g., B.sub.i) comprises
vertically or p-polarized light, i.e., light which has a polarization
vector oriented along a Z axis that extends perpendicular to the plane of
the figure (here the plane defined by the X and Y axes) and represented by
the solid dots on the exemplary light rays shown therein. A collimating
input lens 16, for example, can conveniently constitute means for dividing
the externally derived light beam B.sub.i into a substantially identical
constituent beam pair, e.g., b' and b".
An optical phase modulating device 20 is optically coupled to input lens 16
to receive the constituent beam pair b' and b". As used herein, "Optically
coupled" refers to an arrangement in which one or more light beams are
directed from one optical component to another in a manner which maintains
the integrity of a signal carried by the light beam. In the embodiment
shown in FIG. 1, phase modulating device 20 has one pixel 22, situated to
operate, for example, on constituent beam b'. By way of example and not of
limitation, pixel 22 may comprise a nematic liquid crystal (LC) cell
controllable by the voltage level of an electrical control signal (not
shown) applied thereto to provide a voltage-dependent phase shift to
constituent beam b'. In contrast, constituent beam b" passes through an
inactive pixel 24 of phase modulating material which introduces a fixed
phase shift to the constituent beam b" passing therethrough. Pixel 24, in
this particular embodiment, may be simply replaced with a suitable glass
film or the like. Thus, in the embodiment of FIG. 1, the phase
differential between constituent beam pair b' and b" is solely selected
in accordance with the voltage-dependent phase shift imparted by pixel 22
to constituent beam b'. For example, if the control signal applied to
pixel 22 is selected to impart a phase shift to constituent beam b' which
is equal to the fixed phase shift introduced by pixel 24, then the phase
differential between constituent beam pair b' and b" is 0.degree..
Alternatively, the control signal applied to pixel 22 can be selected to
impart a predetermined phase shift to constituent beam b', which causes
the phase differential between the constituent beam pair to become
180.degree.; this mode of operation, i.e., a single pixel (e.g., 22) being
operated to cause a selected shift in the phase differential (e.g.,
180.degree.) between the constituent beam pair is represented by the .pi.
symbol therein.
Interferometric switch 10 further comprises an output unit 26 optically
coupled to receive the constituent beam pair passing from the
phase-modulating device 20. The output unit 26 may comprise a cube beam
splitter 28 having first and second mutually orthogonal output axes (here
the X and Y axes respectively) and a total internal reflection (TIR) prism
30 situated to direct incident light into cube beam splitter 28. It should
be appreciated that the cube beam splitter 28 being a non-polarizing beam
splitter, i.e., a beam splitter which does not alter the polarization of
incident light, such as a polarizing beam splitter, can operate equally
effectively on either polarized or unpolarized light. Therefore, as will
be explained in further detail in the context of FIGS. 2A and 3A, the
interferometric switch can be conveniently designed to be either
polarization-dependent or independent depending on the characteristics of
phase-modulating device 20.
Cube beam splitter 28 and prism 30 of the output unit 26 cooperate to
combine along coincident collinear paths respective components of the
received constituent beam pair. The collinear paths are situated such that
respective combined components of the received constituent pair mutually
interfere (either constructively or destructively) to form a respective
output light beam (b.sub.o1 or b.sub.o2 shown in phantom lines) directed
along at least a selected one of the first and second output axes, (here
the X and Y axes respectively). In particular, when the constituent beam
pair b' and b" has a selected phase differential of 0.degree., as selected
via the phase-modulating device 20, the respective components thereof when
combined along coincident collinear paths in output unit 26 interfere
constructively along one of the output axes (e.g., the X axis) and
destructively along the other output axis (e.g., the Y axis), thereby
forming an output beam bo.sub.1 along the constructively interfered, or X,
output axis. Conversely, when the constituent beam pair has a phase
differential of 180.degree., as selected via the phase-modulating device
20, the respective components thereof when combined in output unit 26 this
time interfere constructively along the other output axis (e.g., the Y
axis) thereby forming an output beam bo.sub.2 along the Y output axis. In
general, the light output relationship between the first and second output
axes can be characterized in terms of a sinusoidal function whose argument
is the phase differential between the constituent beam pair received by
output unit 26. Thus, in the general case, interferometric switch 10 is
not only capable of an on-off mode of operation, but is also capable of
spatially multiplexing an output beam along both of the first and second
output axes in accordance with the selected shift in the phase
differential between the constituent beam pair imparted by the
phase-modulating device 20. FIG. 1 further illustrates by way of example
graded-index (GRIN) lenses 32 and 34 each disposed to optically couple a
respective output light beam to an associated output port such as an
optical fiber 36 and 38.
FIG. 2A illustrates an embodiment wherein an input unit 50 provides
alternative convenient means for dividing an externally derived beam
(e.g., B.sub.i or B.sub.ii) of substantially coherent, linearly polarized
light (hem vertically or p-polarized) received along either of first and
second input axes (here the X and Y axes respectively). Input unit 50
comprises a respective cube beam splitter 52 and a TIR prism 54 which
cooperate to form a substantially identical constituent beam pair b' and
b" and which are optically coupled to phase-shifting device 20 so as to
allow the constituent beam pair to directly pass therethrough. Further,
those skilled in the art will appreciate that input unit 50 advantageously
permits each constituent beam b' and b" to propagate substantially
colinearly (e.g., along the X axis), and with substantially equal
intensity as the corresponding externally derived light beam. Similar to
the exemplary embodiment of FIG. 1, it is assumed that the externally
derived light beam B.sub.i or B.sub.ii comprises substantially vertically
or p-polarized light. Since the cube beam splitter 52 is a non-polarizing
beam splitter, the assumed polarization state of each constituent beam b'
and b" remains substantially identical to one another, i.e., each
constituent beam maintains the same original polarization had by the
externally derived light beam (here vertically or p-polarized).
As previously stated, interferometric switch 10 can be designed to operate
either on polarized or unpolarized light depending on the characteristics
of phase modulating device 20. The exemplary embodiments illustrated in
FIGS. 2A and 2B, as well as FIG. 1, in particular incorporate a
polarization-dependent phase-modulating device 20, as explained hereafter.
Briefly, the basis of a nematic LC pixel being able to provide a selected
phase shift to light passing therethrough is its electrically controllable
index of refraction. As is well understood by those skilled in the art, in
an unexcited mode of operation, i.e., when the voltage of the control
signal applied to the LC pixel has a zero value, parallel rub LC molecules
can have a predetermined axial alignment which determines a reference
optical axis, also generally referred as nematic molecular director. As
used herein, reference optical axis refers to the axial alignment
exhibited by the LC molecules in an unexcited mode of operation. In
particular, if linearly polarized light passing therethrough has its
polarization vector aligned substantially parallel with the reference
optical axis of the LC, then such linearly polarized light can experience
a selected phase shift determined by the index of refraction change
induced by the control signal applied to the LC. In the exemplary
embodiments of FIGS. 1, 2A and 2B, pixels illustrated with a right-tilt
cross hatch specifically represent pixels whose LC molecules have a
predetermined reference optical axis (here aligned parallel to the Z axis)
substantially parallel to the assumed polarization vector orientation of
constituent light beams respectively passing therethrough. Accordingly,
the phase modulating device 20, as specifically described herein, is a
polarization-dependent device capable of imparting a selected phase shift
to linearly polarized light (here vertically or p-polarized light) having
the assumed polarization vector orientation specifically indicated in
FIGS. 1, 2A and 2B (here oriented parallel to the Z axis). Alternatively,
if the externally derived light beam comprises horizontally or s-polarized
light, i.e., light which has a polarization vector oriented parallel to
the plane of the figure and along the Y axis, then the predetermined
reference optical axis of the LC pixels in phase-modulating device 20
would require a like orientation, i.e., parallel to the plane of the
figure and along the Y axis, for them to provide a selected phase shift to
horizontally or s-polarized light.
It will be appreciated that phase-modulating device 20 shown in FIG. 2A
comprises one pixel stage 20.sub.1 having one substantially coplanar
associated pixel pair 22.sub.1 and 24.sub.1 which cooperates in response
to respective, suitably biased control signals to provide a selected phase
shift to each respective constituent beam respectively passing
therethrough. For example, if a phase differential of 180.degree. is
desired between constituent pair b' and b", pixel 22.sub.1, can be
operated to provide a +90.degree. phase shift (as indicated by the .pi./2
designation) while pixel 24.sub.1, can be operated to provide a
-90.degree. phase shift (as indicated by the -.pi./2 designation) to
constituent beams b' and b" respectively passing therethrough. This mode
of operation is particularly useful to improve the switching speed of the
interferometric switch. Analysis by S. T. Wu and C. S. Wu, in "Small Angle
Relaxation of Highly Deformed Nematic Liquid Crystal," Applied Physics
Letters, Vol 53, No. 19, 1988, pp 1794-1796, indicates that the on-off
response time of a nematic LC based optical switch is directly
proportional to the square o | | |
