U.S. patent application number 10/337549 was filed with the patent office on 2003-07-31 for liquid crystal planar non-blocking nxn cross-connect.
Invention is credited to Leslie, Thomas M., Lindquist, Robert G..
Application Number | 20030142262 10/337549 |
Document ID | / |
Family ID | 23711908 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030142262 |
Kind Code |
A1 |
Leslie, Thomas M. ; et
al. |
July 31, 2003 |
Liquid crystal planar non-blocking NxN cross-connect
Abstract
A non-blocking N.times.N cross-connect is provided that has an
array of liquid crystal (LC) switches in a grid of planar optical
waveguides within a light optical circuit (LOC). LC filled trenches
are used in a planar optical waveguide and each trench provides the
functionality of a waveguide polarization splitter, a transverse
electric (TE) switch cross point, a transverse magnetic (TM) switch
cross point, or a waveguide polarization combiner. By combining
these elements, a cross-connect system is fabricated.
Inventors: |
Leslie, Thomas M.;
(Horseheads, NY) ; Lindquist, Robert G.; (Elmira,
NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
23711908 |
Appl. No.: |
10/337549 |
Filed: |
January 7, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10337549 |
Jan 7, 2003 |
|
|
|
09431430 |
Nov 1, 1999 |
|
|
|
6559921 |
|
|
|
|
Current U.S.
Class: |
349/198 |
Current CPC
Class: |
G02F 1/133738 20210101;
G02F 1/133742 20210101; G02F 1/1326 20130101; G02F 1/3137
20130101 |
Class at
Publication: |
349/198 |
International
Class: |
G02F 001/13 |
Claims
What is claimed:
1. An optical device for directing a light signal, comprising: an
optical path for propagating the light signal; a trench formed in
the optical path, said trench including a surface region; an
alignment layer disposed on said surface region; and a liquid
crystal material disposed in said trench, said liquid crystal
material having a plurality of molecules that are aligned in a
first direction by said alignment layer.
2. The optical device according to claim 1, wherein the alignment
layer is a homogeneous alignment layer, such that the first
direction is parallel to the surface region.
3. The optical device according to claim 1, wherein the alignment
layer is a homeotropic alignment layer, such that the first
direction is orthogonal to the surface region.
4. The optical device according to claim 1, wherein the optical
path comprises: at least one first waveguide; and at least one
second waveguide intersecting said at least one first waveguide at
a cross-point, wherein the trench is disposed at said
cross-point.
5. The optical device according to claim 4, further comprising a
switching device coupled to the liquid crystal material.
6. The optical device according to claim 5, wherein the switching
device is a pair of electrodes.
7. The optical device according to claim 4, wherein the alignment
layer comprises at least one of a copolymer, a polymer, obliquely
evaporated SiO, and silane couple agents.
8. The optical device according to claim 1, wherein the trench is a
slot in the optical path.
9. The optical device according to claim 1, wherein the trench is a
canal substantially extending the length of the optical device.
10. The optical device according to claim 1, wherein the optical
path comprises a substrate and an optical waveguide structure.
11. The optical device according to claim l, wherein the optical
device is disposed within one of a polarization splitter, a
polarization combiner, a TM switch, a TE switch, a variable optical
attenuator, a signal splitter, and an N.times.N TE-TM array.
12. A method of directing a light signal in an optical device, said
optical device having a first optical path and a second optical
path, said method comprising: forming a trench in a cross-point,
wherein said cross-point is a location where the first optical path
intersects the second optical path; forming an alignment layer on a
surface region of the trench; disposing a liquid crystal material
having a plurality of molecules in the trench, wherein said
alignment layer causes said plurality of molecules to align in a
first direction; and applying a voltage to said liquid crystal
material to thereby change an alignment of said plurality of
molecules from said first direction to a second direction to cause
a portion of the light signal to be directed from the first optical
path into the second optical path.
13. The method according to claim 10, wherein the alignment layer
is a homogeneous alignment layer, such that the first direction is
parallel to the surface region.
14. The method according to claim 10, wherein the alignment layer
is a homeotropic alignment layer, such that the first direction is
orthogonal to the surface region.
15. A method of directing a light signal in an optical device, said
optical device including an optical path, a trench formed in said
optical path, and an alignment layer disposed on a surface of said
trench, said method comprising: disposing a switch element in the
trench, said switch element including a plurality of liquid crystal
molecules that are aligned in a first direction by the alignment
layer when no electrical energy is applied to said switch element;
and applying electrical energy to said switch element to thereby
cause said plurality of molecules to align in a second
direction.
16. The method according to claim 15, wherein the first direction
is parallel to the surface of the trench.
17. The method according to claim 15, wherein the first direction
is orthogonal to the surface of the trench.
18. The method according to claim 15, wherein the plurality of
molecules comprises liquid crystal molecules.
19. An optical device for directing a light signal, said optical
device including a substrate having an optical waveguide layer
disposed thereon, said optical device comprising: at least one
first electrode disposed between the substrate and the optical
waveguide; a trench formed in the optical waveguide, said trench
having a surface area; a first alignment layer disposed on the
surface area of said trench; a liquid crystal material disposed in
said trench and covering said first alignment layer; a top plate
connected to the substrate; and a second alignment layer disposed
on the top plate and adjacent to said liquid crystal material.
20. The device according to claim 19, wherein the top plate
comprises: a cover glass connected to the substrate; and a second
electrode disposed between said cover glass and the second
alignment layer.
21. The device according to claim 20, wherein the first electrode
is a grounding electrode and the second electrode is an address
electrode.
22. The device according to claim 19, wherein the first and second
alignment layers each comprise at least one of a copolymer, a
polymer, obliquely evaporated SiO, and silane coupling agents.
23. The device according to claim 19, wherein the liquid crystal
material is disposed in a layer that has a thickness of less than
about 25 .mu.m.
24. The device according to claim 19, wherein the liquid crystal
layer comprises at least one of a nematic class liquid crystal
material and a ferroelectric class liquid crystal material.
25. The device according to claim 19, wherein the liquid crystal
layer comprises liquid crystal molecules that are oriented
responsive to an applied voltage.
26. The device according to claim 25, wherein the liquid crystal
molecules align with the electric field when a voltage exceeding a
predetermined threshold voltage is applied.
27. The device according to claim 25, wherein the liquid crystal
molecules align substantially perpendicular to any interface coated
with the first alignment layer when substantially no voltage is
applied.
28. The device according to claim 25, wherein the liquid crystal
molecules align substantially perpendicular to any interface coated
with the second alignment layer when substantially no voltage is
applied.
29. A liquid crystal cross-connect device, comprising: an input
port for receiving light; a polarization splitter to split the
received light into transverse magnetic (TM) and transverse
electric (TE) components; a TM switch array connected to receive
the TM components from the polarization splitter; a TE switch array
connected to receive the TE components from the polarization
splitter; a polarization combiner coupled to the TM switch array
and the TE switch array to combine the outputs of the TM switch
array and the TE switch array; and an output port coupled to the
polarization combiner.
30. The device according to claim 29, wherein each switch array has
a plurality of paths, each path comprising a switching element.
31. The device according to claim 29, wherein each of the switch
arrays is in the range between about 50 and 500 .mu.m center to
center.
32. The device according to claim 29, wherein each of the
polarization splitter, the TM switch array, the TE switch array,
and the polarization combiner comprises a liquid crystal trench
device comprising: a substrate; at least one first electrode
disposed on the substrate; a first cladding layer disposed on the
first electrode; a core layer disposed on the first cladding layer;
a second cladding layer disposed on the core layer; a trench formed
in the first cladding layer, the core layer, and the second
cladding layer; a first alignment layer disposed in the trench and
on the second cladding layer; a liquid crystal layer disposed on
the first alignment layer; a second alignment layer disposed on the
liquid crystal layer; and a top plate layer disposed above the
second alignment layer.
33. The device according to claim 32, wherein the top plate layer
comprises a second electrode disposed above the second alignment
layer above the trench and above a portion of the liquid crystal
layer over the second cladding layer; and a cover glass disposed
above the second electrode and the second alignment layer.
34. The device according to claim 33, wherein the first electrode
is a grounding electrode and the second electrode is an address
electrode.
35. The device according to claim 32, wherein the first and second
alignment layers each comprise at least one of a copolymer, a
polymer, obliquely evaporated SiO, and silane coupling agents.
36. The device according to claim 32, wherein the first cladding
layer has a thickness between about 12 and 50 .mu.m, the core layer
has a thickness between about 4 and 10 .mu.m, and the second
cladding layer has a thickness between about 12 and 50 .mu.m.
37. The device according to claim 32, wherein the liquid crystal
layer has a thickness less than about 25 .mu.m.
38. The device according to claim 32, wherein the liquid crystal
layer comprises at least one of a nematic class liquid crystal
material and a ferroelectric class liquid crystal material.
39. The device according to claim 32, wherein the liquid crystal
layer comprises liquid crystal molecules that are oriented
responsive to an applied voltage.
40. The device according to claim 39, wherein the liquid crystal
molecules align with the electric field when a voltage exceeding a
predetermined threshold voltage is applied.
41. The device according to claim 39, wherein the liquid crystal
molecules align substantially perpendicular to any interface coated
with the first alignment layer when substantially no voltage is
applied.
42. The device according to claim 39, wherein the liquid crystal
molecules align substantially perpendicular to any interface coated
with the second alignment layer when substantially no voltage is
applied
43. The device according to claim 32, wherein the TM switch array
is an N.times.N waveguide having N.sup.2 trenches, N.sub.2 being an
integer.
44. The device according to claim 43, wherein all but one of the N
trenches are set in a transmission state.
45. The device according to claim 43, wherein the TE switch array
is the voltage dual of the TM switch array.
Description
FIELD OF THE INVENTION
[0001] The present invention relates in general to optical
switching devices. More particularly, the present invention relates
to a liquid crystal cross-connect for an optical waveguide.
BACKGROUND OF THE INVENTION
[0002] One of the current trends in telecommunications is the use
of optical fibers in place of the more conventional transmission
media. One advantage of optical fibers is their larger available
bandwidth handling ability that provides the capability to convey
larger quantities of information for a substantial number of
subscribers via a media of considerably smaller size. Further,
because lightwaves are shorter than microwaves, for example, a
considerable reduction in component size is possible. As a result,
a reduction in material, manufacturing, and packaging costs is
achieved. Moreover, optical fibers do not emit any electromagnetic
or radio frequency radiation of consequence and, hence, have
negligible impact on the surrounding environment. As an additional
advantage, optical fibers are much less sensitive to extraneous
radio frequency emissions from surrounding devices and systems.
With the advent of optical fiber networks, flexible switching
devices are needed to direct light signals between fibers to
all-optical domain fiber networks.
SUMMARY OF THE INVENTION
[0003] One aspect of the invention is an optical device for
directing a light signal, including an optical path for propagating
the light signal. A trench is formed in the optical path, the
trench including a surface region. An alignment layer is disposed
on the surface region, and a liquid crystal material is disposed in
the trench, the liquid crystal material having a plurality of
molecules that are aligned in a first direction by the alignment
layer.
[0004] Another aspect of the invention is a method of directing a
light signal in an optical device, the optical device having a
first optical path and a second optical path, the method including
forming a trench in a cross-point, wherein the cross-point is a
location where the first optical path intersects the second optical
path. The method includes forming an alignment layer on a surface
region of the trench. The method also includes disposing a liquid
crystal material having a plurality of molecules in the trench,
wherein the alignment layer causes the plurality of molecules to
align in a first direction, and applying a voltage to the liquid
crystal material to thereby change an alignment of the plurality of
molecules from the first direction to a second direction to cause a
portion of the light signal to be directed from the first optical
path into the second optical path.
[0005] Another aspect of the invention is a method of directing a
light signal in an optical device, the optical device including an
optical path, a trench formed in the optical path, and an alignment
layer disposed on a surface of the trench, the method including
disposing a switch element in the trench, the switch element
including a plurality of liquid crystal molecules that are aligned
in a first direction by the alignment layer when no electrical
energy is applied to the switch element, and applying electrical
energy to the switch element to thereby cause the plurality of
molecules to align in a second direction.
[0006] Another aspect of the invention is an optical device for
directing a light signal, the optical device including a substrate
having an optical waveguide layer disposed thereon. The optical
device also includes at least one first electrode disposed between
the substrate and the optical waveguide, a trench formed in the
optical waveguide, the trench having a surface area. A first
alignment layer is disposed on the surface area of the trench. A
liquid crystal material is disposed in the trench covering the
first alignment layer. A top plate is connected to the substrate,
and a second alignment layer is disposed on the top plate and
adjacent to the liquid crystal material.
[0007] Another aspect of the invention is a liquid crystal
cross-connect device, including an input port for receiving light,
a polarization splitter to split the received light into transverse
magnetic (TM) and transverse electric (TE) components, a TM switch
array connected to receive the TM components from the polarization
splitter, a TE switch array connected to receive the TE components
from the polarization splitter, a polarization combiner coupled to
the TM switch array and the TE switch array to combine the outputs
of the TM switch array and the TE switch array; and an output port
coupled to the polarization combiner.
[0008] The foregoing and other aspects of the present invention
will become apparent from the following detailed description of the
invention when considered in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1A shows an exemplary cross-connect system in
accordance with the present invention;
[0010] FIG. 1B shows a more detailed cross-connect system of FIG.
1A with the exemplary cross-connect being a 4.times.4
cross-connect;
[0011] FIG. 1C shows an exemplary switch path for two input fibers
of FIG. 1B;
[0012] FIG. 2 shows a cross-sectional side view of an exemplary
liquid crystal (LC) filled trench or canal in a planar waveguide in
accordance with the present invention;
[0013] FIGS. 3A and 3B illustrate the orientation of the LC
molecules in an exemplary trench or canal without and with a
voltage applied to an address electrode, respectively;
[0014] FIG. 4 shows a top view of an exemplary trench or canal in a
polarization splitter in accordance with the present invention;
[0015] FIG. 5 shows an exemplary switch matrix in accordance with
the present invention;
[0016] FIGS. 6A and 6B show a top view of an exemplary LC trench or
canal in the transmission and TIR states, respectively, for a TM
wave in an exemplary TM switch array in accordance with the present
invention;
[0017] FIGS. 7A and 7B show a top view of an exemplary LC trench or
canal in the transmission and TIR states, respectively, for a TE
wave in an exemplary TE switch array in accordance with the present
invention;
[0018] FIG. 8 shows the combined transmission state for a TE wave
and the TIR state for a TM wave in accordance with the present
invention;
[0019] FIG. 9 shows an exemplary switch path for an exemplary
variable optical attenuator in accordance with the present
invention; and
[0020] FIG. 10 shows an exemplary switch path for an exemplary
power distributor/splitter in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention is directed to a non-blocking
N.times.N cross-connect having an array of liquid crystal (LC)
switches in a grid of planar optical waveguides within a; light
optical circuit (LOC). LC filled trenches or canals are used in a
planar optical waveguide and each trench or canal can provide the
functionality of a waveguide polarization splitter, a transverse
electric (TE) switch cross point, a transverse magnetic (TM) switch
cross point, a waveguide polarization combiner, a filter, variable
optical attenuator, or a signal splitter. By combining these
elements, a cross-connect system can be fabricated. The LC material
in the trench or canal can be electrically addressed to create an
index change that can either match the waveguide conditions or
create a total internal reflection condition.
[0022] FIG. 1A shows an exemplary cross-connect system in
accordance with the present invention. The exemplary system
comprises an input port 10, a polarization splitting segment 20, a
TM switch array 30, a TE switch array 40, a polarization combining
segment 50, and an output port 60. The input port 10 is a linear
array of planar waveguides to which an array of fibers can be
pigtailed. The spacing between waveguides is determined by
pigtailing capabilities. Light from the fibers enters the input
port 10 and is passed to the polarization splitter 20. The
polarization splitter separates light into its TM and TE
components. The TM components are then passed to the TM switch
array 30, and the TE components are passed to the TE switch array
40. As described in further detail below, the outputs of the switch
arrays 30 and 40 are combined by the polarization combiner 50 and
passed to the output port 60. Similar to the input port 10, the
output port 60 is a linear array of planar waveguides to which an
array of fibers can be pigtailed.
[0023] It should be noted that TM and TE are defined herein by
convention at the LC trench or canal interfaces, and not by
waveguide convention.
[0024] FIG. 1B shows a more detailed cross-connect system of FIG.
1A with the exemplary cross-connect being a 4.times.4
cross-connect, and FIG. 1C shows an exemplary switch path for input
fibers 1 and 3 of FIG. 1B. Light from the input port 10 enters the
polarization splitter 20 that reflects the TE waves to the TE
switch array 40 while passing the TM waves to the TM switch array
30. The switching arrays 30, 40 are preferably between about 50 and
500 .mu.m center to center, and more preferably about 250 .mu.m
center to center. Each switch array 30, 40 has a plurality of
switching elements 35, 45 in each path 31-34 and 41-44,
respectively. A single switching element in each path is set to a
reflecting state to pass the light onto the polarization combiner
50. It should be noted that the path difference for the TE and TM
waves are substantially identical. The polarization combiner 50
allows the TE wave to pass while reflecting the TM wave to
recombine, as shown, for example, in FIG. 8, described below. Thus,
the beams are recombined and passed to the appropriate path in the
output port 60.
[0025] The polarization splitter 20, the switching arrays 30, 40,
and polarization combiner 50 are preferably formed within the same
fundamental element, which is preferably a liquid crystal (LC)
filled trench or canal in a planar waveguide, as shown in FIG. 2.
An LC film 160 is sandwiched between two plates or substrates
having patterned deposited thin films thereon. The structure
functions as the desired element depending on an applied voltage,
as described below.
[0026] The bottom plate or substrate 110 on which the waveguide is
patterned preferably has five deposited layers thereon. The first
layer is a first electrode 120, such as a grounding electrode that
comprises an unpatterned conductive coating such as gold, aluminum,
or indium tin oxide. The grounding electrode can be either a single
electrode or a segmented electrode. A cladding layer 130 is
deposited on the first electrode 120, and comprises a cladding
layer having a thickness between about 12 and 50 .mu.m thick for
the planar waveguide. A core layer 140 is deposited on the cladding
layer to a thickness between about 4 and 10 .mu.m. The layers 130
and 140 are etched or otherwise patterned to form trenches or
canals 135 that provide the grid structure of FIG. 1B. A cladding
layer 150, similar to the cladding layer 130, is formed above the
patterned core layer 140 to a thickness between about 12 and 50
.mu.m. The trench can either be a slot in the optical path, or a
canal that runs the length of the device.
[0027] A first alignment layer 155 is disposed above the cladding
layer 150 and in the trench or canal 135 directly over the portion
of the first electrode 120 that is exposed by the etching or
patterning of layers 130 and 140. The alignment layer 155 comprises
a thin copolymer layer or other material, such as an obliquely
evaporated SiO, silane coupling agents, or a polymer, to assist in
homeotropic alignment of LC material in the liquid crystal layer
160. The alignment layer 155 is preferably deposited to a thickness
of between one monolayer and about 100?. The alignment layer 155
should be thin enough to avoid creating an optical effect due to
its refractive index.
[0028] The liquid crystal layer 160 is then deposited over the
first alignment layer 155 both in the trench or canal 135 and over
the cladding layer 150. The thickness of the liquid crystal
material above the first alignment layer 155 is preferably less
than about 25 .mu.m. Any liquid crystal material can be used,
including those from the nematic class (preferred) and the
ferroelectric class. One liquid crystal material that can be used
is EM Chemicals BL009, having a .DELTA.n of about 0.28. It should
be noted that the larger the .DELTA.n for the LC material, the more
preferable the material is for use with the system of the present
invention. The index matching and the angles of the LC crystals are
responsive to the .DELTA.n.
[0029] A second alignment layer 165 is disposed on the liquid
crystal layer 160. The second alignment layer 165 is preferably
substantially similar in composition and thickness to the alignment
layer 155, although this does not have to be the case. For
instance, the second alignment layer 165 may be disposed such that
a homogeneous or parallel alignment of the LC molecules occurs. The
second alignment layer 165 can be either homeotropic
(perpendicular) or homogeneous (parallel) and still provide the
desired alignment in the trench or canal.
[0030] A top plate preferably has two deposited layers. A first
plate layer is a second electrode, such as an address electrode
layer 170 that comprises a patterned conductive coating such as
gold, aluminum, or indium tin oxide. A second plate layer is a
cover glass 180. The thickness of the layers 170 and 180 is not
critical, as would be known by those skilled in the art, and each
can have a thickness between about 0.3 and 300 .mu.m or even
greater. It should be noted that the first and second electrodes
can act as the grounding and address electrodes, respectively, or
the first and second electrodes can act as the address and
grounding electrodes, respectively.
[0031] FIGS. 3A and 3B illustrate the orientation of the LC
molecules of the LC layer 160 in the trench or canal 135 without
and with a voltage applied to the second electrode 170,
respectively. With substantially no voltage applied, as shown in
FIG. 3A, the LC molecules tend to align substantially perpendicular
to any interface coated with a properly chosen first alignment
layer. The LC will align parallel or perpendicular to the surface
treated with the second alignment layer depending on what type has
been chosen. Therefore, the director axis inside the trench or
canal 135 lies in the plane of the waveguide fabric and
perpendicular to the trench interface or canal interface. When a
sufficient voltage is applied to the address electrode, as shown in
FIG. 3B, the LC molecules rotate to align with the electric field.
In this case, the director axis lies substantially perpendicular to
the substrate. A typical voltage that is sufficient to turn on the
address electrode is between about 3 and 5 volts. The threshold
voltage depends on the liquid crystal material being used and the
distance between the electrodes. It should be noted that the higher
the applied voltage above the threshold voltage, the faster the
switch.
[0032] Because the birefringence of an LC material is typically on
the order of about 0.1 to 0.3, the optical properties for the TE
and TM guided modes are substantially different and can be
significantly changed with an applied field. As described below,
the trenches or canals can be used for polarization splitting, TE
and TM switching, polarization combining, signal splitting, and
variable optical attenuating.
[0033] LC switches are polarization dependent and thus, the TE and
TM waves are handled separately. The polarization splitting element
20 splits the TM and TE guided waves into independent paths. To
accomplish this, each guide from the input port 10 passes through a
polarization splitting element 20 that is an LC filled trench or
canal. A top view of an exemplary trench or canal used in a
polarization splitting element is shown in FIG. 4. As shown in FIG.
4, a trench or canal intersects the waveguide at an angle .theta.
that is typically about 60 to 70 degrees depending on the
characteristics of both the waveguide and the LC. With no voltage
applied across the trench or canal, the director axis of the liquid
crystalline material lies substantially perpendicular to the trench
wall or canal wall. The TM wave at the interface will couple
directly into the extraordinary wave inside the LC layer. The
effective refractive index will have a value determined by 1 n eff
( ) = [ n o 2 n e 2 n e 2 cos 2 ( ) + n o 2 sin 2 ( ) ] 1 / 2
[0034] in which n.sub.o (.about.1.5) is the ordinary refractive
index of the LC, n.sub.e (.about.1.6 to 1.8) is the extraordinary
refractive index, and .PHI. is the angle between the wave
propagation vector and the LC director axis. If the guide index
(.about.1.7) and LC materials are properly matched via the Fresnel
coefficients for an anisotropic interface, the TM wave passes
through the trench or canal with little or no reflection. The TE
wave, however, does not. The TE wave couples directly into the
ordinary ray in the LC layer which has an index of n.sub.o
(.about.1.5). This index is considerably lower than the effective
guide index, thus resulting in total internal reflection (TIR) at
the interface. Thus, the TM wave passes through the trench or canal
and the TE wave is totally internally reflected at the interface
into a separate guide. In other words, both the TE and TM waves
move in the direction of arrow A in FIG. 4, the TE wave is
reflected at the interface and travels in the direction of arrow B
due to index mismatching, and the TM wave passes through in the
direction of arrow C due to index matching. The guide is desirably
offset from the interface to account for the Goos Hanschen effect.
It should be noted that the index values used herein are based on
commercially available material.
[0035] The TM switch array is an N.times.N waveguide with N.sup.2
trenches or canals at the intersections. FIG. 5 shows an exemplary
switch matrix. All but one of the trenches or canals are set in the
transmission state using index matching, and one intersection in
each row is set to a TIR state using index mismatching. To insure
that one input fiber is exclusively assigned to one output fiber,
preferably only one trench or canal in any column is set to the TIR
state. By controlling which trenches or canals are set in the TIR
state, a non-blocking N.times.N switch matrix is achieved for a set
of input TM waves.
[0036] A top view of exemplary LC trenches or canals in both the
transmission and TIR states are given in FIGS. 6A and 6B,
respectively, for a TM switch array. FIG. 6A is substantially
identical to FIG. 4 and thus it is expected that the TM wave,
moving in the direction of arrow A, passes through the trench or
canal with minimal reflections, to travel in the direction of arrow
C. When an electric field above a predetermined threshold voltage
characteristic (typically about 3 to 5 volts) of the LC material is
applied from top to bottom through the trench or canal, the LC
director axis rotates to align with the field lines giving an
orientation as shown in FIG. 6B. In this state, the TM wave no
longer couples to the extraordinary wave, but rather to the
ordinary ray. Because the ordinary ray has an index of n.sub.o
(.about.1.5), the TM. wave travelling in the direction of arrow A
will totally internally reflect to travel in another waveguide in
the direction of arrow B. Thus, a Avoltage-off@ state across the LC
trench or canal is a transmission state and a Avoltage-on@ state
across the LC trench or canal is a TIR state.
[0037] The TE switch array is the Avoltage-dual@ of the TM switch
array. In other words, the TE switch array is also an N.times.N
waveguide array with N.sup.2 trenches or canals at the
intersections. It has a single TIR state for input path with
remaining trenches or canals in the transmission state. However,
the TIR state is the Avoltage-off@ state and the transmission state
is the Avoltage-on@ state. FIGS. 7A and 7B illustrate the
transmission and TIR states, respectively, for the TE wave in a TE
switch array. In the transmitting state, the TE wave couples into
the extraordinary wave with an effective index equal to n.sub.e. By
matching the index with the guide, the TE wave travels in the
direction of arrow A', passes through the trench or canal with
little or no reflection, and thus continues to travel in the
direction of arrow B. In the TIR state, the TE wave couples into
the ordinary ray inside the LC material, thereby travelling in the
direction of arrow C. The ordinary ray has an index of n.sub.o
(.about.1.5) resulting in a TIR state for the TE wave.
[0038] The polarization combining element combines the TM and TE
waves prior to their being sent to the output port. The
polarization combining element is the Avoltage-dual@ of the
polarization splitter. The trenches or canals have an RMS voltage
applied to align the LC director axis perpendicular to the
substrate. As described above, the Avoltage-on@ state is the
transmission state for the TE and is the TIR state for the TM wave.
The combined feature is illustrated in FIG. 8. The TE wave travels
in the direction of arrow A, and the TM wave travels in the
direction of A'. At the trench or canal, the TE wave passes through
and the TM wave is totally internally reflected, resulting in both
the TE and TM waves combining to travel in the direction of arrow
C.
[0039] It should be noted that crosstalk depends primarily on the
quality of index matching between the effective guide index and the
effective index of the extraordinary wave. For the upper section of
the cross-connect system, that includes the splitting and TM
switching, the guide is preferably matched to a value slightly less
than n.sub.e. For the lower section (the dual for the upper: the
combining and the TE switching), the guide index is preferably
matched to n.sub.e. To satisfy both conditions, several options
exist: (1) electrical compensation (one can tune into the
appropriate index with voltage), (2) different LC materials for the
upper and lower section (requires two LC materials, but is
relatively easy to manufacture), (3) differing guide index for
upper and lower section (unattractive option due to complexity), or
(4) a different angle of the trench or canal. Preferably, option
(2) is used in which the LC layer comprises two different layers of
LC material that will give improved matching.
[0040] The planar cross-connect is compact, less complex, and low
cost. Moreover, the device is scaleable to large arrays. Electrical
compensation and/or different LC materials can be used to improve
crosstalk and compensate for thermal effects.
[0041] Thus, an N.times.N non-blocking cross-connect based on LC
and planar waveguide technologies has been described. An N.times.M
cross-connect system is also within the scope of the present
invention. The technology is scalable, compact, and low cost.
Additionally, the LC trenches or canals can be electrically
compensated for thermal effects to insure low crosstalk.
[0042] In addition to switching, the LC trench or canal can act as
a partial reflector of the TE or TM wave in a variable optical
attenuator and a variable broadcast element. A voltage range exists
between the transmitting and total internal reflection states in
which the LC molecules do not fully rotate to align with the
electric field. In this case, the beam is partially transmitting
and partially reflecting. This effect provides the additional
functionality of a variable optical attenuator (VOA) and a power
distribution/splitter. An exemplary variable optical attenuator
operation is illustrated in FIG. 9. In both the TE and TM switch
arrays 30, 40, a voltage is applied such that only a portion of
light is reflected. Thus, the output beam is attenuated in a
controlled manner through output port 1. In a similar manner, an
exemplary power distributor/splitter is illustrated in FIG. 10 in
which a beam from the input port 1 is split between the output
ports 1 and 3.
[0043] Although illustrated and described herein with reference to
certain specific embodiments, the present invention is nevertheless
not intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the
invention.
* * * * *