Liquid crystal cells – elements and systems – Liquid crystal optical element – Beam dividing switch formed from liquid crystal cell
Reexamination Certificate
1998-11-23
2001-06-26
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal optical element
Beam dividing switch formed from liquid crystal cell
C349S158000
Reexamination Certificate
active
06252644
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to liquid-crystal optical devices. In particular, the invention relates to a mechanical structure for establishing the gap of the cell into which the liquid crystal is filled and to a method of optimizing the optical performance of a liquid-crystal cell.
BACKGROUND ART
Liquid-crystal modulators are well known. They are most prevalently used in displays ranging in size from wrist watches to flat-panel displays on lap top computers. In such displays, the bias applied to the pixel of the multi-element cell, when used in combination with polarizers, determines whether the pixel absorbs or passes light. Since the output is directly viewed, the ratio of the light passed in the transmissive mode to the light passed in the absorptive mode need not be very high. This ratio is referred to as the extinction ratio for a liquid-crystal cell.
Specialized liquid-crystal optical modulators are also known in which a single, well defined beam strikes the modulator and its intensity is modulated according to the electrical bias applied across the liquid-crystal cell. Many applications of optical modulators require a high extinction ratio.
A relatively new application of liquid crystals involves optical switches in a multi-wavelength optical communication. Brackett et al. in “A scalable multiwavelength multihop optical network: a proposal for research on all-optical networks,”
Journal of Lightwave Technology,
vol. 11, no. 5/6, 1993, pp. 736-753 describe an all-optical communication network based on optical fibers, each carrying multiple optical signals of different carrier wavelengths. The all-optical network requires for its most useful applications switching nodes connecting multiple fibers that can switch the different optical signals between three or more fibers or other optical paths according to their wavelength, all the while the signals are maintained in the optical domain, that is, without any electro-optical conversion.
One type of such optical switch is the liquid-crystal switch described by Patel and Silberberg in U.S. Pat. Nos. 5,414,540 and 5,414,541, both incorporated herein by reference, and in “Liquid Crystal and Grating-Based Multiple-Wavelength Cross-Connect Switch,”
IEEE Photonics Technology Letters,
vol. 7, no. 5, May 1995, pp. 514-516. A schematic representation of a 2-wavelength switch based on this technology is illustrated in perpendicularly arranged views in
FIGS. 1 and 2
. A two-wavelength optical beam
10
, assumed in this simple example to be polarized in they-direction, strikes a frequency-dispersive element
12
, such as a Bragg grating to produce two optical beams
14
,
16
separated according to their wavelengths. A lens
18
may be required to produce the required optical configuration. The two beams
14
,
16
strike respective segments
20
,
22
of a segmented liquid-crystal modulator
24
after passing through a first polarization-dispersive element
26
, such as a calcite crystal or Wollaston prism. The calcite crystal
26
is arranged such that the y-polarization corresponds to the ordinary polarization of the calcite. The utility of the first polarization-dispersive element
26
is not readily apparent in this simple explanation, but its need become more obvious when two input beams are being switched in an add/drop circuit.
Many aspects of the invention are not directly dependent upon the use of a liquid-crystal modulator, but that example will be used here for definiteness. Each segment
20
,
22
of the liquid-crystal modulator
24
constitutes a separately controllable liquid-crystal modulator. More details will be given later, but the liquid-crystal cell
24
has been previously used in configurations which typically include two glass plates with a gap between them which is filled with a nematic liquid crystal. In one embodiment, one side of the segmented modulator
24
has a uniform biasing electrode while the other has an array of electrode fingers. In this configuration, states of polarization are use for switching, as discussed in the cited Patel and Silberberg patents. Depending upon whether electrical bias is applied to the respective segment
20
,
22
the polarization of the beam
14
,
16
striking the segment either is left in its y-polarization or is rotated by 90° to the x-polarization, which is the extraordinary polarization with respect to the two calcite crystals
26
,
28
.
After the beams
14
,
16
have passed through the liquid-crystal modulator
24
with perhaps the polarization state of one or the other of the two wavelength signals being rotated, the beams pass through a second polarization-dispersive element
28
. As shown in
FIG. 2
, the polarization-dispersive element
28
distinguishes the polarization states of the beams
14
,
16
and accordingly transmits the ordinarily polarized light into beams
32
,
36
(
FIG. 2
) and transmits the extraordinarily polarized light into beams
34
,
38
. Following focusing by a second lens
30
, a second wavelength-dispersive element
40
recombines the two beams into either first output beam
42
or second output beam
44
, the two output beams
42
,
44
being of different polarizations. If the beams exiting the second polarization-dispersive element
28
are of different polarizations, one is directed to the first output beam
42
and the other to the second output beam. It is understood that the two segments
20
,
22
allow this switching to be performed independently for each wavelength. Thus, the electrical biasing conditions determine onto which output beam
42
,
44
each of the two wavelength-differentiated signals
14
,
16
are switched.
This explanation is intended only as an example of the type of multi-wavelength optical switching that is provided by liquid-crystal cells. The example will be used to illustrate some problems addressed by the invention. Many other configurations of liquid-crystal switches and modulators are included within the invention.
The above optical switching networks do not depend critically upon the modulator being based upon a liquid crystal. Such a switching network, particularly when applied to multiple input beams and to beams of mixed polarization, depends upon a selective polarization converter that in one state can pass the light with its polarization unchanged and in another state simultaneously converts TE-polarized light to TM-polarized light and vice versa.
A schematic cross-sectional view of a conventional segmented liquid-crystal modulator
20
is shown in FIG.
3
. On one transparent glass plate
50
are formed two semi-transparent electrode fingers
52
,
54
, for example, of indium tin oxide (ITO), which are connected to respective biasing sources. On the other transparent glass plate
56
is formed a semi-transparent planar counter-electrode
58
, also of ITO, typically grounded or biased to a fixed potential. Alignment layers
62
,
64
of an organic dielectric material are deposited over the electrodes on both glass substrates
50
,
56
. The alignment layers
62
,
64
are buffed in predetermined directions that are perpendicular to each other when the substrates
50
,
56
are assembled together. Typically, the buffing direction on the first substrate
50
is along the long direction of the finger electrodes
52
,
54
. The two glass substrates
50
,
56
are then assembled into a liquid-crystal cell with the buffing directions perpendicular between them and with a gap
66
of thickness d between the two alignment layers
62
,
64
.
A nematic liquid crystal
68
is then filled into the gap
66
. Because of the perpendicularly buffed alignment layers
62
,
64
, the director of the liquid crystal (i.e., the direction of the long axis of the molecules constituting the liquid-crystal
68
) is fixed at the surfaces of the respective alignment layers
62
,
64
to lie along the respective buffing directions. In the absence of other forces, the director smoothly varies between the two alignment layers
62
,
64
. That is, its vector head follows a helix, and the liquid-crys
Morgan & Lewis & Bockius, LLP
Parker Kenneth
Tellium Inc.
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