Dual liquid-crystal wavelength-selective optical switch

Details Dual liquid-crystal wavelength-selective optical switch Dual liquid-crystal wavelength-selective optical switch

1998-02-03

2001-12-04

Parker, Kenneth (Department: 2871)

Liquid crystal cells, elements and systems

Liquid crystal optical element

Beam dividing switch formed from liquid crystal cell

C359S199200

Reexamination Certificate

active

06327019

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to liquid-crystal devices. In particular, the invention relates to such devices incorporated into telecommunications systems.
BACKGROUND ART
Liquid-crystal modulators have become commercially widely known, particularly in two-dimensional display applications such as wrist watches and flat-screen displays. In most of its applications, a liquid-crystal modulator is in fact a polarization converter which somehow affects the polarization of light incident upon it. Other optical components are used to present the correct light polarization to the liquid-crystal modulator and then to filter out the undesired polarization components. In its most commercially popular form, the liquid-crystal modulator uses a twisted nematic liquid crystal. Alignment layers applied to the two electrodes sandwiching the liquid crystal cause the liquid crystal to twist 90° when the electrodes are in the unbiased state. The twist may be an odd multiple of 90° and have small angular increments to account for other effects. Light incident upon the twisted liquid crystal are waveguided along the twisting liquid crystal, whereby the polarization of the light changes from one side to the other of the liquid crystal cell. However, if a sufficiently high voltage is applied to the electrodes, the twisted waveguiding structure in the liquid crystal is destroyed, and the polarization of light traversing the cell is maintained essentially constant. Thus, an electrical signal applied to the liquid-crystal cell modulates the polarization of light transmitted through the cell.
Patel and Silberberg have disclosed in U.S. Pat. Nos. 5,414,540 and 5,414,541, incorporated herein by reference in their entireties, that liquid-crystal devices can be used to switch individual channels of a multi-wavelength signal, such as is common with an optical wavelength-division multiplexed (WDM) communication network. In such a WDM network, multiple data channels are impressed upon separate lasers or other light sources to produce multiple optical signals having different data signals and different optical carrier wavelengths. By various means, the different optical signals of differing optical wavelength are then impressed upon a single optical channel, such as an optical fiber now very commonly used in telecommunication systems. Thereby, multiple data channels are conveyed along a single optical path.
One of the most fundamental elements in a high-speed telecommunications network is an add/drop multiplexer (ADM). By various means, multiple signals are impressed upon a single physical channel, whether wire, coaxial cable, or fiber. In general, the multiplexing may assume different forms, such as time-division multiplexing, wavelength-division multiplexing, and others. As the physical channel passes through various intermediate nodes in the network, an ADM at that node must be able to extract one or more of the signals multiplexed on the channel and reinsert onto the channel a substitute signal without affecting the other signals not associated with that node.
Wavelength-division multiplexed optical systems have multiple-wavelength signals impressed on a single optical fiber, and an optical ADM for use in a WDM network must be able to extract from the fiber one or more signals at respective wavelengths and to impress upon the fiber other signals at those same wavelengths. In the '540 patent, Patel and Silberberg have disclosed a liquid-crystal add/drop multiplexer
8
, illustrated in FIG.
1
. This figure is meant to be explanatory only and does not necessarily accurately represent the optical paths or placement of elements. The signal received from the network fiber is designated IN and the signal transmitted to the fiber is designated OUT. The signal extracted from the fiber is designated DROP, and the signal impressed upon the fiber is designated ADD. The fibers are not illustrated in
FIG. 1
, and other well known and fairly simple optical components couple the free-space and bulk-optics optical paths illustrated in FIG.
1
.
The IN and ADD signals are received on optical paths
10
,
12
that are incident upon a frequency-dispersive element
14
such as a grating. The frequency-dispersive element
14
spatially divides each of the multiple-wavelength signals on the IN and ADD paths
10
,
12
into multiple and separate signals having respective wavelengths and respective paths
16
,
18
. Note that
FIG. 1
illustrates the paths
16
,
18
for only one wavelength. The paths for the other wavelengths are arranged in the perpendicular direction, that is, into the plane of the illustration. The optical processing to be described hereafter is performed in parallel for the multiple wavelengths.
The illustrated embodiment is designed to be insensitive to polarization of the input signals. Especially on fiber communication lines, it is nearly impossible to control the signal polarization, which may be changing over time due to environmental and other conditions. The frequency-separated signals on paths
16
,
18
strike a first polarization-dispersive element
20
, such as a block of properly oriented calcite crystal, which spatially separates each of the signals according to two perpendicular linear polarizations. However, the relative positions of the grating
14
and calcite crystal
20
are not clearly defined in the cited patents. Half-wave plates
21
,
22
are placed in the path of the beams output from the calcite crystal
20
and having a first polarization. The half-wave plates
21
,
22
thus convert those signals to the perpendicular second polarization. As a result, the two polarization components of both the IN and ADD signals are converted to respective signals that have the same polarization and are spatially separated.
Which polarization is rotated is not of primary importance. A third half-wave plate
24
is placed in the path of both parts of one of the signals, here the ADD signals. As a result, the two beams of the ADD signal are made to have a single polarization perpendicular to the polarization of the two beams of IN signal. It is appreciated that the number of half-wave plates can be reduced to two by combining the effects of plates
21
,
24
.
A lens
26
focuses all four beams toward a second polarization-dispersive element, preferably a Wollaston prism
28
, which has the characteristic, in overly simplified language, that two beams of perpendicular polarization entering the prism
28
at the correct angles are spatially combined into a single beam.
The beam then strikes a segmented liquid-crystal array
30
. A segmented liquid-crystal array
30
is similar to a standard twisted nematic polarization converter, but one of its electrodes is divided into multiple sub-electrodes, each separately controlled by respective electrical control signals. The beam illustrated in
FIG. 1
strikes one of the segments. Beams corresponding to optical signals of different carrier wavelength (frequency) strike other segments and are each separately controlled.
Depending upon whether the electrical signal applied to the liquid-crystal segment of the wavelength illustrated in
FIG. 1
is active or inactive, the IN and ADD signals both either pass through the liquid-crystal cell
30
with their polarizations unchanged or with their polarizations rotated by 90°. If their polarizations are rotated by 90°, the effect is to interchange the polarizations of the IN and ADD signals.
After exiting the liquid-crystal modulator
30
, the two signals pass through another Wollaston prism
32
, a lens
34
, half-wave plates
36
,
40
,
42
, a polarization-dispersive element
44
, and a frequency-dispersive element
46
, all arranged symmetrically to corresponding elements on the other side of the liquid-crystal cell
30
. Thereby, the optical operations performed on the input side are undone on the output side. The result is that one set OUT of WDM signals is carried on one output path
48
, and another set DROP is carried on another output path
50
. Which signal is on which output pa

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