Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
2001-12-28
2004-07-20
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C349S077000, C349S102000, C349S117000, C349S180000, C349S193000
Reexamination Certificate
active
06765635
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
The present invention relates generally to optics, fiber optics, optical networks, and the like. More particularly, the present invention relates to variable polarization controllers and light modulators that have applications in communications, optical detection, optical instrumentation, information display and related areas.
BACKGROUND OF THE INVENTION
Optical fibers are replacing copper cables at a rapid pace as the transition medium for communication systems. Optical fibers are used in the long-haul telecommunication backbone, as well as in regional and metropolitan systems to service the fast growing need of wider bandwidth and faster speed fueled by Internet usage. Switches and attenuators are two key components that are required by current optical communications systems.
Optical switches are used for a number of functions in a communications network, including routing or rerouting of signals and add/drop for multiplexers/demultiplexers. There are currently a number of technologies being employed or evaluated for these functions, including electromechanical, microelectro-mechanical (MEMs), liquid crystal, and acousto-optic.
Today, electromechanical switches are the most common switching elements in use for telecommunications applications. One of their primary advantages is their weak polarization dependence: they have low polarization dependent losses (PDL) and require no additional optical elements to compensate for potential PDL. However, they are bulky and slow (switching times are approximately 50 msec) since they require mechanical movement of a mirror or prism to reroute the signal. They are also less reliable than other technologies because of their moving parts.
MEMs are faster and less bulky, but still rely on mechanical motion so reliability remains an issue. MEMs are micro-mirrors that are constructed using micro-lithographic techniques. The mirrors are deformed or reoriented using electrostatic forces. Switching speeds are currently limited to >10 msec. Since MEMs rely on steering a reflected beam, controlling the angle of reflection is paramount. At high signal power levels, absorptive heating of the mirror can cause distortion of the mirror surface, which results in the beam being routed in an erroneous direction. Small variations (<1 degree) in signal direction can dramatically increase the coupling losses to the outgoing channel (fiber).
Acousto-optic switches use ultrasonic waves in a birefringent crystal to steer the optical signal to different channels. Such switches are fast (submicrosecond switching is achievable). They also are potentially more reliable than the electromechanical and MEMs technologies since they have no mechanically moving parts. At present, signal losses are relatively high (>2 dB) for a single 1×2 switch that routes an incoming signal to one of two exit ports. The cost of such devices is also high since they require an acoustic (piezoelectric) generator in addition to the optical crystal. Power consumption is also a concern since sound waves must be continuously generated to maintain steering of the signal beam.
Liquid crystal technology has a relatively long history in the prior art for optical switching applications. Liquid crystals are fluids that derive their anisotropic physical properties from the long-range orientational order of their constituent molecules. Liquid crystals exhibit birefringence and the optic axis can be reoriented by an electric field. This switchable birefringence is the mechanism underlying all applications of liquid crystals to optical switching and attenuation.
Reorientation of the liquid crystal molecules under the influence of an applied field introduces elastic strains in the material. These strains stem from constraints imposed on the molecular orientation at the boundaries confining the liquid crystal. These surface constraints are given the term “surface anchoring”. In most practical applications, the surface anchoring is strong, so that molecules in the vicinity of a surface are not free to reorient but remain substantially along some preferred direction. In fact, it is this ability to control the liquid crystal surface alignment that makes optical devices employing liquid crystals feasible. As a result of the strain energy, when the field is removed the molecular orientation typically relaxes back to the configuration it had prior to the application of the field. (This is not always the case; there are situations where a liquid crystal has two stable (or metastable) states. In such liquid crystal devices, the electro-optic properties will exhibit hysteresis and possibly bistability in zero fields.)
Two mechanisms have been proposed in the prior art for optical switching using liquid crystals: polarization modulation and total internal reflection (TIR). Note in this context that we are referring to signal redirection to one of at least two channels (1×N switch where N>1). On/off liquid crystal optical switches can also be constructed on the principle of switchable scattering, but this is not the focus of the current invention. (However, it is obvious that a 1×N switch can function as an on/off switch by blocking off N−1 output channels.)
TIR liquid crystal switches rely on the difference in refractive index between the liquid crystal and the confining medium (e.g. glass). By proper choice of materials and angle of incidence of the light at the liquid crystal interface, it is possible to totally internally reflect the light when no field is applied to the liquid crystal. The effective index of the liquid crystal may be changed by reorienting the optic axis of the liquid crystal so that the total internal reflection criterion is no longer met; light then passes through the liquid crystal rather than reflecting from the interface. As with other types of reflective devices, such as MEMs, controlling the reflection angle is critical. Also, since unwanted surface reflections are always present to some degree, crosstalk can be a significant problem.
Polarization modulation is the most common mechanism used in liquid crystal devices for optical switching and attenuation. Switching is achieved between two orthogonal polarization states, for example, two orthogonal linear polarizations or left and right circular polarization. By way of illustration, a simple liquid crystal polarization modulator is illustrated in
FIGS. 1
a
-
1
c.
FIG. 1
a
illustrates a layer of nematic liquid crystal sandwiched between two transparent substrates
2
and
3
. Transparent conducting electrodes
4
and
5
are coated on the inside surfaces of the substrates. The electrodes are connected to a power source (e.g., a voltage source)
6
through an electrical switch
7
. Directly adjacent to the liquid crystal surfaces are two alignment layers
8
and
9
(e.g., rubbed polyimide) that provide the surface anchoring required to orient the liquid crystal The alignment is such that the optic axis of the liquid crystal is substantially the same through the liquid crystal and lies in the plane of the liquid crystal layer when the switch
7
is open.
FIG. 1
b
depicts schematically the liquid crystal configuration in this case. The optic axis in the liquid crystal
11
is substantially the same everywhere throughout the liquid crystal layer.
FIG. 1
c
shows the variation in optic axis orientation
12
that occurs when the switch
7
is closed as a result of molecular reorientation.
To act as a switch, the modulator must produce two orthogonal polarizations at the exit to the modulator that can then be differentiated with additional optical components. This can be achieved if the liquid crystal layer functions as a switchable half wave retardation plate. To do this, the liquid crystal layer thickness, d, and birefringence, &Dgr;n, are chosen so that
Δ
nd
λ
=
1
2
(
1
)
where &lgr; is the wavelength of the incident light. In this situation, if linearly polarized light with wavevector
13
is incident normal to the liquid crystal layer
Kelly Jack R.
Li Qingyu (Tom)
Yuan Haiji J.
CoAdna Photonics, Inc.
Cooley & Godward LLP
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