Frequency modulated liquid crystal beamsteering device

Details Frequency modulated liquid crystal beamsteering device Frequency modulated liquid crystal beamsteering device

2000-09-29

2002-09-24

Ben, Loha (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S253000, C359S315000, C359S316000, C359S209100, C359S573000, C359S245000, C349S140000, C349S193000, C349S202000, C250S227170, C385S014000, C385S130000

Reexamination Certificate

active

06456419

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to spatial light modulation and electro-optical devices generally, and more specifically to high speed, liquid crystal diffractive beamsteering devices and adaptive optics.
2. Description of the Related Art
A high speed, non-mechanical beamsteering device finds applications in optical fiber and laser communications, laser radar or other fields which require fast adaptive optics. High switching speed, wide steering range, fine angular resolution and high optical efficiency are particularly desirable in such a device.
One conventional approach for high speed diffractive beamsteering exploits the electro-optical properties of liquid crystals (LCs). This approach is analogous to the use of phased-arrays to direct microwave radiation, and has been clearly explained in several publications: see, for example, Paul F. McManamon, Edward A Watson, Terry A. Dorschner and Lawrence J. Barnes, “Applications Look at the Use of Liquid Crystal Writable Gratings for Steering Passive Radiation,”
Optical Engineering
Vol. 32, No. 11, pp. 2657-2664, (November 1993); D. P. Resler, D. S. Hobbs, R. C. Sharp, L. J. Friedman and T. A. Dorschner, “High Efficiency Liquid Crystal Optical Phased-array Beam Steering,”
Optics Letters
, Vol. 21, No. 9, pp. 689-691 (May 1, 1996); Paul F. McManamon, Terry A. Dorschner, David L. Corkum, Larry J. Friedman, Douglas S. Hobbs, Michael Holz, Sergey Liberman, Huy Q. Nguyen, Daniel P. Resler, Richard C. Sharp, and Edward A. Watson, “Optical Phased Array Technology,”
Proceedings of the I.E.E.E
., Vol. 84, No. 2, pp. 268-298 (February 1996); and O. D. Lavrentovich, D. Subacius, S. V. Shiyanovskii, and P. J. Bos, “Electrically Controlled Cholesteric gratings,”
SPIE
Vol. 3292, pp. 37-43 (1998).
The principle behind diffractive beamsteering by liquid crystal phase shifting is illustrated in FIG.
1
. For simplicity, unidirectional (single angle) steering is shown. An incident coherent optical beam
20
is shown by its equi-phase surfaces. If we consider a hypothetical prism
22
inserted into the beam path, we can see that such a prism would introduce a linear gradient of optical path delay (OPD) across the beam, shown by phase delay profile
24
. Because the prism has thickness which varies linearly with displacement in the direction x, it introduces corresponding linear phase delay profile
24
, with constant gradient. The introduction of constant gradient of phase delay results in refraction of the beam
20
, so the resulting output beam has equiphase fronts
26
, propagating in a new direction as shown by direction vector
28
.
In the arrangement of
FIG. 1
a phase shift of 2&pgr; can be subtracted periodically from the phase front without influencing the far-field pattern produced (because it corresponds to exactly one wavelength of the light beam). Thus, to produce refraction equivalent to that produced by the OPD gradient
24
, it is sufficient to introduce a periodic, sawtooth-like or “folded” phase profile as shown by the periodic OPD profile
30
. The phase profile
30
is equivalent to that of
24
except that the phase is reset whenever the cumulative phase shift reaches 2&pgr; or an integer multiple thereof. The sawtooth phase profile
30
is also essentially equivalent to that produced by a conventional blazed grating.
FIG. 2
shows a simplified reflection mode device which uses liquid crystals to produce phase shifts approximating the blazed grating profile shown in
FIG. 1
, to steer a coherent beam. The illustration is a simplistic idealization of that device described in U.S. Pat. Nos. 5,098,740 and 5,093,747 (to Dorschner et al. and Dorschner, respectively). A layer of nematic liquid crystals
40
is sandwiched between a reflective groundplane electrode
42
and an array of discrete, electrically distinct transparent electrodes
44
. The elements of the electrode array
44
are electrically connected to a plurality of drive voltages which vary stepwise across the array according to a staircase-like voltage ramp. The variation in electrode potential produces a corresponding variation in electrical field intensity at points within the liquid crystal layer
40
. Manifestly, the electric field will vary with position within the layer, in accordance with electrostatic principles, but the average field will vary across the device in an approximate staircase profile. The material of the liquid crystal layer
40
is a nematic liquid crystal with the property that its orientation is dependent upon the field strength locally applied; therefore, the effective refractive index of the LC (for a particular polarization) will vary with distance x; and the resulting phase delay introduced during light's transit across the LC layer will also vary approximately as the staircase-like ramp
48
.
To implement electrically controlled beam steering in the above described device, the reflective electrode elements of
44
are controlled through addressing electronics to allow application of pre-determined voltages to the elements as required to produce a sawtoothed optical phase delay function. If the voltages are controlled so that the phase delay is reset periodically by subtracting 2&pgr;, then the resulting sawtooth OPD profile
48
approximates a linear phase delay gradient across the device in the x direction. Comparing this function to the phase functions in
FIG. 1
, we can see that the resulting function approximates the phase delay gradient of a refractive prism. The effect of such a gradient, together with the reflective electrodes
44
, is such that incident (polarized) radiation
20
is reflected at an adjustable angle &thgr;, in relation to the voltage profile applied to the electrode array
44
.
While the device of
FIG. 2
seems to hold promise as a beamsteering device, it is limited in several important performance parameters. Most significantly, switching speeds currently achievable by this device at the important communication wavelength of 1.55 nanometer are limited to below approximately 500 Hz. This limitation results from the relatively slow relaxation of nematic crystals as they settle from driven to relaxed states. Attempts have been made to improve switching speeds by increasing the liquid crystal birefringence, thereby reducing the cell gap (the thickness of LC layer
40
); however, any such increase in birefringence is accompanied by an increase in viscosity, which in turn increases the relaxation time of the LC.
In addition to slow switching speeds, conventional nematic LCs have weak elastic anchoring forces which forbid very high phase gradients. Such gradients would be particularly desirable for low grating pitch and high steering angles (pitches of less than approximately 5 microns).
Another problem with prior LC optical phased array steering devices is their undesirable departure from ideal sawtoothed OPD characteristics.
FIG. 3
compares the ideal and actual OPD characteristics of a typical LC optical phased array beamsteering device. The OPD function
58
represents the idealized, desirable sawtooth pattern. Note that the reset portion
60
of the waveform is ideally vertical, which signifies that the phase is reset from 2&pgr; to zero over infinitesimal distance in the x direction. This idealized characteristic is not realizable by physical LC devices. Waveform
62
represents a more realistic, attainable waveform. In practice, the gradient of the phase delay is limited by the finite “fly-back” distance
64
. The optical efficiency of the real device is in inverse relation to the length of the fly-back distance
64
. As this distance becomes longer, ever greater fractions of the input beam are diffracted into undesired grating modes (secondary modes or higher). The attainable fly-back distance is limited by the elastic anchoring forces of the liquid crystal and by the field gradients obtainable within the device. Consequentially, the optical efficiency of a real device is limited by the elastic anchoring forces of the liquid crystal and the electric fringe

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