2008

Nematic Cells for Digital Light Deflection

Smectic A (SmA) prisms can be made in a variety of shapes and are useful for visible spectrum and infrared beam steerage.

Smectic A (SmA) materials can be used in non-mechanical, digital beam deflectors (DBDs) as fillers for passive birefringent prisms based on decoupled pairs of electrically controlled, liquid crystalline polarization rotators, like twisted nematic (TN) cells and passive deflectors. DBDs are used in free-space laser communications, optical fiber communications, optical switches, scanners, and in-situ wavefront correction.

Depending on the applied voltage, the TN cell rotates the polarization of incident light by π/2 (no field, OFF state) or leaves the polarization intact (when the applied electric field reorients the liquid crystal molecules perpendicular to the plates of the cell, ON state). The decoupled pair of a rotator and a deflector has no moving parts, and can be cascaded into N stages, making 2N addressable beam directions. This approach allows for the separation of time response and beam deflection angles, and the optimization of these two parameters separately. A 0.5-ms response time of dual- frequency nematic 90° TN cells was achieved by implementing an overdriving scheme of electrical switching, where an electrical signal is a sequence of high-amplitude pulses (64 V RMS, at 2 kHz and 50 kHz) and holding voltages (6 V and 4 V RMS at 1 kHz and 50 kHz, respectively).

Deflection angles can be optimized by the design of the birefringent prisms. SmA-filled prisms are attractive in low-cost applications where one needs large apertures, large angles of deflection, and/or non-trivial geometries. Mixtures of homologues of 4,4'-n-dialkylazoxybenzene produce SmA phases with a broad temperature range of SmA existence (from 10–20 °C to 40–50 °C) with a relatively small number of residual defects, such as focal conic domains, and high transmission characteristics. In this innovation, the typical magnetic fields needed to remove director distortions around the mechanical inclusions and focal conic domains have been determined. For the SmA prism, the optical axis (and thus the preferred orientation of the SmA molecules) should be aligned along the edge of the wedge. In this geometry, the director field is uniform everywhere. A passive birefringent prism separates the beam into two channels, depending on the beam polarization. Inside the prism, the beam propagates as ordinary or extraordinary mode. As the ordinary and extraordinary refractive indices are different, the two modes of propagation through the prism result in a different angle of deflection.

The SmA prisms are easier and cheaper to form than solid birefringent crystals, such as yttrium vanadate (YVO4) or calcite (CaCO3). The optical axis of SmA prisms can be controlled by surface alignment. They can be prepared as relatively thick prisms (up to 7 mm) or as arrays of microprisms. Light scattering in SmA birefringent prisms can be reduced by proper alignment to levels that are significantly lower than light scattering at the director fluctuations in the nematic samples of the same thickness. As the light scattering is caused mostly by focal conic domains that have a fixed size, it becomes smaller with the increase of the wavelength of light; the infrared (IR) part of the spectrum is less sensitive to these losses. Thus, the SmA prisms are suitable candidates for beam steering not only in the visible part of the spectrum, but in the IR part as well. While SmA prisms can only be used in the temperature range of the SmA phase, it can be expanded significantly by using mixtures.

The TN cells are used to realize the fast switching of the linearly polarized light. The fast switching is achieved because the dual-frequency nematic is always driven by the operational voltage. To reorient the director along the field, operational voltage is applied at a frequency of 1 kHz. To reorient the director perpendicular to the field, the operational voltage is applied at the higher frequency of 50 kHz. The amplitude of the driving voltage determines the switching time between the states. In regular liquid crystal cells, the field is applied only to reorient the director in one state; the reverse transition is achieved by simply switching the field OFF; the director relaxation is slow in this case as the active reorienting bulk torque is absent. Finally, with their high birefringence, SmA prisms can be constructed in a variety of shapes, including single prisms and prismatic, blazed gratings of different angles and profiles.

This work was done by Oleg Pishnyak, Andrii Golovin, and Oleg Lavrentovich of Kent State University; Liubov Kreminska of Truman State University; Bruce Winker of Rockwell Scientific Company; and John Pouch and Felix Miranda of Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18215-1.

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