A digital beam deflector based partly on liquid crystals has been demonstrated as a prototype of a class of optical beam-steering devices that contain no mechanical actuators or solid moving parts. Such beam-steering devices could be useful in a variety of applications, including free-space optical communications, switching in fiber-optic communications, general optical switching, and optical scanning. Liquid crystals are of special interest as active materials in nonmechanical beam steerers and deflectors because of their structural flexibility, low operating voltages, and the relatively low costs of fabrication of devices that contain them. Recent advances in synthesis of liquid-crystal materials and design of the nematic-liquid-crystal cells have resulted in significant improvements in properties (e.g., short response times and birefringence) that are important for effective beam steering.
A beam deflector of this type is a multistage device. Each stage consists mainly of (1) a passive birefringent prism made of yttrium orthovanadate (YVO4) [alternatively, it could be made of a uniaxial smectic A liquid crystal] and (2) a switchable polarization rotator in the form of a cell containing a twisted nematic liquid crystal. A linearly polarized laser beam that one seeks to deflect travels through the liquid-crystal cell on the way to the passive birefringent prism. If no voltage is applied across the cell (“off” state), passage though the cell changes the direction of polarization by 90°. If a suitable non-zero voltage is applied across the cell (“on” state), then the polarization direction remains unchanged after passage through the cell. Therefore, by virtue of birefringence, depending on which of the two selectable polarizations has been imparted to the beam by the liquid- crystal cell, the beam propagates through the crystal in either of two different directions.
If a beam deflector of this type contained N stages, then it would be possible to switch the input laser beam to any of 2N different output directions through electrical switching of the liquid-crystal cells. The prototype device operates with an incident 633-nm-wavelength beam from a helium/neon laser. It contains 4 stages and, hence, can deflect the beam to any of 24 = 16 output directions. In this case, the directions are separated by increments of 8 milliradians. To obtain a relatively short response time (0.5 ms), the cells are made from so-called dual-frequency nematic liquid crystals and operated in a special addressing scheme that features amplitude- and frequency-modulated driving voltage.
This work was done by John J. Pouch and Felix A. Miranda of Glenn Research Center; Liubov Kreminska, Oleg Pishnyak, Andrii Golovin, and Oleg D. Lavrentovich of Kent State University; and Bruce K. Winker of Rockwell Scientific Company LLC.
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