Reliability is the main challenge facing Free Space Optical Communications today. Fog plays a key role here, but so does range and optical power throughput. Presently, the FSOC beam has a divergence in the milliradian range in order to compensate for beam wander caused by platform drift and vibrations and atmospheric index of refraction fluctuations. This large beam divergence limits the power throughput and the maximum range for communication. A beam that is collimated to the microradian level would greatly improve the power throughput (by three to four orders of magnitude) and thus, greatly improve the range, but this would also require a fast (> 1 kHz), accurate (microradiam) beam pointing system. Stress-Optics technology can provide that system.
In Stress-Optics a stress field can be imposed on almost any optical material to modulate the index of refraction within that material in a predetermined manner. In general, stress can bring about a greater change in the index than can other fields that might be imposed. Stress-Optics accomplishes uniform beam steering in two dimensions to moderate angles, as well as beam shaping, including spherical and cylindrical lensing. The method is simple, solid-state, fast (< 5 mseconds), precise (< 1 microradian), inexpensive and durable.
This patented Stress-Optic technique, when operated in conjunction with CMOS imaging feedback from a reference beam, can compensate for beam wander and defocusing from both platform and atmospheric sources. The stabilization is to microradian accuracy with kHz response times and milliradian 2-D collimation control.
In general, stress can create about the same change in the mdcx of refraction of an optical modulator as can a thermaloptic approach, but with a faster response, a smaller, simpler configuration and with less power needed to operate. Also, S-O's creates a greater change in the index than do other fields that are commonly used for modulation, such as with Electro-Optic's. The Stress—Optic technique is also durable. A 2 x 2 switch has been in operation at SeaLite Engineering since 1991; it has 1O'S cycles at 1 KHz. To implement this technology SeaLite Engineering is developing a Stress-Optic guided wave switch and technique as an add-on to a photonic integrated circuit substrate. Stress-Optic beam switching within a photomc integrated circuit can be fabricated in a nearly automated process where the S-O films are applied during the assembly of the planar waveguides. The S-O technique is fast (kHz to MHz, depending upon the circuit dimensions), low power (10's ofmicrowatts per cycle) and rugged (< lO"8cycles to date).