Fine angle non-mechanical beam steering can be accomplished using Liquid Crystal on Silicon (LCoS) Optical Phased Array (OPA) devices.
These reflective devices can use a unique backplane with a linear (one dimensional) array of tens of thousands of long thin electrodes. By varying the refractive index of the liquid crystal above each electrode, one can produce a diffractive phase grating. The resulting device can be thought of as a programmable prism, capable of non-mechanical laser beam steering.
To understand how this works, it is helpful to think of how a prism would steer light, using what is called refractive beam steering. As seen in the graphic, with an index of refraction higher than the surrounding material (a glass prism in air, for example), an incident beam of light is steered proportionally to the angle of the prism wedge.
In general, however, it is not feasible to refractively steer a light beam with liquid crystals in the same way as a prism. To reproduce the thousands of waves of phase delay provided by an ordinary wedge prism would require a similarly thick liquid crystal cell, which would be prohibitively difficult. To avoid this, beam steering with liquid crystal typically is accomplished diffractively, through the use of phase-wrapping with 2π resets.
The diffractive optical phased array can be thought of as a quantized multiple level phase grating, an idealized version of which is shown in the figure to the right. Each ramp shown in the figure would be comprised of several LCoS pixels, each with an increasingly larger phase delay, ranging from zero to 2π. The more phase levels used in the array, the closer the ramp comes to an ideal ramp (instead of stair steps), and the higher the diffraction efficiency.
For example, a binary phase grating ideally provides a diffraction efficiency of 40.5% in each of the two first order diffracted beams. For a quantized phase grating using three phase levels/pixels the ideal first order diffraction efficiency is 68.4%, while for 4 phase levels/pixels, it increases to 81%. For more than four levels the improvement in diffraction efficiency with increasing number of phase levels slows. At 5 levels/pixels the percentage of light diffracted into the first order is ideally 87.5% and for 8 phase levels/pixels the ideal first order diffraction efficiency is 94.9%.
Current devices can steer precisely within a few degrees. Because no scanning is involved, the device can rapidly reconfigure to a new position to provide random access beam steering. Testing has proven the devices to be both radiation tolerant and capable of withstanding high peak and average laser power levels.
Phased Array of Phased Array (PAPA) configurations combine multiple OPAs each individually illuminated by separate sources, into a single high-power coherent beam. Boulder Nonlinear Systems (BNS) is researching PAPA techniques to support high-power beam combining and laser communication architectures.
Contact us for more information about how BNS can develop new fine angle beam steering devices and systems for your application