Until very recently, practical wavefront control systems have been limited by the lack of high-resolution, rapidly programmable spatial light modulator (SLM) technology. The application areas for high-resolution wavefront correction are varied, ranging from military imaging and laser communications to microscopy and astronomy.
|(A) Zernike Pattern, (B) Simulated Interferogram, (C) Actual Interferogram|
Liquid crystal on silicon (LCoS) SLM devices provide a programmable phase shift at each pixel. Because the backplane of the device is a VLSI circuit, thousands of individually controlled elements are available, enabling not only low-order aberration control (e.g. tilt, power) but also the creation of extremely complicated wavefronts with high spatial frequency content. These devices capitalize on the fact that phase modulation is cyclical in nature, and uses phase-wrapping with 2π resets to replicate phase distortion greater than the wavelength of the incident light. As such, the total phase stroke of a SLM device is dependent on the number of pixels – with hundreds of waves of stroke possible.
LCoS SLM technology can also be used in an analogous manner to compensate for the distorting effects of scattering in turbid samples. Numerous techniques for turbidity suppression, such as optical phase conjugation, have been introduced in recent years that capitalize on the deterministic nature of elastic light scattering. In these techniques, SLMs are used to “pre-aberrate” a wavefront such that the effects of scattering are reversed, enabling tight focusing of light millimeters below the surface of scattering samples.
Although SLM technology is normally used to correct for wavefront aberrations, these systems can also be used to produce aberrations, providing a means to simulate atmospheric turbulence. In contrast to phase wheels, oil films or hot plates, the turbulence generated by a SLM can be calibrated with standard turbulence parameters, is repeatable, and dynamic. Segmentation or multiple SLM devices can even be used to simulate a layered turbulent medium.
For adaptive optics in systems that use unpolarized light, Boulder Nonlinear Systems (BNS) has developed polarization independent SLM technology and is currently developing a new high-voltage 512 × 512 pixel SLM backplane capable of driving polarization independent SLMs. Adaptive optics systems can also benefit from fast operating speeds, which BNS can provide through high-voltage backplanes, novel addressing schemes, and low latency 16-bit PCIe drivers. As a result, BNS is unique in developing SLMs with closed-loop SLM response speeds approaching 1 kHz.