Researchers at the Mechanical Engineering Laboratory (Tsukuba, Japan), the University of Auckland (Auckland, New Zealand), and Industrial Research Limited (Lower Hutt, New Zealand) have built an adaptive wavefront-correction system based on an all-optical feedback interferometer. The two-dimensional output fringe pattern from a Mach-Zehnder interferometer with large radial shear is optically fed back to an optically addressed nonpixelized phase-only liquid-crystal spatial light modulator (SLM)--an interferometric setup that does not require a separate aberration-free reference wave.
For testing purposes, the researchers sent a plane wavefront from a 19.5-mW HeNe laser through a plate with several waves of aberration to produce a distorted input wavefront. To increase feedback-loop gain to the point where the system worked, the researchers adjusted the SLM for a response time of 60 ms and a resolution of 60 line pairs/mm. Without feedback, the wavefront was highly distorted--a focused spot exhibited a Strehl ratio of 0.22. When the loop was closed, the Strehl ratio reached 0.92. Contact Tomohiro Shirai at [email protected].