Harnessing caustics for wave-front sensing

Scintillation in measured wave fronts adds spurious dislocations and deform ations to their reconstruction. The source of the problem is caustics forme d by aberrations in intermediate planes. I propose to use intentional caust ics to measure wave fronts under severe conditions such as low light level, fast scale variations, large aberrations, and discontinuities in the wave front. A simple realization is based on the Hartmann-Shack sensor, which sa mples the wave front with a lenslet array. Movement of the lenslets' foci i s linear with slope changes. Here the lenslets are effectively formed in an acousto-optic device: Two standing waves are launched perpendicularly to t he light beam and to each other. At some distance down the beam, each wave creates a comb of caustics, and the two orthogonal combs add up to an array of caustic spots, The spatial frequency of the array is linear with the te mporal frequency of the standing sound waves. A simple Fourier demodulation scheme supplies the two wave-front gradients. (C) 2001 Optical Society of America.