High-speed, sub-Nyquist interferometry.

The velocity measurement limit in dynamic interferometry is vNyq, the velocity at which the interferogram is sampled at the Nyquist limit. We show that vNyq can be exceeded by assuming continuity of the surface motion and unwrapping the velocity modulo 2vNyq. The technique was demonstrated in a high-speed speckle pattern interferometer with spatial phase stepping. Surface velocities of 4vNyq were measured experimentally. With a reduced exposure, high-speed sub-Nyquist interferometry could be implemented up to a maximum acceleration of vNyq/ts, where ts is the detector frame period.