Cyclic shearing interferometer for collimating short coherence-length laser beams

Until now there has not been an accurate method for measuring the radius of curvature, R, of a short coherence-length light source, such as a short-pulse or broadband laser. We show that the easily aligned cyclic shearing interferometer (CSI) solves this problem. The CSI produces a stable fringe pattern from which R can be determined and can be used on beams with short coherence times down to 300 fs because the two beams in the interferometer follow nearly the same path. Comparison with data from a broadband XeCl laser (30-ps coherence time) confirms that the CSI performs as theory predicts.