Transfer function characterization of laser Fizeau interferometer for high spatial frequency phase measurements

Large, high power laser systems such as that being constructed by Lawrence Livermore National Laboratories for the National Ignition Facility (NIF) require accurate measurement of spatial frequencies of up to 2.5 lines/mm over a 100mm field of view. In order to ensure accurate measurements of the parts, the test apparatus must be well characterized. The system transfer function (STF) of the interferometer under development to perform these measurements was calculated by comparing the power spectra of measurements of known phase objects to their theoretical power spectra. Several potential problem areas were identified and studied. Of primary concern was the effect on the STF of the rotating diffuser and incoherent relay system employed in most commercial laser Fizeau interferometers. It was determined that such an arrangement degraded the transfer function beyond acceptability. The other major concern was possible inability to measure certain frequencies due to propagation between the test piece and alignment of the system optics. Use of strictly coherent imaging and small propagation distances between the test piece and return flat, the system transfer function could be kept at acceptable levels within the range of interest.