Digital heterodyne holographic interferometry of flow fields

Abstract

Flow medium inhomogeneities associated with the gain region of large scale laser systems can result in significant degradation of the outcoupled laser beam quality. To achieve beam quality goals, relative density fluctuations will have to be below 10−5. An optical technique called digital heterodyne holographic interferometer (HHI) is presented which measures the wavefront distortion of a visible probe beam by the medium. HHI is similar to conventional holography in that both use very short averaging times to study the homogeneity fluctuations. HHI, however, derives an increase in sensitivity of an order of magnitude over traditional holography by the use of an electronic phase readout technique during reconstruction. This increase in measurement sensitivity is required to diagnose low level perturbations. Experimental results are presented of the flowfields from a set of supersonic nozzles.