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Measuring laser beam quality by use of phase retrieval and Fraunhofer diffraction

We demonstrate the use of phase retrieval and Fraunhofer diffraction as a method for the measurement of laser beam quality. This technique involves using two CCD cameras to record a pair of conjugated light intensity images in defocus plane and one near field measurement instrument to record the light intensity image in near field. The wavefront is then retrieved using an optimization jointly constrained by them. Thereafter, combining with the known light intensity image in near field, light intensity image in focus plane can be figured out. After that, laser beam quality will be obtained by comparing with ideal light intensity distribution in focus plane. As light intensity images in defocus plane can be measured with higher resolution and lower CCD dynamic range than that in focus plane, this method is expected to give a precise laser beam quality.