The coherence properties of microwatt (average power) level picosecond laser pulses have been measured, using photorefractive beam coupling, in the conventional pulse width measurement geometry of noncollinear second harmonic generation (SHG). This technique is particularly useful for diagnostics of fiber compressed pulses, where significant energy may reside in the uncompressed pedestal. Semi-insulating GaAs:Cr was used as the photorefractive nonlinear element.1,2 A 1-mm thick LiIO3 SHG crystal was simply replaced by a 1.5-mm thick GaAs:Cr crystal. In beam coupling or two-wave mixing,2 a photorefractive index grating is formed by the interference of a modulated pump beam and a delayed probe beam. The coherence function (electric field correlation as opposed to intensity correlation) is obtained by detecting the fraction of modulated pump beam scattered by the index grating into the probe beam as a function of delay between the two beams, resulting in a {(sint)/t)2 time dependence for the interference fringes. Only for the case of a transform limited pulse would the width of the coherence function equal the width of the intensity autocorrelation function.

© 1989 Optical Society of America

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