Spectral Pulse Distortion from Two-Beam Coupling of Sub-Picosecond Pulses in a Photorefractive Crystal,
UNIVERSITY OF SOUTHERN CALIFORNIA LOS ANGELES
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Consider the coupling between two trains of mode-locked pulses in a photorefractive crystal. Let each optical pulse be so weak that it takes a large number of pulses to built up a quasi-steady-state refractive-index grating in the crystal. In this case the shape of the pulses will change as the result of beam coupling. The temporal pulse shape changes for two reasons. First, interference between the diffracted pulse and the transmitted pulse alters the pulse shape, as shown. Second, only a portion of each pulses full frequency spectrum is diffracted by the volume photorefractive grating. In the simplest case, in which the laser pulses are long enough so that they can be considered quasi-monochromatic, there is only one photorefractive grating formed in the crystal, and its grating wavevector is k2Omegac sin 2 theta. All of the frequency components in each pulse will be perfectly phase matched to this grating. In Eql, omega O is the center frequency of the laser pulses, c is the speed of light and 20 is the external crossing angle between the two interfering beams.