Compact, efficient, and spatially uniform spectral broadening by combined Kerr and cascaded quadratic nonlinearities

We introduce a method for efficient and spatially uniform spectral broadening of ultrashort pulses that is simple, compact and versatile, based on cross-phase modulation and combined cascaded quadratic and Kerr nonlinearities. An intense drive beam propagates without self-phase modulation and self-focusing in a frequency-doubling crystal via exact cancellation of the material's nonlinear index of refraction by cascaded second-order nonlinearity. A smaller, weaker, and perpendicularly polarized target beam acquires a significant nonlinear phase shift predominantly from the drive beam through cross-phase modulation. This configuration allows very high drive intensity and spatially uniform spectral broadening of the target beam without self-focusing. In a preliminary experiment, using a 3-mJ, 410-fs drive pulse with 195 GW/cm2 intensity in a 10-mm bulk BBO crystal, a 200-μJ target pulse spanning 1026.5-1036.5 nm is broadened to 1015-1055 nm with good spatio-spectral homogeneity. The spectral broadening method is particularly suited when the drive beam and target beam are to be used as a pump/seed pair in a subsequent nonlinear interaction, since (1) the drive beam is undistorted by the process, and so remains viable for further applications, (2) the pump and target beam may be spectrally similar or distinct, and (3) the target beam can be arbitrarily weak.