Simulations of spectral broadening by cross-phase modulation (XPM) with chaotic light pulses

Spectral broadening of single-frequency laser pulses by optical cross-phase modulation (XPM) with chaotic laser pulses in birefringent single-mode optical fibers is investigated numerically and results are compared with experiments. By this process we have generated laser pulses of variable bandwidth (1-25 Å) at the fundamental wavelength (1053 nm) for amplification in high power solid-state Nd:glass lasers used for inertial confinement fusion research. Simulations indicate that a temporally smooth XPM pulse can be generated with intensity fluctuations of less than 10% and spectral width greater than 50 Å using a short length (∼5 m) of special low dispersion and low birefringence fiber, e.g. D = 10 ps/nm-km (normal dispersion) and Δn = 2 × 10^-5. Readily available fibers of similar length, with parameters of D = 40 ps/nm-km and Δn = 6 × 10^-5, can give spectral widths exceeding 25 Å, but the noise will range from 25 to 60%. Broadband laser pulses generated by XPM are now routinely used at Lawrence Livermore National Laboratory for active smoothing of the laser irradiance on targets by the technique of smoothing-by-spectral dispersion.