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

Abstract

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 angstrom) 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 angstrom using a short length (approximately 5 m) of special low dispersion and low birefringence fiber, e.g., D equals 10 ps/nm-km (normal dispersion) and (Delta) n equals 2 X 10⁻⁵. Readily available fibers of similar length, with parameters of D equals 40 ps/nm-km and (Delta) n equals 6 X 10⁻⁵, can give spectral widths exceeding 25 angstroms, 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.