Abstract

Fast self-phase modulation (FSPM) and slow self-phase modulation (SSPM) cause nonlinear chirp of ultrashort pulses. FSPM originates from the optical Kerr effect, while SSPM occurs in amplifiers of low saturation energy. Either effect or both can be present in laser amplifiers. We present a theoretical numerical study of the amplitude distortion and chirp of ultrashort pulses and of their compression by frequency-domain phase compensation. Various shapes (Gaussian, sech² and asymmetrical pulses) are used in numerical integration of the nonlinear equations in the time domain. The distorted and chirped output pulses are Fourier transformed and the phases in the frequency domain are compensated by subtraction of combinations of various phase functions (second-order, third-order,¹ V-shape, etc.). The phases in the frequency domain are sensitive to the shape and chirp of the output pulses resulting from the nonlinear time-domain effects. The shape and width of the compressed pulse depends on the input pulse shape, on the nonlinear gain medium, and on the compensator we use. The maximum amount of compression is related to the linear portion of the chirp. These results are relevant to dye laser, diode laser and other systems, and may be useful for understanding and improving mode-locking as well.

© 1991 Optical Society of America