Physics of Femtosecond Optical Pulses

We examine recent advances in femtosecond optical pulse technology with an emphasis on the physics of the processes which have enabled us to generate pulses as short as 6 fs, or only 3 cycles of the optical field. The sequence of events which has led to production of these short pulses is briefly reviewed. Current work on generation, amplification, and compression of femtosecond pulses is also discussed. We examine, in particular, the importance in generating short pulses of establishing the correct balance of the four basic pulse shaping mechanisms of saturable gain, saturable absorption, self phase modulation and group velocity dispersion. We discuss how balancing these mechanisms in a laser oscillator has resulted in pulses as short as 27 fs. We also examine how altering the relative importance of these different shaping mechanisms can change the laser oscillator mode of operation from one in which the pulses resemble higher order solitons in optical fibers, to one in which the pulses resemble lowest order solitons. We show, in particular, that these latter shaping mechanisms provide one of the effective means of generating stable pulse trains and ultrashot pulses in a laser oscillator. We also examine current work using similar pulse shaping techniques in a laser amplifier. A novel technique for correcting higher order phase distortion of femtosecond pulses is mentioned.