The third-order nonlinear process in femtosecond time-resolved four-wave mixing and femtosecond lasers

Some basic properties of higher order nonlinear optical processes are examined in this thesis. Two novel applications of third order nonlinear optical processes to device physics are also discussed here. The mechanisms of the nonlinear optical processes were explored, in particular, in four-wave mixing experiments. Applications of the third order nonlinear effect were demonstrated by generation of intense ultrashort pulses in an erbium-doped fiber laser, and also by control of the dynamics of two coupled mode-locked laser oscillators.; Basic properties of higher order nonlinear optical processes were studied in femtosecond time-resolved four-wave mixing experiments. The temporal and spectral properties of the third order, the cascaded third order, and the fifth order nonlinear processes in GaAs were investigated. Both the theoretical and experimental results indicate the cascaded third order process dominates the fifth order process in the regime of band gap resonance, however, the fifth order process dominates the cascaded third order process in the regime removed from the band gap resonance. The destructive interference between the third order and the fifth order processes is also demonstrated clearly in the experiment.; As regards device applications the combined action of a nonlinear optical loop mirror and a nonlinear coupler have been used to mode lock an erbium-doped fiber laser. The laser produces stable pulses trains at 3.8 MHz. The pulses have the duration as short as 1 ps and the peak intensity as high as 2.5 kW. These, to our knowledge, are the most intense pulses generated directly from an erbium-doped fiber laser. At these high intensities, the Raman frequency shift within the laser is not negligible. This significantly influences the transmission of the loop mirror. However, the inefficient transmission of the loop mirror due to the Raman frequency shift can be compensated by the nonlinear phase shift inside the coupler. To induce the nonlinear phase shift inside a coupler, the optical intensity must be much higher than the transmission threshold of the loop mirror.; A pair of colliding pulse mode-locked ring lasers have been coupled by sharing a common saturable absorber. The shared bleaching of the saturable absorber provides an attractive mechanism to synchronize the pulses from two oscillators in time. However, the Kerr lens induced in the shared absorber also provides a repulsive mechanism that tends to separate pulses in time. The relative magnitude of these two opposing mechanisms can be balanced by adjusting the pulse intensity. The experimental work has shown that adjustment of the laser oscillator parameters can result in: pulse synchronization, controllable pulse duration switching, and a bistable state in which the particular pulse that dominates can be determined by prior conditions.