A theoretical model of ultrashort pulse difference-frequency generation (DFG) is presented and developed with quasi-phase matching (QPM) gratings in the undepleted-pump, unamplified-signal approximation. An analytic solution to a case of ultrashort pulse pumped DFG process, including groupvelocity mismatching effects and input pulses with frequency chirp and time delay, is obtained. The influences of group-velocity mismatching effects and input signal pulse parameters on the conversion efficiency and output converted pulse characteristics are analyzed and discussed. The results indicate that the conversion efficiency is increased due to the enhancement of pump and signal interaction. The output pulse is inevitable compressed under pulsed pumping and can be compensated by introducing the initial frequency chirp and time delay.

Bandwidth and tunability enhancement of difference frequency generation (DFG)-based wavelength conversion by introducing various distributing formats of p-phase shifting domains into the quasi-phase-matching (QPM) gratings is demonstrated, and the concise expressions describing the tuning curves in these cases are obtained for the first time. The results show not only the pump and signal bandwidth can be obviously broadened and flattened, but also diverse comblike pump-wavelength tuning curves with controllable channel spacing can be achieved by introducing periodically distributed p-phase shifting domains into the QPM gratings, which would greatly improve the tunability and practibility of DFG-based wavelength converters.

Multiwavelength oscillation is demonstrated with a novel laser scheme, in which cascaded second-order nonlinearity in a periodically poled LiNbO3 (PPLN) waveguide is used to double the lasing lines. Twenty-two wavelengths approaching S- and L-bands are simultaneously achieved with a channel spacing of 1.6 nm, and the band separation is changeable. The number of wavelengths and channel spacing can be controlled by the adjustment of the tunable Fabry-Perot (FP) etalon and light polarization states in the laser cavity.

With the consideration of cross-gain modulation (XGM) and probe-depletion (PD) effects, an analytical solution of four-wae mixing (FWM) between picosecond optical pulses in semiconductor optical amplifiers is developed based on our "splitting model" The results indicate that the effect of PD plays an important role in the FWM process for lower pump pulse energy and smaller pump-probe frequency detuning, while the effect of XGM dominates for the pump pulse with higher energy, a shorter pulsewidth, and a larger pump-probe frequency detuning.

Simultaneous multiwavelength lasing is demonstrated exploiting intracavity polarization hole burning in an erbium-doped fiber laser. Experimental results indicate that polarization hole burning can be enhanced by the changes of the light polarization states and bias current of optical MQW waveguide which serves as reflector in the laser. Eleven wavelengths with 0.9 nm spacing are generated at room temperature.