The synthetic effects of group-velocity mismatch and cubic-quintic nonlinearity on cross-phase modulation induced modulation instability in loss single-mode optical fibers have been numerically investigated. The results show that the quintic nonlinearity plays a role similar to the case of neglecting the group-velocity mismatch in modifying the modulation instability, namely, the positive and negative quintic nonlinearities can still enhance and weaken the modulation instability, respectively. The group-velocity mismatch can considerably change the gain spectrum of modulation instability in terms of its shape, peak value, and position. In the normal dispersion regime, with the increase of the group-velocity mismatch parameter, the gain spectrum widens and then narrows, shifts to higher frequencies, and the peak value gets higher before approaching a saturable value. In the abnormal dispersion regime, two separated spectra may occur when the group-velocity mismatch is taken into account. With the increase of the group-velocity mismatch parameter, the peak value of the gain spectrum gets higher and shorter before tending to a saturable value for the first and second spectral regimes, respectively.

It is observed that the pulse duration of loosely focused pulses is self-compressed during the nonlinear propagation in argon when the input pulse peak power is close to the threshold power for self-focusing, and the results are confirmed by the numerical simulation. From the simulation we find that, near the lens focus the central part of the beam moves forward to the outer part owing to the plasma generation, and produces a leading peak in the temporal profile. And then, with the decrease of the plasma density, the spatio-temporal focusing and self-steepening effects predominate and promote a shock beam structure with a steep trailing edge. It is also found that, for our calculation case with the input pulse power close to the critical value of self-focusing, group velocity dispersion and multiphoton absorption effect have little influence on the propagation process, but the spatio-temporal focusing and self-steepening effects play a significant role in promoting the final pulse shortening.

We have observed a hole in the absorption profile of the homogeneously broadened absorption band of an Alexandrite crystal. With the spectral hole-burning technique that resulting from the periodic modulation of the ground state population at the beat frequency between the pump and the probe fields, we observed the slowdown the light propagation with the group velocity as slow as 12.5 m/s at room temperature. And the group velocity in the Alexandrite crystal depends on the amplitude modulation frequency, the power of laser beam and the orientation of the crystal lattice. The lower frequency or higher power leads to slower group velocity of light.

A Nd:YAG master oscillator power amplifier (MOPA) system, pumped by a pulse flash-lamps as the pump source of optical parametric oscillator (OPO), is employed to improve the pump beam quality of OPO pump source. A back amplifying configuration with stimulated Brillouin scattering (SBS) phase conjugation mirror is used. OPO pump laser energy of 611 mJ/pulse with 30-ns pulse duration is obtained, and near diffraction limited beam quality is achieved. Based on the type II degenerate non-critically phase-matched KTP crystal, the OPO is used to convert pump beam from 1.064 μm to 1.57 μm, eye-safe near infrared laser range source. 1.57-μm output energy of 209 mJ/pulse with 18-ns pulse duration is attained with a short cavity KTP OPO, when pump laser energy is approximately 611 mJ. OPO conversion efficiency is up to 38.7% when pump laser energy is approximately 200 mJ.

Expressing the perturbation optical field in terms of module and phase, using the linearized nonlinear Schrodinger equation governing the evolution of perturbations, we have deduced the analytical expressions of the modules, phases, and gain coefficients of the perturbations with zero or cut-off frequency, and studied the evolutions of the two perturbations travelling along lossless optical fibers in the negative dispersion regime. The results indicate that the phase of the perturbation with zero (or cut-off) frequency increases (or decreases) with the propagation distance monotonously and tends to its asymptotic value nπ+π/2 (or nπ) eventually. The evolution rates of the phases are closely related to the initial phase values. Although the asymptotic values of the field gain coefficients of the above mentioned two perturbations are equal to zero, and the increasing fashion of the modules is different from the familiar exponential type, it still suggests that the perturbations have a divergent nature when the propagation distance goes to infinity, indicating that the two kinds of perturbations can both lead to instability.

Based on Fresnel-Kirchhoff diffraction theory, we set up a diffraction model of nonlinear optical media to Gaussian beam, which can interpret the Z-scan phenomenon from a new way. This theory is not only well consistent with the conventional Z-scan theory in the case of small nonlinear phase shift, but also can fit for the lager nonlinear phase shift. Numeric computations indicate the shape of the Z-scan curve is greatly affected by the value of the nonlinear phase shift. The symmetric dispersion-like Z-scan curve is only valid for small nonlinear phase shift (|Δφ0|<π), but with increasing the nonlinear phase shift, the valley of the transmittance is severely suppressed and the peak is greatly enhanced. Further calculations show some new interesting results.