Research On High Brightness Multi-stage Laser Amplifier Systems And Optical Parametric Technology

This dissertation is composed of two parts:the design and realization of high power, good beam quality Master Oscillator Power-Amplifier (MOPA) based on compensation of spherical aberration in the wavefront; analysis and realization of high power, high slope-efficiency Optical Parametric Oscillator (OPO).A high power OPO needs a pump laser source with high power and good beam quality. In this dissertation, a MOPA is employed as the pump laser source, which is very common in high power OPO systems. The beam quality of lasers would always be deteriorated as the amplification in MOPAs. It is shown in this dissertation by the analysis of thermal induced spherical aberration that the spherical aberration is one of the most important reasons for the deterioration of laser beams. It is also demonstrated by the numerical simulation that the spherical aberration in the wavefront of a laser beam would be varied as the beam propagates. As a laser beam propagates, a positive spherical aberration in the wavefront could turn into a negative one. Using this conclusion, the spherical aberration in the wavefront of a laser beam could be compensated by a thermal induced spherical aberration. In the experiments, a side-pumped laser and a fiber laser are used as the master oscillators separately, and the power-amplifiers are end-pumped. Two end-pumped amplifiers are set to be a group. In the group, a positive spherical aberration would be introduced into a laser beam by the first amplifier, and the spherical aberration would turn into a negative one as the beam propagates. Hence the spherical aberration of the laser beam could be compensated by the second amplifier. Employing the technology of spherical aberration compensation, a side-pumped and end-pumped hybrid MOPA as well as a fiber-solid hybrid MOPA are achieved, both of which are high power MOPAs with good beam quality. By two fiber pre-amplifiers and four solid-state amplifiers, the output power of the fiber-solid hybrid MOPA reaches 100 W output power with 100 MHz repetition frequency and 1 ns pulse width at 1.064 (am and the beam quality M2< 1.5.A high power OPO is formed by using the side-pumped and end-pumped hybrid MOPA as the pump source. A numerical model is established based on three wave mixing equations and the equations are solved by the split-step Fourier transform method. During the numerical simulations, the loss of the OPO cavities can be tuned by changing the absorption coefficient of the idler beam or the reflectivity of the resonant mirrors. Both pulsed OPO and continuous wave (cw) OPO have been simulated. The results of the simulations show that the back conversion would strongly depress the efficiency of high power OPOs. According to the three wave mixing equations, cavity loss could be used to suppress the back conversion. It is shown by a series of numerical simulations that there are different best values of the cavity loss for different pump powers. A pulsed OPO and a cw OPO are formed in the experiments based on PPLN crystals. A side-pumped laser with M2<1.3 is used as the pump source of the two OPOs. The experimental results support the numerical results. The idler wavelength could be changed by tuning the temperature of PPLN crystal. Simultaneously, the idler wave absorption coefficient would be changed. In the experiment of the pulsed OPO, it is shown that the output power of an OPO would benefit from a proper idler wave absorption coefficient. In the experiment of the cw OPO, the reflectivity of the resonator mirror is tuned by the idler wavelength as well. The slope efficiency is changed with the reflectivity of the cavity mirrors. When a proper reflectivity is selected, the slope efficiency of the cw OPO can be reached to as high as 62.1%. Based on the numerical and experimental results, a high power 2.9 μm output cw OPO is established. The pump laser source is the side-pumped and end-pumped hybrid MOPA mentioned above. The idler wave power reaches to 8.08 W when the pump power is 40 W, and the idler wave slope efficiency is 29.13%.