Study On Compact High Peak Power End-Pumped Solid-State Lasers

High power fiber coupled LD end-pumped solid-state lasers, characterized by compact structure and high peak power output, can be perfectly used as light sources of scanning imaging laser radars, whose output characteristics such as the pulse repetition rate, the pulse width, the peak power and so forth, which determine key performance parameters of lidars including pixel resolution or frame frequency, ranging accuracy, operating distance and so on respectively, have crucial influence on practical applications of laser radar systems.This dissertation deals with acoustooptical(AO) Q-switched end-pumped Nd:YAG lasers. Much more attention is paid to how to achieve continuous pulse output with high repetition rate, short pulse width and high peak power in compact AO Q-switched lasers. For an end-pumped laser, the output performance is seriously affected by 3-D spatial coupling of the pumping light and the oscillating light in the gain medium, and the basic prerequisite for high peak power output is to improve pumping power, which will lead to strong thermal effect in the crystal and too high intensity of the oscillating light in the cavity to be effectively switched off by an AO Q-switch. In this dissertation, all the problems mentioned above are studied theoretically and experimentally in great detail, in summary, the major studies include four parts as follows.The first part focuses on the study of the coupling effectiveness between the pumping light and the oscillating light. The output intensity distribution of common fiber coupled semiconductor lasers with both single-core fiber and multi-core fibers or fiber bundle is measured and analyzed particularly, furthermore, the numerical calculation results of the intensity distribution are given after the fiber output light travels through a simple double-lens coupling system. The self-reproductive mode of the optical resonator is solved numerically using Fox-Li iterative algorithm, during the formation of which both the diffraction loss and beam quality factor are calculated. Based on the rate equations, the expressions of threshold power, slope efficiency and output power are all deduced, then the effect on output features is studied of the spatial distribution of pumping light and oscillating light, the transmittance of output mirror and verified experimentally as well. It is found experimentally that the output intesity distribution of some fibercoupled semiconductor lasers shows ring structure with multiple extreme values and the central dip, a comparison of the pumping effect of two kinds of fiber coupled laser diodes with different levels of central dip is made experimentally. It concludes that it is better to use plano-convex pumping light in the end-pumped laser, and not only the measuring method but also the judging criteria of plano-convex pumping light are proposed.The second part concentrates on the calculating, analyzing and controlling of thermal effect in the gain medium. The distribution of crystal temperature field is solved numerically by Finite Volume Method, and a detail analysis is made of the influence of doping concentration, pumping beam average radius and pumping power on it, then the symmetry of the temperature field is discussed with the off-axis or asymmetric pumping light, the asymmetric or non-uniform boundary condition respectively. The distribution of crystal thermal strain and stress is numerically calculated, and a comparison is made of the contribution to optical path difference between the thermal induced end-face deformation and thermal stress, then the effect of the thermal induced diffraction loss on the pumping threshold and the slope efficiency is discussed. A new kind of algorithm of thermal lensing in end pumped solid state lasers is presented with its longitudinally non-uniform nature considered, by which the effects of thermal lensing on the cavity modes are discussed. A new method for heat dissipating directly from the pumped end of crystal rod is proposed, and the heat sink and its assembly process are designed specially. The effects on the heat dissipation and temperature controlling are compared between the new method and the traditional side surface cooling one, the results indicate that the heat concentrating in the pumped end of the crystal rod can be efficiently removed, and the thermal effect is obviously reduced, which shows that the method is very suitable for solving the problem of heat dissipation in a high-power end-pumped solid-state laser.The third part is to primarily study high power end pumping solution. The scheme of the ladder doped crystal is presented, and ladder doped crystals consisting of two or three sections are designed and optimized according to the principles of the nearly same absorption and focal length of thermal lensing in each section. On this basis, the concept of gradient doped concentration crystal is proposed, and then the design and analysis results of both the linear gradient doped concentration crystals and the uniformlyabsorptive ones are given. With the application of the both ends pumping scheme, thermal density inside the crystal can be dispersed and mitigated. The residual pumping power after absorption reduces effectively when the center wavelength of pumping light is tuned to match the absorption peak of Nd: YAG crystal by the means of adjusting LD temperature. As experimental results indicated, 8.75 W and 8.35 W CW output are obtained with 28.7 W and 28.5 W pumping power individually injected into crystal rod ends respectively, and with the total 57.2W pumping power injected simultaneously, the output power arises to 15.9 W, approximating to the sum of maximum power of individually pumping. The scheme of single-end pumped lower doping concentration crystal is studied, in which silver foil is used as thermal interface material to fill the gap between the crystal rod and heat sink, the pumping beam profile is uniformed and several new kinds of high efficient heat sink are particularly designed to improve efficiency of heat dissipation. In practice, the maximum pumping power is raised to 187.7W with a 0.1at.% doping concentration Nd:YAG crystal rod used, and the CW output power reaches 60.1 W, the optical conversion efficiency is calculated to be 32%.The finally part mainly studies the Q-switching techniques for high peak power output. Based on the rate equations of Q-switching lasers, the factors influencing the Q-switching output characteristics are discussed and verified experimentally. In order to effectively shut off the high intensity oscillating light within the cavity, we proposed three kinds of solution including choosing high diffraction efficiency channel for the oscillating beam transfer through with the inhomogeneous distribution of the sound field within the AO crystal considered, placing the AO Q-switch at the oscillating beam waist to obtain high diffraction efficiency and meantime to reduce the ultrasonic wave’s transit time which have important influences on the pulse output, and it is found and confirmed that the best diffraction efficiency, which is just enough to shut off the laser oscillation, exists in AO Q-switched lasers.Using two orthogonally placed Q-switches to generate diffraction in two orthogonal directions simultaneously can shut off the oscillation of high order modes with large divergence angle and reduce the requirements of diffraction efficiency for a single AO Q-switch. The laser head shell with periodical fin structure is specially designed, outside which five fans adhere to the left and right sides and the bottom to force air cooling for heat dissipation, and the maximum unsaturated end pumping power reaches 142.6W with a low doping concentration Nd:YAG rod used as gain medium, the average output power, pulse width and the peakpower output are 32.6 W, 6ns, and 543.3 k W respectively, the M2 factor is measured by 8.7775 using moving knife edge method as well.