Research On Thermal Effects In The LD End-pumped Tm, Ho: YLF Solid-state Laser

LD end-pumped all-solid-state lasers have extensively potential applications due to its compactness, high-energy storage, simplicity, high reliability and solid durability. But the energy density of the pumping laser is high for small beam radius which induces a great deal of heat to deposite in the laser crystal. It results in the thermal effects in the laser crystal which make the laser output performance bad.The thermal effects in a laser diode end-pumped Tm,Ho:YLF solid state laser are investigated in this thesis. Firstly, the development of the laser and the background of the thermal effects in the solid state laser are introduced. Then the fractional thermal loading of the laser crystal is investigated by the rate equation theory. Under considering the Gaussian distribution of the pumping laser and output laser, a set of rate equations with 8-level are given and the rate equations are made discrete. So the rate equations can be solved under the condition of steady state to obtain the population distribution in the crystal. The thermal effects are investigated in detail, and the relationships between the fractional thermal loading and the system parameters are obtained by solving the rate equations. The energy transfer upconversion is a very important process for the fractional thermal loading, so the relationships between the energy transfer upconversion coefficient and the thermal effects are investigated.Secondly, some analytical results related to the thermal effects are deduced by the heat conduction equation and the theory of elasticity. With the analytical results and the fractional thermal loading, the influences of the pumping power and the pump-to-mode size ratio on the crystal temperature, optical path difference and thermal focal length are demonstrated. The thermal focal length as a function of the pump power is obtained by the knife edge method experimentally. Furthermore, the experimental results are compared with the theoretical results, and it is found that the theoretical results agree with the experimental results.At last, the influences of the thermal effects on the laser crystal are simulated with Ansys software which can deal with the thermal effects through the method of finite element. The influences of the pumping power, the beam radius and the cooling temperature are investigated. The temperature distribution and thermal strain distribution are also obtained in the crystal. It is found that the increase of the pumping power and the decrease of the pumping beam radius can increase the temperature and thermal strain of the crystal, even make the temperature distribution more nonuniform. While the decrease of the cooling temperature can decrease the temperature in the crystal, but the nonuniformity of the temperature and the thermal strain can not be changed. At the same time, the thermal effects of the composite crystal and double-end-pumped lasers are simulated. It is found that the composite crystal can not only decrease the crystal temperature, but also restrain the end-deformation; the double-end-pumped configuration can decrease the crystal temperature and make the temperature distribution more uniform.