Study Of Heat Dissipation And Thermal Compensation Technology In LD End Pumped ND³⁺ Doped Quasi-three-level Laser

In recent 20 years, diode pumped solid-state lasers (DPSSL) have become a highlight in the laser field for the high efficiency, compactness and long lifetime, etc. Because the output laser is aroud 0.9μm, Nd³⁺doped quasi-three-level lasers also obtain much attention. But due to the non-uniform temperature distribution and serious thermal effect, it is difficult for the lasers to export high power and good beam profile laser. The purpose of this paper is mainly to analyze the thermal effect and progress of heat dissipation in Nd³⁺doped quasi-three-level lasers. Based on this, we used indium-solder and unsteady cavity to improve the capacity of heat dissipation of the lasers.This paper first introduces the energy level of Nd³⁺in Nd:GdVO₄ crystal and shows mechanism of heat generation in it. Temperature and stress distribution in laser crystal is simulated, and the result shows the difference between four-level and quasi-three-level laser. Based on these, we utilize three methods to measure the focal thermal length in Nd:GdVO₄ quasi-three-level laser. The methods consist steady method of resonant cavity, method of interference fringe and inverse method with beam quality (M²). Synthesized with the experimental results, the shortest focal thermal length is 20 ⁴0 mm, which is much shorter than four-level's.The temperature distribution in crystal with different boundary coefficient of heat transfer is also compared and shows the necessity to enhance the coefficient of heat transfer for improving the capacity of heat dissipation. According to the principle of diffusion welding, the microcosmic progress of indium-solder is illustrated detailed. By plenty of experiment, the progress of indium-solder is optimized. With the indium-solder technology, the maximum output Nd:GdVO₄ 912 nm laser power is 14.3W in plano-concave cavity, and it is raised by 1.5W compared with indium-wrapped one. If in plano-plane cavity with indiums-solder heat sink, the maximum output power is 12.5W and it has better spatial beam profile (nearly Gaussian distribution) and beam quality (M²=3.02).Based on the theory of resonant cavity and measured thermal length, series of un-steady cavities are designed to compensate its thermal lens effect. The relation between laser beam radius and input pump power is simulated to test the compensation character of un-steady cavity. The character of 912 nm output laser between steady cavity and un-steady cavity in 25mm long cavity is compared. In f=150 mm convex-concave cavity (concave radius R=200 mm), the maximum output power is raised to 14.2 W. The 912 nm output laser character between steady cavity and un-steady cavity in 45 mm long cavity is also compared. With the plane output mirror, it could easily observed that the steady region move to direction of high power. When the convex mirror is f=25 mm, the maximum 912 nm laser output is 6.5W and is higher that of 5.3W by plane mirror.At last, the preliminary experimental study is also performed in characteristics of high peak power laser output at 912 nm with high repetition rate and short duration realized by acousto-optical Q-switching. When the repetition is 10 kHz in f=100 mm convex-concave cavity, the maximum average output power is 1.4 W, pulse width is 23 ns and the peaking power is 6.1 kW.