Objective The mid-infrared 2 μm solid-state laser has unique characteristics in terms of output wavelength: it is in the atmospheric window, the absorption band of water and the human eye safety zone. Based on these special properties, it can be widely used in laser imaging radar, doppler coherent wind radar, differential absorption radar and other laser sources used to measure the concentration and temperature changes of the earth's atmosphere. In addition, the 2 μm band laser can also be used as a pump source for optical parametric oscillators to achieve longer wavelength infrared laser output. With the deepening of research on 2 μm solid-state lasers, thermal effect has become a major problem limiting laser output power and beam quality improvement, so it has attracted much attention. During the operation of solid-state lasers, the heat generation is due to quantum deficit, energy conversion between the lower laser level and the ground state, and laser quenching, which leads to uneven temperature distribution inside the laser crystal, resulting in thermal lensing effect. However, by using bonding technology to bond YAG crystals with Tm:YAG crystals to form composite crystals as the working material of lasers, the influence of thermal effect can be effectively reduced. Lasers using composite crystals have advantages such as high reliability, peak power and excellent spot quality

Methods In this paper, the thermal effect of laser crystals is reduced by introducing two composite crystal models, namely single-ended bond and double-ended bond. By analyzing the working characteristics of continuous LD end-pumped square composite Tm:YAG crystal, a heat source heat model of laser crystal (Square Tm:YAG crystal model and its heat sink experimental device structure schematic diagram as shown in Fig.2) is constructed. Considering the boundary conditions of thermal convection between the crystal rod end face and air and the surrounding constant temperature, the finite element analysis method is used. The thermal effect of laser diode end pump square composite Tm:YAG crystal is studied.

Results and Discussions  The thermal model of laser crystal is established which is more suitable for the actual working conditions. The temperature field, thermal stress field and end shape variables of the composite crystal are numerically calculated by finite element analysis. The effects of single/double end bonding, undoped crystal length and gain crystal length on the internal temperature field and end shape variables of the square composite crystal are discussed. The bonded crystals can significantly improve the thermal effect of crystal end faces.

Conclusions The thermal model of laser crystal is established which is more suitable for the actual working conditions. When the pump power of the laser diode is 30 W, the radius of the pump spot is 400 μm, the thickness of YAG crystal is 1 mm, and the thickness of gain crystal is 1.5 mm, the maximum internal temperature rise of the square single-ended and double-ended Tm:YAG composite crystal are 81.2 ℃ and 77.9 ℃ respectively. The maximum internal stresses are 146 MPa and 104 MPa respectively. The thermal shape variables are 0.468 μm and 0.172 μm. It can be seen that the composite crystal can effectively alleviate the temperature rise of the crystal and the thermal deformation of the crystal end face, and the double-end bonding has a better effect on reducing the thermal effect of the crystal. When the thickness of the gain crystal is more than 2.6 mm, the influence of the two bonding methods on the maximum temperature rise inside the composite crystal is basically the same.