Objective The poor thermal conductivity of Er:Yb:glass leads to poor heat dissipation in the laser system and the increased thermal load leads to severe thermal effect, which limits the laser output of high power, but there are few studies on the thermal effect of Er:Yb:glass. When the laser interacts with the gain medium, a portion of the pump light absorbed by the crystal will be converted to heat and stored in the crystal, resulting in a rise in its temperature, which becomes one of the factors limiting the output energy. The influence of thermal effect on the laser has two main aspects. On the one hand, as the laser crystal temperature increases, the fluorescence spectral line will broaden and the quantum efficiency will decrease, which will eventually lead to the decrease of conversion efficiency. On the other hand, the thermal stress and thermal lens effect generated by the temperature gradient will seriously affect the output stability and beam quality of the laser. Therefore, it is necessary to investigate the heat treatment capability of Er:Yb:glass as gain medium in order to develop a scheme for further optimization of the output performance.
Methods The heat accumulation process inside Er:Yb:glass as the gain medium is calculated in detail by finite element analysis method. Assuming that the surface temperature of the crystal is constant with the ambient temperature, the heat generated inside the medium is mainly dissipated by heat conduction, and the outer surface is affected by natural heat convection of air. When the LD is end-pumped with Er:Yb:glass, the pump source acts as an internal heat source and operates with a temperature gradient, which triggers the heat conduction process (Fig.2). In order to compare the heat dissipation performance of the gain medium under different conditions, the simulation is carried out after reaching steady state. The crystal model was divided according to the ultra-fine grid, and the effects of non-bonded and bonded crystals as well as different pump wavelengths, powers and beam waist radius on crystal temperature distribution, thermal stress and deformation were quantitatively analyzed.
Results and Discussions Co:MALO in the bonded crystal not only acts as a Q-switched crystal, but also as a heat sink. The bonded crystal significantly reduces the influence of thermal effects, especially the temperature of the surface after the gain medium (Fig.4). The pump wavelength of 940 nm is more penetrating to the gain medium than 976 nm, but the temperature diffusion range at 976 nm is less than 940 nm (Fig.6). The temperature of the crystal pumped at 976 nm is higher than that of 940 nm (Fig.7), but the temperature drops faster at the central wavelength of 976 nm. At this wavelength, the most dangerous effect of the temperature rise is on the surface. The increase of pump power of 100 mW corresponds to the increase of temperature of 9 K (Fig.8).The increase of the beam waist radius leads to the decrease of the optical power density and the decrease of the temperature of the bonded crystal, with only a large difference in temperature at the center of the crystal surface. The pump power is proportional to the thermal deformation. The increase of pump power of 100 mW corresponds to the increases of the thermal deformation of the crystal of 0.5 μm (Fig.17). At the same pump power, the thermal deformation will decrease with the increase of the beam waist radius, but the resulting change is not significant compared to the pump power. Co:MALO crystal bonding on the pump side can effectively reduce temperature (Fig.10), thermal stress (Fig.14) and deformation variables (Fig.18).
Conclusions The finite element method was used to study the LD pumped Er:Yb:glass/Co:MALO based on the theory of heat conduction. Due to the heat sink effect of CO:MALO, the bonded crystal can reduce the maximum temperature, thermal stress and thermal deformation of the laser crystal. On this basis, the pump with a central wavelength of 940 nm will reduce the maximum laser temperature, while the 976 nm pump structure is a safer structure for diffusion bonding. Increasing the pump power and decreasing the beam waist radius will also lead to the thermal effect of the crystal. In the design of the laser system, the thermal effects on the output characteristics should be mitigated by preventing excessive crystal temperatures and deformations. This study provides an optimized condition for the further design of Er:Yb:glass laser with better thermal performance, and also provides a theoretical basis for the output of 1.5 μm laser with high power and beam quality.
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