Objective Inertial Confinement Fusion (ICF) is one of the technical approaches to realize controllable nuclear energies. The key to ICF is the high-peak power solid-state laser system. A large-sized, Nd-doped phosphate laser glass disk is the core gain material of the high-peak power solid-state laser system. In the gain media, the amplified spontaneous emission (ASE) and parasitic oscillation (PO) are generated, which can affect the energy storage efficiency and laser output capability of the high-peak power solid-state laser system. Now, the main method to absorb ASE and suppress PO is to clad the Cu ²⁺-doped glass at the peripheral edge of the Nd-doped phosphate laser glass. The edge-cladded Cu ²⁺-doped glass can absorb the reflected or scattered light at 1 μm and suppress the onset of parasitic oscillation.

At first,the sealing edge-cladding method was applied to the elliptical slab of N21 Nd-doped phosphate laser glass and the N21 rectangular Nd-doped phosphate laser glass, immersed in the organic cooling medium in the high-power laser system. The sealing edge-cladding process involves first mixing a low melting temperature glass powder with a dispersant to form a slurry, then coating the slurry on the edge of the Nd-doped phosphate laser glass, and finally heat-treating the coated glass at below a temperature at which the Nd-doped phosphate laser glass is softened and deformed to bond the low melting temperature glass to the edge of the Nd-doped phosphate laser glass. However, this cladding method can create lots of defects such as bubbles, pits, and carbide at the cladding interface. These defects can increase the residual reflection (0.1 × 10^-2-25 × 10^-2), which can lead to a considerable increase in ASE and PO. With a rapid advancement in the laser technologies, the Nd-doped phosphate laser glass with better performance is obtained, such as the types of N31 and N41. They have relatively low soften temperatures. Also, the temperature for heat-treating coated glass is much lower than that of N21, which can result in more defects at the cladding interface. Therefore, the sealing edge-cladding method is not suitable for new types of laser glass. The monolithic edge-cladding method is developed which involves pouring the melted edge-cladding glass around the edge of the Nd-doped phosphate laser glass preheated in the mold. Now, we studied the monolithic edge-cladding of the N31 Nd-doped phosphate laser glass. The residual stress is a very important parameter of monolithic edge-cladding for engineering applications. In this paper, we discussed the residual stress in the monolithic edge-cladding of the Nd-doped phosphate laser glass.

Methods We studied the influence factors on the residual stress in the monolithic edge-cladding of the Nd-doped phosphate laser glass with the simulation and experimental methods. The finite element analysis software of COMSOL Multiphysics 5.5 was used to simulate the melt bonding process of monolithic edge-cladding. We did some experiments on the cladding for various cladding temperatures and different thermal expansion coefficients of edge-cladding glass. Stress distributions of these cladded samples were measured with the high-precision imaging polarimeter, and the stress distributions were studied.

Results and Discussions By simulation, the residual stress distributions between the Nd-doped phosphate laser glass and edge-cladding glass for different thermal expansion coefficients are shown in Figs. 4 and 5. They indicated that a mismatch between the thermal expansion coefficients of the Nd-doped phosphate laser glass and the edge-cladding glass can cause residual stress. Higher difference in the thermal expansion coefficients can give rise to higher residual stress. The residual stress distributions are similar but different in magnitude. The Nd-doped phosphate laser glass cladded with glass with a thermal expansion coefficient of α₂ is chosen to demonstrate the characteristics of residual stress distribution, as shown in Figs. 6, 7, and 8. The residual stress distributions for different cladding temperatures are shown in Figs. 9, 10, 12, and 13. They indicated that the cladding temperature can also lead to residual stress. Higher cladding temperature can give rise to higher residual stress. The residual stress distributions are also similar but different in magnitude. The Nd-doped phosphate laser glass cladded with T=1273.15 K is chosen as a representative to demonstrate the characteristics of residual stress distribution shown in Figs. 14, 15, and 16. By experiment, the residual stress distributions for cladding with different thermal expansion coefficients are shown in Fig. 18 for α₁, Fig. 20 for α₂, and Fig. 22 for α₃. They demonstrated that the larger mismatch of thermal expansion coefficients between these two glasses can give rise to the higher residual stress. The residual stress distributions for cladding with different temperatures are shown in Fig. 23 for T=1073 K, Fig. 24 for T=1173 K, Fig. 25 for T=1273 K, Fig. 26 for T=1373 K. They demonstrated that the higher cladding temperature can give rise to the higher residual stress. The experimental results are consistent with the simulation results. So, the best strategy to minimize the residual stress of monolithic edge-cladding includes two aspects: the thermal expansion coefficient of the Nd-doped phosphate laser glass and the edge-cladding glass should be approximately equal and the cladding temperature should be as low as possible.

Conclusions The residual stress mainly comes from the melt bonding process of monolithic edge-cladding involving three aspects: the transformation of cladding glass from liquid to solid on the Nd-doped phosphate laser glass surface produces compressive stress, the high temperature of cladding glass on the Nd-doped phosphate laser glass surface creates a temperature gradient from the edge to the center, which can produce residual stress, and the difference in the thermal expansion coefficients between the edge-cladding glass and the Nd-doped phosphate laser glass can give rise to residual stress. The experimental data and simulated data indicated that the matched thermal expansion coefficients of the Nd-doped phosphate laser glass and the edge-cladding as well as the appropriate cladding temperature can reduce residual stress. However, a fine annealing treatment is needed for the monolithic edge-cladding of the Nd-doped phosphate laser glass to further reduce residual stress for an engineering application.