OH_x free radical LIDAR detection usually requires a single-frequency ultraviolet wavelength, such as 308 nm, and an optical parametric oscillator (OPO) is one of the main devices to achieve nonlinear frequency conversion. However, most traditional OPOs use wide linewidth or multi-mode lasers as pump sources, which leads to problems such as high phase noise and low spectral purity of the final output laser and limits the application of OPOs in high-precision spectral detection and quantum optics. In addition, increasing the power of single-frequency lasers faces challenges such as nonlinear effects and thermal effects in laser gain media. Therefore, it is essential to use a high-quality and high-power single-frequency pump source for pumping OPOs. In this paper, based on the efficient one-dimensional heat dissipation characteristics and spherical aberration self-compensation technology of Innoslab, we designed the Innoslab amplifier to increase the single-frequency power while keeping the beam quality unchanged, providing a good pump source for OPOs.

The thermal effect of the laser crystal is essential to the design of the Innoslab laser amplifier. We first used finite element analysis to simulate the multi-dimensional thermal effects of the laser crystal and explored the temperature distribution and thermal stress distribution of the laser crystal from each dimension. Then, we investigated whether Nd∶YVO₄ would undergo thermally induced stress fracture at the theoretical pump power and determined the focal length of the thermal lens of the laser amplifier at the same theoretical pump power. Another key design factor of the Innoslab laser amplifier was the pattern matching between the seed light and the pump light. For the known pump light size, we used multiple cylindrical lenses to flexibly shape the dimensions of the seed light along the fast axis and slow axis directions to match the pump spot. In addition, while realizing the high-power single-frequency laser output, we should also avoid the deterioration of beam quality caused by the thermally induced spherical aberration effect. Therefore, we designed spherical aberration self-compensation structure based on the principle of the Fourier transform 4f imaging system.

Through finite element analysis and simulation, the temperature distribution and multi-dimensional thermal stress distribution of the laser crystal are clarified, and the focal length of the thermal lens is further determined. We numerically determine the thermal effect of the laser crystal accurately and provide theoretical support for the design of the Innoslab laser amplifier (Fig. 3 and Fig. 4). Thermally induced spherical aberration has always been a key factor affecting the performance of Innoslab laser amplifier. The realization principle of spherical aberration self-compensation is explained from the perspective of the 4f imaging system based on Fourier optics (Fig. 5), and the beam quality before and after amplification remains the same. The design of the Innoslab laser amplifier requires that when the seed light passes through the laser crystal multiple times, its size in the fast axis direction remains unchanged, and its size in the the slow axis direction increases evenly. This purpose is achieved through the flexible design of the magnifying end-pump and multiple cylindrical lenses (Fig. 7). The engineered Innoslab laser amplifier designed has been operating continuously and stably for several months (Fig. 10).

The proposed high-power single-frequency laser is successfully applied to the OH_x free radical LIDAR light source, and the pumped OPO module successfully outputs a 731 nm laser, which provides the basis for subsequent nonlinear frequency conversion. Compared with previous laser amplifiers, the principle of spherical aberration self-compensation is theoretically explained, ensuring that the beam quality before and after laser amplification is unchanged. In addition, the designed Innoslab laser amplifier has been applied in engineering and exhibits stability. So far, it has been operating stably for several months.