The first international aerosol carbon detection LiDAR (ACDL), which was developed by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, was successfully launched and has been continuously operated in orbit since April 2022. This Lidar uses a high-energy single beam pulsed laser with frequency stabilization at three wavelengths (532 nm/1064 nm/1572 nm). High-power space lasers typically produce a large amount of heat during operation; however, heat concentration in the laser causes the temperature of the laser diode (LD), which is a key device in the laser, to rise, resulting in a drift in the LD output wavelength that causes the overall efficiency of the laser to decrease or fail. Simultaneously, large amounts of heat also increases the temperature of the internal optical structure, resulting in a large temperature gradient, which causes the main laser structure to deform and stress accumulation inside the core optical components, thereby resulting in extremely serious effects on the laser output power, beam pointing, divergence angle, and polarization characteristics. An efficient and stable thermal control technology is one of the core technologies in the development of space laser loads. To meet the requirements of on-orbit applications, it is necessary to design, simulate, and test the thermal control system of the space high-energy pulsed solid-state laser used in the system.

Generally, the LiDAR is operated on a sun-synchronous orbit with an orbital altitude of 705 km and an orbital inclination of 98.1°. The laser is preferably installed inside the main body of the LiDAR and insulated from its main structure, and the connection is done in series through a heat pipe to transfer heat to a radiant cooling plate (Fig. 4). The thermal control states of the main and standby lasers are consistent when operated separately. Simultaneously, a single laser can transfer the heat to the second laser through a heat pipe to ensure an effective storage temperature. The temperature fluctuation and temperature gradient of the laser are controlled using the temperature-control form of multichannel active compensation heating, and the heating temperature control circuit is arranged on the top surface, lateral face, and external heat pipe of the laser shell to independently control the local temperature. The radiant cooling plate is installed on the nonilluminated surface, which is less influenced by the external heat flow when dissipating the high-power heat generated by the laser.

The space vacuum environment and space radiation cold background are simulated using a space environment simulator, and an infrared light array is used to simulate the external heat flow environment of the LiDAR in different directions (Fig. 8). High-temperature and low-temperature tests are conducted under thermal vacuum equilibrium conditions, where the temperature of the laser internal amplifier is maintained between 18.9 ℃ and 26.8 ℃, the temperature of the laser shell is maintained between 19.2 ℃ and 21.5 ℃, and the maximum temperature fluctuation is ±0.67 ℃ (Fig. 11). After the LiDAR is launched into orbit, an on-orbit test is conducted and the laser and laser thermal control system operate normally. By analyzing the correlation between the laser energy fluctuation and temperature measurement value, the laser energy fluctuation cycle is found to be consistent with the external heat flow fluctuation cycle. Additionally, the laser telemetry energy fluctuation is 4.9%, and the temperature control parameters of the laser external heater are adjusted using the on-orbit injection number to improve the temperature control accuracy of the laser during the on-orbit test. For the on-orbit thermal control parameter optimization process, the duty cycle of the heat pipe heater is reduced by reducing the temperature threshold of the heat pipe heater and opening the laser shell temperature compensation backup heater to increase the temperature control threshold of the heat pipe mounting surface heater. After on-orbit adjustment, the temperature fluctuation of the laser in orbit is ±0.033 ℃ (Fig. 12), and the fluctuation of laser energy telemetry is 1.2%.

Using simulation calculations and space environment thermal experiments, the design verification index is completed to ensure the usage requirements are met. After the LiDAR is launched into orbit, the laser thermal control system operates normally and meets the long-term stable working requirements of lasers in orbit. Therefore, the laser thermal control technology used in this study is reasonable, feasible, and has high reliability and design margin, making it an important reference for the thermal design of high-power space laser loads.