The high energy spaceborne all-solid-state lasers based on fluid-loop cooling system

We demonstrate a 532.2 nm high-power all-solid-state pulse laser with a repetition rate of 85 Hz, a single pulse energy of 215.8 mJ. The lasers spot size is (3.8, 3.6) mm, and the divergence angle is (0.57, 0.69) mrad. The stable working state of this laser last for 402 s and the pointing stability of the output beam is better than 50 μrad. A fluid loop system based on phase-change energy-storage heat exchanger is designed in this paper in order to solve several key problems induced by the heat dissipation. The laser is working normally in orbit, and it lays the foundation for the development of spaceborne all solid-state laser with high-energy, high-repetition rate.


Introduction
With the increasing requirements of spaceborne lidar for detection parameters and detection efficiency, the demands for multi-wavelength, high repetition rate and high-energy solidstate lasers also grow dramatically [1][2][3][4][5][6].However, for a highenergy solid-state laser, the improvement of the repetition rate and the laser energy will be greatly affected by the thermal effect.In such lasers, only a small part of the total input energy of the solid-state laser can be converted into laser output, while the residual most energy will be converted into the heat which is dissipated on the laser crystal.This heating effect has a Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.great impact on the laser output.The temperature gradient and stress distribution caused by the heating effect will change the refractive index of the laser crystal, resulting in the thermal lens effect of the whole crystal which declines the quality of the laser output beam.In addition, excessive thermal stress may also cause the laser crystal to burst.The thermal stress and thermal lens effect caused by the heating effect are collectively referred to as the thermal effect of the laser crystal.The larger the input power of the laser, the more serious the thermal effect [7][8][9][10].Therefore, eliminating the heat converted by the power dissipation in time is one of the most important technologies in the development of high-power solid-state lasers.
The heat of the lasers used on the ground can be controlled by forced convection, radiation, conduction as well as active or passive thermal control methods.However, there is no airflow on satellite and the specific thermal control requires more satellite resources.Therefore, the most popular thermal control method for spaceborne lasers is mainly conduction and radiation [11,12].The Ice Cloud and land Elevation Satellite-2 (ICESat-2) launched in 2018 utilized the thermal control method of conduction cooling to realize 532 nm 0.9 mJ pulse laser output with 10 kHz repetition rate [13].The international first atmospheric lidar, the aerosol and carbon dioxide detection lidar (ACDL), used a 532 nm laser as the laser source which had 40 Hz repetition rate and pulse energy of 150 mJ [14].The pulse energy of the lasers mentioned above is hard to further increase by utilizing conventional methods of conduction and radiation.Therefore, the active thermal control method such as forced convection heat transferring is an alternative to mitigate the heat effect in developing spaceborne high-energy solid-state lasers.
We propose a spaceborne all-solid-state laser with a fluid loop system based on the phase-change energy-storage heat exchanger (PEHE).This thermal control technology has several advantages, such as strong heat transfer ability, excellent safety and high reliability.Compared with the lasers used in ACDL, both the 532.2 nm laser output pulse energy (215.8mJ) and the pulse repetition rate (85 Hz) are higher.The laser can achieve a stable working state for 402 s and the pointing stability of the output beam is better than 50 µrad.The laser was launched into space in January 2023, and is working well for almost 14 months in orbit.

Experiment setup
The laser adopts the scheme of the main oscillation power amplification, as shown in figure 1.First, the non-planar ring cavity laser (NPRO laser) provides a continuous single frequency laser output at 1064 nm and the laser beam is chopped into a pulse laser by an acousto-optic modulator (AOM).Then, the optical pulse is amplified to 1 µJ by a Nd:YOV 4 crystal after the AOM.Next, the pulse laser is coupled into the amplification system which includes the preamplifier and the three-stages amplification system based on Nd:YOV 4 and Nd:YAG crystals.Finally, the amplified laser pulse with the energy of 500 mJ enters the second harmonic generation (SHG) system to generate 532 nm laser.To improve the convention efficiency, we use two sets of SHG systems.In the first SHG system, a Lithium Triborate crystal (LBO) coverts most 1064 nm optical pulse into 532 nm.The residual 1064 nm fundamental frequency light is reflected by a mirror and injected into the second LBO.The 532 nm laser generated by two sets of frequency doubling crystals enters the adjustable beam expander after polarization beam combination, and finally goes out through the output window.
As the key elements in the laser amplifier system, the Nd:YAG and Nd:YVO 4 pump heads demand extraordinary high thermal management to achieve high system performance, including the extraction efficiency of the laser crystal, laser beam quality, the lifetime of diode bars, and etc.For instance, the pump heads employ diode bars as pump source, whose pumping shots and lifetime straightly depend on their working temperature.The diodes' lifetime decreases with the increase of the interface temperature.Considering the demanded working lifetime and the room temperature during assembling and adjustment, the operation temperature of the mounting surface of the bar is designed not to exceed 24.0 • C. Due to the temperature gradient between the bar and the cold plate, the interface temperature of the cold plate is proposed to be 15.0 ± 2.0 • C for more than 308 s.
As shown in table 1, the main heat sources of the laser are four laser power amplifiers, the heat output of the laser reaches 520 W during the working time and it needs to be dissipated in time.The need for the heat flow density ranges from 1.9 W cm −2 to 2.6 W cm −2 .As the temperature of the inlet fluid is lower than that of the outlet fluid, the flow in the cold plate is designed to go through from the last stage amplification to the pre-amplification to ensure the temperature uniformity.The rapid collection of waste heat with high heat flow density should be realized.

The thermal control design
As an important active thermal control technology, a fluid loop system uses the pump to drive the forced fluid-convectioncirculation for temperature control, has been widely applied in many domestic and foreign spacecraft [15][16][17][18].In order to meet the heat dissipation requirements of this laser mentioned above, the fluid circuit system based on the PEHE is designed, and the composition schematic diagram is shown in figure 2. It is mainly composed of pump components, laser heat sink, compensating heater, PEHE, fluid circuit controller, fluid pipeline and circulating working medium.During the operation of the laser, the heat of the internal devices is collected in time through the active fluid circuit and transported to the PEHE for heat storage.During the intermission period, the slow discharge of stored heat is realized by passive heat dissipation of phase change heat exchanger, and the initial state is restored at the next working cycle.The thermal management system for these heat sources from the laser heat sink employs a cold plate within the fluid circuit.The cooling capacity of the cold plate primarily depends on the heat transfer coefficient of the fluid flowing inside it.The cold plate designed in this work features an internal plate-fin structure, which allows for a larger heat exchange surface area within a limited volume, thereby enhancing the heat transfer capability of the cold plate.The material chosen for the cold plate is aluminum alloy (6061), which offers advantages such as good machinability, high thermal conductivity, and compatibility with the working fluid.Heat from the system is collected, transported, and dissipated through the circulation of a working fluid within the loop.From a performance standpoint, the working fluid should have high specific heat capacity, high thermal conductivity, low viscosity, low vapor pressure, good chemical compatibility, and a low freezing point.Taking into account factors such as temperature, safety, and legacy, a 40% glycol solution has been chosen as the working fluid for the loop, which is also widely used in manned space engineering projects.
The PEHE is a pure passive thermal control methods with high reliability and it is primarily filled with phase change materials (PCMs), which absorb or release a significant amount of heat during the phase-change process, thus extending the duration of stable temperatures within the system [19,20].Fin structures are designed inside the PEHE to enhance heat dissipation efficiency.The PCM selected for the system is n-tetradecane, and the PEHE is required to have a heat storage capacity of more than 160 kJ.Considering the heat generation of the pump components and the heat loss of the system, 1.8 kg of n-tetradecane energy is charged in the PEHE, which can store 407 kJ of heat.The main parameters of n-tetradecane is shown in table 2.
As shown in figure 3, the laser uses a fluid circuit to dissipate heat from the pump modules with high heat generation, including pre-amplification and three-stage amplification.The heat-dissipation cold plate is heat-insulated inside the laser shell, and the fluid input and output ports are sealed.
The fluid circuit is a single-phase liquid working medium, which circulates in the fluid circuit driven by the pump.The working medium absorbs energy through the cold plate which induces the temperature increase.The energy is transferred to the PEHE, and then flows to the radiator.Finally it discharges into space.The heat transferred by the working medium in the fluid circuit is: where Q s is the heat transferred from the heat source to the fluid; c is the specific heat of the fluid; ṁ is the mass flow rate of the working medium.T ho and T hi are the temperature of the working fluid flowing out and into the cold plate, respectively.When the initial temperature of the fluid and PCM is 2 • C, the initial temperature of the laser is 20 • C, and the flow rate of the fluid circuit is 3.5 l min −1 .The four temperature test points are set near the cold plate where the four amplifiers are mounted on, respectively as shown in figure 3. Combined with the heat source distribution in table 1, the thermal analysis of the laser working at rated power is carried out in figure 4.
It can be seen from the fluid thermal analysis results in figure 5 that the laser can work continuously for 450 s under the condition that the temperature of the laser heat source mounting surface is 15.0 ± 2.0 • C, and the maximum temperature of the mounting surface of the bar is below 24.0 • C. The designed fluid circuit system based on PEHE meets the requirements of the thermal control design.

Experiment setup
In order to further verify the laser performance and thermal control design, a test system is built, as shown in figure 6.The output of the laser is split into two beams.One is sent into the energy meter to measure the optical energy variance, and the other one is illuminated on an charge-coupled device for the pointing stability test.
Figure 7 shows the temperature curve of different position of the cooling plate during the laser test, from which it can be seen that the laser can work continuously for 402 s under the condition that the temperature of the heat source mounting surface is 15.0 ± 2.0 • C.
From the main light energy curve in figure 8, it can be seen that the laser energy tends to be stable after 85 s from the startup.After stabilization, the average energy value is 215.8 mJ   while the standard deviation is 4.8 mJ. Figure 9 shows the nearfield and far-field intensity distributions of the laser beam, this spot size is (3.8, 3.6) mm, and the divergence angle is (0.57, 0.69) mrad.From the pointing stability curve in figure 10, the pointing of the laser tends to be stable after 150 s from the startup.The maximum changes in the horizontal (x direction) and pitching (Y direction) directions are 44.1 µrad and 42.0 µrad, respectively, and the standard deviations are 8.0 µrad and 8.6 µrad, respectively.The pointing stability is comparable to that of the ALADIN laser (40 µrad) which is also the spaceborne highenergy laser but with lower output energy of 120 mJ [21].These changes may be caused by the thermal lensing effect resulted from the high pump light absorption of the laser crystal in the last stage amplifier.This thermal lensing effect leads to a wavefront distortion, and then induces a variety of aberration in the beam propagation through the crystal which cause the optical axis jitter.

Conclusion
In conclusion, the fluid circuit system based on PEHE is designed to solve the heat dissipation problem of the all-solidstate laser on satellite.Under the working conditions of fluid circuit flow of 3.5 l min −1 and the initial temperature of PCM at 2 • C, the temperature of laser-heat-source mounting surface is kept at 15.0 ± 2.0 • C for 402 s.The output energy of the laser tends to be stable after 100 s with an average value of 215.8 mJ with a repetition rate of 85 Hz.The laser pointing stability is less than 50 µrad after 150 s from the startup.Up to now, the laser is working well in orbit, and it paves the way for the development of spaceborne all solid-state laser with highenergy, high repetition rate.

Figure 1 .
Figure 1.Schematic diagram of the laser system.

Table 2 . 07 Figure 3 .
Figure 3. Schematic diagram of the active thermal control design.

Figure 4 .
Figure 4. Temperature cloud map from thermal analysis.

Figure 5 .
Figure 5. Temperature curve of the fluid thermal analysis.

Figure 6 .
Figure 6.Schematic diagram of testing methods for lasers energy and directional stability.

Figure 7 .
Figure 7. Temperature curve of the test points.

Figure 8 .
Figure 8. Energy output of the laser system.

Table 1 .
Lasers heat source distribution.
Figure 2. Schematic diagram of fluid circuit system.