A concept for cooling side-pumped laser crystals, using a thin film of evaporating fluid, was investigated for use on future space-based light detection and ranging (LIDAR) instruments. Analyses of the solid crystal domain show that the thin-film cooling scheme will result in improved thermal management of the lanthanide crystal material investigated. Several candidate flow configurations are presented to produce the desired fluid flow across the crystal surface. Onedimensional, axial flow, two–fluid evaporation models were then solved to characterize the fluid and thermal performance of two of the proposed flow configurations. In addition, an incipience model is presented to determine the film thickness constraints necessary to suppress nucleate boiling. The bulk flow model indicates that excellent thermal management of the crystal, low liquid velocities, and low liquid pressure drops are possible with the two axial flow configurations analyzed. The incipience model indicates that liquid film thickness less than 10 microns may be necessary to ensure the complete absence of vapor bubbles in the liquid flowfield. This result indicates a need to develop three-dimensional fluid models for future studies so that more complex flow geometries may be studied.
NASA Langley Research Center has been actively developing a series of side-pumped, 2-micron output lasers as part of a long-term LIDAR risk-reduction program. A typical arrangement of the pump diodes and luminescent oscillator crystal are shown in Figure 1.
The current LIDAR concepts at NASA rely upon conductive cooling as the baseline design for the thermal management of the side-pumped flight instruments. While totally passive, conductively-cooled configurations result in non-uniform temperature distributions across the crystal cross section, result in higher than desired crystal operating temperatures, and are sensitive to minor variations in contact pressure at the conductive interface. Improvements in laser output energy are gained with lower operating temperatures.
To promote improved cooling, provide nearly uniform exterior crystal temperature, and to allow lower operating temperatures, a concept for cooling the crystal by means of a thin evaporating liquid film flowing across the crystal surface was proposed and investigated. Several candidate flow morphologies were analyzed using a steady state, onedimensional, two-fluid flow model. Analyses using this model were conducted using the properties of R507A and R508B azeotropic refrigerant fluids. Preliminary analyses of critical film thicknesses required to suppress nucleate boiling were performed using these same fluids. The combined results of the analyses indicate that temperatures below 223K (-50oC) are possible at the crystal surface, core temperatures below 237K (-36C), and concentric isotherms may be attainable. The results also indicate that liquid film thicknesses on the order of 10 microns may be necessary to ensure the complete suppression of nucleate boiling. More rigorous modeling and analysis are needed, and guidance for subsequent research is presented.
More information and detail can be found in below paper:
- Stewart, B. K., M. K. Akbar, and S. M. Ghiaasiaan. “Thin-film evaporative cooling of a side-pumped solid-state laser diode oscillator for space-based lidar applications.” In Lidar Technologies, Techniques, and Measurements for Atmospheric Remote Sensing, vol. 5984, p. 598405. International Society for Optics and Photonics, 2005.