Thermal considerations for the SPARCLE optical system

Abstract

The SPAce Readiness Coherent Lidar Experiment (SPARCLE) is the first demonstration of a coherent Doppler wind lidar in space. Coherent lidars can accurately measure the wind velocity by extracting the Doppler frequency shift in the back-scattered signal from the atmosphere through optical heterodyne (coherent) detection. Coherent detection is therefore highly sensitive to aberrations in the signal phase front, and to relative alignment between the signal and the local oscillator beams. The telescope and scanning optics consist of an off-axis Mersenne telescope followed by a rotating wedge of silicon and a window of fused silica. The wedge is in very close proximity to the experiment window, and is essentially in contact with the scanner motor/encoder system. The can environment temperature is nominally 20 degrees Celsius, the window ranges from -20 degrees Celsius to 0 degrees Celsius, and the scanner motor/encoder system alone could generate temperatures as high as 35 degrees Celsius. This thermal environment, coupled with the relatively large sensitivity of silicon's refractie index to temperature, has required careful thermal design and compensation techniques. This paper discusses the optical issues of these thermal effects and a variety of methods used to ameliorate them.