Spectral and spatial filtering for daytime deep-space optical communications

In deep-space optical communications, one particular challenge encountered at the ground receiver side are the wavefront deformations caused by atmospheric turbulence. This gives rise to reduced signal-to-background ratios and signal fades, especially during daytime. Background rejection is therefore a fundamental requirement for optical links operating in the low-photon-count regime with a strong solar background. Both, spectral and spatial filtering subsystems are essential for this application. To this end, we have analyzed several spectral filters and wavefront sensing approaches. In laboratory experiments, a combination of a bandpass filter and a Fabry-Pérot etalon delivered the required bandwidth of 0.17 nm and transmission of 90%, while a SWIR Shack-Hartmann sensor, combined with custom-built wavefront reconstruction software, directed the adaptive optics loop. We have obtained improvements in the Strehl ratio for signal-to-background ratios down to 0.2.