Evaluation of FSO-technology as a Candidate for Reliable Long-distance Communication Links for Deep Space Applications

In this work an evaluation breadboard for preliminary tests of deep space FSO technology is shown. The testbed is based on a transmission path (operating on 1550 nm wavelength) realized with fibre optics technology. It contains a self-developed channel emulator, background noise module and a Superconducting Nanowire Single-Photon Detector (SNSPD) as receiver. Tungsten-Halogen light sources are proposed as the most accurate source for representing the real noise conditions. Moreover, the current scheme involves a Variable Optical Attenuator (VOA) with voltage control, which is used as a main device for dynamic optical power changes. The adjustable attenuation setup is based on a self-developed attenuator control unit with a software-defined Digital Analog Converter (DAC). The accomplished calibration measurements show that various atmospheric effects, such as turbulence-induced fading and Mie scattering, can be implemented through preliminary prepared lookup tables containing desired attenuation values. As demonstrated in the LLCD mission to the moon, the implementation of the optical receiver will be a SNSPD characterized with single-photon sensitivity and high detection efficiency. Addressing the VOA unit, which can dynamically change the optical power, the testbed allows sophisticated tests of the SNSPD system in artificial laboratory environments. Once the Poisson channel is successfully emulated, various measurements for testing a stable operation mode of the SNSPD system are performed. The shown analysis supports the search for an optimum SNSPD configuration in relation to the chosen communication scenario. The testbed was developed and established within the frame of a running ESA contract and would be used for future activities with Joanneum Research in Graz.