Experimental characterization of SNSPD receiver technology for deep space FSO under laboratory testbed conditions

Deep space Free Space Optical (FSO) systems are an emerging technology, characterized with very high performance in terms of information capacity and communication distances. However, this novel technology demands preliminary experiments, which are unmanageable in real conditions. Regarding this issue, an innovative approach for testing deep space optical communication links in controlled laboratory environment is developed. The proposed testbed is based on fibre optics technology and combines a couple of units, which represent a real deep space FSO link. The parameters of several different photon-counting receivers are discussed. Similar to already demonstrated deep space missions, the implemented optical receiver is Superconducting Nanowire Single Photon Detector (SNSPD) characterized with high single photon sensitivity and detection efficiency. Consequently, in this paper an authentic deep space Poisson channel is theoretically defined, emulated and examined. The description of the Poisson point process is supported by real SNSPD measurements in terms of high efficiency single-photon detection. Moreover, a self-developed unit including Variable Optical Attenuator (VOA) and software controlling the attenuation in the communication loop of the breadboard is applied. Apart from possibility for representing atmospheric-induced fading due to turbulence and Mie scattering effects, the module is utilized as a precise attenuator changing dynamically the optical power based on lookup tables. Addressing the capabilities of VOA unit, detection intensity, detector noise and efficiency versus SNSPD's bias current is measured. In addition, VOA is used as a very slow optical modulator for testing the restrictions regarding On-Off extinction ratio of the implemented SNSPD detector.