Gert Freiberger’s research while affiliated with Graz University of Technology and other places

Free space optics (FSO) is a wireless optical communication technology enabling extremely high data transmission rates, which can be used for a wide range of emerging applications. Nevertheless, FSO system reliability can be easily deteriorated in the presence of various weather-induced disruptions. The main two atmospheric effects influencing FSO links are fog and turbulence. Their investigation is based on real data and simulations—separately performed for two different locations in Austria. Based on the aforementioned weather-induced disruption analysis and existing knowledge about the link margin of the selected FSO communication measurement scenario, both outage probability and availability parameters are evaluated. Considering these outcomes, the most prominent and well-established atmospheric mitigation techniques for FSO technologies are explained. To address these techniques, a special emphasis is placed on two emerging applications: modern deep space communications as well as vehicle-to-vehicle (V2V) wireless optical communications. Both are examined based on their susceptibility to the investigated weather-induced disruptions. In order to improve the deep space FSO system resilience against long-distance and atmospheric effects, an approach using the superconducting nanowire single-photon detector (SNSPD) technology is considered. Furthermore, a hybrid V2V communication solution based on radio frequency (RF) and FSO is described.

Future ground to deep space communication systems should support Gbps data links, which are resilient to atmospheric impairments. An alternative and in the same time complementary to the conventional RF technology is the emerging deep space Free Space Optical (FSO) system, which can successfully face the challenge. Aligned with this future possibility, the current paper reports on a performance of the deep space FSO system based on detailed simulations considering single-photon communication scenario. Among the several available optical receiver units, a state of the art Superconducting Nanowire Single Photon Detector (SNSPD) technology is implemented. The SNSPD is characterized with a quantum efficiency, dead time, dark count rate and number of channels. The investigation is based on the Poisson channel and well-known Pulse Position Modulation (PPM) deployed as a modulation technique. Moreover, the selected receiver aperture diameter provides the necessary resilience to atmospheric turbulence effect. Addressing the sophisticated parameterization of the current simulations, BER performance of the deep space FSO link based on the SNSPD receiver is provided. On the top of it, an output of the SNSPD in RF domain is considered. The sequence of RF output pulses is presented before and after the applied ideal low-pass filter.

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.

Performance of a deep space FSO-link incorporating SNSPD parametrized with deadtime, QE and N-array is addressed. Considering atmospheric turbulence fading ≤3.5 dB, data rate up to 20 Mbps for -17 (ph/ns)[dB] received signal is reached.