Hristo Ivanov

Free Space Optics (FSO) technology enabling next-generation near-Earth communication is prone to severe propagation losses due to atmospheric-turbulence-induced fading and Mie scattering (clouds). As an alternative to the real-time evaluation of the weather effects over optical signal, a state-of-the-art laboratory testbed for verification of slant APD-based (Avalanche Photodiode) FSO links in laboratory conditions is proposed. In particular, a hardware channel emulator representing an FSO channel by means of fiber-coupled Variable Optical Attenuator (VOA) controlled by driver board and software is utilized. While atmospheric scintillation data are generated based on Radiosonde Observation (RAOB) databases combined with a statistical design approach, cloud attenuation is introduced using Mie theory together with empirical Log-Normal modeling. The estimation of atmospheric-turbulence-induced losses within the emulated optical downlink is done with an FSO IM/DD prototype (Intensity Modulation/Direct Detection) relying on two different data throughputs using a transmitter with external and internal modulation. Moreover, the receiver under-test is a high-speed 10 Gbps APD photodetector with integrated Transimpedance Amplifier (TIA) typically installed in OGSs (Optical Ground Stations) for LEO/GEO satellite communication. The overall testbed performance is addressed by a BER tester and a digital oscilloscope, providing BER graphs and eye diagrams that prove the applied approach for testing APD-TIA in the presence of weather-based disruptions. Furthermore, the testbed benefits from the used beam camera that measures the quality of the generated FSO beam.

Optical wireless networks realized by means of gigabit optical wireless communication (OWC) systems are becoming, in a variety of applications, an important alternative, or a complementary solution, to their fiber-based counterparts. However, performance of the OWC systems can be considerably degraded in periods of unfavorable weather conditions, such as heavy fog, which temporarily reduce the effective capacity of the network. In this paper, we focus on optical wireless mesh networks that use terrestrial links (called FSO - free-space optical - links) composed of several parallel full-duplex FSO systems, and present two complementary solutions that together provide a means to maximize network traffic performance in various weather conditions encountered during network operation. The first solution is a method for estimating the degradation of the effective FSO link capacity in adverse weather conditions such as fog, rain and snow (called the weather states in this paper). The second solution is an optimization model aiming at maximizing the network traffic throughput for a given list of weather states, derived from the conducted measurements. The model assumes the so-called affine flow thinning (AFT) traffic routing and protection mechanism capable of controlling the end-to-end traffic flows in response to fluctuations of capacity available on FSO links caused by changing weather conditions. The proposed link capacity modeling approach and the elaborated optimization model are verified through an exhaustive numerical study, illustrating the trade-off between the increase of traffic performance of the FSO networks and the corresponding cost of additional OWC systems.

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.

A novel vehicle network design, based on optical technologies, is presented to handle all the issues of a growing wiring harness. Electromagnetic interferences, weight, and insufficient data rates of the transmission lines are just a few of those problems. The wiring harness of autonomous vehicles is far too complex, heavy, and prone to errors and must be adapted or entirely replaced to support future systems. Consequently, Reconfigurable Optical Add/Drop Multiplexers (ROADMs) can be used to reduce the overall complexity, while optimizing wiring and weight. Furthermore, a flexible optical network structure can be achieved, which is immune to electromagnetic interferences.

Autonomous vehicles are sophisticated self-sufficient systems which employ hundreds of sensors. To improve road safety, the number of Advanced Driver Assistance Systems (ADAS) steadily increases. Consequently, the applied systems are growing in complexity and connectivity. The network architecture within vehicles, however, is only adapted and extended to support emerging high-bandwidth applications while redesigning the whole network would yield many benefits. As a result, modern wiring harnesses are too complex, heavy and prone to errors.In this paper, existing bus systems are analysed for their capabilities to handle sophisticated network traffic which will emerge in autonomous vehicles. Furthermore, problems are identified and discussed, and a potential solution based on optical technologies is presented. This solution combines existing optical technologies in a novel way to cope with the upcoming bandwidth requirements of autonomous vehicles, while keeping the costs to a minimum.

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.