Experimental verification of fiber coupling characteristics for FSO downlinks from the International Space Station

Abstract

Free-space optical (FSO) systems are compulsory to realize high capacity and interference-free communication links from low-Earth orbit (LEO) satellite constellations as well as spacecraft and space stations to the Earth. To be integrated with high-capacity ground networks, the collected portion of the incident beam should be coupled into an optical fiber. To accurately evaluate the signal-to-noise ratio (SNR) and bit-error rate (BER) performance metrics, the probability density function (PDF) of fiber coupling efficiency (CE) must be determined. Previous studies have experimentally verified the CE PDF for a single-mode fiber, however, there is no such investigation for the CE PDF of a multi-mode fiber (MMF) in a LEO-to-ground FSO downlink. In this paper, for the first time, the CE PDF for a 200-μm MMF is experimentally investigated using data from an FSO downlink from the Small Optical Link for International Space Station (SOLISS) terminal to a 40-cm sub-aperture optical ground station (OGS) supported by a fine-tracking system. An average CE of 5.45 dB was also achieved given that the alignment between SOLISS and OGS was not optimal. In addition, using the angle-of-arrival (AoA) and received power data, the statistical characteristics such as channel coherence time, power spectral density, spectrogram, and PDFs of AoA, beam misalignments, and atmospheric turbulence-induced fluctuations are revealed and compared with the state-of-the-art theoretical background.

Low-complexity turbulence resilience enabled by a multi-mode bi-directional transceiver

“…FSO systems built upon widely-available commercial components, such as SFPs and industry standard optical fibers, could allow for the rapid and effective deployment of high-speed communication links. Multi-mode (MM) fibers offer an interesting alternative, as these have a larger acceptance angle and spatial bandwidth, allowing for the efficient coupling of a received beam with an aberrated PSF due to turbulence [11,13,[26][27][28][29], as well as increased resilience to pointing errors [30][31][32]. However, intermodal coupling within MM fibers is commonly considered to limit their usage as effective transmitters for FSO applications because the higher-order output field will be considerably more divergent than an equivalent field created by an SM fiber based transmitter.…”

Low-complexity turbulence resilience enabled by a multi-mode bi-directional transceiver

“…FSO systems built upon widely-available commercial components, such as SFPs and industry standard optical fibers, could allow for the rapid and effective deployment of high-speed communication links. Multi-mode (MM) fibers offer an interesting alternative, as these have a larger acceptance angle and spatial bandwidth, allowing for the efficient coupling of a received beam with an aberrated PSF due to turbulence [11,13,[26][27][28][29], as well as increased resilience to pointing errors [30][31][32]. However, intermodal coupling within MM fibers is commonly considered to limit their usage as effective transmitters for FSO applications because the higher-order output field will be considerably more divergent than an equivalent field created by an SM fiber based transmitter.…”