C. Chen, A. Grier, M. Malfa, E. Booen, H. Harding, C. Xia, M. Hunwardsen, J. Demers, K. Kudinov, G. Mak, B. Smith, A. Sahasrabudhe, F. Patawaran, T. Wang, A. Wang, C. Zhao, D. Leang, J. Gin, M. Lewis, B. Zhang, D. Nguyen, D. Jandrain, F. Haque, K. Quirk
Coherent, free-space optical communication technology offers near-quantum-limited receiver sensitivity and high spectral efficiency compared to conventional direct detection systems. In this paper, we will present the initial results from a bidirectional air-to-ground demonstration of a coherent optical link.
C. Chen, A. Grier, M. Malfa, E. Booen, H. Harding, C. Xia, M. Hunwardsen, J. Demers, K. Kudinov, G. Mak, B. Smith, A. Sahasrabudhe, F. Patawaran, T. Wang, A. Wang, C. Zhao, D. Leang, J. Gin, M. Lewis, D. Nguyen, K. Quirk
High speed optical backbone links between a fleet of UAVs is an integral part of the Facebook connectivity architecture. To support the architecture, the optical terminals need to provide high throughput rates (in excess of tens of Gbps) while achieving low weight and power consumption. The initial effort is to develop and demonstrate an optical terminal capable of meeting the data rate requirements and demonstrate its functions for both air-air and air-ground engagements. This paper is a summary of the effort to date.
KEYWORDS: Signal processing, Photon counting, Sensors, Digital signal processing, Signal detection, Single photon, Field programmable gate arrays, Signal attenuation, Telecommunications, Single photon detectors
Early applications driving the development of single photon sensitive detectors, such as fluorescence and
photoluminescence spectroscopy, simply required low noise performance with kiloHertz and lower count rate
requirements and minimal or no timing resolution. Newer applications, such as high data rate photon starved free space
optical communications require photon counting at flux rates into megaphoton or gigaphoton per second regimes
coupled with sub-nanosecond timing accuracy. With deep space optical communications as our application driver, we
have developed and implemented systems to both characterize gigaHertz bandwidth single photon detectors as well as
process photon count signals at rates beyond 100 megaphotons per second to implement communications links at data
rates exceeding 100 megabits per second with efficiencies greater than two bits per detected photon. With these
systems, we have implemented high bandwidth real-time systems using intensified photodiodes, visible light photon
counter detectors, superconducting nanowire detectors, Geiger-mode semiconductor avalanche photodiodes, and
negative avalanche feedback photon counters.
In this paper we discuss recent progress on the implementation of a hardware free-space optical communications
test-bed. The test-bed implements an end-to-end communications system comprising a data encoder, modulator,
laser-transmitter, telescope, detector, receiver and error-correction-code decoder. Implementation of each of
the component systems is discussed, with an emphasis on 'real-world' system performance degradation and
limitations. We have demonstrated real-time data rates of 44 Mbps and photon efficiencies of approximately 1.8
bits/photon over a 100m free-space optical link.
A canonical deep space optical communications transceiver which makes synergistic use of advanced technologies to
reduce size, weight, power and cost has been designed and is currently under fabrication and test. This optical
transceiver can be used to retire risks associated with deep space optical communications on a planetary pathfinder
mission and is complementary to ongoing lunar & access link developments. Advanced technologies being integrated
into this transceiver include use of a single photon-sensitive detector array for acquisition, tracking and communications;
use of two-photon absorption for transmit beam tracking to vastly improve transmit/receive isolation; and a sub-Hertz
break frequency vibration isolation platform is used to mitigate spacecraft vibration jitter. This article will present the
design and current test results of the canonical transceiver.
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