A robust pointing, acquisition, and tracking approach is critical to closing and maintaining a free space optical communications link. Many fiber-coupled terminal architectures use a beamsplitter to direct a portion of the received light onto a quadrant detector and generate an error signal. A feedback control loop uses this error signal to adjust a fine steering element to maximize power into the collection fiber. However, this approach produces additional insertion loss due to transmission through a beamsplitter and tracking loss due to inevitable boresighting errors between the quadrant detector and the collection fiber. We present an alternative architecture that makes use of a hexagonally packed, seven-fiber bundle for data beam tracking. The outer fibers are inherently co-boresighted position sensors used to sense displacement of the received beam from the central data fiber. We present field test data directly comparing performance between a fiber bundle-based terminal and a quadrant detector-based terminal. Our results show nearly an order of magnitude of improvement in control loop tracking stability and a 2.8× improvement in tracking performance as seen in Strehl ratio for the fiber bundle system.
Laser communications (Lasercomm) systems have gained increasing interest for both defense and commercial applications due to their ability to provide secure, long-distance, high-bandwidth communications on the move, without the need for RF spectrum management. This paper will present field test results from the U.S. Navy’s Trident Warrior 2017 fleet exercise, where a compact Lasercomm system was evaluated. As compared to previously demonstrated long range Lasercomm terminals, this compact terminal design leverages simultaneous transmit and receive spatial diversity to mitigate scintillation fades in a smaller form factor. In addition, a 10 Gbps retransmission capability has been integrated and tested to assure error-free data transport even through short duration path blockages and optical fades. The system was operated at full functionality over seven test days with network traffic loads ranging from 1 - 7.5 Gbps in bidirectional configurations. The system was exercised to a line of sight limited range of 45 km and showed capability through haze and even some levels of fog on multiple days.
Lasercomm technology continues to be of interest for many applications both in the commercial and defense sectors because of its potential to provide high bandwidth communications that are secure without the need for RF spectrum management. Over the last decade, terrestrial Lasercomm development has progressed from initial experiments in the lab through field demonstrations in airborne and maritime environments. While these demonstrations have shown high capability levels, the complexity, size, weight, and power of the systems has slowed transition into fielded systems. This paper presents field test results of a recently developed maritime Lasercomm terminal and modem architecture with a compact form factor for enabling robust, 10-Gbps class data transport over highly scintillated links as found in terrestrial applications such as air-to-air, air-to-surface, and surface-to-surface links.
In recent years, various terrestrial free-space optical (FSO) communications systems have been demonstrated to achieve high-bandwidth communications between mobile platforms. The terminal architectures fall into three general categories: (1) single aperture systems with tip/tilt control, (2) multi-aperture system with tip/tilt control, and (3) single aperture systems with tip/tilt control and higher order adaptive optics correction. Terrestrial modem approaches generally use direct detection receivers because they provide high bandwidth capability (0.1-10 Gbps) without the complexity of coherent detection. Modems are often augmented with a mix of forward error correction (FEC), interleaving, and/or retransmission for improved data transport. This paper will present a terminal and modem architecture for a low-SWAP FSO communications system that enables robust, high-bandwidth communications under highly scintillated links as found in terrestrial applications such as air-to-air, air-to-surface, and surface-to-surface links.
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