Free-space optical communications (FSO) systems have gained increasing interest for both defense and commercial applications due to their ability to provide secure, long-distance, high-capacity communications on the move. In terrestrial environments, because clouds and strong weather effects can limit FSO systems performance, integrating them with directional radio frequency (RF) links can yield a system that leverages the best of both modalities - the high capacity of FSO when available with the reliability of the RF link to ensure the highest priority data can be sent even during degraded weather conditions. This paper will present the development of a highly integrated FSO/RF link architecture implementing three key functionalities: (1) operation at data transfer rates up to 10 Gbps, (2) seamless failovers between the FSO and RF modalities, and (2) the necessary quality of service (QoS) mechanisms to handle the rate disparity between the two links while providing priority to critical data. This architecture utilizes a network transport system that provides layer 2 data transport and QoS arbitration across the FSO and RF modalities. Results from testing in lab as well as at outdoor ranges of up to 30 km will be presented.
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.
A free-space optical communications link spanning 147 km between the islands of Hawaii and Maui was studied as
part of AFRL's IRON-T2 program and in support of risk reduction efforts for DARPA's ORCA program in
September/October 2008. Over 14 days, the performance of a 10-Gbps bi-directional link was tested during different
periods of the day. This paper will present the test configuration, discuss the effects of atmospheric turbulence on the
10 Gbps link, and compare its performance to the available turbulence measurements. Additionally, modeling of the link
configuration will be presented and comparisons will be made to collected data including local Cn2 to understand the
impact of atmospheric turbulence on future long distance links.
Free Space Optical (FSO) Communications channels can exhibit high percentage availability, yet are subject to frequent
intensity fades due to turbulence effects. For gigabit class links, tremendous amounts of data can still be transported
through a fading channel, but an efficient network protocol is required to overcome the effects of fades. We describe a
custom error detector that can process a digital signal from a channel which has frequent fades below system sensitivity
and can provide data link statistics with bit-level timing accuracy. The statistics measured by the instrument include bit
level counters that allow the device to be used as a traditional bit error rate tester (BERT), as well as block-based
counters, which provide insight to the channel for packet based transmission formats. Synchronization parameters are
adjustable to accommodate different link dynamics. Additionally, stretched error and sync pulse outputs provide useful
live indicators of link performance when plotted against optical channel power. This paper will discuss the performance
of the custom bit error rate tester (cBERT) in testing a 2.5 Gbps channel over a maritime FSO link trial conducted off the
mid-Atlantic coast near Wallops Island, VA, in July and September 2009. Additionally, the overall design of the cBERT
will be presented.