Free space optical (FSO) communication has enjoyed a renewal of interest in the past decade driven by
increasing data rate requirements and decreasing amounts of radio frequency spectrum. These needs exist in
both the commercial and military sectors. However military communications requirements differ in other
ways. At the U.S. Naval Research Laboratory (NRL) we have been conducting research on FSO
communications for over ten years with an emphasis on tactical applications. NRL's FSO research has
covered propagation studies in the maritime domain, new component development, and systems
demonstrations. In addition NRL has developed both conventional, direct, laser communications systems and
retro-reflecting systems. In this paper we review some of this work and discuss possible future applications of
Small robots are finding increasing use for operations in areas that may be dangerous to humans.
These robots often have needs for high bandwidth communications to return video and other data.
While radio frequency (RF) links can be used in may cases, in some circumstances they may be
impractical due to frequency congestion, reflections off surfaces, jamming or other RF noise. In
these cases an optical link may be advantageous, particularly when a clear line of sight exists.
However, a conventional optical link has limitations for this application. For example, a
conventional optical link operating at rates of megabits per second at ranges of 1 Km requires
about a 1 degree pointing accuracy. This implies a need for active pointing and tracking, which
maybe be unacceptable for a small platform. We explored an optical modulating retroreflector
(MRR) link for these cases. An array of 6 MRRs and photodetectors with a field of view of 180 degrees (azimuth)x 30 degrees (elevation) was constructed and mounted a small robot, the iRobot
PackbotTM. An Ethernet modem designed to work with MRR links was also part of the system.
Using a tracking laser interrogator at the other end of the link, a 1.5 Mbps free space optical
Ethernet link was established that completely replaced the normal RF Ethernet link. The link was
demonstrated out to ranges of 1 Km down a road, exceeding the range of the RF link. Design
issues and measurements of performance will be described.
For Free Space Optical Communication (FSOC) systems, employed on moving platform or communicating
with a moving remote terminal, the quality of the communication channel strongly depends upon the tracking
performance. In these systems quadrant Position Sensitive Detectors (PSD) are commonly used for beam tracking.
This paper presents the results of significantly improved performance in acquisition and tracking of the FSOC system
using a custom made 8-segments PSD and minimizing the tracking spot size on the detector.