In free space optical communication, photodetectors serve not only as communications receivers but as position sensitive detectors (PSD) for pointing, tracking, and stabilization. Typically, two separate detectors are utilized to perform these tasks but recent advances in the fabrication and development of large area, low noise avalanche photodiode (APD) arrays have enabled these devices to be used both as PSDs and as data communication receivers. This combined functionality allows for more flexibility and simplicity in optical assembly design without sacrificing the sensitivity and bandwidth performance of smaller, single element data receivers. This work presents a large area, five element concentric avalanche photodiode array rated for bandwidths beyond 1GHz with a measured carrier ionization ratio of approximately 0.2 at moderate APD gains. We discuss the integration of this array in a bi-static optical interrogator where it acts as a data receiver and provides position information for pointing and stabilization. In addition to front-end and digital electronics design, we also describe the optical assembly design and the development of a pointing and stabilization algorithm.
Free space optical communication uses photodetectors for two purposes: as communications receivers and, in the form of
a quadrant cell or a position sensitive detector, for tracking. Generally two separate detectors are used. In this work we
describe combining these functions into one device through the use of heterostructure avalanche photodiode (APD)
arrays. Combined functionality more efficiently uses the available light and allows for large area communications
detector arrays that maintain the bandwidth and sensitivity of smaller, single-element, devices. In this paper we describe
a prototype 2x2 arrays and associated electronics and processing. The design tradeoffs in balancing both functions are
explored and future geometries that are more effective than square arrays are described.
As military sensors and systems become more sophisticated, tactical situations will require reliable, high data rate
communications. The current RF communication systems are increasingly competing for the limited amount of RF
spectrum and bandwidth. One possible way to augment the current RF communication systems is by the use of free
space lasercomm in tactical networks for links in which direct line of sight is possible. Free space lasercomm has
been demonstrated over horizontal distances greater than 10 nautical miles and at data rates greater than 1
gigabit/sec. Lasercomm links do not require any RF frequency allocation, nor do they have an RF signature. They
are inherently low probability of intercept and detection and they are very difficult to jam due to the very narrow
divergence of the communication beams and the very narrow acceptance angle of the receivers.<sup>1-6</sup> The U.S. Naval Research Laboratory has demonstrated the use of free space lasercomm in tactical networks at
Trident Spectre 2009 and Empire Challenge 2010. This paper will discuss these lasercomm demonstrations and
present packet error rate test data captured at both.
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
Packbot<sup>TM</sup>. 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.
In free space optical communication systems, atmospheric turbulence makes it very difficult to focus transmitted laser
power onto small, low capacitance photodetectors. The obvious challenge, therefore, is to take advantage of larger area
photodiodes without sacrificing a great deal of bandwidth and sensitivity in the process. In this work, we report on a
high sensitivity, high speed adaptive avalanche photodetector array for free-space optical communication. The receiver
consists of a 2×2 InGaAs APD array with each 100um element in the array having its own dedicated trans-impedance
amplifier and buffering stage. The corresponding voltage outputs for each element are processed through a four channel
digital, fast switching and summation circuit. The resulting signal is selectable to be either that of the element in the
array with the greatest signal response or the sum of multiple or all channels. Design requirements, laboratory
sensitivity measurements, and field testing results are presented.
This paper presents the results of a successful bidirectional free-space optical link across 16 km to a modulated retroreflector array. The link was implemented at the Naval Research Laboratory's Chesapeake Bay Detachment laser test range. A 6-W cw 1550-nm class 1 M interrogation beam was used to illuminate an array of three modulated cat's-eye retroreflectors located on a tower across the Chesapeake Bay on Tilghman Island. The modulated retroreflectors had a diameter of 16 mm and were arranged in a triangular pattern with a spacing of 30 cm. The interrogating terminal employed a 100-µrad divergence and a high-speed pointing and tracking system to maintain link alignment. Link testing occurred over 12 days in the months of September, October, and November of 2006. Topics presented in this paper include the link scenario for the 16-km free-space optical link, the link budget, and terminal designs, as well as link acquisition and performance. Link performance results presented include data transmission throughput, scintillation data, and pointing and tracking results.
An experimental study has been made on the contribution to the effective scintillation index due to two retroreflectors, as a function of retroreflector spacing. For closely spaced retroreflectors the effect of coherent interference at the receiver is seen to increase the effective variance of the received signal, whereas spatial averaging is apparent for more widely spaced retroreflectors. The scintillation index, probability density functions, power spectral densities and fade rates are all affected by the interference.
The range, under which these experiments were conducted, was typically 500 - 800m over mixed water/land terrains. The interrogator used a monostatic, 1550nm laser probe beam with a divergence of 0.4 mrad and had a 50mm diameter receiver aperture. Data sets of received power were recorded for durations of 10s each, using a photodiode with a bandwidth of 100kHz. For comparison, the received power from a single retroreflector at various radial positions in the probe beam was recorded. Knowledge of the fade rates and fade durations is of practical importance in considerations regarding the optimal transmission of data packets.
NRL's Chesapeake Bay lasercom test facility (LCTF) offers a variety of ranges for researching free-space optical laser communication (FSO lasercom) links in a maritime environment. This paper discusses link performance over the 16 km one-way range at the LCTF. There are several methods to determine the link quality in FSO lasercom. Bit-error-rate (BER) testing and packet testing are two possible methods. Since errors generally tend to occur in bursts in FSO channels, packet testing may offer a better indication of the quality of service (QoS) rather than BER testing. Link performance measured via packet testing is being investigated in a variety of atmospheric conditions. Results of these experiments will be presented.