Wade T. Freeman

Free-space optical communication link across 16 kilometers over the Chesapeake Bay to a modulated retroreflector array

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 Laboratorys 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 cats-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.

Modulating Retro-Reflector Lasercom Systems for Small Unmanned Vehicles

Lasercom, also known as free space optical (FSO) communication, has enjoyed a renewal of interest driven by increasing data rate requirements and the crowding of the RF spectrum, affecting both commercial and military sectors. Military communications must also deal with intentional or unintentional jamming, as well as frequency allocation restrictions, neither of which affects lasercom. The U.S. Naval Research Laboratory (NRL) has been conducting research on lasercom since 1998 with an emphasis on tactical applications. NRLs lasercom research has covered propagation studies in the maritime domain, component development, and systems demonstrations. NRL has developed both conventional lasercom systems and retro-reflecting systems for small platforms. This paper reviews some of the retro-reflecting work, discusses applications of lasercom in the areas of explosive ordnance disposal (EOD) unmanned ground vehicles (UGVs), and unmanned aerial vehicles (UAVs), and describes future directions.

Free space optical communications research at the U.S. Naval Research Laboratory

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. NRLs 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 FSO communications.

Interference effects and aperture averaging in retroreflected light

An experimental study has been made on the contribution to the effective scintillation index due to aperture averaging and interference effects when using multiple retroreflectors in free-space optical links. These studies are of relevance to asymmetric data links where modulating retroreflectors are used at the remote end of the free-space optical link. 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, and fade rates are all affected by the interference. Experiments were conducted over both a short-range (500 to 800 m) and a long-range (16 km) link. The long-range link used three retroreflectors having a spacing that was large enough that interference effects contributed minimally to the observed variance. However, the variance clearly depended on the number of retroreflectors. In addition to measuring the scintillation index, images of the returned light were recorded using a fast framing camera. These experiments were conducted over land and mixed water-land terrains in the case of the short-range links, and exclusively over water in the case of the 16-km link from Chesapeake Beach to Tilghman Island.

Free-space optical data link to a small robot using modulating retroreflectors.

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.

Large area adaptive avalanche photodetector arrays for free-space optical communication

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.

InGaAs avalanche photodiode arrays for simultaneous communications and tracking

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.

Modulating Retro-Reflector Devices and Current Link Performance at the Naval Research Laboratory

Free-space optical communication is often desired between two nodes having different capabilities. Using a modulating retroreflector (MRR) shifts most of the power, weight, and pointing requirements to one end of the link, allowing the other end to be extremely small, low-power, and requiring only rough pointing (to within +/-15 degrees). For a 5 Mb/s link at 2 km, our entire MRR package including drive electronics weighs only 8.5 g and requires 60 mW. This same device has also been arrayed to further relax the pointing requirements. In the simplest MRR link, an unmodulated (CW) laser interrogates a MRR comprised of an absorptive modulator and a retroreflector. If the interrogation beam is within the retroreflectors field of view (FOV), the beam will return to the interrogator with data impressed on it. In this discussion, we present the range of MRRs developed since 1998 at the Naval Research Laboratory (NRL). NRLs MRRs include both corner cube retroreflector and cats eye retroreflector types, both individually and in arrays. Each variation has unique characteristics which may be beneficial or disqualifying in different situations. Size, weight, FOV, power consumption, cost, speed × range product, modulator type, ruggedness, time-to-market, and requirement for angle division multiplexing are all determining factors in MRR design and retroreflector choice. We compare strengths, weaknesses, and give current link performance data for several different systems. Links demonstrated include shore-to-shore, boat-to-shore, and UAV-to-ground over a wide range of distances and data rates.

Development of a large area InGaAs APD receiver based on an impact ionization engineered detector for free-space lasercomm applications

The U.S. Naval Research Laboratory (NRL) is developing a small size, weight and power (SWaP) free space lasercomm terminal for small unmanned airborne platforms. The terminal is based on a small gimbal developed by CloudCap Technology. A receiver with a large field of view and with sensitivity sufficient to meet the program range goals is required for this terminal. An InGaAs Avalanche Photodiode (APD) with internal structures engineered to reduce excess noise and keff in high gain applications was selected as the detector. The detector is a 350 micron diameter impact ionization engineered (I2E) APD developed by Optogration, Inc. Results of development and characterization of the receiver will be presented.

Characterization of InGaAs avalanche photodiode arrays with varying geometries for free-space optical communication

Photodiode arrays are instrumental in providing pointing and tracking information for free space optical communication systems. Recent advances in the fabrication and development of low noise, high bandwidth avalanche photodiode (APD) arrays have enabled these devices to be used not only as position sensitive detectors (PSD) for tracking but also as communications receivers. In a collaborative effort with Optogration, Inc., the U.S. Naval Research Laboratory has developed avalanche photodiode arrays with three different geometries: a 3x3 square pixel array, a centered hexagonal pixel array, and a 5 pixel concentric array configuration with a center pixel and four periphery pixels. The characterization and performance of each array geometry will be described along with associated front-end and digital electronics. Design tradeoffs for maximizing the performance of a given array geometry will also be discussed.