M. R. Suite

45-Mbit/s cat’s-eye modulating retroreflectors

Modulating retroreflectors (MRRs) couple passive optical retroreflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing-acquisition-tracking system required on only one end of the link. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large-area MQW placed in front of the aperture of a corner cube. For a MQW modulator, the maximum modulation rate can range into the gigahertz, limited only by the RC time constant of the device. Most MRR systems have used corner-cube retroreflectors with apertures of about 1 cm, which require large, and hence high-capacitance, modulators. Thus data rates exceeding a few megabits per second are not possible. We describe a new kind of MQW MRR that uses a cats-eye retroreflector with the MQW in the focal plane of the cats-eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 45-Mbit/s free space link over a range of 7 km is demonstrated.

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

Free-space optical communication can allow high-bandwidth data links that are hard to detect, intercept, or jam. This makes them attractive for many applications. However, these links also require very accurate pointing, and their availability is affected by weather. These challenges have limited the deployment of free-space optical systems. The U.S. Naval Research Laboratory has, for the last 15 years, engaged in research into atmospheric propagation and photonic components with a goal of characterizing and overcoming these limitations. In addition several demonstrations of free-space optical links in real-world Navy applications have been conducted. This paper reviews this work and the principles guiding it.

45 Mbps cat's eye modulating retro-reflector link over 7 Km

Modulating retro-reflectors (MRR) allow free space optical links with no need for pointing, tracking or a laser on one end of the link. They work by coupling a passive optical retro-reflector with an optical modulator. The most common kind of MRR uses a corner cube retro-reflector. These devices must have a modulator whose active area is as large as the area of the corner cube. This limits the ability to close longer range high speed links because the large aperture need to return sufficient light implies a large modulator capacitance. To overcome this limitation we developed the concept of a cats eye MRR. Cats eye MRRs place the modulator in the focal plane of a lens system designed to passively retro-reflect light. Because the light focuses onto the modulator, a small, low capacitance, modulator can be used with a large optical aperture. However, the position of the focal spot varies with the angle of incidence so an array of modulators must be placed in the focal plane, In addition, to avoid having to drive all the modulator pixels, an angle of arrival sensor must be used. We discuss several cats eye MRR systems with near diffraction limited performance and bandwidths of 45 Mbps. We also discuss a link to a cats eye MRR over a 7 Km range.

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.

Latest results from the 32 km maritime lasercom link at the Naval Research Laboratory, Chesapeake Bay Lasercom Test Facility

The Naval Center for Space Technology at the Naval Research Laboratory reports the latest results from the long-range, maritime, free-space lasercom test facility located between Chesapeake Beach, MD and Tilghman Island, MD. The two sections of the facility are separated by 16.2 km of the Chesapeake Bay. Using a new OC-48 receiver developed by NRL’s Optical Science Division with a sensitivity of -33dBm for 10-9 bit error rate at 2.5 Gbps, we have closed a 32.4 km maritime lasercom link (round trip across the Chesapeake Bay) and performed bit error rate testing while transmitting 1.13 Terabytes of data. Bit error rate testing was also performed at lower data rates when atmospheric conditions were not favorable for high speed (2.5 Gbps), including testing at 150 Mbps through light fog and rain. In addition, we have set up a system for digitizing and transmitting full-color, uncompressed, video along with six audio channels and three RS-232 data channels over the maritime link. The digital link operated at 311 Mbps and could be maintained indefinitely, depending on atmospheric conditions. Several complete videos were transmitted in entirety or in part as well as live video from a handheld camcorder to test the system operation and robustness. The transmitter and receiver were co-located on the western shore of the bay at the NRL Chesapeake Bay Detachment. The data for both the bit error rate testing and the video was transmitted across the bay and returned from an array of retroreflectors located on a tower at Tilghman Island on the eastern shore. The lasercom links were closed with static pointing and with no active atmospheric aberration mitigation such as adaptive optics or fast steering mirrors on the receiver optics.

Large Diameter, High Speed InGaAs Receivers for Free-Space Lasercom

The U. S. Naval Research Laboratory (NRL) and OptoGration, Inc. have collaborated in the development and testing of large area, high speed InGaAs avalanche photodiode (APD) receivers for use in free-space lasercom systems. A 200 micron diameter InGaAs APD receiver has been tested in a free-space lasercom testbed and has demonstrated sensitivities of -42.4 dBm at 622 Mbps and -44.8 dBm at 155 Mbps. A 100 micron diameter receiver has been tested with a resulting sensitivity of -35.75 dBm at 2.4883 Gbps. These receivers are made possible due to OptoGrations capability to manufacture a large area, high speed InGaAs APD with an effective ionization ratio of < 0.2 and by matching the APD device with an appropriate transimpedance amplifier and limiting amplifier. Development and testing of the APD receivers will be described below.

Packet testing in free-space optical communication links over water

NRLs 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.

Optical scintillation measurements in a desert environment II: retroreflector links

The NRL Transportable Atmospheric Testing Suite (TATS) system was used to measure time resolved scintillation over a variety of different ranges at China Lake in December 2010. In this paper an analysis of scintillation effects on retro-reflector links is presented. Scintillation index, power spectral density and probability distribution functions are deduced from the measured irradiance fluctuations. Effects of aperture averaging using multiple retro-reflectors is examined as well as the contribution to the irradiance fluctuations due to enhanced backscatter.

Statistical properties of a short, analog RF free-space optical link

Free-space optical (FSO) links typically carry digital data over short to moderate distances (100m - 50 km). From a military perspective, FSO links have the advantages of low probability of interception, inherent anti-jam capability and a reasonable degree of covertness. In certain applications it is desirable to minimize size, weight and power of the transmitter and this pushes the designer to eliminate power-hungry digitizers and to transmit raw analog, instead of digital, information. However, the transmission of analog signals presents significant technical challenges due to strong fluctuations in received optical power resulting from atmospheric turbulence. In this case the standard RF link properties of gain, noise factor, and linearity must be described entirely in statistical terms. In this paper we present preliminary experimental results from a short (500m), single-tone, 5 MHz analog FSO link and compare the data to theoretical predictions. Theory suggests, and our preliminary experimental results confirm, that the statistical properties of all the RF performance metrics of a FSO analog link are determined entirely by the statistical properties of the received optical power. We show that the distribution of values of RF link parameters can be obtained directly from the distribution of received optical power, without the need for modeling the received power with a continuous function.

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.