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.

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

Mobile free-space laser communications is the next frontier for net-centric connectivity, as bandwidth, spectrum and security issues drive its adoption as an adjunct to RF communications. However, key technology challenges must be addressed and mission requirements understood for it to emerge as a meaningful capability for dynamic tactical communications environments. We present a brief overview of the current military communication infrastructure and future needs, before highlighting terrestrial Lasercomm technology status and trends, and lessons learned from several experiments and field tests. Optical link service parameters and metrics are assessed in the context of available architecture and techniques, while the effectiveness of various technologies to mitigate atmospheric turbulence, poor weather and pointing/tracking inaccuracies is analyzed for improved reliability. Finally, the ramifications of cost and SWaP (size, weight and power), critical to tactical Lasercomm deployment, are considered in narrowing down these technology options.

Requirements and challenges for tactical free-space Lasercomm

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 FSO. The U.S. Naval Research Laboratory (NRL) has been conducting research on FSO communications since 1998 with an emphasis on tactical applications. NRL's FSO research has covered propagation studies in the maritime domain, component development, and systems demonstrations. NRL has developed both conventional laser communications systems and retro-reflecting systems for small platforms. This paper reviews some of the retro-reflecting work, discusses applications of FSO in the areas of explosive ordnance disposal (EOD) and unmanned aerial vehicles (UAVs), and describes future directions.

Modulating retro-reflector lasercom systems at the Naval Research Laboratory

An optical system for remotely optical communications at a high data rate between a base station and a remote station under atmospheric turbulence conditions is disclosed. The remote station includes an entirely different type of retroreflector that does not use the conventional type of retroreflection, but instead consists of two sets of lenslets coupled with single-mode fiber array, called fiber “retro”. Amplified retromodulation is achieved requiring only one single optical amplifier and one single modulator. A transmitter located at the base station sends an interrogating optical beam to the fiber “retro” which modulates the optical beam according to the input signal/data, and redirects the modulated optical beam to the base station for detection by a receiver. The present invention includes the capabilities of providing Identification of Friend-or-Foe (IFF), secure communication, and a means of achieving a wide field-of-view (FOV) with a fiber-coupled lenselet array.

Wide field-of-view amplified fiber-retro for secure high data rate communications and remote data transfer

In this letter, we present the design and implementation of a pixelated electro-absorption modulator-based modulating retroreflector (MRR) for high-speed optical wireless communications. The modulator is based on a multiple quantum well structure embedded in an asymmetric Fabry–Perot cavity. This MRR was used in an outdoor link, operating at 150 Mb/s with a bit error rate (BER) of $1.22\times 10^{-6}$ at a range of 200 m. The system was also tested in laboratory-controlled conditions achieving a data rate of 200 Mb/s with a BER of $2\times 10^{-4}$ . To the best of our knowledge, this is the fastest retroreflective free-space optics demonstration in both the indoor and outdoor environments.

/pdf/high-speed-electro-absorption-modulator-for-long-range-2y7l0t5dyu.pdf

High Speed Electro-Absorption Modulator for Long Range Retroreflective Free Space Optics

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 cat's-eye retroreflector with the MQW in the focal plane of the cat's-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.

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

Optical propagation in a marine environment is important to understand for many applications. In this work we compare measurements of the distribution function to simulation. Measurements were conducted on the Naval Research Laboratory’s Chesapeake Bay Lasercom Testbed. The experimental data is compared to wave optics simulations, which also produces a distribution function. The dependence of the distribution function on turbulence and aperture size is studied. 

Scintillation distribution functions in a marine environment

Free space optical propagation affects many application areas, including communication, LIDAR, and directed energy. Optical turbulence causes dynamic changes in the propagated beam that can impact these applications. A primary measure of these effects is the optical scintillation index. In this work, we report on a comparison of experimental measurements of optical scintillation, conducted on a 16 km range across the Chesapeake Bay over a period of three months, to model predictions. Models for turbulence parameters were based on NAVSLaM, a Monin Obhukov similarity theory, and used environmental measurements taken on the range simultaneously with the scintillation measurements. These parameters were then used in two different classes of optical scintillation models: Extended Rytov theory, and wave optic simulation. We show that wave optics simulation matched the data much better than Extended Rytov theory, and that prediction of scintillation using environmental parameters is possible. In addition, we show that optical scintillation over water has different characteristics in stable versus unstable conditions.

Optical scintillation in a maritime environment.

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

Scintillation distribution functions in a marine environment

Optical scintillation in a maritime environment.