Linda M. Thomas

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.

Analysis of long-term measurements of laser propagation over the Chesapeake Bay

Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were measured for the months of January through June of 2007 on a continuous basis, as conditions allowed. Both the scintillation index sigmaI(2) and the atmospheric structure constant Cn(2) are found to have a strong dependence on the air-minus-water temperature difference. There is no obvious diurnal variation of Cn(2) or of sigmaI(2) akin to the reduction in turbulence level seen in terrestrial links in the hour before sunrise and the hour after sunset. Results from the analysis of these data highlight a need for new approaches to modeling beam propagation in a maritime environment.

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.

Power spectra of a free space optical link in a maritime environment

Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were measured over a 6 month period in 2007, on a continuous basis as conditions allowed. Both the scintillation index and the atmospheric structure constant were found to have a strong dependence on the air - water temperature difference. Temporal-frequency spectra were also generated from the received intensity fluctuations. In this paper the frequency components of the laser irradiance are studied in order to ascertain the degree of correlation with prevailing meteorological conditions. In addition, the high frequency behavior of the power spectra is documented and shows an extensive range of values for the power-law scaling index.

InAlAs/InGaAs avalanche photodiode arrays for free space optical communication.

In free space optical communication, photodetectors serve not only as communications receivers but also as position sensitive detectors (PSDs) 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 communications receivers. This combined functionality allows for more flexibility and simplicity in optical system design without sacrificing the sensitivity and bandwidth performance of smaller, single-element data receivers. This work presents the development of APD arrays rated for bandwidths beyond 1 GHz with measured carrier ionization ratios of approximately 0.2 at moderate APD gains. We discuss the fabrication and characterization of three types of APD arrays along with their performance as high-speed photodetectors.

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.

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.

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.

Miniature lasercomm module for integration into a small unmanned aerial platform

The U.S. Naval Research Laboratory (NRL) is developing a small lasercomm terminal for both direct lasercomm and communication to a modulated retroreflector (MRR). A gimbal from Cloud Cap Technology serves as the terminal positioner, which is to be integrated onto a small, unmanned airborne vehicle. A lasercomm module must be developed that meets size, weight, power, and optical performance requirements. The final module incorporates the shared optics, the tracking optic and detector, and the lasercomm receiver optics and detector. This module was designed, fabricated, assembled, and integrated into the Cloud Cap T2 gimbal. After integration, the tracking signals were measured in the lab to establish the field of view and response to pointing errors.

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.