Frederick G. Walther

Experimental observations of channel reciprocity in single-mode free-space optical links

This article describes observations of near-unity signal correlations obtained during a recent series of single-mode lasercom experiments in which links were established between a small aircraft and a ground station separated by ranges up to 80km.

Architecture overview and data summary of a 5.4 km free-space laser communication experiment

MIT Lincoln Laboratory designed and built two free-space laser communications terminals, and successfully demonstrated error-free communication between two ground sites separated by 5.4 km in September, 2008. The primary goal of this work was to emulate a low elevation angle air-to-ground link capable of supporting standard OTU1 (2.667 Gb/s) data formatting with standard client interfaces. Mitigation of turbulence-induced scintillation effects was accomplished through the use of multiple small-aperture receivers and novel encoding and interleaver hardware. Data from both the field and laboratory experiments were used to assess link performance as a function of system parameters such as transmitted power, degree of spatial diversity, and interleaver span, with and without forward error correction. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Measurements of Natural Radiation Effects in a Low Noise Avalanche Photodiode

Low noise avalanche photodiodes were irradiated to determine their sensitivity to the natural space environment. Radiation effects on important APD parameters were determined by exposure to 1.5 MeV electrons from a Van de Graaff generator and to gamma rays from a Co60 source, both to a total dose of 300 krad (Si). During irradiation, the DC dark current and an associated 1/f noise were found to increase linearly with dose only if bias voltage was applied. A t-x annealing behavior was observed. Radiation damage coefficients, threshold dose rates, and threshold fluences are calculated.

Differential Phase-Shift Keying in Spatial Diversity Transmitters for Fade Mitigation

We investigate the use of differential phase-shift keying (DPSK) in multiwavelength spatial diversity transmitters for mitigation of atmospheric fading. By selecting the transmitter wavelengths to coincide with the transmission peaks of the delay-line interferometer in the receiver, the only modification required to a standard DPSK receiver is a wider optical filter. We examine the wavelength separation required to minimize penalties from beating between the wavelengths and find that a separation of twice the data rate is sufficient for a four-wavelength system with narrow filtering. We also demonstrate a reduction in scintillation loss for a four-wavelength DPSK system in a fading channel.

A process for free-space laser communications system design

We present a design methodology for free-space laser communications systems. The first phase includes a characterization through numerical simulations of the channel to evaluate the range of extinction and scintillation. The second phase is the selection of fade mitigation schemes, which would incorporate pointing, acquisition, tracking, and communication system parameters specifically tailored to the channel. Ideally, the process would include sufficient flexibility to adapt to a wide range of channel conditions. We provide an example of the successful application of this design approach to a recent set of field experiments. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Comparison of Scintillation Measurements from a 5 km Communication Link to Standard Statistical Models

As part of a free-space optical communications experiment over a 5km horizontal path, an extensive database of tilt-stabilized receiver data was collected for Cn2 n conditions ranging from benign to very strong. This paper focuses on the scintillation measurements made during those tests. Ensemble probability distributions are compiled from these results, and are subsequently compared with standard channel models such as the log-normal and gammagamma distributions. Statistical representations of temporal behavior are also developed from this database. Accurate statistical models of atmospheric channel effects have proved to be invaluable in the development of high-performance free-space transceivers.

A free-space optical terminal for fading channels

This paper describes a lasercom terminal using spatial diversity to mitigate fading caused by atmospheric scintillation. Multiple receive apertures are separated sufficiently to capture statistically independent samples of the incoming beam. The received optical signals are tracked individually, photo-detected, and summed electrically, with measured diversity gain. The terminal consists of COTS components. It was used in successful demonstrations over a 5.4km ground-ground link from June through September 2008, during which it experienced a wide temperature range. Design overview and hardware realization are presented. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Mitigation of Self-Phase Modulation in High-Peak-Power Lasercom Systems

We demonstrate a reduction in the power penalty caused by self-phase modulation in free-space lasercom systems with peak powers of 400 W by pre-compensation with a phase modulator driven sinusoidally at the slot rate.

Differential Phase-Shift Keying in Multi-Wavelength Spatial Diversity Links

We investigate the use of differential phase-shift keying in multi-wavelength spatial diversity transmitters that mitigate atmospheric fading.

Power penalty from amplified spontaneous emission in spatial diversity links

We investigate the power penalty caused by excess amplified spontaneous emission in an optically preamplified receiver for use with a multi-wavelength spatial diversity transmitter to mitigate atmospheric fading.