EXPERIMENTS IN OPTIMIZATION OF FREE SPACE OPTICAL COMMUNICATION LINKS FOR APPLICATIONS IN A MARITIME ENVIRONMENT

Abstract

The United States Navy relies heavily on radio frequency communication networks and this reliance generates two major operational limitations: bandwidth, and lack of contingency capability in the event of jamming or detection by adversaries. One possible complementary solution to current radio frequency systems is through the use of free-space optical communication links. Free-space optical communication links are inherently high-bandwidth and highly directional, making them hard to detect or jam. These links have drawbacks as well. A laser beam propagating in a maritime environment can experience significant random intensity fluctuations due to optical turbulence, which can lead to power loss at the receiver and degraded performance. Understanding the effects of the maritime environment on the propagating laser beam is critical to the improvement of laser communication in this environment. For example, the probability density function of the intensity for a given detector is vital for estimating the fade statistics of an optical signal and its effect on the bit-error rate of a communication system. Understanding the evolution and form of the probability density function as it relates to distance, turbulence level, and detector type holds great benefit for optimizing the maritime communication link in a given optical channel.

Our research focuses on how to modify the transmit characteristics of the laser beam in order to minimize the intensity fluctuations and time and depth of fades – which can be on the order of milliseconds and tens of decibels. Specifically, modifying the partial spatial coherence properties of the beam at the transmitter offers significant potential in minimizing the deep fades at the receiver.

Also, field experimentation is critical. To that end, two field tests off the Atlantic Coast and five field experiments at the United States Naval Academy were performed. Additionally, working in a controlled laboratory setting capable of simulating some of the scaled effects of the environment holds great advantages in cost, testing methods, and optimization. We built an in-laboratory hot-air turbulence emulator with a modular and extendable design, allowing us to research comparisons between the field and laboratory experiments. This has greatly enhanced our ability to project theory into practice.

Thesis Adviser: Dr. Frederic M. Davidson Thesis Readers: Dr. Frederic M. Davidson and Dr. Jacob Khurgin