Time-dependent underwater optical propagation measurements using modulated light fields

摘要

Optical imaging and optical communications systems are limited in the underwater environment due to optical scattering. For an imaging system, light that scatters back to the receiver before reaching an underwater object degrades image contrast. Light that scatters multiple times on its way to and from the object of interest tends to blur the image and further reduce its contrast. This forward-scattered light may also ultimately limit the bandwidth of a point-to-point optical communications link. While continuous wave sources with low frequency modulation (<100MHz) and pulsed sources with several nanosecond pulse durations have been used to mitigate the backscatter problem, little has been done to study the effect of higher modulation frequencies (>100MHz) or short (<2nsec) pulse durations on forward-scattered light. The challenge has been the lack of hardware and theoretical models required to examine events at these short time scales. Fortunately, short pulse and modulated pulse sources have now been developed, along with optical receivers with sufficient bandwidth and sensitivity to measure the response of the water to these sources. The purpose of this work is to use these tools to study the propagation of modulated light fields at frequencies up to lGHz. Results from laboratory tank experiments and their impact on future underwater optical imaging and communications systems will be discussed.