Brian M. Concannon

Amplitude-modulated laser imager

Laser systems have been developed to image underwater objects. However, the performance of these systems can be severely degraded in turbid water. We have developed a technique using modulated light to improve underwater detection and imaging. A program, Modulated Vision System (MVS), which is based on a new theoretical approach, has been developed to predict modulated laser imaging performance. Experiments have been conducted in a controlled laboratory environment to test the accuracy of the theory as a function of system and environmental parameters. Results show a strong correlation between experiment and theory and indicate that the MVS program can be used to predict future system performance.

The latmix summer campaign: Submesoscale stirring in the upper ocean

AbstractLateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast diff...

Modulated laser line scanner for enhanced underwater imaging

Current Laser Line Scanner (LLS) sensor performance is limited in turbid water and in bright solar background conditions. In turbid water, backscattered and small angle forward scattered light reaching the receiver decreases underwater target contrast and resolution. Scattered solar energy reaching the detector also decreases detection sensitivity by increasing receiver noise. Thus, a technique which rejects unwanted, scattered light while retaining image-bearing photons is needed to improve underwater object detection and identification. The approach which we are investigating is the application of radar modulation and detection techniques to the LLS. This configuration will enable us to use optical modulation to discriminate against scattered light. A nonscanning mock-up of an existing LLS, the Electro-Optic Identification sensor, has been developed with off-the-shelf components. An electro-optic modulator will be added to this system to create a modulated LLS prototype. Laboratory tank experiments will be conducted to evaluate the performance of the modulated LLS as a function of water clarity and solar background levels. The new system will be compared to its unmodulated counterpart in terms of target contrast.

Ocean water clarity measurement using shipboard lidar systems

Experiments with two laser radar systems were conducted off the coast of Key West Florida in May of 2001. The purpose of the test was to observe the effect of the water optical properties on the Lidar return signal decay rate and compare the performance of the two systems. The first lidar system, the Shipborad K-meter Survey System (KSS) was configured to transmit linearly polarized light and to receive backscattered light in both channels. The second system, the Airborne KSS, is designed to conduct global surveys from patrolling P3-C aircraft. For this test the Airborne KSS was specially configured to operate from the deck of a ship and both systems were operated in conjunction with each other. The shipboard KSS was configured with a remotely controlled mechanical iris in both receiver channels to allow the use of different fields of view in each channel. Several oceanographic in-situ instruments were used to measure such water properties as optical transmission and absorption, backscatter coefficient, diffuse attenuation , temperature, and salinity as functions of depth. This in-situ dat was then compared with the lidar measurements.

Nanosecond gated optical sensors for ocean optic applications

Nanosecond gated photosensitive modules with wide dynamic range has been design for ocean optic applications (1) on base R7400U Hamamatsu (5mm. active area) photomultiplier tube. The photomultiplier tubes R7400U series have two kinds of photocathode: low resistance semitransparent multialkali photocathodes that can be gated with nanosecond response and semitransparent bialkali photocathodes with large resistance and slow gate response time (2). Different kinds of photocathodes require different approach to gating circuits design. High-speed pulse gating (gating rise time 10 nsec, setting time 40 nsec for 99%) has been used for enhancing of target contrast at ocean optic application for both kinds: semitransparent bialkali and semitransparent multialkali photocathodes. Wide dynamic range (50 dB of optical power) has been achieved by optimizing of applied to dynodes voltages. Compression up to 30 dB has been used for following output signal digital processing.

Observations and numerical simulations of large-eddy circulation in the ocean surface mixed layer

Two near-surface dye releases were mapped on scales of minutes to hours temporally, meters to order 1 km horizontally, and 1–20 m vertically using a scanning, depth-resolving airborne lidar. In both cases, dye evolved into a series of rolls with their major axes approximately aligned with the wind and/or near-surface current. In both cases, roll spacing was also of order 5–10 times the mixed layer depth, considerably larger than the 1–2 aspect ratio expected for Langmuir cells. Numerical large-eddy simulations under similar forcing showed similar features, even without Stokes drift forcing. In one case, inertial shear driven by light winds induced large aspect ratio large-eddy circulation. In the second, a preexisting lateral mixed layer density gradient provided the dominant forcing. In both cases, the growth of the large-eddy structures and the strength of the resulting dispersion were highly dependent on the type of forcing.

Nanosecond gated PMT for LIDAR-RADAR applications

Wide dynamic range gating photosensor modules has been design for LIDAR-RADAR applications on base R7400U (active area 8 mm. diameter) R7600U (active area 18x18 mm.) Hamamatsu photomultiplier tubes. The photomultiplier tubes R7400U, series have two kinds of photocathode: low resistance semitransparent multialkali photocathodes and semitransparent bialkali photocathodes with large resistance. Different kinds of photocathodes require different approach to gating circuits design. High-speed pulse gating (gating rise time 10 nsec, setting time 40 nsec for 99%) has been used for enhancing of target contrast at ocean optic application for both kinds: semitransparent bialkali and semitransparent multialkali photocathodes. Wide dynamic range (50 dB of optical power) has been achieved by optimizing of applied to dynodes voltages. Compression up to 30 dB has been used for following output signal digital processing. Hamamatsu photosensitive modules were used in the two system receivers in pulsed LIDAR system. The system was mounted on the bow of the R/V New Horizon and collected data from August 25 thru September 8, 2005 as part of the LOCO field test in Monterey Bay. Approximately 4 million LIDAR profiles were collected during this period. During the field test the profiles were processed to show relative changes in water optical properties and to reveal water column structure in real time.

Adjustable dynamic signal compression by photomultiplier space charge control

In pulsed blue-green laser radar applications, the signal return amplitude can have a dynamic range of 9 orders of magnitude in 200 ns. Signal compression is required to match this range to the input capabilities of the digitizing or processing devices, typically 2 or 3 orders of magnitude. The compression of the dynamic range of the signal by photomultiplier space charge control (P5CC) results in an adjustable compression range, variable gain, bandwidth in excess of 100 MHz, and extended input dynamic range, with no loss of photomultiplier sensitivity (i.e. photon counting operations are still achievable).