State-of-the-art tools for next-generation underwater optical imaging systems 1 Linda Mullen a, Shawn O’Connor a, Brandon Cochenour a, Fraser Dalgleish b a Naval Air Systems Command, NAVAIR, Electro-Optics and Special Mission Sensors Division, Cedar Point Road, Patuxent River, MD 20670; b Harbor Branch Oceanographic Institute, Ocean Visibility and Optics Lab, Fort Pierce, FL For Official Use Only - Unclassified

Application: Underwater optical imaging Paper Nonscattered Forward-scattered Backscattered Receiver d TR d TO = d RO >> d TR Laser Above-water Underwater BackscatterForward-scatter = + Amplitude time Amplitude time Amplitude time Bandpass Filtered *Need a way to discriminate against backscattered and forward-scattered light*

New approach – modulated pulse Paper U. S. Patent No. 5,822,047, 13 October, Macro pulse (pulse envelope): –Coarse target ranging –Enables receiver to be operated in gated mode Micro pulses (modulation): –Backscatter/forward scatter suppression –Fine target ranging Nonscattered Forward-scattered Backscattered Receiver d TR d TO = d RO >> d TR Laser

Discussion – frequency spectrum Paper frequency 100MHz 1000MHz Backscatter Forward scatter Solar ambient Shot noise By modulating light and AC coupling receiver : Shift operating point away from these low frequency signals Optimize A/D dynamic range However, optical receiver still ‘sees’ these signals – consumes optical dynamic range, generates shot noise Modulated pulse return Non- modulated pulse return For DC-coupled receiver – low frequency signals can limit performance

State-of-the-art tools Paper Modulated Pulse Source: Navy SBIR Topic N – Modulated Pulsed Laser Sources for Imaging Lidars –3 Phase II projects –Fibertek, SA Photonics – MOPA configuration (low energy, high rep rate) –AdvR – electro-optic beam deflector modulator (high energy, low rep rate applications) –SPAWAR Phase II.5 for Fibertek source for dual use (comms/imaging) Modulated Pulse Receiver: Navy SBIR Topic N – Hybrid lidar-radar receiver for underwater imaging applications –2 Phase II projects – SA Photonics and ATG –High speed, high sensitivity, large area photodetector and radar processing Modulated Pulse performance prediction model – developed by Dr. Eleonora Zege (National Academy of Sciences, Belarus)

Modulated pulse receiver Paper Objective of SBIR Phase II: develop photoreceivers with high speed (GHz), large aperture (>1cm), and high sensitivity (gain >1000) that can also be gated quickly (ns) – Photonis 5 stage Photomultiplier tube (PMT): –Bandwidth: 1GHz –Aperture: 12mm diameter –Gain: –Gate rise time: 50ns –Gate extinction: 10dB – Average photocurrent: <100  a –Photek MCPPMT: –Bandwidth: 6GHz – Aperture: 10mm diameter – Gain: 5000 – Gate rise time: 20ns – Gate extinction: >15dB – Average photocurrent: <1  a

Performance prediction modeling - underwater Paper

8 Performance prediction modeling – above water

Laboratory experiments (April 2011) Paper Translation stage White/black target Fibertek 532nm Modulated Pulse Laser High speed Digital Scope Modulation depth of target return Filtered return (LPF, BPF) Target contrast (black/white) Processing Photonis PMT iris 1.2m 19ns f = 521MHz

Experimental results Paper Clean water No backscatter Target return modulation depth 100% Turbid water (scattering agents added ~ 5 AL) Large backscatter return But – backscatter is not modulated Target return modulation depth reduced due to forward scattering But - cross-correlation peak still at target location Backscatter

Experiment vs. model (turbid water) Paper E E E E E E time, t (ns) Power (W). Experiment Model

Coded pulse experimental results Paper Clean water Turbid water (scattering agents added ~ 5 AL)

Modulated pulse imaging – new Phase II hardware (Feb 2013) Paper White/black target Fibertek 532nm Modulated Pulse Laser Photonis PMT 1.2m 19ns f = 521MHz Polygon scanner 6deg FOV High-speed Digitizer/ Coherent RF Processor Diplexer HPLP Macro pulse sync/ Modulation clock Gate trigger Polygon Scanner PC Facet edge trigger LPF (U.S. Patent 11/857,039 Developed under ONR funding to Harbor Branch)

Preliminary results Paper WITH GATE cd=0.56 cd=3.78 cd=5.18 NO GATE cd=5.18 cd=3.78 cd=0.56

Preliminary results - cd = 0.56 Paper DC RF NO GATE DC RF WITH GATE

Preliminary results – cd = 3.78 Paper NO GATE DC RF WITH GATE DC

Preliminary results – cd = 5.18 Paper NO GATE DC RF WITH GATE DC RF

Preliminary results – cd = 6.09 Paper WITH GATE RF DC RF DC NO GATE

Comparison – NO GATE Paper cd=0.56 cd=3.78 cd=5.18 cd=6.09 DC RF

Comparison – WITH GATE Paper cd=0.56 cd=3.78 cd=5.18 cd=6.09 RF cd=0.56 cd=3.78 cd=5.18 cd=6.09 DC

Conclusions Paper Bench-top modulated-pulse gated laser line scan imager was developed hardware delivered from Navy SBIR projects system was tested in a controlled laboratory environment to evaluate the benefits of the modulated pulse technique. Results show that although gating out the backscatter enhances the contrast of the ‘DC’ image, the contrast never exceeds that of the ‘RF’ image. This suggests that the high frequency modulation helps suppress the contribution from both backscatter and forward- scattered light. Future work will focus on quantifying the differences between the ‘DC’ and ‘RF’ images and generating additional data for different targets and system geometries.

Any questions? Paper