1. 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, 22347 Cedar Point Road, Patuxent River, MD 20670; b Harbor Branch Oceanographic Institute, Ocean Visibility and Optics Lab, Fort Pierce, FL 34946. For Official Use Only - Unclassified

2. Application: Underwater optical imaging Paper 8724-1 2 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*

3. New approach – modulated pulse Paper 8724-1 3 U. S. Patent No. 5,822,047, 13 October, 1998. 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

4. Discussion – frequency spectrum Paper 8724-1 4 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

5. State-of-the-art tools Paper 8724-1 5 Modulated Pulse Source: Navy SBIR Topic N07-036 – 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 N08-032 – 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)

6. Modulated pulse receiver Paper 8724-1 6 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: 10000 –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

7. Performance prediction modeling - underwater Paper 8724-1 7

8. 8 Performance prediction modeling – above water

9. Laboratory experiments (April 2011) Paper 8724-1 9 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

10. Experimental results Paper 8724-1 10 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

11. Experiment vs. model (turbid water) Paper 8724-1 11 -2.0E-10 0.0E+00 2.0E-10 4.0E-10 6.0E-10 8.0E-10 020406080100120140 time, t (ns) Power (W). Experiment Model

12. Coded pulse experimental results Paper 8724-1 12 Clean water Turbid water (scattering agents added ~ 5 AL)

13. Modulated pulse imaging – new Phase II hardware (Feb 2013) Paper 8724-1 13 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)

14. Preliminary results Paper 8724-1 14 WITH GATE cd=0.56 cd=3.78 cd=5.18 NO GATE cd=5.18 cd=3.78 cd=0.56

15. Preliminary results - cd = 0.56 Paper 8724-1 15 DC RF NO GATE DC RF WITH GATE

16. Preliminary results – cd = 3.78 Paper 8724-1 16 NO GATE DC RF WITH GATE DC

17. Preliminary results – cd = 5.18 Paper 8724-1 17 NO GATE DC RF WITH GATE DC RF

18. Preliminary results – cd = 6.09 Paper 8724-1 18 WITH GATE RF DC RF DC NO GATE

19. Comparison – NO GATE Paper 8724-1 19 cd=0.56 cd=3.78 cd=5.18 cd=6.09 DC RF

20. Comparison – WITH GATE Paper 8724-1 20 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

21. Conclusions Paper 8724-1 21 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.

22. Any questions? fdalglei@hboi.fau.edu linda.mullen@navy.mil fdalglei@hboi.fau.edu linda.mullen@navy.mil Paper 8724-1 22