State-of-the-art tools for next-generation underwater optical imaging systems

1. Linda Mullena, Shawn O’Connora, Brandon Cochenoura, Fraser Dalgleishb a Naval Air Systems Command, NAVAIR, Electro-Optics and Special Mission Sensors Division, 22347 Cedar Point Road, Patuxent River, MD 20670; bHarbor Branch Oceanographic Institute, Ocean Visibility and Optics Lab, Fort Pierce, FL 34946. State-of-the-art tools for next-generation underwater optical imaging systems For Official Use Only - Unclassified

2. Application: Underwater optical imaging Underwater Above-water = + Forward-scatter Backscatter *Need a way to discriminate against backscattered and forward-scattered light* Nonscattered Forward-scattered Backscattered Laser dTR Amplitude Receiver time dTO = dRO >> dTR Bandpass Filtered Amplitude Amplitude time time Paper 8724-1

3. New approach – modulated pulse Nonscattered Forward-scattered Backscattered 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 Laser dTR Receiver dTO = dRO >> dTR U. S. Patent No. 5,822,047, 13 October, 1998. Paper 8724-1

4. Discussion – frequency spectrum Solar ambient Modulated pulse return Non- modulated pulse return Backscatter Shot noise Forward scatter frequency 1000MHz 100MHz • For DC-coupled receiver – low frequency signals can limit performance • 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 Paper 8724-1

5. State-of-the-art tools • 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) Paper 8724-1

6. Modulated pulse receiver • 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: <100ma • Photek MCPPMT: • Bandwidth: 6GHz • Aperture: 10mm diameter • Gain: 5000 • Gate rise time: 20ns • Gate extinction: >15dB • Average photocurrent: <1ma Paper 8724-1

7. Performance prediction modeling - underwater Paper 8724-1

8. Performance prediction modeling – above water Paper 8724-1

9. Laboratory experiments (April 2011) Translation stage White/black target High speed Digital Scope iris Photonis PMT 1.2m Fibertek 532nm Modulated Pulse Laser Processing 19ns Modulation depth of target return Filtered return (LPF, BPF) Target contrast (black/white) f = 521MHz Paper 8724-1

10. Experimental results Clean water Turbid water (scattering agents added ~ 5 AL) Backscatter • No backscatter • Target return modulation depth 100% • 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 Paper 8724-1

11. 8.0E-10 . 6.0E-10 Power (W) 4.0E-10 2.0E-10 0.0E+00 -2.0E-10 0 20 40 60 80 100 120 140 t time, (ns) Experiment vs. model (turbid water) Model Experiment Paper 8724-1

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

13. Modulated pulse imaging – new Phase II hardware (Feb 2013) (U.S. Patent 11/857,039 Developed under ONR funding to Harbor Branch) Polygon Scanner PC High-speed Digitizer/ Coherent RF Processor Facet edge trigger Gate trigger White/black target LP HP Macro pulse sync/ Modulation clock Photonis PMT Diplexer LPF 6deg FOV Polygon scanner 1.2m Fibertek 532nm Modulated Pulse Laser 19ns f = 521MHz Paper 8724-1

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

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

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

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

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

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

20. Comparison – WITH GATE DC RF cd=0.56 cd=0.56 cd=3.78 cd=3.78 cd=5.18 cd=5.18 cd=6.09 cd=6.09 Paper 8724-1

21. Conclusions • 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. Paper 8724-1

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