Active Imaging and Remote Optical Power Beaming using Fiber Array Laser Transceivers with Adaptive Beam Shaping

Transcription

1 Active Imaging and Remote Optical Power Beaming using Fiber Array Laser Transceivers with Adaptive Beam Shaping Thomas Weyrauch, 1 Mikhail Vorontsov, 1,2 David Bricker 2, Bezhad Bordbar 1, and Yoshihiro Masui, 2 1) University of Dayton, Department of Electro-Optics and Photonics, 300 College Park, Dayton, OH ) Optonicus, 711 E. Monument Ave. Ste 101, Dayton, OH

2 Outline Active imaging Fiber array illuminator system with coherence control Image quality metric Experiments over 1.3 km and 7 km Power beaming Problem statement Beam shaping with fiber array transceivers Experiments Simulations 2

3 Mitigation of Turbulence-Induced Scintillations in Active Imaging 7-km atmospheric propagation path Target board Fiber array-based laser illuminator Side-by-side comparison of target image quality with a conventional laser illuminator using a Cassegrain telescope and a fiber-array with 21-subapertures C n 2 = m 2/3 Short-exposure image obtained with target illumination by conventional (Cassegrain telescope) beam director Short-exposure image obtained with target illumination by fiber array-based beam director [uncontrolled phase) Short-exposure image obtained with target illumination by fiber array using randomized piston phases 3

4 Image Quality Metric for Active Target Illumination Compute pixel-wise variance of irradiance (normalized by the square of the pixel-wise averaged irradiance) over a sequence of short exposure frames (variance "image"). Sum over the region of interest to get the variance metric 2 I N 1 1 ( r) 2 In( r) I ( r) N I () r n 1 1 I ( r) In( r) Long-exposure N frame In( r ) I () r 2 () r Conventional illuminator N n 1 2 I J v = 1.4 Variance image J v 1 S PDF ROI 2 2 I () r d r Fiber array with phase randomization Image quality metric (variance metric) PDF s of the normalized irradiance variance for different illuminator types Fiber array w/o phase control J v = 0.97 Fiber array without phase randomization Conventional illuminator Fiber array with randomized phase J v = I 4

5 Active Imaging with Fiber Array Illuminator over 1.3 km Experimental Setting Concrete target board with resolution chart imaging targets. Targets used are in the lower right side Fiber array transceiver with 21 transmitter channels at 20W output power each Test range at Naval Surface Activity, Crane, IN) Electro-optics testing tower: Location of active imaging system. Double pass and pupil plane imaging receivers using 9.25 Schmidt Cassegrain telescopes 5

6 Active Imaging with Fiber Array Illuminator over 1.3 km Single short-exposure (0.2 ms) images of resolution charts over 1.26 km Fiber-array w/o phase control Fiber array with phase randomization Sunlight illumination 0.3 inch per line Imaging telescope 0.4 inch per line Imaging systems' field of view for target at 1.26 km distance 0.5 inch per line Resolution limit (Rayleigh criterion): 7.2 mm = 0.28" 6

7 Analysis of Image Quality Metrics Evaluation of double pass image sequences of the 0.5 inch resolution targets Calculation of the variance metric J v Fiber array without phase control Fiber array with phase randomization Variance of intensity fluctuations for a sequence of N short-exposure frames N I ( r) 2 In( r) I ( r) N I () r n 1 PDF PDF of image irradiance variance for fiber array based illuminator with and without piston phase randomization J v = 0.42 J v = 0.11 Fiber array with phase randomization Fiber array without phase randomization Multiple frame average Multiple frame average 2 I 7

8 Intelligent Optics Laboratory, UD, R. L. Fitz Hall Active Imaging with Fiber Array Illuminator over 7 km UD Intelligent Optics Laboratory VA Medical Center (VA-MC) Double-pass imaging telescope Fiber-array transmitter Setup in the IOL outdoor support lab Pupil-plane imaging telescope Double pass imaging telescope: Imaging of the remote target Diffraction limited resolution 40 mm Target stripes ~ 50 mm Pupil-plane imaging telescope: Afocal setup with camera in image plane of pupil Both telescopes 9.25" Schmidt- Cassegrain Setup at the target site Stripe-pattern target made from retro-tape Instrument shed Targetplane camera 8

9 Active Imaging with Fiber Array Illuminator over 7 km Short-exposure double pass images Short-exposure pupil-plane speckle field irradiance Conventional laser illuminator 4 PDF 3 J v = 1.21 J v = 0.75 J v = 0.33 Fiber array FALI_ABS with phase illuminator randomization Fiber array w/o phase control Fiber array with phase randomization PDF of image irradiance variance Conventional laser illuminator 60 PDF 50 J v = 0.59 J v = 0.42 J v = 0.11 FALI_ABS illuminator Fiber array w/o phase control Fiber array with phase randomization Fiber array with phase randomization PDF of pupil-plane speckle field irradiance PDF, 2 Fiber array Fiber array w/o without phase randomization randomization PDF, Conventional illuminator Conventional laser illuminator Fiber array without Fiber array phase w/o randomization Conventional illuminator Conventional laser illuminator I 2 I (r) I 2 I (r) 9

10 Goal: Optical Power Beaming Through the Atmosphere: Objectives and Challenges Efficient power transfer from an electrical power source to a remote electrical load using laser beams Example: enabling enduring flight of an electrically powered UAV Requirements: Low losses in power conversion and delivery Focus here: Efficiency of optical power transfer from the laser source to the photovoltaic converter (PVC) Atmospheric propagation Electrical power source Laser Transmitter with beam forming and adaptive optics P V C Heat sink Challenges: Power losses from a non-optimal match between photovoltaic cell and the projected laser beam footprint Errors in laser beam pointing, turbulence-induced beam spread and wander Spatial inhomogeneity of the irradiance distribution at the PVC assembly reduces efficiency of photovoltaic power conversion Charge controller Battery DC Load It is not sufficient to concentrate the beam in the smallest possible spot at the PVC; the beam should fill the PVC area homogeneously at any perspective/attitude of the PVC and distance to the PVC Spatio-temporal fluctuations (scintillations) of the laser beam require tailored power conversion circuitry 10

11 Laser Power Converter Array & Electrical Circuit Design Laser power converter: Photovoltaic converter array optimized for single wavelength Challenges Due to scintillations laser power converter cells are illuminated unequally Voltage at maximum power output depends on cell irradiance Optimal combination of cells' electrical output Laser Power Converter Maximum Power Point Controller Voltage Converter Battery Series Configuration Parallel Configuration Increased output voltage Less internal loss Voltage multiplier may not be required to charge battery (or use more efficient voltage converter) Darkest cell limits the total output current Dark-cell bottleneck may be overcome or mitigated using special circuitry (e.g. capacitors) Not limited by least illuminated cell Low output voltage (~0.6 V) Higher internal loss (high current) Requires high-ratio voltage up conversion to charge battery (low efficiency) - 11

12 Power Beaming with Fiber Array: Concept Patent pending 12

13 37 mm Proof-of-principle Experiments Over the UD 7 km Test Range Fiber array-based laser illuminator used in the experiments Maximizing PIB signal received from retroreflectors shapes beam at the PVC/retro-reflector array To verify the adaptive beam shaping concept retro-reflector arrays were built on foam boards A fiber-array transmitter was used to project 21 beams over a 7 km path Beam footprints on foam board with and without adaptive beam shaping were recorded with a camera (a) (b) (c) 22 mm = 7 mm 13

14 Proof-of-principle Experiments: Results (a) (b) (c) Adaptive beam shaping OFF Retro-reflectors Adaptive beam shaping ON 1.5 r Airy Experiments verify beam shaping concept Distance between retro-reflectors should not exceed Airy radius r Airy = 40 mm (over 7 km) 14

15 225 mm Numerical Simulations of Beam Shaping over 7 km Adaptive beam shaping OFF Fiber array tiled aperture 230 mm C 2 n = m 2 3 r 0 = 9.38 cm C 2 n = m 2 3 r 0 = 6.19 cm C 2 n = m 2 3 r 0 = 2.36 cm Adaptive beam shaping ON Retro-reflector Pixel size: 1 mm 15

16 Laser Power Converter with Retro-reflector Array Custom laser power converter (LPC) assembly optimized for = 1064 nm Retro-reflectors were inserted into holes in an acrylic glass plate The acrylic glass plate was mounted in front of the arrays of LPC cells The acrylic glass plate reduces irradiance by < 10% (~8% due to reflections, because no anti-reflection coating was used) Retro-reflectors were located at borders between cells to avoid blocking too much light from individual cells Two different configurations were evaluated Acrylic glass plate Retro-reflectors Retro-reflectors 16

17 DAQ DAQ Optical-to-Electrical Power Conversion Experiments over 7 km U L,G1 U L,G5 R L,G1 R L,G5 Division of the 5x5 array into five groups... Measurement of short-circuit current of each group Measured average shortcircuit current Only four retroreflectors used in this experiment Short-circuit current from DAQ voltage: Isc UL / R L Beam shaping OFF Beam shaping ON 17

18 Capacitors mitigate current bottlenecks caused by weakly illuminated cells OFF Optical-to-Electrical Power Conversion Experiments over 7 km Three groups of LPC cells (G1, G3, G5) connected in parallel Received irradiance estimated with optical power meter Output power measured with source measuring unit (Keithley SourceMeter) Without capacitors Electrical power output / estimated efficiency Twelve retroreflectors used With capacitors connected 18

19 Power Conversion Experiments over 4.9 km at NSA Crane LPC powering motor with propeller Displayed numbers indicate rotational speed Video starts with SPGD phase control off (no rotation), then adaptive beam shaping with SPGD control is turned on Target site: 4.9 km Tower 19

20 Instantaneous Irradiance Distribution at Retro-reflector Array Short exposure video sequence T int = 0.75 ms, 234 frames/s Averaged irradiance (2000 frames) Adaptive beam shaping OFF Adaptive beam shaping ON 9.5 cm 9.5 cm 9.5 cm 0.75 ms, 234 frames/sec 9.5 cm 9.5 cm 9.5 cm 9.5 cm 9.5 cm 9.5 cm 9.5 cm 9.5 cm 0.75 ms, 234 frames/sec 20

21 Power Conversion Experiments over 4.9 km at NSA Crane Combination of adaptive beam shaping using SPGD phase control and coherence control through phase randomization at RF frequencies No phase control SPGD beam shaping with RF phase randomization Retro-reflectors replaced by patches of retro-tape 21

22 This research was funded by Acknowledgements Air Force Office of Scientific Research (MURI: Wave Optics of Deep Atmospheric Turbulence) Army RDECOM CERDEC Air Force Research Laboratory Colleagues from the University of Dayton and Optonicus Svetlana Lachinova Han Li Vladimir Ovchinnikov Ernst Polnau 22