Multi-beam Photon-counting Laser Imaging

Light detection and ranging(Lidar)is a remote sensing technique,which detects the distance and the other information by illuminating the target with a laser.Due to the advantages of small volume and high precision,the Lidar shows considerable significance with applications in geography,forestry,atmospheric physics and so on.The breakthrough of the measurement distance and the detecting rate is the research focus of Lidar.The measurement can be improved to the quantum limit through the techniques of single-photon detection and time-correlated single-photon counting(TCSPC),which increase the detection ability of the Lidar greatly.On the other hand,with more laser beams in use,the Lidar system can rapid scan the target,which provides an important method for increasing the detecting rate.So,we combined the two techniques of photon counting and multi-beam,and developed a multi-beam photon-counting Lidar system.Finally,long-distance high-speed three-dimension laser imaging was realized in the experiment.This paper focuses on the research of the key techniques in the photon-counting laser ranging and imaging system,such as the high performance multi-channel single-photon detector,multi-beam laser transmit-receive,and the multi-channel high precision time-of-flight(TOF)measurement.We achieved a 100-channel single-photon detector of Geiger-mode silicon avalanche photodiodes(Si APDs)through developing a multi-channel active-quench circuit based on a Field-Programmable Gate Array(FPGA)board,and techniques of multi-channel fiber coupling and cooling.The average detection efficiency of the 100-channel single-photon detector was higher than 30%at 532 nm,and its average dark counts was less than 3000 counts per second(cps).It was one of the largest multi-channel single-photon detector with discrete APDs.By means of the diffractive optical element(DOE)beam-divider and the fiber array,the 100-beam laser transmitting and receiving was achieved,solving a problem of the high precision receiving matching alignment of a large number of beams.The coupling consistency of different channels was about 90%and the crosstalk between the adjacent fibers in the fiber array was less than 1%.Based on 7 FPGA boards,we developed the 100-channel high precision time-to-digital(TDC)measurement with a minimum time resolution of 64 ps.Taking advantage of the parallel operation of the FPGA,the TCSPC was realized on the board.The 100 channel TOF data was uploaded to the computer through the USB interface,and realized the 100-beam laser imaging.The maximum measurement distance of the system could be more than 2.5 km with an average distance precision of 26mm,while the average reflected photons number was only 0.0065 per pulse.In addition,to solve the background noise and the range ambiguity in the photon-counting laser ranging and imaging system,we developed the techniques on the chirped amplitude modulation(CAM)laser ranging and the solar-blind ultraviolet laser imaging technique.Based on sine wave and low-pass filtering detection technology,we developed 1.5-GHz gated InGaAs/InP APD single-photon detector.The dead time was decreased to 6.4 ns.Thus,the modulation bandwidth of the CAM signal was extended to 200 MHz,realizing a distance precision better than 0.12 m without ambiguity distance.A three-dimensional laser imaging system at the solar-blind ultraviolet band was achieved based on a Geiger mode Si-APD single-photon detector and a micro-pulse laser source at 266 nm.Due to the less solar radiation of the solar-blind band on the earth's surface,the problem of the background noise was solved,leading to the laser imaging at solar-blind ultraviolet band with a distance precision of several centimeters within hundreds of meters in the sunlight.The main research contents and innovations of this paper are introduced as follows:1.A 100-channel Si-APD single-photon detector was realized by active quenching from a FPGA board.And a 100-beam photon-counting laser imaging system was realized with the multi-beam transmit-receive and multi-channel TCSPC measurement.1)The 100-channel Si APD single-photon detector was realized by improving the techniques of cooling,optical fiber coupling,fiber array,and active quenching circuits triggered by FPGA.The detector was divided into several modules,including avalanche signal discrimination,active quenching,power supply,and cooling.Among the 100 channels of single-photon detector,the average detection efficiency was 36.8%and most of them were operated at low dark count noise less than 3×103 cps.The average timing jitter was about 800 ps.And there was no cross talking between the channels,as the 100 single-photon detectors were operated independently.2)At the laser transmitter,the laser beam was divided into 100 beams in a linear array by a DOE.Meanwhile,at the receiver,the echo photons were coupled into a linear fiber array consisted of 100 fibers.In this way,100-beam laser high-precision transmit-receive was realized.3)The 100-channel single-photon detector and the TCSPC module were utilized in the multi-beam photon-counting laser imaging system.The system could detect the arear in a wide swath at one time,and obtain the dense point cloud of the targets quickly with a two-dimensional scanning platform.Thus,the efficiency of the system was improved.Finally,we achieved multi-beam photon-counting laser imaging system with a distance precision of several centimeters in a range of several kilometers.2.We achieved a high-speed single-photon detection based on a 1.5-GHz sine-wave gated InGaAs/InP APD,which was applied in the CAM laser ranging.1)A InGaAs/InP APD was operated in 1.5-GHz sine-wave gated Geiger mode to realize the high speed quench and reset.The avalanche signals could be discriminate from the spike noise efficiently with low-pass fliting at suitable cutoff frequency.The 1.5 GHz sine-wave gated InGaAs/InP APD single photon detector was operate at the "quasi-continuous" mode with an effective detecting width of?200 ps and a dead time of 6.4 ns.2)In the CAM laser ranging system,the distance precision was mainly determined by the modulation bandwidth of the CAM signals.Owing to the short dead time of the single-photon detector,the modulation bandwidth of CAM signals was increased to 200 MHz with 30-dB SNR of the IF signals.And the direct distance precision was improved to 0.12 m.3.A three-dimensional laser imaging system at the solar-blind ultraviolet band was realized with a compact Si-APD single-photon detector and the time-correlated single-photon counting technique.The detection efficiency at 266 nm was?3%.Due to the strong absorption of ozone gas in the stratosphere,the solar radiation in the solar-blind band can hardly reach the Earth's surface.Therefore,the system could work in the daylight and achieve the all-day three-dimensional laser imaging.