| Single photon detection,a new type of lidar technology,has a sensitivity as high as single photon energy level,and has incomparable performance advantages in weak signal detection compared with the traditional lidar technology.When single photon detection is applied in underwater scenes,the backscattering noises generated by laser beams propagating in water is more serious than that in atmosphere,which lead to a sharp decline of the echo SNR received by the detector,and adversely affect the image reconstruction afterwards.Range gating technology can separate the target echo signals from the backscattering noises at the time level by applying high-frequency gating signal to the detector,so it is an effective method to improve the SNR.This thesis focuses on the requirements analysis,schemes design,simulations and experiments of the gating module for underwater range-gated single photon imaging system,and the following research results have been achieved:(1)The gating signal requirements of range-gated single-photon imaging in a variety of underwater scenes were quantitatively analyzed.Mathematical models of the distance delay and slice delay were established for the gating requirements of static and several dynamic range-gated imaging scenes,including typical scenes of the imaging system moves at different speeds and different modes in view of the target,and an appropriate simplification was made for better application considering the speed of underwater vehicle.(2)Aiming at the requirements of underwater single photon range-gated imaging under various static scenes,a dual-gear gating design scheme combining the coarse delay and fine delay was proposed.On the one hand,the high frequency clock generated by MMCM in FPGA chip was used as the counter’s clock source,and the coarse delay part was generated after setting counter’s parameters.On the other hand,IDELAYE2 primitive was utilized as the fine delay unit,and the fine delay part was generated after the number of taps was set.The combination of coarse delay and fine delay was realized by the logical operation of the designed signals,forming a configurable gating module with large dynamic range and high stepping accuracy in both delay time and gate width.Based on the design scheme,a prototype verifying the basic operating principles and an engineering prototype considering the characteristics of other equipments in the imaging system were developed successively,the timing simulations in multiple scenarios were carried out.(3)Three types of signal outputs from the gating module with engineering prototype program was fully tested,and the quality and compatibility of all signals generated were quantitatively analyzed.The hardware system of gating module was built,including the selection of FPGA chip and the design of modules of power supply,clock crystal oscillator,reset and start,status monitoring and output ports.By configuring parameters properly,the gating module produced laser trigger signal,TCSPC reference signal and the SPAD gating signal under medium-range,short-range and long-range scenarios,and the relative delay,positive and negative pulse width,high and low voltage of each signal were measured multiple times.According to the abundant data obtained,the matching degree between the actual signal outputs and the theoretical results were evaluated.(4)The single photon ranging experiment was practiced to prove the practicality and effectiveness of the designed gating module for underwater single photon detection.According to the requirements of laser devices,the external trigger mode was adapted for the gating module based on the internal trigger mode.After the gating module was added to the underwater single photon detection system,experiments were carried out on the same target scene under the conditions of applying the gating signal or not.By comparing the difference of peaks’ characteristics from echo data and analyzing the experimental results,the range gating ability of the designed gating module was effectively verified. |
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