| Pulse lidar system is widely used in military reconnaissance,earth sciences,traffic safety and many other practical applications due to its flexibility in both operating principle and system structure.For a pulse lidar system,the SNR is proportional to the operating range of the lidar system,and there i s a positive correlation between the SNR and ranging precision of the system.As a result,the limitation of the SNR becomes a obstacle for the further development of lidar system.Convential pulse lidar systems usually adopt electronics modulation technology to improve the SNR by modulation of the pulse signal,which has encountered the bottleneck.Photonics modulation technology is a signal modualtion technology which modulate and demodulate the laser pulse signal with photonic devices such as optical waveguide,fiber gratings,etc.Photonics modulation technology can achieve signal modulation with high speed and large modulation bandwidth,and benefits with concise structure,low consumption and tunability,which makes a strong support to the further development of pulse lidar.Aimming at the limited SNR of pulse lidar system,we introduce the photonics modulation technology to pulse lidar system to modulate and demodulate the signal before photoelectric conversion,which is supposed to break through the bottleneck of electronics modulation technology and improve the SNR and other performances of pulse lidar system.In order to enhance the SNR of pulse lidar,firstly we propose a pulse heterodyne lidar based on dual acousto-optic modulation according to photonics high frequency carrier frequency modulation technology.The SNR of the proposed modulated pulse-modulated lidar system is theoretically derived,and ayalysis of the improvement of SNR is conducted with different influence factors.Numerical simulation result indicates that the SNR of the proposed system is 6 ~12 d B higher than the conventional pulse lidar system.A test system is set up to verify the ability of the dual acousto-optic modulation module to generate radio frequency modulated pulses,and a improvement of 6.48 d B of the SNR is obtained.By combining the pulse compression lidar theory and the method of generating extreme chirp laser pulses with fiber gratings in photonics modulation technology,a full optical pulse compression lidar system is developed to improve the SNR of pulse lidar.Analysis on the operation principle of the proposed lidar is made with transmission matrix method,and the SNR and ranging resolution of the lidar are also simulated.The simulation result indicates a SNR improvement of 16.14 d B with the help of the generation and compression of a linear frequency modulated pulse with a large time-bandwidth product.A scheme of bandwidth enhanced stepped frequency pulse lidar system with heterodyne detection is proposed to improve the SNR of pulse lidar,which combines stepped frequency radar and the arbitrary waveform generator based on optical frequency comb using a dual parallel MZM in photonics modulation technology.In addition,a centroid algorithm is used in the receiver to process the signal with high precision,and the feasibility of the proposed algorithm is verified with a simulated SNR improvement of 20.74 d B as well as a theoretical ranging precision of 2.09 cm@10 m.A prototype is set up in the lab and a verification experiment is conducted with the prototype,and the obtained experimental ranging precision is 2.38 cm@10 m.Finally,we improve the design of stepped frequency lidar and propose a linear frequency modulated pulse lidar system based on a dual parallel Mach-Zehnder modulator.The proposed pulse lidar system generates linear frequency modulated signal with large time-bandwidth product by introducing the arbitrary waveform generator based on optical frequency comb using a dual parallel MZM in photonics modulation technology,which is supposed to enhance the unambiguous distance as well as to improve the SNR and ranging precision of the lidar system.Theoretical analysis on the generated linear frequency modulated broad pulse with a large time-bandwidth product is made,and a specific signal processing method is proposed according to the phase discontinuity of the generated linear frequency modulated pulses.Numerical simulation of the proposed LFM pulse lidar system is conducted and the simulation result indicates a SNR improvement of 21.2 d B and a ranging precision superior to 7.5 cm@300 m under practical conditions with physical disturbance. |
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