Research On High-power Laser Modulation System And Technology For Fm/cw Imaging Lidar

Frequency modulation/continuous wave (FM/cw) nonscanning laser radar is a 4D imaging sensor with the characteristics of miniaturization, all-solid-state, high frame-rate and high ranging accuracy. It has significant practical application in the commercial and military field. At present, the system has been established by the foreign researchers and its function has been implemented, while native research is still sticking on the initial stage. The key element for the lidar system is the broadband chirped amplitude modulation technique of high-power laser transmitter. However, the current technique has limited the detecting ability of lidar system to a certain extent. Thus it needs to be improved.Focusing on the above issues, the dissertation has theoretically and experi-mentally explored and researched on the broadband chirped amplitude modulation system of 1.5μm FM/cw scannerless imaging lidar transmitter. A new modulation scheme of high-power lidar transmitter with linearity, broadband and high modulation depth was proposed based on the electro-absorption modulated laser (EML) technique. The modulation system has been developed and experimental validated. Based on the high integrating characteristic of EML, an achievable scheme of high-power and miniaturized EA modulated array lasers with non-coherent and uniform illumination was also put forward. These researches have significant application in the long range FM/cw imaging lidar, and provide with an improvement in the miniaturization and integration of the imaging lidar system.Firstly, the dissertation analyzed the state of arts and the facing difficulties of the FM/cw scannerless imaging lidar, and a new method of high-power laser broadband modulation was proposed. Secondly, intermediate frequency (IF), target range, range resolution and ranging accuracy of the FM/cw scannerless imaging lidar were investigated. Besides, the laser modulation factor influencing these parameters was also analyzed. Aim at the requirement for high-power broadband modulation, a study was done on the key performances and problems in main laser modulation techniquesIn order to improve the laser power level and modulation depth of the existing schemes, a broadband modulation scheme for the high-power FM/cw lidar was proposed. Based on the EML technique, the scheme is proposed to achieve high-power output through multilevel optical fiber amplification after the linear and high-depth modulation of low-power seed laser. The various subsystem schemes were presented and their performances were studied. The tunable linear frequency modulation (LFM) generator with a bandwidth of 600MHz was designed based on direct digital synthesis (DDS) of AD9910. We theoretically analyzed the performances of this LFM generator and its influence on the ranging accuracy of FM/cw lidar combing with the experiment results. And the performances of EML and Mach-Zehnder:LiNbO₃ (M-Z:LN) modulated seed laser were explored, respectively. The multilevel structure and properties of fiber amplifier were also discussed, and then the output performances of the double-level amplifier were experimentally tested.Utilizing the presented modulation scheme in the dissertation, experimental and theoretical study was done on the main performances of high-power, broadband and linearly laser modulation based on the EML technique. A linear modulation scheme was set up to address the inherent nonlinear of EA modulator. In order to improve linear optical modulation depth (OMD), a new method based on the extension of linear modulation region was proposed. We proposed and established the performance evaluation systems based on evaluating the acquisition and processing of modulated laser to make up the current evaluation criteria gaps of high-power modulation performance in the FM/cw lidar. Spectral analysis and the evaluation criteria were applied to analyze the power spectral density, time-frequency relation, signal to noise ratio, modulation correlation coefficient, linear OMD, harmonic noise components and modulation flatness in band for the performances of high-power laser modulation system. Besides, modulated high-power laser source was established based on the M-Z:LN modualtion seed laser, and comparison between EML and M-Z: LN was done.Finally, as a prospect of the presented research, a conception along with its implement scheme of miniaturized and integrated high-power EA modulated lasers was,along with brought forward for the future miniaturized lidar. In order to get better nonscanning imaging, several considerations were put forward, which could make the array lasers achieve an incoherent laser with the far-field uniform illumination. Combining this array lasers with focal plane detectors array is hopeful to realize a comparatively ideal scannerless imaging in the future.The research could provide a broadband, high-power amplitude modulated laser source for FM/cw imaging lidar transmitter, and the obtained theoretical and experimental results could promote the research on nonscanning lidar and space optical communication.