Research On Fitting Working Manner And Phase Modulation Detection Method Of Doppler Lidar

Doppler Lidar is a powerful tool for velocity measurements of a moving target and wind. According to its working principle, it can be mainly divided into direct detection Doppler Lidar and coherent detection Doppler Lidar. Each of them has obvious advantages and insurmountable disadvantages as well. Coherent Doppler Lidar is difficulty to be applied in engineering practice widely, because its detective method requires a rigorous laser line width, a frequency stability, and optical alignment. Direct detection Lidar, for its edge filter transmittance characteristics restriction, is hard to reach high measuring accuracy and wide measuring range. According to this situation, this paper investigates two aspects of contents as followed:Firstly, this paper proposed a fitting type working manner for direct detection Mie Doppler wind Lidar that utilizes the Fabry-Perot interferometer double-edge technique, and theoretical research will be done at the same time. Based on the measuring error characteristics of the Doppler wind Lidar, the fitting type working manner, by changing the working point of outgoing laser on Fabry-Perot transmission curve for the requirement of real wind measurement, always uses the maximum accuracy curve party to measure the wind for increasing the measuring accuracy and range.Theoretically, it has been proved that normal Fabry-Perot double-edge Mie direct detection Doppler wind Lidar employing the fitting working manner, can improve the measuring accuracy as high as possible with no need to consider wind measuring range restrict when designing the system parameters, and can adjust its own state to suite the wind requirement for the best measuring result.It has been proved theoretically that the special angle tuned Fabry-Perot double-edge Mie direct detection Doppler wind Lidar designed with fitting working manner, comparing with the normal Fabry-Perot double-edge Mie direct detection Doppler wind Lidar, has the same minimum measuring error, but its measuring error range is5.76times less than the latter.Secondly, this paper proposed a new kind of Doppler shift detection method. It uses the phase modulator to make the signal light generate the first order sidebands from the original frequency component (carrier wave), then utilizes the Fabry-Perot interferometer to tune the amplitude and phase of them for generating the beat frequency signal with fixed frequency. This beat frequency signal is used to measure Doppler frequency shift. The beat frequency signal is generated from the different signal light frequency components, which undergo the same space-time and will not reduce coherence due to increase of detection range. No needing a tall order for optical structure and light source, extramly high beat frequency efficiency can be obtained spontaneously. This fundamentally solves the problems of coherent detection method.Theoretically and experimentally, it can be proved that the beat frequency signal amplitude and phase of phase modulation signal light can be used to measure the Doppler shift; measuring range of the Doppler shift measuring method using the beat frequency signal phase is twice as that of the Fabry-Perot single edge technique, and no needs to detect the energy of signal light.Experimentally, utilizing the virtual digital phase-locked amplifier based on LabView visual programming languages and Data Acquisition Card to extract the phase and amplitude of beat frequency signal of phase modulation has been realized; it has been proved that the Doppler shift measuring method of the beat frequency signal amplitude has a higher ability to resist external electromagnetic interference than the normal edge technique Doppler shift measuring method, the measuring accuracy of it is10times higher than that of normal edge technique under the condition of high external electromagnetic interference.In theory, Measurement error formula of phase modulation beat frequency signal Doppler shift measuring methods have been derived out; the simulation results of them fit the experimental results very well; by using these formula, the best phase modulation frequency and depth for this kind Doppler shift measuring method are gotten.