Experimental Research On The Enhancement Of Signal To Noise Ratio Using Coherent Doppler Lidar

Civil Aviation Organization considers that lidar to be the only efficient methods remote sensing in clear sky. Along with the development of photo-communication, the devices of 1.5μm Coherent Doppler Lidar is getting mature and easy to get, which makes the 1.5μm Coherent Doppler Lidar a research hotspot. The fibers using in the transmission medium and the laser device, can make lidar’s structure is simple, miniaturization an engineered, but the power of the 1.5μm Coherent Doppler Lidar is several orders of magnitude lower than the power of the laser devices at other wavelengths. This limits the detection range of the 1.5μm Coherent Doppler Lidar. Therefore 1.5μm Coherent Doppler Lidar improve signal to noise ratio of the detection method is studied.First of all, the research in Coherent Doppler Lidar at home and abroad is reviewed. The solution aiming at the low pulse energy of 1.5μm Coherent Doppler Lidar. The method is to improve the structure of the laser. Typically the MITSUBISHI group in Japan changed the structure of the laser amplifier into batten and increases the signal pulse energy to 10.9mJ. The method are all improve the emission signals of the single pulse energy. But solutions have limitations: give up the whole structure of optical fiber. This paper presents the phase-coherent accumulation method to increase the signal to noise ratio. In the scheme the method of improving the structure of the system.Secondly, the echo signal of the system is with random initial phase. Due to the requirement of the coherence in frequency and phase, this paper studies the scheme of coherent accumulation for the lidar echo signals. the phase detector and the phase alignment is studied in the scheme, which can eliminate the phase and achieve coherent accumulation. According to the research of the proposed scheme of simulation and experiment platform is established using MATLAB.Thirdly, the research scheme of the phase detector is proposed. In the simulation platform, the simulation analysis using the method of quadrature mixing. In the simulation platform, for several kinds of typical IIR and FIR filter is analyzed, the equivalent ripple manner low pass filter is chosen to be used in the quadrature mixing, and the parameters of the filter are set. By using the experimental, the phase detecting is realized for the real echo signal.Finally, the research scheme of the phase alignment method is proposed. This scheme includes an oversampling phase alignment method and interpolation re-discrete phase alignment method. Wherein the first way is to sample the signal at a high sampling rate, align the phase and accumulate the signal in the time domain. The second way is to sample the signal at a lower rate which can meet the sampling theorem. Interpolate the signal into a continuous signal, and then discrete the signal at a high sampling rate, and then accumulate the signal in the time domain. Complete the phase alignment method using two coherent integration for real signals using the experimental platform. The results of the improvement for the signal to noise ratio using the two methods of processing conform to the theoretical value. The influence of the improvement in SNR using the two coherent integration methods on the phase alignment is comparatively analyzed, the increase of the SNR using the interpolation and re-discrete phase align method is slightly lower than that of the oversampling phase alignment method.