Experimental Studies On 1.55μm Optical Heterodyne For Coherent Doppler Wind Lidar

Coherent Doppler wind lidar is currently important instrument for wind field measurement which is essential to weather forecast, aeronautic security, national defence. It is capable of obtaining high spatio-temporal resolution, high signal-noise ratio near ground wind field in clear atmosphere. High resolution optic collimation is requested for the Coherent Doppler wind lidar, especially for short wave Coherent Doppler wind lidar. So the method to improve heterodyne efficiency and signal-noise ratio is still requested.This paper deals with the two aspects reasons which affect optic heterodyne efficiency. One factor is the phase matching which means the phase matching between signal beam and the local oscillate beam. Mathematics model has been given for each factor, and the calculations of heterodyne efficiency show the results as following:The heterodyne efficiency will be reduced as the angle between signal beam and the local oscillate beam increases.When the wave vectors angle is 52.3 mrad, the heterodyne efficiency has been change to 0.2.it will also reduced as parallel offset increased. There is an optimal distance between fiber collimator and detector that make an optimal area of laser facula on photo-sensitive for detector by which the heterodyne efficiency can be rised up to a maximum. The polarization mismatch will cause the heterodyne efficiency reduced with cosine function.The other factor is the power of local oscillate beam. The optimal power of local oscillate beam which make the maximizes signal-noise ratio is researched. The optimal power of local oscillate beam is researched from two kinds of photo-detector, photodiodes and photoconductors. Using the small-signal equivalent circuit, the optimal power of local oscillate beam is calculated. The two bias case for photoconductors, voltage bias case and current bias case are all calculated detailedly. For temperature is 300K,load-resistance is 75Ω,Si-PIN photodiodes, the calculated optimal power of local oscillate beam is 0.26mW.When photoconductors parallel resistance is 1MΩ, load-resistance is 1MΩ,the optimal power of local oscillate beam in voltage bias case is 2. 3mW ,in current bias case is 2.8mW.The polarization-maintenance fiber 1.55μm optical heterodyne receiver experimental systems has been designed. The system include its optical fiber passive device, sound frequency shifter and optoelectronic devices receiver module. Maintaining full use of optical fiber are intended to enhance optical alignment system performance and heterodyne efficiency, reduce wave vectors angle mismatch. parallel offset mismatch, the polarization state mismatch, and enhance laser radar system stability. Observe the stability of the 100 MHz IF heterodyne signal as experimental measurement.Finally, the experiment of optimal local oscillate power has been given, received the heterodyne signal waveform local oscillate power vibration at 0.18 mW, 0.27 mW, 0.49 mW, 0.63 mW, 0.98 mW, Experimental results fitting curve and theoretical results is agreement.