Investigation On Co₂ Laser Heterodyne Detection Characteristic Using Rectangular Detector

Laser heterodyne detection is a well accepted quantum limited detection technology, the detection power of which is 10-11W magnitude and also the quantity is some order higher than that of the direct detection. It has higher receiving sensitivity and rate resolution factor. Its better capability of detect weak signal has attracted research and application of heterodyne detection more and more abroad. Heterodyne efficiency is an important parameter to evaluate the performance of heterodyne detection system. How to enhance heterodyne efficiency, i.e. to enhance the signal to noise, is a key technology for research and attention. Heterodyne efficiency is directly related to the field distribution of the signal and the local oscillation on the active area of detector. The amplitude, phase as well as polarization can affect the heterodyne efficiency. Only when the amplitude, phase and polarization of the local oscillation match strictly with that of the signal, can high heterodyne efficiency be obtained. The shape, size and performance of detector are also important factors that affect heterodyne efficiency. Most detector active area in existence is circular or square, and there're few research reports on rectangular active area. Accordingly, based on rectangular detector existed in laboratory, this thesis preliminarily studies and analyzes heterodyne characteristic of system.According to the theory of heterodyne efficiency, based on formula of signal to noise, this thesis generalizes the deduction process of heterodyne efficiency formula. Heterodyne efficiencies of systems using circular detector and rectangular detector are theoretically compared and analyzed. Effects of signal light axis offset and tilt to heterodyne detection are analyzed. In this thesis, by employing Matlab software, numerical simulation for the heterodyne efficiency is carried out when the field distribution of local oscillation and the signal on the active area of detector follows the Airy pattern. Effects of signal light axis offset, length to wide of rectangular detector, signal light axis tilt to heterodyne efficiency are all analyzed. For different size of signal spot, heterodyne efficiencies of circular detector and rectangular detector are compared and analyzed when signal light axis is offset and tilt. Numerical simulation for the heterodyne efficiency is analyzed when the field distribution of local oscillation is Gauss pattern and the signal is Airy pattern. The effect of Gauss beam waist and imaging lens F, signal light axis offset, length to wide ratio of rectangular detector and signal light axis tilt to heterodyne efficiency have been analyzed and discussed. Heterodyne efficiencies of circular detector and rectangular detector are compared and analyzed when signal light axis is offset and tilt.According to the result of simulation and existing condition of laboratory, by employing ZEMAX software the local oscillator beam expander system and imaging lens system of heterodyne experiment needed are designed. According to Ray Fan curve, OPD Fan curve, Filed Curve\Dist curve, Spot Diagram and MTF curve, design result of Aberration is simply analyzed. Using existing rectangular detector and circular detector in laboratory, heterodyne characteristic is studied by preliminary experiment when the offset of signal light axis exists, and the result are analyzed and discussed.