激光光源线宽对外差探测性能的影响

Effect of laser linewidth on the performance of heterodyne detection

本文根据统计理论分析了激光线宽对外差探测结果的影响,并讨论了激光线宽对杨氏干涉条纹对比度的影响.本文基于解析结果做了数值仿真,所得结果表明激光线宽增加时,外差探测方式仍可以检测到中频信号,但在线宽较大时,受噪声影响无法准确地提取到中频频率.为验证理论分析结果,使用线宽为1 MHz的激光光源进行了8.1 km外场实验,实验结果与数值分析结论一致,即不会因为线宽增加而无法检测到中频信号.文中所得结论对于外差探测光源的选择有重要指导意义,因此根据测量目标的特性和测量要求,可按照文中结论评估光源的线宽指标.

According to the statistical theory, the influence of laser linewidth on the performance of optical heterodyne system is studied. Also, the effect of laser linewidth on the visibility of fringe pattern is discussed. The expressions of selfcorrelation function power-spectrum and definition of visibility of fringe patterns are obtained in this paper. Based on the analytical expressions, the numerical simulation is performed. The obtained results demonstrate that the laser linewidth influences the visibility of fringe patterns according to the result shown in Fig. 3, and that the intermediate frequency can be still detected by heterodyne detection technique as the laser linewidth increases. For different linewidths,the measurement of intermediate frequency is accurate without the influence of noise as the delay τdbetween the received signal and locally generated signal is less than the coherence time τc of laser source. If the delay τd is greater than the coherence time τc, the full width at half maximum of intermediate frequency in the frequency spectrum of the output signal of photodetector will broaden as the laser linewidth increases. However, for a wide linewidth, the measurement of intermediate frequency is inaccurate due to the influence of noise when the delay τdis greater than the coherence timeτc. The wider the linewidth, the less accurate the measurement of intermediate frequency will be. In order to check the correctness of theoretical results, an experiment is carried out by using a laser with a linewidth of 1MHz, which has an 8.1km channel path. In our experimental set-up, a cooperative target is employed to modulate and reflect the transmitted beam. In this way, an echo signal is received. The mixing process of the received signal and local signal on the photodetector surface produces an electrical current known as the photomixing current. A spectrum analyzer is used to observe the output signal of detector. The obtained spectrum shows that intermediate frequency can be checked,which is in agreement with the theoretical result. In this work, the obtained conclusions can be directly used to choose a proper laser for optical heterodyne system. According to the target characteristics and measurement requirements, and by following the conclusions obtained in this paper, the laser linewidth can be evaluated.