光外差检测条件1.ppt

习题 一光探测器应用于光外差检测。假设入射到光混频器上的本振光功率50毫瓦,光混频器的量子效率为0.5。入射光信号功率为1微瓦, 波长1微米。负载电阻为50欧姆。系统中频带宽155MHz。背景光功率，暗电流忽略不计,系统温度为300K。 试:(1).画出系统方框图。 (2).求输出信号功率。 (3).求该系统的信噪比。 (4).该系统是否于量子声限工作。（工程上认为如果A比B大十倍时，B 可忽略不计） 这一要求意味着信号光与本振光的波前应重合，在空间上 实现角准直。 1、Requirements: From the discussion on the mixing frequency efficiency of the heterodyne detection scheme Eq.(2.2.19), we know that: Only when the signal light field and local light field have identical polarized directions、comparable amplitudes and phases at all spots on the detector’s sensitive surface, we have optimum mixing frequency efficiency, which is called that the spatial phase matching is realized. This means that the wave fronts of the signal and local lights should be coincident to realize spatial collimation of the incident angles. 2、两光场不重合对信噪比的影响 The influence on the SNR when the two optical fields are not coincident 设：探测器光敏面为边长为d 的正方形；信号光场和本振光场 均为平面波，其中，本振光垂直入射到光敏面；二波前夹角 ， Suppose: Detector’s photosensitive surface is a square with a side length of d; both of the signal and local lights are plane waves, the local light is normally incident on the photosensitive surface of the detector; and the angle between the two wave fronts is shown as following figure 如图所示 注意到信号光同一波前上各点到达光敏面的时间不同， 设其在 x 方向的速度分量为 ，则在光敏面上不同点处形成波前的相位差，此时，信号光场可写为 Noticing that the arrival time of each spot on the same wave front of signal light is different at the photosensitive surface, suppose the x-component of its velocity is , then the phase difference between the different spots of the wave-fronts is formed on the photosensitive surface. In this case, the signal optical field can be written as (2.2.29) where (see above figure) and notice 是信号光波矢 在 x 方向 的分量。 is the x-component of signal light wave vector . 将其代入