【正文】 sulting the deviation from the spatial phase requirement of the heterodyne detection, and the mixing efficiency will be reduced, which will lead to the reducing of the intermediatefrequency output and the dropping of the detecting sensitivity. 理論分析結(jié)論 The conclusions of theoretical analysis： 存在大氣湍流時(shí)外差檢測系統(tǒng)的信噪比為 The SNR of heterodyne detection system with atmospheric turbulence is where: : 入射信號光強(qiáng) incident signal optical intensity : 接收孔徑 receiving aperture : 橫向相干長度 transverse coherent length : 歸一化信噪比 normalized SNR 關(guān)系曲線如下 The curve of is as follows 由圖可知 : 當(dāng) 時(shí) ,曲線近似為一直線 , 大氣湍流影響不明顯； When , the curve is approximately to a straight line, the influence of the atmospheric turbulence is not obvious。 隨 增大， 增加減慢，脫離直線關(guān)系，到 處曲 線明顯彎曲； As the increasing of the , the increase of is slowed down， deviating from the linearity, and the curve is bended obviously at ； 當(dāng) ， 趨于 1, 表明增大接收孔徑已不能改善信噪比； When the ， approaches to 1, indicating that increasing the receiving aperture will not improve the SNR again in this situation； 通常認(rèn)為 時(shí)的 D為最佳接收孔徑，此時(shí)可得最大信 噪比，此即 的物理含義。 The D in the case of is usually taken as the optimum receiving aperture, using which the maximum SNR can be obtained. This is the physical meaning of the . The formula of the is given as： () 式中： 為不完全 函數(shù)， 為頂角， 為接收高度。 where: is the inplete function, is the apical angle, is the receiving height. 上式僅適用于白天，夜晚的 值約為上式的 2倍； Above formula is only suitable for daytime, because the value of is about two times of that in Eq.(). 還和大氣的結(jié)構(gòu)常數(shù) 、傳播距離 L 有關(guān)，在水平傳輸時(shí)， 有： The is also relevant to atmospheric structural constant and propagation distance L . In cases of horizon transmission, it is given as (5。 ) 外差檢測與直接檢測性能比較 (可自己歸納 ) The Comparison of the performances between heterodyne amp。 direct detection 一、一般比較 A general parison 直接檢測 : 僅響應(yīng)入射光的輻射功率，檢測靈敏度低，裝置相 對簡單，適用于強(qiáng)光檢測。 外差檢測：響應(yīng)入射光的輻射功率、頻率、相位，既適用于強(qiáng) 光檢測，也適用于弱光檢測，輸出信噪比和靈敏度 均較高，裝置復(fù)雜。 Direct detection: only responding to the radiation power of incident light。 low sensitivity。 simple equipments。 suitable for strong light detection. Heterodyne detection: responding to the radiation power、 frequency and phase of incident light。 not only suitable for strong light, but also for weak light。 detection。 high output SNR amp。 sensitivity。 plex equipments. 二、光電轉(zhuǎn)換功率增益 Photoelectric transform power gain 直接檢測輸出信號功率依（ ）為 According to Eq.(), direct detection’s output signal power is 外差檢測輸出信號功率依（ ）為 (不考慮探測器自身增 益 ) According to Eq.(), heterodyne detection’s output signal power is (not considering detector itself gain.) 若二者輸入光功率及負(fù)載電阻均相同 , 則 外差檢測相對于直 接檢測的光電轉(zhuǎn)換增益 為 If the incident optical powers and load resistances of the two are the same, the photoelectric transform gain of heterodyne detection to direct detection is given as 外差檢測中 量級； 量級；代入上式， 知： 倍。 In heterodyne detection, order of magnitude。 order of magnitude。 substitute into above formula, we know times. 三、信噪改善比 SNIR 直接檢測系統(tǒng)信噪比依（ ）為 According to Eq.(), direct detection system’s SNR is For weak light detection: , and For strong light detection , and 則 信噪改善比 為 Then the SNIR is 最小可探測光功率為 The minimum detectable optical power is 外差檢測系統(tǒng)信噪比（僅考慮本振光霰彈噪聲） Heterodyne detection system’s SNR is (only the shot noise of the local light Is considered) () 最小可探測光功率 Minimum detectable optical power is () 為直接檢測系統(tǒng)最小可探測光功率的 1/2 。 It is a half of that of the direct detection system. 習(xí)題 ? 一光探測器應(yīng)用于光外差檢測。假設(shè)入射到光混頻器上的本振光功率 50毫瓦 ,光混頻器的量子效率為 。入射光信號功率為 1微瓦 , 波長 1微米。負(fù)載電阻為 50歐姆。系統(tǒng)中頻帶寬 155MHz。背景光功率，暗電流忽略不計(jì) ,系統(tǒng)溫度為 300K。 ? 試 :(1).畫出系統(tǒng)方框圖。 ? (2).求輸出信號功率。 ? (3).求該系統(tǒng)的信噪比。 ? (4).該系統(tǒng)是否于量子聲限工作。（工程上認(rèn)為如果 A比 B大十倍時(shí)， B 可忽略不計(jì)）