Detection of Weak Optical Signals with Photodetecting Assemblies Based on Avalanche Photodiodes

Abstract

In the very beginning of the book (Sect. 1.2) we observed that the internal gain of photodetector is required to “pull” the signal and noise of photodetector itself out of the noise of amplifier. In Sect. 2.5 we calculated the optimal multiplication of avalanche photodiode for the most simple case, when the spectral density of the noise is fixed. Now we are going to consider the general case of detection of pulsed signal, when the frequency-dependent high-frequency noise component is noticeable. We shall see that there is no alternative for the avalanche photodiode in the detection of short pulsed signals in threshold systems (Sect. 6.1). In some cases, the avalanche photodiode is able to detect an optical signal so weak that one has to take into account the fluctuation of the number of quanta in this signal (Sect. 6.2). In a special mode (photon count mode) the avalanche photodiode is capable of detecting single photons, approaching the fundamental quantum detection threshold (Sect. 6.3)