What is Avalanche Photo diode. Mention its applications.

An avalanche photodiode is a semiconductor-based photodetector (photodiode) which is operated with a relatively high reverse voltage (typically tens or even hundreds of volts), sometimes just below breakdown. In this regime, carriers (electrons and holes) excited by absorbed photons are strongly accelerated in the strong internal electric field, so that they can generate secondary carriers, as it also occurs in photomultipliers. The avalanche process, which may take place over a distance of only a few micrometers, for example, effectively amplifies the photocurrent by a significant factor. Therefore, avalanche photodiodes can be used for very sensitive detectors, which need less electronic signal amplification and are thus less susceptible to electronic noise.

Typical applications of avalanche photodiodes include 

• receivers in optical fiber communications, 
• range finding, imaging, 
• high-speed laser scanners, 
• laser microscopy, 
• and optical time domain reflectometry (OTDR).

Geiger Mode for Single Photon Counting :

When operated in the so-called Geiger mode with carefully designed electronics, avalanche photodiodes can be used even for single photon counting with dark count rates well below 1 kHz and with a quantum efficiency of several tens of percent, sometimes even well above 50%. The Geiger mode means that the diode is operated slightly above the breakdown threshold voltage, where a single electron–hole pair (generated by absorption of a photon or by a thermal fluctuation) can trigger a strong avalanche. In the case of such an event, an electronic quenching circuit reduces the voltage at the diode below the threshold voltage for a short time, so that the avalanche is stopped and the detector is ready for detection of further photons after some recovery time of e.g. 100 ns. That dead time constitutes a substantial limitation of this technology. It limits the count rate to the order of 10 MHz, whereas an avalanche diode in linear mode (i.e., operated with lower reverse voltage) may be operated with a bandwidth of many gigahertzes.

Photon-counting APDs are also called SPADs = single-photon avalanche diodes. They can be used in quantum optics experiments (for example, for quantum cryptography) and in some of the applications mentioned above if an extremely high sensitivity is required.