photodetectors etc.

I can't answer it any better than wiki does, so I'll just quote the wiki article.

If very high gain is needed (10^5 to 10^6), certain APDs (single photon avalanche diodes) can be operated with a reverse voltage above the APD's breakdown voltage. In this case, the APD needs to have its signal current limited and quickly diminished. Active and passive current quenching techniques have been used for this purpose. APDs that operate in this high-gain regime are in Geiger mode. This mode is particularly useful for single photon detection provided that the dark count event rate is sufficiently low.

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Some information about current quenching is given here: (single-photon avalanche diodes) To get more details, you would probably need to do a literature search on these " active and passive current quenching techniques". But, basically, the voltage must be precisely controlled to control the gain and a current quenching must be implemented to reset the device and prevent damage. In this way you can have current pulses corresponding with single photon events and occasional shot noise events. If the photon rate is much greater than the shot noise rate, you have a useful photon counter.

Thank you, Steve.

So, there are two kinds of APDs: SAPD and APD. It looks like only APD is used in optical communication because its avalanche current is directly related to light signal intensity and further APD does not have self-sustaining avalanche breakdown. Kindly correct me if I have it wrong. Thanks.

Regards
PG

PG, you are correct. The normal APD just has an effective amplification gain. It is important to control the voltage to get the correct gain.

Hi,

Just to add a little practical experience to the mix...

In the past i have used PIN diodes to test IR remote controls. I have used them in reverse bias mode. What is nice about this is that they are very fast responding compared to the photo transistors so i was able to read the remote codes for the different remotes even though some remotes use a higher pulse frequency above 100kHz. With some care and tinkering, the codes can be read on a regular oscilloscope although it is a little tricky because some of the codes are long. Pressing each button on the remote to be tested, the codes can be viewed one by one and recorded.
Of course if you feel like programming a uC to read the codes you can do that too and in that way even construct an automatic code reading remote, but for a quick test the scope works ok.

Hi

Could you please help me with these queries? This is the copy of reference I have used. Thanks.

Regards
PG

Hi,

Try reading the following part of that text:
"Reverse voltage separates photogenerated electrons and holes and moves them toward the pn junction..."

like this intead:
"Reverse voltage separates photogenerated electrons from the holes and moves them toward the pn junction..."

In other words, the charge carriers get separated in the larger area and move away from each other, so only one type of charge carrier gets moved toward the pn junction.

I'm not an expert on this but that's the way i read it.

It depends on the mode that the diode is operated in PG.
If it is run in an avalanche mode it then requires a snubbing circuit.
This is not much different to old geiger tubes. A single photon causes the avalanche. When the current exceeds a threshold, the circuit shuts it down and resets the diode/tube.