The function of the photodiode in the laser head
The active region of the laser diode is in the intrinsic (I) region, and the carriers (electrons and holes) are pumped into that region from the N and P regions respectively. This photodiode converts a fraction of the backward-traveling laser light into an electrical signal, which is sent as feedback to the current regulator. This phenomenon describes how light interacts with matter, causing the transfer of energy from photons to electrons.
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