Optical detectors used in fiber optic communication system

Avalanche photodiode, PIN photodiode and pn junction photodiode are the main optical detectors employed in fiber optic communication. These devices converts light signal to electrical waveforms. Pn junction photodiode consists of a p-type layer deposited on an n-type substrate. Minority carriers are responsible for photocurrent. In the reverse biased mode, larger portion of the depletion layer occupies the n-layer around the junction. The reason for this is attributed to smaller impurity concentration in the n-layer. Positive-Intrinsic-Negative (PIN) photodiode consists of an intrinsic or lightly doped n-layer between two heavily doped n and p layers. Here depletion region is wide and this enhances capture area of the photons. Avalanche photodiodes are variations of PIN photodiodes. More responsivity in avalanche photodiodes is due to avalanche multiplication effect.