How do photodiodes detect light in fibre-optic telecommunications networks?

In fibre-optic telecommunications networks, photodiodes function as optoelectronic receivers that convert incident optical power into electric current, effectively extracting baseband signals from modulated optical carriers.1

These semiconductor-based devices detect light by utilizing the photoelectric effect within a semiconductor material. When photons from the optical signal strike the semiconductor, they possess sufficient energy to excite electrons from the valence band to the conduction band. This process creates mobile electrons and corresponding vacancies, or "holes," in the valence band.234

Under an externally applied electric field, these charge carriers are moved through the material and collected at electrodes, where they are sensed as a photoinduced current. The two primary types of photodiodes used in these networks are:3

• PIN photodiode: Known for having a very fast response time, making it ideal for high-speed data transmission.1
• Avalanche photodiode (APD): Characterized by high gain, which allows for greater sensitivity, though it typically operates with a slower response than the PIN photodiode.1