A photodiode may be used to detect optical signals as we observe the change in current with change in light intensity when a reverse bias voltage is applied. Under a reverse biased condition, it has a very high resistance, but when light falls on it, the resistance becomes negligible. Therefore, a larger change in the resistance is easier to see why it is held in the reverse direction.
When the photodiode is reverse biased, its current is the sum of its leakage currents and photocurrent and capacitance lower than forward bias – which is convenient when measuring RF modulated light. The associated circuit serves to amplify this current. This is the photoconductive mode.
Light emitted by the transmitter exits the singlemode optical fiber at a receiver location and is coupled into the receiver module via connectors. Within the receiver module, a high speed photodiode performs optical to electrical conversion to deliver an RF electrical signal. Photodiodes are most commonly used to capture optical signals.
When properly biased, they will flow about one milliamp of current for every milliwatt of optical power that falls on the transition. They are usually in metal cans with lead and a transparent lid. Some have optical connectors for fiber optic signals, others have a lens for collecting light. The behavior of a typical photodiode is represented by its response curve. Instead of photodiodes, phototransistors can also be used.