Many of today’s fiber optic system is an electric and fiber optic combined network, usually the signal arrive and leave the fiber optic link in electrical form. At the transmitting side, transmitter convert the signal from the form of electric to fiber optical, then at the receiving side, receivers convert the signal back from optical to electrical thus closing the optical patch for information traveling along the fiber optic communications link. The receiver consist of photodiodes and electronic parts, photodiode is the heart of the receivers.

Although photodiode is heart of receiver, it is not the photodiode alone but the entire receiver acquires the information and transmits it to the customer. Several characteristics of a photodiode should be used when evaluating the quality of this device. These are responsivity, bandwidth, capacitance, dark current and reverse voltage.