Light detectors perform the opposite function of light emitters. Emitters, as we already know, are electro-optic devices. They convert electrical pulses into light pulses. Detectors are opto-electric devices. They convert the optical signals back into electrical impulses that are used by the receiving end of the fiber optic data, video, or audio link. The most common detector is the semiconductor photodiode, which produces current in response to incident light.

In an LED, the energy emitted during the recombination of electron-hole pairs is in the form of light. In a photodiode, the opposite phenomenon occurs. Light striking the photodiode creates a current in the external circuit. Absorbed photons excite the electrons and the result is the creation of an electron-hole pair. For each electron-hole pair created, an electron is set flowing as current in the external circuit. Like light emitters, detectors operate based on the principle of the p-n junction. An incident photon striking the diode gives an electron in the valence band sufficient energy to move to the conduction band, creating a free electron and a hole. If the creation of these carriers occurs in a depleted region, the carriers will quickly separate and create a current. As they reach the edge of the depleted area, the electrical forces diminish and current ceases. While the p-n diodes are insufficient detectors for fiber optic systems, both PIN photodiodes and avalanche photodiodes (APD's) are designed to compensate for the drawbacks of the p-n diode.