The photodetector can detect the light power coming in and out of its surface, and convert the change of the light power into the corresponding current. Because the optical signal has loss and distortion in the optical fiber, the performance of the optical detector is required to be very high. The most important requirements are high sensitivity, low noise and fast response speed within the wavelength range of the light source used to adapt to speed transmission.
The light detector shall generally meet the following requirements:
(1) It has high quantum efficiency within the wavelength range required by the system;
(2) Fast response speed;
(3) It has good linear input-output property;
(4) Able to work reliably under normal conditions.
Photomultiplier tube , thermoelectric detector, semiconductor photodetector, etc. Photomultiplier tube
It consists of a photocathode and a multiplier installed in a vacuum tube. It has high gain and low noise, but it is not suitable for optical fiber communication systems because of its large size and high bias voltage.
Thermoelectric detector
Including the conversion from heat energy to light energy, the response of this detector is flat in a relatively wide spectral range, but the response speed is very slow and is not suitable for optical fiber communication systems.
Semiconductor photodetector
In semiconductor photodetectors photodiode Small size, high flexibility and fast response speed have been widely used in optical fiber communication systems. There are two common photodiodes: PIN photodiode and Avalanche photodiode (APD) 
