LED : Light-emitting diode
Light sources are used to generate input signals of the optical communications systems. Optical communication systems often use semiconductor optical sources such as LEDs (light emitting diodes) and semiconductor LDs (laser diodes).
LASER : Light Amplification by Stimulated Emission and Radiation
These kinds of semiconductor optical devices offer high efficiency and reliability. Moreover, they allow an accurate selection of the wavelength range and emissive areas compatible with optical fibre core dimensions. The following table summarizes main characteristics and structures of LEDs and LDs used in optical communication systems through optical fibres.
Semiconductor optical sources |
Characteristics |
Structures |
LEDs |
LEDs used in optical communications must have a high radiance (light intensity), fast response time and high QE (quantum efficiency). |
Planar, dome, edge-emitting led or surface-emitting led. |
LDs |
LDs used in optical communications should have coherent light, narrow beam width and high output power. |
Spontaneous emission. Stimulated emission |
At the end of the optical communication systems optical sensors (detectors of light) are used in order to recover the transmitted information and convert it again into an electrical signal through the photoelectric effect. The role of a photodetector is to recover the data transmitted through the optical fibre communication system.
Photodetectors are optoelectronic devices that convert an incident radiation (light) to an electrical signal, such as voltage or current.
Light detectors or photodetectors are usually based on PDs (photodiodes), photoconductive detectors and phototransistors. Photoconductive detectors have the simplest structure of this family of light detectors and can be obtained by attaching two metal electrodes to a semiconductor material. The conductivity of the semiconductor increases when some incident photons are absorbed in the semiconductor. As result, an increase of the external current appears when a voltage bias is applied to the electrodes. Solar cells are a specific type of photodetectors used in photovoltaic solar energy generation systems, not in communication systems.
A photodiode is a semiconductor diode that functions as a photodetector. It is a p-n junction or p-i-n structure. When a photon of sufficient energy strikes the diode, it excites an electron thereby creating a mobile electron and a positively charged electron hole.
Phototransistors are BJTs (bipolar junction transistors) that operates as photodetectors and offer as well photo-current gain. These devices are semiconductor light sensors formed from a basic transistor with a transparent cover.
Semiconductor optical detectors |
Characteristics |
Examples of structures |
Photodiodes |
Based on pn junctions. |
pn or p-i-n diodes. APDs (Avalanche photodiodes). Heterojunction photodiodes. |
Schottky junction |
Junction formed by an n-type semiconductor in contact with a metal. |
Schottky contacts. |
Solar cells |
Solar cells convert the incident radiation energy into electrical energy. |
cSi ( crystalline silicon) aSi:H (amporphous silicon). HiT (heterojunction intrinsic layer thin film solar cell ). GaAs |
Phototransistors |
Light-sensitive transistors. Phototransistors amplify variations in the light striking it. |
npn BJTs pnp BJTs |
Photoconductive detectors |
Conductivity variation due to absorption of light. |
LDR (light-dependent resistor). PbS ( lead sulfide) IR ( infrared detector). Lead selenide (PbSe) IR detectors. |