Optical fibers are wired channels that transmit optical signals.
We refer to the unnecessary electrical signals in the channel as noise. The noise in the communication system is superimposed on the signal, and there is also noise in the communication system when there is no transmission signal, and the noise always exists in the communication system. Noise can be seen as a kind of interference in the channel, also known as additive interference, because it is superimposed on the signal. Noise is harmful to signal transmission, it can distort analog signals, make errors in digital signals, and limit the transmission rate of information.
According to the classification of sources, noise can be divided into two categories: human-made noise and natural noise. Anthropogenic noise is generated by human activities, such as spark from electric drills and transient electrical switches, spark from automotive ignition systems, interference from fluorescent lamps, and electromagnetic radiation from other radio stations and appliances. Natural noise is a variety of electromagnetic radiation that exists in nature, such as lightning (light-ning), atmosphere noise, and cosmicnoise from the sun and the galaxy (galaxy). In addition, there is a very important natural noise, that is, thermal noise. Thermal noise comes from the thermal movement of electrons in all resistive components. For example, wires, resistors, and semiconductor devices all generate thermal noise. Therefore, thermal noise is everywhere and inevitably exists in all electronic devices, unless the device is at a thermodynamic temperature OK. In resistive components, free electrons are in constant motion due to their thermal energy, colliding with other particles in motion and moving randomly in a polygonal path, that is, appearing as Brownian motion. In the absence of external forces, the average current resulting from the Brownian motion of these electrons is equal to zero, but an AC current component is generated. This AC component is called thermal noise. The frequency range of thermal noise is very wide, it is evenly distributed at approximately from near zero frequency up to 102Hz.
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