The optical fiber cable includes a core, a cladding, and a protective coating on the outer layer. The core and the fiber shell with a high refractive index are usually made of high-quality silica glass, but now there are also optical fibers that use plastic as the material. Also, because the outer layer of the optical fiber is coated with an acrylate polymer cured by ultraviolet light, it can be buried underground like a copper cable without too much maintenance cost. However, if the fiber is bent excessively, there is still a risk of breaking. fiber cable，Moreover, because the connection of the two ends of the fiber requires very precise alignment, it is difficult to re-splice the broken fiber.

Multimode fiber and single-mode fiber are mainly used for optical communication. The core diameter of multi-mode fiber is larger (≥50 microns), and the requirements for transmitters and connectors are lower. However, multimode fiber introduces multimode dispersion, which limits the bandwidth and length of the system. In addition, due to the higher impurity content, multimode fiber technology usually has a higher attenuation. Single-mode fiber has a smaller core diameter (< 10 microns) and requires more transmitters and connectors, but can establish longer transmission distances and better performance. There are different grades of single-mode and multi-mode fibers.

Optical amplifier

Main article: Optical amplifier

In the past, the distance limitation of optical fiber communication was mainly caused by the attenuation and distortion of the signal in the optical fiber. The repeater converts the optical signal back to an electrical signal, Microarray，amplifies it, and then converts it into a stronger optical signal for transmission to the next repeater.

receiver

The main component of the optical receiver is the photodetector, which uses the photoelectric effect to convert the incident optical signal into an electrical signal. The photodetector is usually a semiconductor-based photo diode, such as a p-n junction diode, a p-i-n diode, or an avalanche diode. In addition, Metal-Semiconductor-Metal (MSM) photodetectors are also used in optical regenerators or wavelength division multiplexers because of their good integration with circuits.

Optical receiver circuits usually use transimpedence amplifiers (TIA) and limiting amplifiers to process the photocurrent converted by the photodetector. The transimpedence amplifiers and limiting amplifiers can convert the photocurrent into amplitude. The small voltage signal is converted into a digital signal through a comparator circuit on the back end. For high-speed optical fiber communication systems, the signal is often attenuated relatively severely. In order to prevent the digital signal output from the receiver circuit from deforming beyond the specifications, a clock and data recovery circuit (clock and data recovery circuit) is usually added to the back of the receiver circuit. Data recovery (CDR) and phase-locked loop (pLL) process the signal appropriately and then output it.

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Optical fiber communication industry application

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