The tiny fiber is encased in a plastic jacket that allows it to bend without breaking. Typically, a transmitter at one end of an optical fiber uses a light emitting diode (LED) or a laser to transmit light pulses to the fiber, and a receiver at the other end of the fiber detects the pulses using a photosensitive element. In daily life, since the conduction loss of light in optical fibers is much lower than that of electricity in wires, optical fibers are used for long-distance information transmission. Usually the terms optical fiber and optical cable are confused. Most optical fibers must be covered with several layers of protective structures before use, and the coated cable is called optical cable. Damage, such as water, fire, electric shock, etc. Optical cable is divided into: optical fiber, buffer layer and coating. Optical fiber is similar to coaxial cable, but there is no mesh shielding layer. At the center is the glass core where light travels. In a multimode fiber, the diameter of the core is 15 μm to 50 μm, which is roughly the thickness of a human hair. The diameter of the single-mode fiber core is 8 μm~10 μm. The core is surrounded by a glass envelope with a lower refractive index than the core to keep light within the core. On the outside is a thin plastic jacket that protects the envelope. Optical fibers are usually bundled and protected by a jacket. The fiber core is usually a double-layer concentric cylinder made of quartz glass with a small cross-sectional area, which is brittle and easy to break, so an external protective layer is required. Optical fiber structure The bare fiber is generally divided into three layers: the central high-refractive-index glass core (the core diameter is generally 50 or 62.5μm), the middle is the low-refractive-index silica glass cladding (the diameter is generally 125μm), and the outermost is for strengthening Resin coating. Numerical aperture The light incident on the end face of the fiber cannot be all transmitted by the fiber, only the incident light within a certain angle range can. This angle is called the numerical aperture of the fiber. A larger numerical aperture of the optical fiber is advantageous for the butt-joining of the optical fiber. Optical fibers produced by different manufacturers have different numerical apertures (AT&T CORNING). Silica fiber Silica fiber is a fiber that uses silica (SiO2) as the main raw material and controls the refractive index distribution of the core and cladding according to different doping amounts. Quartz (glass) series of optical fibers have the characteristics of low consumption and broadband, and are now widely used in cable television and communication systems. The advantage of quartz glass optical fiber is that the loss is low. When the light wavelength is 1.0 ~ 1.7μm (about 1.4μm), the loss is only 1dB/km, and the lowest at 1.55μm, only 0.2dB/km. Fluorine Doped Fiber Fluorine Doped Fiber is one of the typical products of silica fiber. Usually, as a communication fiber in the 1.3Pm wave band, the dopant controlling the core is germanium dioxide (GeO2), and the cladding is made of SiO. However, the core of the fluorine fiber is mostly SiO2, but fluorine is doped in the cladding. Because, Rayleigh scattering loss is a phenomenon of light scattering caused by the change of refractive index. Therefore, it is desirable to reduce the amount of dopant that forms a factor of refractive index variation. The main function of fluorine is to reduce the refractive index of SIO2. Therefore, it is often used for doping of cladding. Because in fluorine-doped fiber, the core does not contain fluorine dopant that affects the refractive index. Because its Rayleigh scattering is small, and the loss is also close to the theoretical minimum. Therefore, it is mostly used for long-distance optical signal transmission. Compared with optical fibers of other materials, silica fiber also has a broad spectrum of light transmission from ultraviolet light to near-infrared light. In addition to communication purposes, it can also be used in fields such as light guide and image transmission.