First of all, there are two kinds of optical fibers used for communication. \x0d\ Ordinary optical fiber and special optical fiber (special optical fiber has different uses, different classifications, various types and different functions, so I won't go into details here. If you want to know which one, you can ask again. ) \x0d\x0d\ Types and manufacturing processes of optical fibers \ x0d \ Optical fibers are divided into multimode optical fibers and single-mode optical fibers. Multimode fiber is divided into step multimode fiber and gradient multimode fiber. The refractive index n 1 of step multimode fiber-core glass must be greater than the refractive index n2 of cladding glass, and the refractive index increases step by step at the interface between glass and cladding glass, and each is constant. This kind of optical fiber is the simplest structure and the easiest to manufacture, but it is rarely used because of its large mode dispersion and narrow bandwidth. Gradient multimode fiber-When the refractive index of the core glass gradually decreases from the maximum value of the fiber spindle n 1 to the refractive index distribution of the cladding glass interface n2, which is made into an accurate parabolic shape (g=2), this kind of fiber reduces the mode dispersion and improves the bandwidth. Single-mode optical fibers include G652, G653, G654, G655 and G656. The core diameter of single-mode fiber is 8-9um, and the outer diameter is 125um. G652 fiber-The longest used ones are simple step-matched cladding type and simple step-down concave inner cladding type. The performance of simply matched clad fiber is slightly worse. Generally, doping Ge is used to improve the refractive index of the core. Excessive doping will increase the attenuation of the fiber due to the dispersion loss of the material, so the relative refractive index difference △ is low (about 0.3%), and the bending characteristics of the fiber are slightly worse. The performance of concave inner cladding fiber is better. Generally, F is used in the inner cladding to produce concave refractive index △-,so as long as a small amount of Ge is doped in the core, a larger total relative refractive index can be obtained, △ =△ +△-. High delta can greatly improve the bending resistance and loss of optical fiber. At the same time, this structure has four design degrees of freedom. The cutoff wavelength, zero dispersion wavelength, mode field diameter and so on can be optimized by properly selecting △+,△-and 2b. G653 fiber-using segmented core and double stepped core. The fiber has successfully achieved low attenuation and zero dispersion at the wavelength of 1550nm, and has good bending resistance and low connection loss. . In particular, multi-core structures have many degrees of freedom in design. By adjusting the refractive index difference and geometric size of each part, it is easy to control the waveguide dispersion and realize the shift of zero dispersion wavelength. But it is not suitable for wavelength division system. G654 fiber —— The refractive index profile structure of this fiber is the same as that of standard single-mode fiber, and it still adopts simple step matching cladding and simple step concave inner cladding. The difference is that the pure silica core is selected to reduce the attenuation of the fiber, and the refractive index difference is obtained by doping F in the cladding. The biggest advantage of this fiber is that the lowest attenuation at the wavelength of 1550nm is 0. 15 dB/km. G655 optical fiber —— The refractive index profile structure of optical fiber is triangular core and double-ring core structure. The first ring in the optical fiber has the function of shifting the zero dispersion wavelength. The outer rings of these two profile structures play a key role in realizing large effective area and micro-bending loss, which can attract light from the central peak, realize larger field distribution and effectively guide the direction on a large radius. Therefore, the effective area can be increased by reducing the peak value, and the microbending performance can be improved by preventing light from leaking to the cladding. The difference between the two structures is that the attenuation of triangular core is slightly lower, and the effective area of double-ring core is slightly larger. G.656 fiber-a new non-zero dispersion-shifted single-mode fiber developed in recent years for DWDM and CWDM systems, has a wider bandwidth. Compared with G.655 fiber, it has a wider working wavelength (1460- 1625nm) and a more optimized dispersion value. Preparation method of optical fiber The preparation of optical fiber can be divided into two categories: vapor deposition and non-vapor deposition. Vapor deposition techniques include: 1. External chemical meteorological deposition (OVD), 2. Axial chemical vapor deposition (VAD)3. Improved chemical vapor deposition (MCVD) 4. Plasma chemical vapor deposition (PCVD)5. Plasma modified chemical vapor deposition. Axial and transverse plasma chemical vapor deposition (ALPD) non-vapor deposition technologies include: 1. Interface Gel Method (BSG) 2. Melting method (DM)3. Glass phase separation (PSG)4. Melting glue. Gas phase process flow of mechanical extrusion molding (MSP): 1. Preparation and purification of raw materials II. Prefabrication 3. Vapor deposition process 1. Raw material preparation and pre-purification. Silicon tetrachloride can be prepared by chlorination of industrial silicon at high temperature. The chemical reaction is Si+2Cl2= SiCl4. This reaction is exothermic, and with the intensification of the reaction, the temperature in the furnace increases. Therefore, it is necessary to control the chlorine flow rate to prevent the reaction temperature from being too high, so as to generate Si2Cl6 and Si3Cl8, and the SiCl4 vapor generated by the reaction flows into the condenser tube, so that SiCl4 liquid raw materials can be prepared. The purity of optical fiber should reach 99.9999%, that is, the impurity content is less than 10-6. General halide materials can't reach such high purity and need further purification. Generally, SiCl4 _ 4 contains four kinds of impurities, including metal oxides, nonmetallic oxides, hydrogen-containing compounds and complexes. Among them, the boiling points of metal oxides and some nonmetallic oxides are quite different from the boiling point of SiCl4 _ 4 (57.6 degrees), which can be removed by distillation, that is, impurities can be removed by using the boiling points of raw materials and impurities. For other impurities whose boiling point is close to SiCl4 _ 4 _ 4, suitable adsorbents can be used to achieve the purpose of purification. For example, hydrides such as OH in SiCl4 _ 4 can be purified by selecting suitable adsorbents according to the different chemical bond properties between the substances to be purified and impurities. High purity of SiCl4 _ 4 can be obtained by distillation-adsorption-distillation mixed purification method. The content of metal impurities is about 5PPb, and the content of hydride SiHCl3 _ 3 is less than 0.2ppm. Preparation of preform: Timely glass with high transparency and best optical properties was prepared by vapor deposition. The refractive index of the preform is obtained by forming dopants from non-reactive glass. These dopants include: GeO2, B203, P2O5, Ti2O2, Al2O3 and F. Deposition is usually on the surface of the substrate target or in a hollow glass tube, which is stacked layer by layer. Therefore, the dopant concentration can be gradually changed to give a gradient refractive profile, or it can be kept constant to give a first-order refractive profile. 3. In the process of vapor deposition, there are six deposition methods. VOD (External Vapor Deposition) was invented by Kapron of American Corning Company in 1970. Its mechanism is flame hydrolysis, that is, hydrogen-oxygen flame or methane flame is used to gradually deposit "dust" produced by halide gas hydrolysis to obtain the required glass composition: sicl 4+2 H2O = SiO 2+4 HCl deposition process is to horizontally place the target rod on the glass lathe along its longitudinal axis and rotate it. Then, high-purity oxygen is used as the carrier, and the formed glass halide gas is sent to the nozzle of the flame torch to generate glass oxide powder under high-temperature hydrolysis reaction, which is deposited on the outer surface of the horizontally rotating target rod. The target rod moves back and forth in the longitudinal direction to generate porous glass layer by layer. By changing the doping type and doping amount of each layer, optical fiber preforms with different refractive index distributions can be made. In the sintering process, the cylindrical hollow preform with certain strength and porosity prepared by deposition method is sent into a sintering furnace and sintered at a high temperature of 1400- 1600 degrees to form a bubble-free transparent solid glass preform. In the sintering process, chlorine gas should be continuously used as a desiccant to blow the porous preform to remove all the water in it, so as to ensure the small attenuation of the optical fiber. The classification, name, IEC and ITU-T of single-mode fiber are as follows: name ITU-T IEC non-dispersion shifted single-mode fiber G.652: A, B B 1. 1 low water peak fiber G.652: C, D B 1.3 dispersion shifted single-mode fiber G.653 B2 cut-off wavelength shift.