Section 1: Structure and Classification of Optical Fibers

2.1.1 Structure of Optical Fibers

---- Optical fibers are fiber waveguides or light guides that transmit light Abbreviation for fiber. Its typical structure is a multi-layer coaxial cylinder, as shown in Figure 2-1, with core, cladding and coating layers from the inside to the outside.

----The core part is the core and cladding. The core is made of highly transparent material and is the main transmission channel for light waves; the refractive index of the cladding is slightly smaller than the core, making the light The transmission performance is relatively stable. The core thickness and the refractive index of the core material and cladding material have a decisive influence on the characteristics of the optical fiber. The coating layer includes primary coating, buffer layer and secondary coating, which protect the optical fiber from water vapor erosion and mechanical scratches, while also increasing the flexibility of the optical fiber and extending the life of the optical fiber.

2.1.2 Classification of optical fibers

---- According to the distribution shape of the refractive index on the cross-section, there are step optical fibers and gradient optical fibers (gradient optical fibers). ) Two types of fiber optics. The refractive index of the step fiber at the interface between the core and the cladding shows a step-shaped mutation, and the refractive index n1 of the core and the refractive index n2 of the cladding are uniform constants

. The refractive index nl of the core of the graded optical fiber gradually decreases according to a certain rule (such as square law, bisecant curve, etc.) as the radius increases. At the junction of the core and the cladding, the refractive index nl of the cladding is n2, and the refractive index of the core is n2. The refractive index is not a uniform constant.

----According to the number of transmission modes in the optical fiber, it can be divided into two categories: single-mode optical fiber and multi-mode optical fiber. Single-mode fiber only transmits one mode and has a thin core diameter, usually in the range of 4μm to 10μm. Multimode optical fiber can transmit multiple modes and has a thicker core diameter, with a typical size of about 50μm.

----According to the materials used in manufacturing optical fibers, there are four types: quartz series, plastic-clad quartz core, multi-component glass fiber, and all-plastic optical fiber. Quartz optical fiber is mainly used in optical communication, and the optical fiber mentioned in the future mainly refers to quartz optical fiber.

----In addition, if divided according to working wavelength, it can also be divided into short-wavelength optical fiber and long-wavelength optical fiber. Multimode optical fiber can adopt step refractive index distribution or gradient refractive index distribution; single-mode optical fiber mostly adopts step refractive index distribution. Therefore, silica optical fiber can be roughly divided into multi-mode step refractive index fiber, multi-mode graded refractive index fiber and single-mode step refractive index fiber.

Their structure,