Exploring The Backstory Of A brief Introduction To Optical Fiber And Tts Types

Light and its characteristics

Light is an electromagnetic wave. The wavelength range of the visible light part is: 390-760nm (millimeters). The part greater than 760nm is infrared light, and the part less than 390nm is ultraviolet light. The three types used in optical fiber are: 850nm, 1300nm, and 1550nm.

Optical fiber structure

Optical fiber (OF) is the abbreviation of optical fiber, which is a transparent medium fiber used to guide light. A practical optical fiber is composed of multiple layers of transparent medium. The bare optical fiber is generally divided into three layers: the central high refractive index glass core (core diameter is generally 50 or 62.5μm), the middle is the low refractive index silicon glass cladding (diameter is generally 125μm), and the outermost is the resin coating for strengthening.

As shown in Figure 1.

Types of optical fiber

Classification by the transmission mode of light in the optical fiber

According to the transmission mode of light in the optical fiber, the optical fiber is divided into single-mode optical fiber and multi-mode optical fiber.

Multimode optical fiber: The central glass core is thicker (50 or 62.5μm), which can transmit multiple modes of light. However, its inter-modal dispersion is large, which limits the frequency of transmitting digital signals, and it will become more serious with the increase of distance. For example: 600MB/KM optical fiber has only 300MB bandwidth at 2KM. Therefore, the transmission distance of multi-mode optical fiber is relatively short, generally only a few kilometers.

Single-mode optical fiber: The central glass core is thinner (the core diameter is generally 9 or 10μm), and can only transmit one mode of light. Therefore, its inter-modal dispersion is very small, which is suitable for long-distance communication, but its chromatic dispersion plays a major role, so single-mode optical fiber has higher requirements for the spectral width and stability of the light source, that is, the spectral width should be narrow and the stability should be good.

Classification by refractive index distribution on the optical fiber cross section

According to the refractive index distribution on the cross section of the optical fiber, optical fibers can be divided into step-index fiber (SIF) and graded-index fiber (GIF) according to the different refractive index distribution on the cross section.

According to ITU-T's recommendations on fiber types

According to ITU-T's recommendations on fiber types, optical fibers can be classified into G.651 fiber (gradient-index multimode fiber), G.652 fiber (conventional single-mode fiber), G.653 fiber (dispersion-shifted fiber), G.654 fiber (cut-off wavelength fiber) and G.655 (non-zero dispersion-shifted fiber). Among them, G.652 and G.655 are widely used in current projects. G.652, also known as conventional single fiber, can work at dual wavelengths, with the smallest dispersion at 1310nM, and its typical attenuation value is 0.34dB/km. At 1550, the lowest attenuation is about 0.2dB/km. G.652 fiber has a low price and mature technology. It is the most widely used fiber at present, accounting for more than 90%.

Common fiber specifications

 Advantages of optical fiber

● The bandwidth of optical fiber is very wide. Theoretically, it can reach 3 billion MHz.

● No repeater segment length. It is tens to more than 100 kilometers, while copper wire is only a few hundred meters.

● Not affected by electromagnetic fields and electromagnetic radiation.

● Lightweight and small size. For example, 900 pairs of twisted pair cables for 21,000 voice channels have a diameter of 3 inches and a weight of 8 tons/KM. The diameter of the optical cable with ten times the communication volume is 0.5 inches and weighs 450P/KM.

● Optical fiber communication is non-electrical and safe to use. It can be used in flammable and explosive places.

● Wide operating temperature range.

● Chemical corrosion and long service life.