Fiber optic cable is the use of one or more optical fibers placed in the cladding sheath as a transmission medium and can be used individually or in groups of communication cable components. It is mainly composed of fiber core, cladding, coating, reinforcing elements outer sheath, and other parts, and is an important information transmission carrier.

The core is the transmission medium of light waves, made of high refractive index materials. The cladding material refractive index is slightly lower than the core, and the core together constitute an optical waveguide, the propagation of light waves plays a decisive role. 

The coating is mainly used to isolate stray light, improve the strength of the fiber, and protect the fiber. Reinforcement elements are usually metal steel wire or non-metallic fiber-reinforced plastics and other materials, its role is to withstand the installation and laying process of the external forces exerted. 

The outer sheath protects the optical fiber of the cable and is required to be pressure-resistant, moisture-resistant, with good humidity characteristics, lightweight, chemical-resistant, and flame-retardant.

By the end of 2022, the total length of China's fiber optic cable lines will reach 59.58 million kilometers, the gigabit optical network can cover more than 500 million households, and built and opened 2.312 million 5G base stations. 

This fully illustrates the important position of fiber optic cable in modern information and communication. Fiber optic cable is widely used in communication, electric power, transportation, and other fields, becoming an indispensable infrastructure in the information age. 

It provides strong support for people's lives and social development with its advantages of high speed, large capacity, and low loss.

Second, the development process (a) origin and breakthroughs

In 1966, Charles Kao, a British Chinese, first proposed transmitting light waves over long distances with quartz fibers. 

History has tested this forward-looking theory for more than 40 years and won the Nobel Prize. 7 August 1970, four doctors from Corning Incorporated, headquartered in New York, successfully developed the world's first low-light-loss single-mode quartz optical fiber samples based on the forward-looking ideas and theories of Kao and Hockham's thesis by adopting the Internal Vapor Deposition (IVD) process for making optical fiber pre-fabricated rods, marking a breakthrough in optical communication technology. 

This marked a breakthrough in optical communication technology.

(II) Domestic and international development

In the world, in 1973, the world's first fiber-optic communication experimental system was built in Bell Labs; in 1976, the U.S. Bell and West Point built an optical communication laboratory, the same year France installed a 19-core fiber-optic cable line, the U.S. installed a 144-core fiber-optic cable line for the test. 1989, the first Pacific Ocean fiber-optic cable line between the U.S. and Japan was opened. 

In 1997, the global fiber optic cable interconnection line network (FLAG) was in operation, with a total length of 28,000 kilometers of fiber optic cable lines.

In China, the development of submarine fiber optic cable started in 1986, and the first submarine fiber optic cable communication system in China was put into operation in 1990. on December 15, 1993, the 1252-kilometer China-Japan submarine fiber optic cable from Nanhui, Shanghai to Miyazaki, Kyushu, Japan was officially opened, which is the first submarine fiber optic cable opened in China, with a total system capacity of 7,560 talk circuits. in October 1993, the longest land-based fiber optic cable in the world was opened, which is the first submarine fiber optic cable in the world. 

In October 1993, the world's longest land cable (made by Chengdu 514 Factory) was opened in China; it was 4,700 kilometers long from Beijing to Hainan. 

Subsequently, and Asia-Pacific nine countries or regions jointly built the Asia-Pacific submarine cable network, a total length of 1,100 kilometers. January 28, 1997, China's first international land cable was officially opened, the China-Russia fiber optic cable from Harbin through Fuyuan across the border of Russia and the Ussuri River to Khabarovsk, Russia, a total length of 970 kilometers.

Third, the working principle (a) based on total reflection

Light propagation in the optical fiber, using the principle of total reflection to achieve efficient transmission. 

Mutant optical fiber, the refractive index of the core medium is greater than the refractive index of the cladding medium. 

When the light entering the core reaches the core and cladding interface, if the angle of incidence is greater than the critical angle of total reflection, the light will be able to occur in total reflection and no light energy out of the core, so that in the interface by countless times the total reflection forward transmission. 

However, when the fiber is bent, the interface normally turns, and part of the incident angle of light becomes smaller, may be less than, at this time, this part of the light can not be fully reflected, but the original angle of incidence of the larger light can still be fully reflected, so the fiber can still be transmitted during the time of bending, but will cause energy loss. 

In general, the bending radius is greater than 50 to 100 mm, and the loss is negligible, while the tiny bend will cause serious “micro-bending loss”.

(ii) a variety of transmission modes

Optical fiber is divided into single-mode fiber and multi-mode fiber according to the transmission mode. 

Single-mode fiber with a thin central glass core, the core diameter is generally 8.5 or 9.5μm, and in the 1310 and 1550nm wavelength operation, allowing only one direction of light through, avoiding the problem of mode dispersion, with an extremely wide bandwidth, especially suitable for large-capacity fiber optic communications. 

Multimode fiber core diameter is generally 50μm/62.5μm, due to the larger core diameter, allowing different modes of light transmission on a fiber, the standard wavelengths of 850nm and 1300nm, respectively, there is also a new multimode fiber standard, known as the WBMMF (Wideband Multimode Optical Fiber), which uses wavelengths between 850nm and 953nm. 

In multimode transmission, due to the larger core size, the intermodel dispersion is larger, i.e., the optical signal “spreads” faster, and the quality of the signal is degraded over long distances. 

Therefore, multimode fibers are typically used for short distances, audio/video applications, and Local Area Networks (LANs), and OM3/OM4/OM5 multimode fibers can support high-rate OM3/OM4/OM5 multimode fiber can support high-rate data transmission.

(iii) There are attenuation factors

Light propagation in the fiber will be due to a variety of factors causing loss. The first is the inherent loss of materials, including Rayleigh scattering, inherent absorption, etc.. 

For example, the basic material for manufacturing optical fiber silicon dioxide (SiO2) which is the absorption of light, there are ultraviolet absorption, and infrared absorption, fiber optic communications generally only work in the 0.8 ~ 1.6μm wavelength region, in this work area, the infrared absorption of quartz optical fiber loss is generated by the infrared region of the material's molecular vibration in the wavelength band of more than 2μm there are a few vibrations of the absorption peak. 

Secondly, it is the loss caused by impurity absorption. Harmful transition metal impurities such as copper, iron, chromium, manganese, etc., as well as hydroxide root (OHˉ) in the material used to manufacture optical fiber will absorb the light energy, resulting in the loss of light energy. Furthermore, the structural causes of loss, such as fiber bending part of the light within the fiber will be lost due to scattering; fiber is squeezed to produce a small bend and cause loss; fiber optic material refractive index is not uniform will also cause loss; optical fiber butt joints, different axes, the end face and the axis is not perpendicular to the end of the surface is not flat, the butt core diameter mismatch and fusion splice poor quality and so on will produce loss. 

These factors make the optical power in the fiber along the longitudinal axis gradually decrease, the optical power decreases with the wavelength, and fiber optic link, the power decreases mainly due to scattering, absorption, and connectors and fusion splicing joints caused by optical power loss, attenuation of the unit for dB.

Fourth, the connection method and characteristics (a) direct connection

Direct connection is to connect the two fiber optic cables directly, this way of operation is simple, high transmission efficiency. 

However, it has certain restrictions on the length of the fiber optic cable, not too long, otherwise, the signal loss rate will increase. 

For example, when the length of the directly connected fiber optic cable exceeds a certain range, it may result in a significant reduction in signal strength, affecting the quality of communication. 

At the same time, if a single-mode fiber optic cable is used, the transmission speed and optical wavelength of the two cables need to be consistent, to ensure stable signal transmission. If not consistent, it will seriously affect the quality of signal transmission, and may even lead to communication interruption.

(B) fiber optic patch cord

A fiber optic patch cord is an effective way to connect two different lengths of fiber optic cable, through which it can be convenient to change and maintain the fiber optic cable. 

The length of the patch cord can be selected according to the actual demand, and different lengths of patch cords can connect different distances of fiber optic cables, providing greater flexibility for the layout of the fiber optic network. 

For example, in a large data center, it may be necessary to use various lengths of fiber optic patch cords to connect different devices and servers to meet the needs of a complex network architecture. 

Moreover, when the network needs to be adjusted or repaired, only the corresponding fiber optic patch cords need to be replaced, without large-scale changes to the entire fiber optic cable route, greatly improving the maintenance efficiency.

(C) fiber optic cable connection box

A fiber optic cable connection box can better protect the fiber optic cable port, but also better resistance to physical damage. In practice, the fiber optic cable connection box usually has good sealing performance, which can prevent water, dust, and other impurities from entering the fiber optic cable, to ensure the transmission quality of the fiber optic cable. 

For example, in the outdoor environment, the fiber optic cable connection box can effectively protect the fiber optic cable from bad weather and other external factors. 

In addition, the fiber optic cable connection box can also provide a stable connection for fiber optic cables, reducing the risk of loosening or damage to the connection due to external pulling or vibration and other factors.