What is an optical fiber?

Ⅰ. What is an optical fiber?

Optical fiber is a fiber made of glass or plastic. It is a light-conducting tool that utilizes the principle of total internal reflection of light transmitted in these fibers. The tiny fiber is encapsulated in a plastic sheath that allows it to be bent without breaking. Typically, the transmitting device at one end of the fiber uses a light-emitting diode or a laser to send light pulses into the fiber, and the receiving device at the other end of the fiber uses a photosensitive component to detect the pulses. Cables that contain optical fibers are called fiber optic cables.

Because the transmission loss of information in optical fibers is much lower than that of electricity in wires, and because the main raw material is silicon, which has a huge reserve and is easy to mine, its price is very cheap, which promotes the use of optical fibers as long-distance information transmission medium. With further reductions in prices, fiber optics are also being used for medical and recreational purposes.

Ⅱ. The history of optical fiber

In 1880, Alexander Graham Bell invented the "optical talking set".

In 1887, British scientist Charles Vernon Boys pulled out the first optical fiber in the laboratory.

In 1956, a student at the University of Michigan made the first glass-clad optical fiber. He melted a tube of glass with a low index of refraction onto a rod of glass with a high index of refraction.

In 1966, British-Chinese scholar Gao Kun pointed out the possibility and technical approach of using optical fiber for information transmission, which laid the foundation for modern optical fiber communication.

In 1970, three researchers from Corning Corporation of the United States, Marel, Capron, and Keck, successfully developed a low-loss silica optical fiber with a transmission loss of only 20dB/km by using the modified chemical phase deposition method (MCVD method).

In 1970, Bell Laboratories of the United States developed the world's first gallium aluminum arsenide semiconductor laser that works in continuous wave at room temperature.

In 1973, Bell Laboratories in the United States made even greater achievements, reducing the fiber loss to 2.5dB/km.

In 1977, Bell Research Institute and Nippon Telegraph and Telephone Corporation almost simultaneously successfully developed a semiconductor laser with a service life of 1 million hours (about 10 years in practical use).

In 1979, Zhao Zisen drew the first practical optical fiber independently developed by my country, so he was known as the "Father of Chinese Optical Fiber".

In May 2023, Chinese scientists realized quantum key distribution over thousands of kilometers of unrepeated fiber optics. This not only sets a world record for the distance of quantum key distribution without relays in optical fiber, but also provides a solution for high-speed backbone links of intercity quantum communication.

Ⅲ. Structure of optical fiber

An optical fiber is a thin, soft, solid glass substance. It consists of three parts: cladding, coating and core.

1. Cladding

It is located around the core, and its composition is also high-purity silica or glass. The cladding provides a reflective surface and light isolation for light transmission, and plays a certain role in mechanical protection.

2. Coating 

It is the outermost layer of the optical fiber and is composed of acrylate, silicone rubber, and nylon. The coating protects the optical fiber from water vapor erosion and mechanical abrasion.

3. Core

It is located in the center of the optical fiber and is composed of high-purity silica.

Ⅳ. How does optical fiber works?

The optical transmission system consists of three parts: light source, transmission medium and detector. At the transmitting end, the optical fiber first converts the medium to be transmitted (such as sound) into an electrical signal, and then transmits the electrical signal to the laser beam through a laser. The intensity of the light varies with the frequency of the electrical signal and is emitted through the optical fiber. At the receiving end, a detector receives the optical signal and converts it into an electrical signal. After processing, the original information is restored.

Ⅴ. The advantages and disadvantages of optical fiber

1. Advantages

(1) Long service lifespan.

(2) The optical fiber is not charged, so it is safe to use. It can be used in flammable, explosive and other places.

(3) Light weight and small size.

Because the optical fiber is very thin, the diameter of the single-mode optical fiber core wire is generally 4um to 10um, and the outer diameter is only 125um. With the addition of waterproof layer, reinforcing rib, sheath, etc., the diameter of the optical cable composed of 4 to 48 optical fibers is less than 13mm. It is much smaller than the 47mm diameter of standard coaxial cable. In addition, the optical fiber is a glass fiber with a small specific gravity, which makes it have the characteristics of small diameter and light weight.

(4) The passband of the optical fiber is very wide, theoretically up to 30T.

The higher the frequency of the carrier, the wider the frequency bandwidth over which the signal can be transmitted. The bandwidth of a single light source only accounts for a small part of it (the frequency band of multimode fiber is about several hundred megahertz, and a good single-mode fiber can reach more than 10GHz). Using advanced coherent optical communication, 2000 optical carriers can be arranged for wavelength division multiplexing in the range of 30000GHz, which can accommodate millions of channels.

(5) Strong anti-interference ability

Because the basic component of optical fiber is quartz. It only transmits light, does not conduct electricity, and is not affected by electromagnetic fields. The transmitted optical signal is not affected by the electromagnetic field, so the optical fiber transmission has strong resistance to electromagnetic interference and industrial interference. Because of this, the signal transmitted in the optical fiber is not easy to be eavesdropped, which is conducive to confidentiality.

(6) Low cost

The material (quartz) from which optical fibers are made is abundant. As technology advances, its cost will be further reduced. The copper raw materials required for cables are limited, and the price will become higher and higher. Obviously, optical fiber transmission will have an absolute advantage in the future.

(7) Reliable working performance

The reliability of a system is related to the number of devices that make up the system. The more equipment there is, the greater the chance of failure. Because the fiber optic system contains a small number of devices (not dozens of amplifiers like the cable system), the reliability is naturally high. In addition, the life of the fiber optic equipment is very long, and its trouble-free working time can reach 500,000 to 750,000 Hour. Among them, the laser in the optical transmitter has the shortest life span, and its minimum lifespan is more than 100,000 hours. Therefore, the working performance of a well-designed, correctly installed and debugged optical fiber system is very reliable.

2. Disadvantages

(1) Low tensile strength

Microscopic cracks are produced on the surface of optical fibers during production. Under the tension of construction, the crack area will expand. Therefore, the actual tensile strength of the fiber is very low.

(2) The connection is difficult

In the connection process, it is very difficult to reduce the loss of the connection. In order to align the two slender optical fibers exactly as required, we need to use expensive professional tools. In addition, quartz has a high melting point, which also increases the difficulty of connection.

Ⅵ. The main ways of laying optical fiber

Due to different construction conditions and construction requirements, communication optical cables will be laid in different ways in different scenarios. Common laying methods include: pipe laying, direct burial laying and aerial laying etc.

1. Pipe laying

The duct laying of optical cables usually refers to laying optical cables in underground communication ducts by traction or air blowing. Underground communication pipelines generally consist of a group of pipes buried under the surface and manholes (hands) at both ends of the group of pipes. Tube groups usually consist of one or more plastic tubes. According to different construction areas and section lengths, underground communication pipelines are usually divided into urban pipelines and long-distance pipelines.

2. Direct burial laying

Direct burial laying refers to the laying method in which the optical cable is directly buried in the underground soil. Usually, in ordinary soil and hard soil, the optical cable needs to be buried below 1.2 meters from the ground surface. When constructing direct-buried optical cables, we need to dig out the cable trenches that meet the requirements, then lay the buried optical cables to the bottom of the cable trenches, and then backfill the soil. In order to facilitate the laying position of the optical cable during maintenance, after backfilling the soil, we bury marker stones on the surface every about 50 meters for marking. Directly buried optical cables can be laid not only manually, but also mechanically. The direct buried optical cable generally adopts the GYTA53 structure. This structure is equivalent to adding chrome-plated steel tape and polyethylene outer sheath to the sheath of ordinary GYTA optical cable, so that the optical cable has better protection performance.

In the 1990s, large-scale construction of long-distance optical cable lines began nationwide. At that time, long-distance optical cable lines were mainly laid by direct burial. The safety of direct buried optical cable lines is better, but the construction and maintenance costs are high and it is difficult to expand. Therefore, it is gradually replaced by long-distance plastic pipe air blowing and other laying methods. At present, it still has a small amount of construction in some areas with vast land and sparsely populated areas (such as grasslands and deserts) where aerial laying is not suitable.

3. Aerial laying

The overhead of the optical cable means that the optical cable relies on poles and other supports, so that the optical cable is suspended in a space of 3.0 meters to 8.0 meters from the ground. The support used for overhead optical cables is mainly poles, and the distance between poles (the distance between adjacent poles) is generally about 50 meters. Aerial optical cables are less expensive to build, but also less secure. It tends to be easily hung up by super-high vehicles when crossing the road. In the event of a flood, it often occurs that the connected pole roads are washed away.

There are two main methods for overhead laying: using steel strands to attach and self-supporting overhead.

(1) Use steel strands to attach

Aerial optical cables are mainly laid by steel strands. In this way, the steel strand needs to be fixed on the pole first, and then the optical cable is attached to the steel strand through the cable hook. Usually, 1 to 4 steel strands can be fixed on each pole, and 1 to more optical cables can be attached to each steel strand. Aerial optical cables generally adopt the GYTS structure. There is a layer of steel tape armor between the sheath and the cable core of the optical cable, which makes the optical cable have a certain ability to prevent birds and rodents.

(2) Self-supporting overhead

Self-supporting overhead refers to the use of a self-supporting optical cable with high tensile strength. The optical cable does not need to be attached by steel strands, it can be directly fixed on supports such as poles. Self-supporting overhead is often used in scenes where it is not convenient to lay steel strands, such as: under power poles, areas with strong air corrosion, or sections that do not need to be expanded for a long time after the laying of optical cables. In the scenario of laying along high-voltage power lines or requiring large spans, self-supporting optical cables usually adopt the ADSS structure. In scenarios where low-cost construction is required, self-supporting optical cables usually adopt a "figure 8" structure.

Ⅶ. Causes of fiber attenuation

The main factors causing fiber attenuation are: inhomogeneity, bending, intrinsic, impurity, extrusion and butt joint, etc.

1. Intrinsic: It is the inherent loss of the optical fiber, including: rayleigh scattering, inherent absorption, etc.

2. Squeeze: The loss caused by the slight bending of the optical fiber when it is squeezed.

3. Docking: The loss generated when the fiber is docked, such as: misalignment (the coaxiality of single-mode fiber is required to be less than 0.8 μm), the end face is not perpendicular to the axis, the end face is uneven, the butt core diameter does not match, and the welding quality is poor.

4. Bending: When the fiber is bent, part of the light in the fiber will be lost due to scattering, resulting in loss.

5. Impurities: Impurities in the optical fiber absorb and scatter the light propagating in the optical fiber, resulting in losses.

6. Inhomogeneity: The loss caused by the inhomogeneity of the refractive index of the fiber material.