Optical fiber transmission principle_Development and application of optical fiber

Optical fiber is a kind of fine, soft solid glass material, which consists of three parts: core, cladding, and coating layer. It can be used as a light transmission tool.

The core of the optical fiber is mainly made of high-purity silicon dioxide (SiO2) and doped with a small amount of dopants to increase the optical refractive index n1 of the core; the cladding is also high-purity dioxide (SiO2), and is also doped with some The dopants, to reduce the refractive index of the cladding n2, n1> n2, total reflection occurs; coating layer using acrylic, silicone rubber, nylon, increase the mechanical strength and bendability.

Optical fiber transmission principle

Total reflection principle: Because the propagation speed of light in different substances is different, when light is radiated from one substance to another, refraction and reflection will occur at the interface between the two substances. Moreover, the angle of the refracted light changes with the angle of the incident light.

When the angle of incident light reaches or exceeds a certain angle, the refracted light will disappear, and the incident light will be reflected back. This is the total reflection of light.

Different materials have different refraction angles for light of the same wavelength (ie, different materials have different light refraction indexes), and the same material has different refraction angles for different wavelengths of light. Optical fiber communication is based on the above principles.

According to the principle of total reflection of the geometrical optics, the ray generates total reflection at the interface between the core and the cladding, and forms a necessary condition for blocking the light from propagating inside the fiber core, even if the routed light does not exit the fiber.

The origin and development of fiber optic technology

In 1966, the Chinese-American Gao Hao and Kok Haham published papers, and the concept of fiber optics emerged. In 1970, the United States Corning Corporation successfully developed a fiber with a loss of 20dB/km for the first time, and the optical fiber communication era began.

In 1977, the United States successfully conducted fiber optic communication experiments with multimode fiber for the first time in Chicago. The 8.5-micron-band multimode optical wave was the first generation of optical fiber communication system. Then in 1981, 1984 and the late 1980s, the optical fiber communication system rapidly developed into the fourth generation. The fifth-generation fiber-optic communication system has reached the standard of application and has realized the long-distance transmission of light waves.

The development stage of optical fiber communication

The first stage: 1966-1976 was the period of development from basic research to commercial application. At this stage, a low-wavelength 0.85 μm low-speed 45 or 34 Mb/s multimode optical fiber communication system is realized, and the non-relay transmission distance is about 10 km.

The second stage: From 1976 to 1986, this was a period of great development with the aim of increasing the transmission rate and increasing the transmission distance as the research goal and promoting the application. During this period, the optical fiber developed from multimode to single mode, and the operating wavelength developed from a short wavelength of 0.85μm to a long wavelength of 1.31μm and 1.55μm, and a single-mode optical fiber communication system with a working wavelength of 1.31μm and a transmission rate of 140565Mb/s was realized. The non-relay transmission distance is 10050km.

The third stage: From 1986 to 1996, this was a period of large capacity and long distance as the goal, and a comprehensive and in-depth development of new technologies. During this period, a 1.55 μm dispersion shifted single mode fiber communication system was implemented. Using external modulation technology, the transmission rate can reach 2.510Gb/s, and the non-relay transmission distance can reach 150100km. The laboratory can reach higher levels.

Type of optical fiber

There are many kinds of optical fibers, and there are various classification methods.

According to the materials used for the manufacture of optical fiber: quartz fiber, multi-component glass fiber, plastic clad quartz core fiber, full plastic fiber and fluoride fiber.

Plastic optical fibers are made of highly transparent polystyrene or polymethylmethacrylate (plexiglass). It is characterized by low manufacturing cost, relatively large core diameter, high coupling efficiency with the light source, high optical power coupled into the fiber, and ease of use. However, because of the large loss and small bandwidth, this kind of fiber is only suitable for short-range low-rate communications, such as short-distance computer network links and intra-ship communications. At present, quartz-based fibers are commonly used in communications.

According to the optical transmission mode in the optical fiber: single-mode fiber and multi-mode fiber.

Single-mode fiber: The center glass core is very thin (the core diameter is generally 9 or 10 μm) and can only transmit one mode of light. Therefore, the dispersion between modules is very small, suitable for long-distance communication, but there are also material dispersion and waveguide dispersion, so that the single-mode optical fiber has higher requirements on the spectral width and stability of the light source, ie, the spectral width should be narrow and stable. Better. Later, at a wavelength of 1.31 μm, the material dispersion and waveguide dispersion of the single-mode fiber were found to be positive, negative, and exactly the same size.

This means that at a wavelength of 1.31 μm, the total dispersion of a single-mode fiber is zero. From the fiber loss characteristics, 1.31μm is just a low-loss window of the fiber. In this way, the 1.31 μm wavelength region becomes a very ideal working window for optical fiber communication, and it is also the main operating band of practical optical fiber communication systems. The main parameters of the 1.31 μm conventional single-mode fiber are determined by the International Telecommunication Union ITU-T in the G652 recommendation, so this fiber is also called G652 fiber.

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

Divided by refractive index profile: Stepped and graded fiber.

Step type: The core refractive index of the optical fiber is higher than the refractive index of the cladding, so that the input light energy continues to generate total reflection at the core-cladding interface and advances. This fiber core has a uniform refractive index and the cladding has a slightly lower refractive index. The index of refraction of the central core of the optical fiber to the glass cladding is catastrophic and has only one step. Therefore, it is called a step index type multimode optical fiber, or step optical fiber for short, which is also called a mutation optical fiber.

This kind of optical fiber has many transmission modes. The transmission paths of different modes are different. The time after reaching the end point after transmission is also different. Therefore, there is a time delay difference and the optical pulse is broadened. Therefore, this kind of optical fiber has high inter-mode dispersion, the transmission band is not wide, and the transmission rate cannot be too high. It is not ideal for communication and is only suitable for short-haul low-speed communications, such as industrial control. However, since single-mode fibers have very small inter-modal dispersion, single-mode fibers use mutations. This is an earlier research and development of optical fiber, which has now gradually been eliminated.

Gradual fiber: In order to solve the drawbacks of step fiber, people have also developed and developed a graded index multimode fiber, referred to as graded fiber. The refractive index of the fiber center core to the glass cladding gradually becomes smaller, so that the light of the high-order mode can be transmitted in a sinusoidal manner, which can reduce the dispersion between the modes, increase the bandwidth of the optical fiber, increase the transmission distance, but the cost is high, and now many Mode fiber is mostly graded fiber.

The gradient refractive index profile of a graded fiber is uniform as a step fiber. The core of the graded fiber has the largest refractive index center and gradually decreases along the core radius. Since the light of the high-order mode and the low-order mode of light respectively refracts at the interface of the different refractive index layers in accordance with the law of refraction and enters into the low refractive index layer, the angle formed by the light traveling direction and the direction of the optical fiber axis will gradually change. small.

The same process continues until the light is totally reflected in a certain refractive index layer, redirects the light, and travels toward the center of the higher refractive index layer. At this time, the angle formed by the direction of travel of the light and the direction of the axis of the optical fiber is increased once for each refractive index layer, and finally reaches the point where the central refractive index is maximized.

After this, the same process as described above is continuously repeated, thereby realizing the transmission of light waves. It can be seen that the light will consciously adjust in the gradient fiber to finally reach the destination. This is called autofocus.

According to the optical fiber operating wavelength: short wavelength fiber, long wavelength fiber and ultra-long wavelength fiber.

The short-wavelength optical fiber refers to an optical fiber of 0.8 to 0.9 μm; the long-wavelength optical fiber refers to an optical fiber of 1.0 to 1.7 μm; and the ultra-long-wavelength optical fiber refers to an optical fiber of 2 μm or more.

At present, the international standards for single-mode optical fibers are mainly ITU-T series: G.650 “Definition and Test Method for Related Parameters of Single-mode Optical Fibers”, G.652 “Single-mode Optical Fiber and Optical Fiber Characteristics,” and G.653 “Dispersion. Displacement single-mode fiber and fiber cable characteristics, G.654 "Cut-off wavelength displacement single-mode fiber and fiber cable characteristics", G.655 "Non-zero dispersion displacement single-mode fiber and fiber cable characteristics" and G.656 "for broadband transmission Non-zero dispersion shifted fiber and cable characteristics." ITU-T's standard for multimode fiber is G.651 "Multimode 50/125μm graded index fiber and fiber cable characteristics."

Single mode fiber

Ordinary single mode fiber

Ordinary single-mode fiber refers to a single-mode fiber with a zero-dispersion wavelength in a window of 1 310 nm. It is also called a dispersion-unshifted fiber or an ordinary fiber. The International Telecommunication Union (ITU-T) specifies this fiber as a G.652 fiber.

G.652 belongs to the first generation of single-mode fiber and is the single-mode fiber with the best performance at 1310 nm wavelength. When the operating wavelength is 1310 nm, the dispersion of the fiber is very small, the chromatic dispersion coefficient D is in the range of 0 to 3.5 ps/nm·km, but the loss is relatively large, and is about 0.3 to 0.4 dB/km. At this point, the transmission distance of the system is mainly limited by the attenuation of the optical fiber.

The loss at 1 550 nm is small, about 0.19 to 0.25 dB/km, but the dispersion is large, about 20 ps/nm·km. The PDH systems traditionally opened on G.652 mostly use a 1310 nm zero-dispersion window. However, the SDH system opened in recent years uses a minimum attenuation window of 1550 nm.

In addition, due to the practical application of Erbium Doped Fiber Amplifier (EDFA), DWDM also operates in the 1550 nm window, making the 1550 nm window the main working window of G.652 fiber.

For DWDM systems based on 2.5 Gb/s and below, G.652 fiber is the best choice. However, due to the large dispersion in the 1550 nm band, if a 10 Gb/s signal is transmitted, an expensive dispersion compensation module is generally used when the transmission distance exceeds 50 km, which increases the total cost of the system. The dispersion compensation module introduces greater attenuation. Therefore, the dispersion compensation module is often operated together with the EDFA, placed between the two levels of EDFA amplification, so as not to take up the power margin of the link.

Some of the optical characteristics of the G.652 fiber and any parameters of the characteristics.

G.652 fiber is further divided into four sub-categories: A, B, C, and D:

The G.652A fiber is mainly applicable to the SDH transmission system specified in ITU-T G.951 and the SDH transmission system with optical amplification specified by G.691 up to STM-16 and can only support 2.5Gb/s and below. system.

The G.652B fiber is mainly applied to the SDH transmission system defined in ITU-T G.957 and the single-channel SDH transmission system with optical amplification specified in G.691 up to the ITU-T G.692 wavelength-division wavelength division with STM-64 Multiplexed transmission system that can support parametrically required 10 Gb/s rates for PMD