The Advantages And Disadvantages of Multimode And Single-mode Fiber

1. The Development of Optical Fiber

1.1 Development of Single-mode Fiber

The late 1970s, people tried to use the successful development of long-life semiconductor laser light source to replace the light tube, in order to get longer communication distance and greater communication capacity. However, the laser pattern noise occurs in the multi-mode transmission fiber. To overcome pattern noise, in 1980 successfully developed the zero dispersion point at 1.31m single-mode fiber (non-dispersion shifted single-mode fiber). The International Telecommunication Union (ITU-T) recommended that such a single-mode fiber is defined as G.652 fiber. Because the design of a single mode fiber is only thinking of a transmission mode, so the pattern noise in the transmission multimode fiber occurred does not occur. Thus, the mid-1980s, by the laser light source and composition of the 140sMbit G.652 Fiber Optic Patch Cables communication systems [2], which, following distance and transmission capacity far exceeds the coaxial cable so that the optical fiber communication gradually replace the use of copper to become the the main means of communication in telecommunications industry.

1.2 Development of Multi-mode Fiber

Thinking of optical fiber communication by Chinese-American Kao paper published in 1966 , " optical frequency dielectric fiber surface waveguides " made of quartz glass optical signal transmission fiber ( referred to as optical fiber ) to communicate. In 1970, the British telecommunications, Bell Labs and Corning Glass Company jointly developed the world's first root attenuation coefficient 20kmdB Multimode Fiber Cables[ 3 ]. It should be noted that the multi -mode fiber as the optical transmission medium of the semiconductor laser and a long life as a prelude optical fiber common carrier communication research. Expand and increase the transmission speed of the optical fiber communication transmission capacity, transmission distance is extended and the fiber attenuation is closely related to dispersion and nonlinear effects. Innovation process is that the people of fiber varieties of fiber attenuation, chromatic dispersion , nonlinear effects in optical fiber communication systems and other performance plays an important role in the recognition process. 1976 Bell Labs in the United States between Atlanta to Washington establish the world's first practical optical fiber communication systems,the transmission speed of 45sMbit, uses a multi-mode fiber . Multimode fiber always is trying to reduce the attenuation and mode dispersion, and further improve the transmission bandwidth of optical fiber as the research center. In recent years, studies have multimode fiber mutagenicity progress, fiber transmission bandwidth has been greatly improved.

2. Principle of Multimode Fiber and Single-mode Communication

Optical fiber communication is the use of light as a carrier to transmit information, and optical fiber as a transmission medium to achieve information transmission, one of the latest communications technology achieve the purpose of communication. Therefore, the principle of the optical fiber and the transmission of information studies is important.

First, optical fibers transmit information use the principle of total reflection of light, i.e., when the light from the optical dense medium (higher refractive index medium) optically thinner medium intake, the refraction angle will be greater than the incident angle, when the incident angle to a certain degree, the angle of refraction of the incident angle of 90 °of this case referred to the total reflection angle. When the angle of incidence continues to increase, will not happen this time refraction, reflection and safety of all incident light is reflected Theorem. This phenomenon is a reflection. To achieve total internal reflection optical fiber to satisfy two conditions: Light must be incident light from the optically denser medium to dense medium; Angle of incidence must be greater than the critical angle. Thus, a practical optical fiber essentially consists of three parts, namely, a high refractive index of the core portion and the refractive index of the lower cladding layer and the outer coating layer.

There are many classification of Fiber Optic Patch Cables, but there is a very important classification, according to the number of conduction-mode fiber to classify, so-called "model" refers to a certain angular velocity of a beam of light into the fiber. Only one mode of conducting could be single-mode fiber, the fiber can be transmitted to a plurality of mold for hundreds of patterns is called a multi-mode fiber. It is possible to determine the number of transmission modes, an important structural parameter is the normalized frequency of the fiber, generally denoted by V, defined as:



2.1 Multi-mode Optical FiberThe number of fiber conduction mode with its normalized frequency, the total number of conduction mode similar to:



Wherein, V is the normalized frequency of the fiber, αis the fiber refractive index profile exponential. For example, the total number of parabolic fiber α=2, the conduction mode is:




2.2 Single-mode Optical FiberOnly one mode fiber transmission is called multimode fiber, Single Mode Fiber Cable can transmit the fundamental mode (lowest-order mode), there is no difference between the mode delay. Therefore, single-mode fiber refractive index α profile exponential impact on its far less bandwidth multimode fiber is generally used large step-index profile.