Types of optical fibers commonly used in optical fiber communication systems
Optical fiber, short for optical fiber, is a fiber made of glass or plastic that functions as a light transmission tool. The transmission principle is "total reflection of light".
Optical fiber communication system is a communication system in which light is used as the carrier wave, and extremely thin optical fibers are drawn from extremely high purity glass as the transmission medium, and information is transmitted optically through photoelectric conversion. Communication using optical fibers is called optical fiber communication.
Fiber optic transmission has many outstanding advantages:
1. Bandwidth: The width of the frequency band represents the size of the transmission capacity. The higher the carrier frequency, the larger the signal frequency bandwidth that can be transmitted;
2. Low loss: when transmitting 1.31um light, the loss per kilometer is less than 0.35dB. If the light of 1.55um is transmitted, the loss per kilometer is smaller, which can reach below 0.2dB;
3. Light weight: The optical fiber is a glass fiber with a small specific gravity, which makes it have the characteristics of small diameter and light weight, and the installation is very convenient;
4. Strong anti-interference ability: Since the basic component of optical fiber is quartz, it only transmits light, does not conduct electricity, and is not affected by electromagnetic fields. The optical signal transmitted in it is not affected by electromagnetic fields. strong resistance;
5. High fidelity: Since optical fiber transmission generally does not require repeater amplification, no new nonlinear distortion will be introduced due to amplification. As long as the linearity of the laser is good, the TV signal can be transmitted with high fidelity;
6. Reliable working performance: The number of devices contained in the optical fiber system is small (unlike the optical cable system which requires dozens of amplifiers), and the reliability is naturally high. In addition, the fiber optic equipment has a long service life, and the trouble-free working time reaches 500,000 times. approximately 750,000 hours;
7. Costs continue to decrease: As the source of materials (quartz) for manufacturing optical fibers is very rich, with the advancement of technology, the cost will be further reduced.
As the mainstream mode of broadband access, optical fiber has the advantages of large communication capacity, long relay distance, good security performance, strong adaptability, small size, light weight, wide source of raw materials, and low price. Applications are becoming more and more widespread.
Common fiber specifications are:
Single mode: 8/125μm, 9/125μm, 10/125μm
Multimode: 50/125μm, (European standard)
62.5/125μm, (US Standard)
Optical fibers currently used in optical fiber communication systems include silica optical fibers, plastic-coated optical fibers, single-mode optical fibers and multi-mode optical fibers.
Silica fiber is based on silica (SiO2) as the main raw material, and the refractive index distribution of the core and cladding is controlled according to different doping amounts. Quartz (glass) series of optical fibers have the characteristics of low power consumption and broadband, and are now widely used in cable television and communication systems. The advantage of silica glass fiber is low loss. When the light wavelength is 1.0-1.7μm (about 1.4μm), the loss is only 1dB/km, and the lowest is 1.55μm, which is only 0.2dB/km.
Plastic clad fiber (Plastic Clad Fiber) is a ladder-type fiber, with high-purity silica glass as the core and plastic with a slightly lower refractive index than silica, such as silica gel, as the cladding. Compared with silica fiber, it has the characteristics of high core rent and high numerical aperture (NA). Therefore, it is easy to combine with the light-emitting diode LED light source, and the loss is also very small. Therefore, it is very suitable for local area network (LAN) and short-range communication.
Single-mode fiber refers to a fiber that can only transmit one propagation mode within the working wavelength, usually called single-mode fiber (SMF: Single Mode Fiber). Currently, it is the most widely used optical fiber in cable television and optical communications.
Multimode fiber refers to a fiber in which the fiber has multiple propagation modes according to the operating wavelength, which is called a multimode fiber (MMF: Mlti ModeFiber). The core diameter is 50 μm, and since the transmission modes can reach hundreds, the transmission bandwidth is dominated by modal dispersion compared to SMF. Historically used for short-distance transmission in cable television and communication systems. Optical cables made of multimode fibers can be used for communications. It has good conduction properties and large transmission information capacity. A channel can hold dozens of people talking at the same time. Can transmit dozens of TV programs simultaneously