Wednesday, December 29, 2010
【 Weak current College 】 optical fibre definition 】
«Ò» ¡ ¢ optical fiber transmission
Until 1960, the United States scientists Maiman invented the world's first laser, optical communications provides excellent light source. Then more than 20 years, people have optical transmission media for research, has finally made a low loss optical fiber, thereby laid the cornerstone of optical communications. Since then, optical communications has entered a phase of rapid development.
Fiber optic transmission has many outstanding benefits:
1. Frequency bandwidth
Band width represents the size of the transmission capacity. The higher the frequency of the carrier, can transmit the signal, the greater the bandwidth. In the VHF band, carrier frequency for 48.5MHz ~ 300Mhz. About bandwidth, you can only transfer 250MHz 27 television and dozens of sets of FM radio. The frequency of visible light up to VHF band 100000GHz, more than $ 100 million times. Although due to fiber optic light on different frequencies are different, the band width is affected, but the minimum loss of bandwidth you can reach 30000GHz. Currently, a single light source of bandwidth only a very small part of them (multimode optical fiber frequency bands around several hundred megahertz, good single-mode fiber up to 10GHz above), the use of advanced coherent optical communication can 30000GHz range arrangement 2000 optical carrier, WDM, can accommodate millions of channels.
2. loss low
In coaxial cable system, the best cable signal transmission 800MHz, each km of the loss is 40dB or more. By contrast, the loss of optical fiber is much smaller, transfer 1, 31um light, the loss per km in 0.35dB following if transmission of light, per 1.55um km, lossy smaller, can reach 0.2dB following. This power loss than the coaxial cable to the small 100 million times, enabling it to transfer much of the distance. In addition, the optical transmission loss, there are two features that are in all cable TV channels have the same loss, don't need like the cable route that must lead a balanced per capita scales; the second is its loss almost do not change with temperature, do not worry because of the ambient temperature changes caused by the fluctuation of the main level.
3. light weight
Because fiber is very thin, single-mode fiber core diameter usually 4um ~ 10um, outer diameter only 125um, plus a waterproof layer, Rebar, sheath, with 4 ~ 48 consisting of fiber optic cable with a diameter of less than a standard coaxial cable 13mm, diameter 47mm are much smaller, and the proportion of glass fiber is small, so that it has a diameter of small, light weight, easy installation.
4. strong anti-interference ability
Because the basic ingredient is optical fibers, optical quartz, is not only spread, are not affected by electromagnetic fields conducting, in which the transmission of optical signals are not affected by the impact of electromagnetic fields, the fiber-optic transmission of electromagnetic interference, industrial interference has a strong defensive ability. Also because of this, fiber transmission signal and, therefore, not easily tapped to remain confidential.
5. high fidelity
Because optical fiber transmission generally do not need to relay amplifier, not because the zoom amusing new nonlinear distortion. As long as the laser, linear, to high definition and transmission of television signals. Actual tests showed that good am fibre system carrier combined third beat ratio C/CTB in 70dB above, alternating index more than 60dB cM also, much higher than the General cable route system for nonlinear distortion metric.
6. work reliable performance
We know that a system's reliability and composition of the number of devices in the system. Equipment, the more chances of failure. Because fibre system contains a number of devices (unlike cable systems that require dozens of amplifier), the naturally high reliability, coupled with optical devices for life is very long, and no fault working time upto 50 000 to 75 000 hours, of which the shortest life is the laser optical transmitter, the lowest life expectancy in 10 thousand hours. It is a well-designed, properly installed debugging behavior of fiber-optic system is very reliable.