Fibre Optical Cabling Systems
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An optical fiber or fibre is a thin, transparent fiber, usually made of glass or plastic, for transmitting light. Fiber optics is the branch of applied science and engineering concerned with such optical fibers. Optical fibers are commonly used in telecommunication systems, as well as in illumination, sensors, and imaging optics.
The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Although fibers can be made out of either transparent plastic or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers, single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.
The light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use. The fiber absorption is minimal for 1550 nm light and dispersion is minimal at 1310 nm making these the optimal wavelength regions for data transmission. A local minimum of absorption is found near 850 nm, a wavelength for which low cost transmitters and receivers can be designed, and this wavelength is often used for short distance applications. Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction.
Since the refractive index of glass is around 1.5, the speed of light in the fiber is around 200,000 km/s, or two thirds of the speed of light in a vacuum.
For modern glass optical fiber, the maximum transmission distance is limited not by attenuation but by dispersion, or spreading of optical pulses as they travel along the fiber. Dispersion in optical fibers is caused by a variety of factors. Intermodal dispersion, caused by the different axial speeds of different transverse modes, limits the performance of multi-mode fiber. Because single-mode fiber supports only one transverse mode, intermodal dispersion is eliminated. For single-mode fiber performance is limited by chromatic dispersion, which occurs because the index of the glass varies slightly depending on the wavelength of the light, and light from real optical transmitters has nonzero spectral width. Polarization mode dispersion, which can limit the performance of single-mode systems, occurs because although the single-mode fiber can sustain only one transverse mode, it can carry this mode with two different polarizations, and slight imperfections or distortions in a fiber can alter the propagation velocities for the two polarizations. Dispersion limits the bandwidth of the fiber because the spreading optical pulse limits the rate that pulses can follow one another on the fiber and still be distinguishable at the receiver.
Because the effect of dispersion increases with the length of the fiber, a fiber transmission system is often characterized by its bandwidth-distance product, often expressed in units of MHz×km. This value is a product of bandwidth and distance because there is a trade off between the bandwidth of the signal and the distance it can be carried. For example, a common multimode fiber with bandwidth-distance product of 500 MHz×km could carry a 500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km.
In single-mode fiber systems, both the fiber characteristics and the spectral width of the transmitter contribute to determining the bandwidth-distance product of the system. Typical single-mode systems can sustain transmission distances of 80 to 140 km (50 to 87 miles) between regenerations of the signal. By using an extremely narrow-spectrum laser source, data rates of up to 40 gigabits per second are achieved in real-world applications.
Using Wavelength division multiplexing (WDM), the bandwidth carried by a single fiber can be increased into the range of terabits per second. This is accomplished by transmitting many wavelengths at once on the fiber. Wavelength division multiplexers and demultiplexers are used to combine and split up the wavelengths at each end of the link. In coarse WDM (CWDM) only a few wavelengths are used. One use of CWDM is to allow bidirectional communications over one fiber. Dense Wavelength Division Multiplexing (DWDM) usually involves transmitting and receiving more than eight "windows" of light. Sixteen, 40, and 80 windowed systems are common. Mathematically, 111 windows are possible over a single pair of optical fibers at the wavelengths used today.
The range of long-range systems is extended by the use of repeaters and optical amplifiers. A repeater is essentially a back-to-back receiver and transmitter, which regenerates the optical signal, eliminating or reducing the degradations resulting from transmission through the fiber. An optical amplifier is typically made by doping a length of fiber with the rare-earth mineral erbium, and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm). Because of their greater reliability, amplifiers have largely replaced repeaters in new installations.
Recent advances in fiber and optical communications technology have reduced signal degradation so far that regeneration of the optical signal is only needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost and reliability of repeaters is one of the key factors determining the performance of the whole cable system. The main advances contributing to these performance improvements are dispersion management, which seeks to balance the effects of dispersion against non-linearity; and solitons, which use nonlinear effects in the fiber to enable dispersion-free propagation over long distances.