1.2 OPTICAL NETWORKS
Optical networks (in which data is converted to bits of light called photons and then transmitted over fiber) are faster than traditional networks (in which data is converted to electrons that travel through copper cable) because photons weigh less than electrons, and further, unlike electrons, photons do not affect one another when they move in a fiber (because they have no electric charge) and are not affected by stray photons outside the fiber. Light has higher frequencies and hence shorter wavelengths, and therefore more "bits" of information can be contained in a length of fiber versus the same length of copper.
Optical glass fibers based on the principle of total internal reflection, which was well known in the 1850s, were developed for endoscopes early in the 1900s. The use of fiber glass for communication was first proposed by Kao and Hockham in 1966 . The manufacture of optical fiber began in 1970s. A variety of optical networks came into existence in the late 1980s and early 1990s which used optical fiber as a replacement for copper cable to achieve higher speeds. Computer interconnects such as ESCON (Enterprise Serial Connection), Fiber Channel, and HiPPI (High Performance Parallel Interface), for interconnecting computers to other computers or peripheral systems, use low bit-rate optical components which are inexpensive. FDDI (Fiber Distributed Data Interface) uses dual, fiber optic token rings to provide 100200 megabits per second (Mb/s) transmission between workstations. SONET/SDH (Synchronous Optical NETwork in North America, Synchronous Digital Hierarchy in Europe and Asia)3 which forms the basis for current high-speed backbone networks and one of the most successful standards in the entire networking industryallows seamless interworking of fibers up to OC-192 rate of about 10 gigabits per second (Gb/s). (OC-n [Optical Carrier-n] specifies an electronic data rate of n × 51.84 Mb/s approximately; so OC-48 and OC-192 correspond to approximate data rates of 2.5 Gb/s and 10 Gb/s, respectively. OC-768 [40 Gb/s] is the next milestone in highest realizable electronic communication speed.)
1.2.1 Optical Fiber Principles
Optical fiber consists of a very fine cylinder of glass (core) through which light propagates. The core is surrounded by a concentric layer of glass (cladding) which is protected by a thin plastic jacket as shown in Fig. 1.1(a). The core has a slightly higher index of refraction than the cladding. The ratio of the indices of refraction of the cladding and the core defines a critical angle, 0c. What makes fiber optics work is total internal reflection: when a ray of light from the core approaches the core-cladding surface at an angle less than 0c, the ray is completely reflected back into the core (see Fig. 1.1[b]).
Since any ray of light incident on the core cladding surface at an angle less than 0c(critical angle) is reflected internally, many different rays of light from the core will be bouncing at different angles. In such a situation, each ray is said to have a different mode and hence a fiber having this property is called a multimode fiber (see Fig. 1.2[a]). Multiple modes cause the rays to interfere with each other, thereby
Figure 1.1 (a) Optical fiber. (b) Reflection in fiber.
Figure 1.2 (a) Multimode fiber (multiple rays follow different paths). (b) Single-mode fiber (only direct path propagates in fiber).
limiting the maximum bit rates that are achievable using a multimode fiber. If the diameter of the core is made very narrow, the fiber acts like a waveguide, and the light can travel in a straight line along the center axis of the fiber. A fiber having this property is called a single-mode fiber (see Fig. 1.2[b]). Single-mode fibers can transmit data at several gigabits per second over hundreds of kilometers and are more expensive. In multimode fibers the core is 50 microns (1 micron is 10-6 meter) in diameter, whereas in single-mode fibers the core is 8 to 10 microns.
1.2.2 Optical Transmission System
An optical transmission system has three basic componentstransmitter, transmission medium, and receiveras shown in Fig. 1.3. The transmitter consists of a light source (laser or LED [light-emitting diode]) that can be modulated according to an electrical input signal to produce a beam of light which is transmitted into the optical fiberthe transmission medium. Typically the binary information sequence is converted into a sequence of on/off light pulses which are then transmitted into the optical fiber medium. At the receiver, the on/off light pulses are converted back to an electrical signal by an optical detector. Thus we have a unidirectional transmission system (operating only in one direction) which accepts an electrical signal, converts and transmits it by light pulses through the medium, and then reconverts the light pulses to an electrical signal at the receiving end.
Figure 1.3 Optical transmission system.
Attenuation in fiber leads to loss of signal power as the signal propagates over some distance. Optical fiber had an attenuation loss of 20 dB/km when it was invented in 1970, but within 10 years fibers with a loss of 0.2 dB/km had become available. The attenuation in decibels is computed as 10 log10(transmitted power/received power). The attenuation of light through fiber depends on the wavelength used. Figure 1.4 shows the attenuation in decibels per (linear) kilometer of fiber. As can be seen from this figure, there are three low-loss windows (bands) centered at 0.85, 1.30, and 1.55 microns. Early optical fiber transmission systems (in the 1970s) operated in the first, that is, 0.85-micron band at bit rates in the tens of megabits per second and used relatively inexpensive LEDs as the light source. Present ones use laser sources and operate in the 1.30- and 1.55-micron bands, achieving rates of gigabits per second. Attenuation is primarily due to impurities (water vapor) in the fiber glass and Rayleigh scattering (when the medium is not absolutely uniform, it causes small fluctuations in the refractive index, which in turn causes the light to be scattered and thereby attenuating the propagating signal). To overcome attenuation, electronic regenerators ("refueling stations"), also called repeaters, are used between fiber sections (see Fig. 1.3), which restore a degraded signal for continued transmission.
Figure 1.4 Attenuation versus wavelength for optical fiber.
As the light pulses propagate through the fiber, the pulses spread out, which means the duration of the pulses broadens. This spreading is called dispersion, and the amount of it is wavelength-dependent. Dispersion limits the minimum time interval between consecutive pulses (because of interference with pulses of light ahead and behind) and hence the bit rate. There are two basic dispersive effects in a fiber. They are intermodal dispersion and chromatic dispersion. Intermodal dispersion occurs in multimode fibers; in these fibers, as the energy in a pulse travels in different modes, each with a different velocity, the pulse gets smeared after it has traveled some distance along the fiber. Chromatic dispersion is caused by the transmitting laser, which is unable to send all the photons at exactly the same wavelength, so different wavelengths travel at different speeds. A special pulse shape, called a soliton, has been discovered that retains its shape as it propagates through the fiber. Thus solitons provide a solution to the dispersion problem. Experiments have shown that solitons can achieve speeds of 80 Gb/s over distances of 10,000 km.