The concept of wavelength division multiplexing (WDM) and DWDM has also been introduced. Finally, the latest trends in optical communication and networks are briefly explained.
Optical communication system is similar to any kind of communication system, as far as the basics are concerned. The major difference in optical and other communication models is the additional requirement of a source (for conversion of signal from electrical to optical domain), receiver (for conversion from optical to electrical domain) and use of fibre as media in place of conventional copper wire or microwave. The use of glass fibre or optical fibre features a number of advantages that make it a formidable media, in comparison to others.
One of the most important assets of optical communication is the availability of enormous potential bandwidth to the tune of hundreds of GHz. The potential cannot be fully harnessed owing to electrical domain limitation. However, technologies like WDM or DWDM are evolving to optimize the available bandwidth usage. Being fabricated from glass, the fibre does no have earth loop or interface problem as are evident in electrical media. The optical communication is immune to interference from radio frequency and electromagnetic radiations or EMP (electromagnetic pulses) as they act as dielectric waveguides. There are practically insignificant radiations from fibre which enhance security of signal in a communication system. Other advantages include low weight, small size, flexible, reliable, easily maintainable, low transmission losses and cost effective.
The requirement for capacity is increasing at a fast rate in the present day networks. Growth of worldwide web and Internet are the primary causes for this increase. Every four to six months, the need is almost doubling. Available broadband access technologies like Digital subscriber line (DSL), cable modems etc. provide bandwidths of 1Mbps and 28 to 56 kbps per user, respectively. With online businesses company networks with their distant locations and the clients, the bandwidth requirement is will soon rise to level where, the only solution would remain to switch to optical networks.
Optical networks can be broadly divided into two generations. The first generation optical networks were solely used to enhance capacity to stand alone communication links while; the second-generation networks encompass the issues of routing and switching.
In the subsequent paragraphs we will discuss optical communication and networks with reference to the various components and devices in greater details. We shall initially cover the basic theory behind transmission, attenuation and distortion of optical signal in the fibre.
Optical Transmission Basics
Optical fibre acts as a cylindrical waveguide with an inner core and outer cladding of differing refractive index as shown in Figure 1, core refractive index being greater than clad refractive index. The light traverses in the core as well as in the cladding. To illustrate the transmission of light, let's consider ray theory.
Figure 1: Structure of optical fibre (adopted from Senior, John M.)
The refractive index of