The salient features are as follows:
(a) Wide Band Width. It can accommodate number of channels through one media. AT&T’s cable can handle 6000 simultaneous telephone calls, through a pair of cable. Latest WT-4 cable which is made by Bell Labs can handle 2,30,000 channels through a single pair of line. It can handle 2MB/Sec (Million Bits/Sec). With this rate 30 volume of 20th Century Chamber’s Dictionary (1700 pages) can be transmitted in less than a sec.
(b) Attenuation. In earlier days from 10 db/ km to 2 db/ km attenuation was brought and it gives satisfactory result. Today lowest attenuation is 0.19 db per km at 1.55 m wavelength of light. Lowest attenuation in co-axial system is 10db/km. 1 db/km is general attenuation without a repeater of 100 km.
(c) Noise Immunity. Fibre cable are inherently immune from any radiation, i.e., electro-magnetic or electrostatic interference, e.g. lighting motors, EHT (Extra High Tension) power lines, RF, cross talk and various pick up, etc, irrespective of data rate.
(d) Size and Weight. It offers greater signal handling capacity accompanied by smaller size and lighter weight. A copper cable, which can handle 36000 channels has a dia of 7.5 cm and weighs 11 kg/m. But a fibre cable carrying 50,000 channels is only 1.25 cm in dia and weighs just 1.2 kg/m. Thus it saves cost, shipping and storage.
(e) Compatibility with Conventional Electronics. It perfectly matches with modern electronic communication system utilizing CMOS/TTL, IC and VLSI circuits.
(f) Modular Design. It can be easily designed into small modules, which can easily be replaced. It eases the servicing and fault finding.
(g) Security. It is an almost perfectly secured system. Because it is extremely difficult to tap the information from cable without detection. If in case the cable is broken, light will escape and will be absorbed into space without any effective use.
(h) Safety. It is totally safe, because as it does not handle large voltages, no sparking or short circuit will be caused. It can be used in hazardous environments.
(j) No scarcity of Raw Material. It is made of silica, which is available on earth in abundance.
(k) Insulating Medium. Silica is a good insulating medium, which is utilized in several applications.
Fibre optical transmission system can be divided into three main functional units:
(a) The transmitter or Source.
(b) The receiver or Detector.
(c) The propagating Media.
Propagating Medium. Plastic and glass fibre are used as propagating medium of light signal from transmitter to receiver. The principle of transmission of energy along with an optical fibre is similar to the concept of total internal reflection, which occurs when light in a glass core strikes the boundary of the glass sheath of lower refractive index at greater than critical angle. It is directly depending on the ratio of the two refractive indices. Plastic fibre cable is used in optical transmission because of their low cost, high source fibre, coupling efficiency and ease of handling. Their attenuation is high.
Critical Angle. It is that incident angle at which refracted light travels along with axis (surface of outer boundary). Striking rate is always greater than the critical angle. Optical fibre cable consists of glass clad with a sheath of different glass; core has a higher refractive index than sheath. Core has got very low optical scattering and absorption for low losses. There are three types of fibre:
(a) Step Index Fibre (SI).
(b) Graded Index Fibre.
(c) Single Mode or Mono Index Fibre.
Thus we can recall that fibre optics communication system is the one in which light is used as carrier to carry various types of signals. It needs an optical transmitter, an optical receiver and a physical optical cable as medium. It is so emerging out as the leader of communication. In India, P&T, Railways, ONGC, Defence Services and various public sectors are extensively using it.
Laser Communication. No other invention in the history of scientific development has made such a far-reaching impact in various areas of science and technology as LASER. It stands for Light Amplification by Stimulated Emission of Radiation.
In 1954 the first low noise microwaves amplifier produced by Professor Towens and his colleagues, called MASER (Microwave Amplification by Stimulated Emission of Radiation). It was extremely low noise amplification of microwave signals by a Quantum-Mechanical Process. The LASER or optical MASER (L stands for light) is a development of this idea, which permits the generation or amplification of coherent light. Coherent means single frequency, in phase, directional and polarized. The Laser is a source of coherent electromagnetic wave at infra red and light frequencies. This ranges from 430 to 750 Tera Hertz (T Hz) (i.e., 430,000 G Hz to 750,000 G Hz) (1 T Hz = 1000 G Hz).
The first Laser, using RUBY was proposed in 1958 and a scientist named Theodore Miaman in 1960 developed practical Laser. The first continuously operating Laser was followed in 1961 and used a mixer of helium and neon gases. Laser has a wide range of applications as in saving life in ophthalmic and other types of surgery and for military purpose.
Principle. Laser is based on the principle of spontaneous and stimulated emission of photon. The atom can without external force, spontaneously emit unwanted energy as photon. The released photon drains off enough energy from the atom to return to the ground state. The process of spontaneous emission takes place very rapidly in a tiny fraction of a second. It can also happen that a photon leaving excited atom strikes another atom stimulating it to give up its photon sooner than it would have been released spontaneously. The collision process between photon and excited atoms starts a chain reaction, which causes more and more atoms to give up, thus releasing vast amount of energy. This energy starts building up a massive wave front, which grows with each collision between a photon and an excited atom. The length of time spent in excited state is called ‘life time’ and varies with atoms of different materials and energy levels.
Application. The application of laser is wide ranged. Few of them are discussed in the following paragraphs:-
(a) Spectroscopy. Spectroscope is an instrument used to study intensity and wave length of a ray of light. Powerful impact of laser beams can be applied in spectroscopy on the principle of Raman scattering to generate high intensity laser lights at many different frequencies.
(b) Power Transmission. A laser beam could be focused to heat a pot of coffee from a distance of 1000 miles. With the improved technology scientists are trying to achieve laser power lines from satellite to operate low power equipment.
(c) Satellite Nudging. Nudging means shifting slightly. Light exerts a tangible pressure. A laser beam exerts a pressure of several pounds/sq inch over a tiny area. When satellite begin to slow down and thus be drawn towards the earth, a laser beam projected from earth can be used like a giant finger gently to push it back into higher orbit.
(d) Radar. Powerful laser pulses are capable of producing measurable reflection over greater distances than microwaves. At the 1.10-inch wavelength (X) of light, radar can detect much smaller object than can be done with inches long wavelength microwaves. Even high velocity measurement can also be made.
(e) Communication. A laser beam is theoretically capable of simultaneously transporting voice of 10 channels. Thus more number of channels can be accommodated than any other communication. Laser beams have become a means of communication between earth and moon or other satellites.