Describe the construction of optical fiber. Explain the principle of propagation of light waves within a fiber. Define acceptance angle and acceptance. How optical fibers can be used in medical and communication fields? Write two prominent applications of optical fibers. What do you mean by numerical aperture of an optical fiber? Derive an expression for one numerical aperture of a step index optical fiber. Explain clearly the propagation of an electromagnetic wave inside an optical fiber. Use proper diagram. Derive the expression for the maximum acceptance angle of optical fiber in air.
Ans.: Light Propagation in Fibers
Let us consider light propagation in an optical fiber. The end at which the light enters the fiber is called the launching end. Let the refracting index of the core be n and the refractive index of the cladding be n2. Let the outside medium from which the light is launched into the fiber have a refractive index. Let a light ray enter the fiber at an angle to the axis of the fiber. Let the refracted ray make an angle with the axis and strikes the core-cladding interface at an angle.
Fractional Refractive Index Change This parameter is defined as the ratio of the difference between the refractive indices of the core and the cladding to the refractive index of core. It is denoted by and expressed as is always positive because refractive index of core is always grater then.
Advantages/Disadvantages of Optical Fiber
Advantages: 1. Cheaper. The main ingredient in glass is sand Hence it is very cost effective to make fiber. 2. Safety: Through optical fibers, photons of light move instead of an electrical current. Therefore, there is no change of a spark flash, which could be dangerous. 3. Radio frequency interference: Since the fiber system carries no electrical current, the energy transmitted through the fiber cannot radiate RF interference, nor can it be contaminated by any external noise or RF fields. 4. Security: Because of the absence of the flow of current through the fiber, criminal intrusion into the system is also prevented. Confidential information cannot be routed to unwanted. Receivers, nor can false information be fed into data stream. 5. Low losses: The transmission loss per unit length of an optical fiber is about 4 dB/km. Therefore, longer cable runs between repeaters are feasible. Examination of 46 fiber types revealed a range of losses from 2 to 385 dB/km, with an average loss lf 27 dB/km. 6. Wider bandwidth: Within a totally closed system, the number of signals that can be modulated on a fiber optic light beam exceeds the number that can be modulated on a very high frequency RF carrier by a factor of about 1000. 7. Deterioration: Glass is immune to corrosive and oxide degradation and will stand up well in harsh environments. Moisture, toxic vapors and acids will not degrade the glass fibers. 8. Small size and light weight: The size of the core and clad of a single fiber conductor is much smaller than the diameter of a common copper wire conductor; however, when the insulation is included, the sizes are similar. Bundles of optical fiber cables are smaller by a factor of 10 and weigh less by a factor of 14 than an equal number of copper wire conductors. 9. Temperature: Excluding the protective insulation, the melting point of glass is much higher than copper and that of copper is much higher than the plastics used for cores and clods. 10. Long life: The life expectancy of glass fibers is predicted to exceed 100 years. (This prediction is based on the history of glass and due to its immunity to harsh environment).
Disadvantages Limited application: All fiber optic systems are limited to fixed point to point ground installations. They cannot leave the ground nor be associated with a mobile communication station. 2. Nuclear radiation: Glass, when exposed to neutron bombardment, will darken. The harder the glass, the more quickly it will discolor. 3. Low power: Popular light emitting sources are restricted to very low power devices. Though, higher power devices are available but they are very costly. 4. Distance: Because of the low power sources, the distance between repeater amplifiers must be relatively short for the high data rates demanded in some systems. 5. Modulation: There are limited ways in which the light source can be modulated. 6. Fragility: The other disadvantage with optical fiber is that these fibers are easily broken or damaged due to age and vibrations.
Uses of Optical Fibers Optical fibers are widely used in many systems. Some of these are as follows: 1. The most important application of optical fibers is in the field of communications as information channel or transmission medium. Other telecommunication networks include telephone, cable TV, videophone, multimedia, desktop teleconferencing etc. 2. In military mobiles such as air-craft, ships tanks etc., fiber guided missiles, short and long distance communication links. 3. Close circuit TV (CCTV) links for traffic controls, security etc. 4. In ophthalmology, a laser beam guided by the fibers is used to reattach detached retinas and to correct defects in vision. 5. In the fabrication of fiberscope in endoscopy for the visualization of internal portions of the human body. 6. In sensors and transducers. 7. The signal multiplexing and transmission in automotive electronics, centralized locking, door lamps power windows; seat mechanism etc. in automobiles is under investigation. The fiberscope is also useful in industry. It could be used to examine welds, nozzles and combustion chambers inside the aircraft engines. These are not easily accessible for observation otherwise.