UNIT III FIBER OPTICS AND APPLICATIONS PART-A MARK QUESTION AND ANSWERS 1. What is optical fiber? Optical fiber is a wave guide made up of transparent dielectric like glass or plastics in cylindrical form through which light is transmitted by total internal reflection. An optical fiber consists of a central core glass about 50μm diameter surrounded by a cladding about 15μm to 00μm diameter which is of slightly lower refractive index than core material.. Define fiber optics? Fiber optics is defined as a branch of physics which deals with the transmission and reception of light waves using optical fibers which acts as a guiding media. 3. Explain the basic principle of fiber optic communication. Total internal reflection is the principle of fibre optic communication. Principle:- When light travels from a denser to a rarer medium, at a particular angle of incidence called critical angle, the ray emerges along the surface of separation. When the angle of incidence exceeds the critical angle, the incident ray is reflected in the same medium and the phenomenon is called total internal reflection. 4. What are the conditions to obtain total internal reflection? Light should travel from denser medium to rarer medium The angle of the incidence on core should be greater than the critical angle. 5. What are the features of optical fiber? It is light in weight It has high band width and low loss There is no internal noise or cross talk It is non-conductive, non-radiative and non-inductive 6. What is the role of cladding in an optical fiber? An optical fiber consists of core which is surrounded by cladding, which has slightly lower refractive index than core. Here the role of cladding is to make the light to suffer total internal reflection inside the fiber, satisfying the condition that the light should travel from denser to rarer medium. 1
7. Define acceptance angle. Acceptance angle may be defined as the maximum angle at or below which the light can suffer total Internal Reflection. 8. Define numerical aperture of a fiber. Numerical Aperture determines the light gathering ability of the fiber. It is a measure of the amount of light that can be accepted by a fiber. Numerical Aperture (NA) can also be defined as the sine of the acceptance angle. If n1 and n are the refractive indices of core and cladding respectively. Then NA sin im n 1 n 9. What is meant by fractional index change? It is the ratio of refractive index difference in core and cladding to the refractive index of core. 10. How will you classify optical fibers? 1. Material Optical fibers are classified into three major categories based on. Number of modes 3. Refractive index profile. 1. Based on the material it can be classified into a. Glass fiber b. Plastic fiber. Based on number of modes they are classified as a. Single mode fiber b. Multimode fiber 3. Based on refractive index profile they can be classified as a. Step index fiber b. Graded index fiber 11. Write differences between single and multimode fiber? S.NO SINGLE MODE FIBRE MULTIMODE FIBRE 1 In single mode fiber only one mode can be propogated. The fiber in this case allows large number modes for light to pass through it. It has small core diameter and difference in refractive index of core and cladding is small. Here both the core and cladding refractive indices differences is large as the core diameter is large 3 No dispersion. Dispersion is more. 4 Since the information transmission Information can be carried to a
capacity is inversely proportional to dispersion T 1.The fiber can D carry information s to longer distance. 5 Launching of the light and connecting to fibers are difficult. 6 Initialization or fabrication is difficult as it is more costly. shorter distance only. Launching of the light and connecting to fibers are easy. Initialization or fabrication is easy as it cost is low. 1. What is called mode of propagation in optical fibers? Mode of propagation represents the number of possible directions or the path of propagation of light through the optical fibers. When single ray of light propagate through a path then it is called single mode and when many rays propagates through different directions it is called multimode. 13. What are the differences between step index and graded index fiber? S.NO. STEP INDEX FIBRE GRADED INDEX FIBRE 1 Difference in refractive index between the core and the cladding is obtained in a single step and hence called as step index fiber. The light rays propagate as meridonial rays and passes through fibre axis. 3 The path of light propagation is in zig-zag manner. Due to non uniform refractive indices their differences in refractive indices between the core and the cladding gradually increases from the centre towards the interface and hence called graded index fiber. Light propagation is in the form of skew rays and does not cross the fibre axis. The path of light propagation is Helical (i.e) spiral manner. 4 It has low bandwidth. It has a high bandwidth 5 Distortion is more in multimode step index fibre. Distortion is very less/no distortion due to self focusing effects. 6 No of modes of propagation No of modes of progation 3
d NA d NA N step 4.9( ) V / 4.9( ) V / (or) N step N Step Ngraded. 14. What is meant by splicing? Mention the two types of splicing. Splicing is the technique used to connect the fiber permanently. In this technique two fiber can be joined with the help of the elastomer (or) four rod splices, using an adhesive or matching gel. Types : 1. Mechanical Splices. Fusion Splices 15. List out the factors that causes loss in optical fibers. During the transmission of light through the optical fiber, three major losses will occur namely attenuation, distortion and dispersion. Attenuation is mainly caused due to the absorption, scattering and radiation of light inside the fibers. Distortion and dispersion occurs due to the spreading of light and also due to manufacturing defects. 16. What is meant by attenuation? It is defined as the ratio of the optical power output (P out ) from the fiber of length L to the power of input pin i.e attenuation (P in ) i.e Attenuation 10 Pin log db / Km L P out 17. What are the different types of light sources for optical fiber? Light emitting diode (LED) (a)planar (b)dome shaped LED Laser diodes (a) Homojunction laser 4
(b) Hetero junction laser (c)injection laser diode. 18. Why is laser much preferred then LED as an optical source? Laser light is much preferred than LED because it is more directional, highly coherent, faster rise time and narrow spectral width than LED. 19. What is meant by LED / give its principles An LED is an abbreviation of light emitting diodes. It is a semiconductor p-n junction diode which converts electrical energy to light energy under for ward biasing. 0. How a LED can be converted into laser diode? In a semiconductor diode, if the emission is not stimulated, the device is called light emitting diode (LED). To convert LED into a laser diode, high current density is required to a achieve population inversion and opposite surfaces should be polished. 1. What is meant by injection luminescence? Give examples? When the majority carriers are injected from the p to n and n to p region becomes excess minority carriers. Then these excess minority carriers diffuses majority carriers in p and a region and emits light. This phenomenon is known as injection luminescence. Example : light emitting diodes(led).. What is meant by a photo detector? Photo detector is a device used to detect the light falling over it. It converts the light falling over it. It converts light energy into electrical energy. i.e when the light falls on the photo detector, it produces the electron hole pairs and thereby generates current in the output. Examples: (i) Pin photo diode (ii) Avalanche photo diode (APD). 3. Give the applications of the fiber optical system. Mention the application of optical fibers in engineering field. 1. It can be used for long distance communication in trunk lines.. A large number of telephone signals nearly 15000 can be passed through the optical fibers in a particular time without interference. 3. It is used in computer networks, especially in LAN. 4. It is also used in optical sensors. 5
4. What are the types of sensors used in the fiber optics? There are two types of sensors used 1. Intrinsic sensors- here fiber itself acts as a sensing element. Extrinsic sensors- separate sensing system collects the light from the fiber. 5. Mention any 4 applications of fiber optic sensors.(dec 94)? 1. Fiber optic sensors are used as optical displacement sensors which is used to find the displacement of a target along with its position.. It is used as fluid level sensor. 3. It is used to sense the pressure and temperature at any environment. 4. It is also used to measure the number of rotations of the fiber coil using instrument called as gyroscope. 6. Give any applications of optical fibers in the industry? 1. Fiber optic endoscopes are used to find minor cracks and pores etc in big machinaries.. They are used in coagulation in chemical industries and labs. 3. It is also used to connect monitoring stations and remote sensor in industries. 7. State some of the application of optical fibers in medical field? 1. Fiber optic endoscopes are used in medical diagnosis.. It is used to visualize the inner organs of the body. 3. Fiber endoscopes are used in various medical fields such as cardiascopy, laparoscopy, cytoscopy. 8. What is meant by endoscope? A medical endoscope is a tabular optical instrument used to inspect or view the internal parts of the human body which are not visible to naked eye. A photograph of an internal part can also be taken using endoscopes. 6