Question and Answer of Optical Fibre

REMEMBER THESE

1. Optical fibres are waveguides that guide light through them on the principle of total internal reflection. 2. Light incident at fibres at angles less than acceptance angle do not propagate into the fibre. 3. Numerical aperture of an optical fibre is a measure of its light gathering ability. It is defined by :

4. An optical fibre may be made of glass or plastic. It consists of essentially three regions. The central region is known as core. The middle region is called the cladding. The outer region is a protective sheath. 5. The refractive index of cladding region is always less than that of the core region. 6. Optical fibres are divided into different groups basing on their refractive index profile. 7. If the refractive index of core is radially constant and abruptly changes at core cladding boundary, it is called step index fibre. 8. If the refractive index of core varies radially, it is known as graded index fibre. 9. Light follows zig zag paths in an optical fibre. The possible paths of light in an optical fibre are called modes. The no. of modes possible in a fibre is related to the core diameter and numerical aperture as

10. A single mode fibre is very thin and allows only axial rays. 11. Glass fibres are made from silica.

12. Light gets attenuated in an optical fibre due to scattering, microbending, impurities etc. 13. An optical signal spreads due to dispersion effects as it travels in the fibre. If the signal pulse rate is too fast, dispersion will cause the pulses to overlap giving rise to distortion. 14. Different source wavelengths and modes are the two main factors which caues dispersion.

15. A light source, an optical fibre and a photodetector are the basic components of a fibre optic system.

SHORT ANSWER TYPE QUESTIONS

Q 1. Define acceptance angle and numerical aperture in optical fibre. Ans. Acceptance angle : It is defined as the half angle of the cone within which the light is totally reflected by the fibre core. Acceptance angle

n1- refractive index of core ; n2 - refractive index of cladding ; no - refractive index of outside medium. Numerical aperture : The light gathering ability of a fibre is related to its acceptance angle and is expressed as the numerical aperture of the fibre.

Q 2. Plot the graphs for type-I and type-Il superconductors. Ans. Graphs for Type I and Type II superconductors

Graph is plotted between external magnetic field H and intensity of magnetism atoms.

Q 3. Define bending losses in OFC’s. Ans. Optical fibres suffer radiation losses at bends on their paths. In this loss, the part of the propagating made which is outside of bend is required to travel faster than that of inside so that a wavefront perpendicular to direction of propagation is maintained. Hence, part of mode is cladding needs to travel faster than velocity of light which is impossible. As a result, the energy associated with this part of mode is lost through radiation. Q 4. What are the advantages of optical fibres in communication systems?

Ans. Optical fibre transmit the light from one point to another, by total internal reflection (TIR). Their main advantages in communication are 1. They are light weight. 2. They have extremely high wide band width. 3. Their raw material is siIica, which is present in plenty. 4. Their communication is more reliable i.e. cross talk is not possible. 5. Since fibers -are made upto glasses of plastic (insulators), so insulating coating between the core and cladding is not required. 6. Signals are delivered at low cost.

Q 5. Distinguish between a step-index fibre and graded index fibre.

Ans. Difference between step-index and graded index fiber: 1. Multi-mode graded index fiber can easily couple light info and single mode step-index fiber but with more difficulty than in multi mode step-Index fiber. 2. The distortion of light pulse while passing through multi-mode fiber is more than single-mode step-index fiber. 3. Multi-mode graded index fiber are easier to construct than single mode step-index fiber.

Q 6. Explain the principle of optical fibre communication. OR What is total internal reflection? Ans. Phenomenon of reflection of light into a denser medium from an interface of denser medium and rarer medium is called, total internal reflection. Two essential conditions for total internal reflection are (i) Light should travel from a denser medium to a rarer medium. (ii) Angle of incidence in denser medium should be greater than the critical angle the pair of media-in contact.

Q 7. Why the information carrying capacity of an optical fibre is very much greater than the conventional radiowaves and microwaves? Ans. Th information carrying capacity of optical fibre is very much greater than the radio and micro-waves because we can send a large number of signals via optical fibres without interfering each other.

Q 8. Write the various modes of an optical fibre. Ans. There are three types of optical fibre configurations 1. Single-mode step index fibre. 2. Multi-mode step index fibre. 3. Multi-mode graded index fibre.

Q 9. What are splicers and couplers? Ans. Splicers are permanent or semi-permanent joints formed between the individual optical fibres which are frequently used to establish optical fibre links.

Coupler is a device that distributes light from main fibre into one or more branch fibre. They are used in data buses, communication network etc.

Q 10. Explain the basic structure of optical fibre. Ans. Basic structure of optical fibre:

Optical fibre is a thin flexible thread of transparent plastic or glass. The inner most part is called core. The core is surrounded by a layer called cladding. The refractive index of core is slightly more than that-of cladding. Around the cladding, there is a silicon coating, which protects the cladding from the moisture and impurities and also provide mechanical strength. Further the buffer coating and black polyurethane outer jacket provides overall mechanical strength to fibre. Q 11. Define normalised frequency.

Ans. Normalised frequency : It gives the maximum no. of modes that a fibre an support, then

Q 12. What do you mean by material dispersion? Ans. The refracture index of core material depends upon the wavelength of the guided mode. As the group velocity of given mode depends upon refractive index of core of fibre, therefore, group velocity of an given mode depends upon the wavelength. The pulse spreading occurs even when different wavelengths follow the same path. The transmitters transmit light over a band of wavelength. Different spectral components of an optical pulse have different transit time and therefore the spectral components of pulse combine to produce broadened pulse with lower peak amplitude at fibre end.

LONG ANSWER TYPE QUESTIONS

Q 1. What is meant of acceptances angle for an optical fibre? Show how it is related to numerical aperture. OR Calculate the expression for NA. for OFC’s. Ans. Acceptance angle may be defined as the maximum angle that a light ray can have relative to the axis of fibre and propagates down. the fibre. Relation between NA and Acceptances Angle : Let the beam enter an angle ‘i’ to the ax.is of the core of fibre and makes an angle 0 with the normal at the core ladding interface. In order that light signal is to be propagated through the core, it should be totally internally reflected at core-cladding interface. For it, angle should be greater than critical angle C.

Let n1 = refractive index of core n2 = refractive, index of cladding n0 = refractive index of outside medium. Applying Shell’s law at point P

Applying Shell’s law at point Q

Put these values in eqn (1) and (2) eqn (1)

eqn (2)

squaring and adding eqn (3) and (4)

Numerical aperature N.A.

Q 2. A step, index fiber has a normalized frequency = 26.6 at 1300 nm wavelength. if core is thick calculate the acceptance angle of the fiber. Ans.

Q 3.The light gathering capacity of an optical fibre is 0.479. If relative core cladding index difference is 0.0005, calculate the refractive index of cladding, if outside medium is air.

Ans.

Q 4. What is splicing? Define its types. Explain optical couplers. Ans. (I) Splicing : The process of joining the two fibres ends permanently through an optical link is called splicing. Splicing is a sort of soldering. Types of splicing 1. Fusion splicing : ln this process of fusion splicing, the ends of fibres are aligned either manually using micromanipulators and a microscope system for viewing the splice, or automatically either using cameras or by measuring the light transmitted through the splice and adjusting the position-of fibres to optimise the transmission. The ends of fibres are then melted together using a gas flame or an electric arc. In this process, perfect splices are obtained with losses as low as 0.02 dB. 2. Mechanical splicing : It is the process in which two fibre ends are held together in a -splice, it consists of some device usually made of glass which automatically brings two fibres into alignment. The openings at each end of device are usually fluted to allow the fibres to be guided into capillary in which alignment takes place. The splice is first filled with an optical cement whose refractive index is same as that of core of fibre. After the fibres have been entered into splice they are adjusted to give the optimum transmission of light. At this point they are clamped in position and whole assembly is exposed to UV light which cures the cement.

(ii) Optical Couplers : An optical fibre coupler is a device that distributes light from a main fibre into one or more branch fibres. Now-a-days, these couplers are used in LANS, telecommunication access networks etc. Optical fibre couplers are passive devices in which the power transfer takes place by (a) the fibre cross-section by butt joining the fibres or by using some form of imaging optics between the fibres or (b) by through the fibres surface and normal to its axis by converting the guided core modes to both cladding and refracted modes which then enable the power sharing mechanism. The multimode fibre couplers are sub-divided into the following main categories : (a) Three and four port couplers which are used for signal splitting, distribution and combining. (b) Star couplers are generally used for distributing a single input signal to multiple outputs. (c) Wavelength division multiplexing, which is a specialized form of coupler designed to permit a no. of different peak wavelength optical signals to be transmitted in parallel on a single fibre.This device either combine the different optical wavelength optical signals or separate the different optical signals at the outputs.

Q 5. What are various kinds of losses in optical fibres? Explain the different mechanisms of dispersion in fibres. Ans. The light pulses that propagate through a fibre suffer several dispersion effects. It causes spreading of output pulse in the time domain and changes its shape. Thus the optiöal signal becomes distorted as it propagates through an optical fibre. This distortion of the signal is due to the two types of dispersion 1. Intramodal dispersion 2. Intermodal dispersion

1. Intramodal Dispersion The intramodal dispersion is also called chromatic dispersion The intramodal dispersion is pulse spreading that occurs within a single mode. This dispersion is due to the fact that group velocity of guided mode is a function of the wavelength. (The group velocity is the speed at which energy in a particular mode travels along a fibre). The intramodal dispersion depends upon the wavelength and therefore, its effect on signal distortion increases with the spectral width of the opticalsource. The spectral width of the optical source is defined as the band of wavelengths over which the optical source emits light. As shown in The fig. is peak emission wavelength and is the spectral width. The dispersion that loccurs due to the core material as a function of a wavelength is called intramodal dispersion. The spectral width is normally characterized by the root mean square spectral width denoted by In case of light emitting Eliode as an-optical source the spectral width is approximately 5 percent of the central wavelength. For example, let the peak emission wavelength of an LED source be 800 mm. Then spectral width is approximately 40 mm. ieans that LED source can emit most of its spectral power in wavelength band 780 n.m. to 820 n.m.

There are two main causes of intramodal dispersion. (a) Material dispersion (b) Wavelength dispersion. (a) Material dispersion : The refractive’ index of the core material depend upon the wavalength of the guided mode. As the group velocity of the given mode depends upon the refractive index of the core material of fibre, therefore, group velocity of an given mode depends upon the wavelength. The above said effect is similar to the deviation of the different wavelengths by different angles by a given prism due to the different refractive index of the prism for different wavelengths. (b) Waveguide dispersion : In a given single-mode optical fibre, 80% of the total optical power is being transmitted through the core. The remaining 20% signal is travelling through the cladding. As the refractive index of the cladding is less as compared to the refractive index of core, light signal propagating through the cladding travels faster than light signal propagating

through core. The dispersion that arises is called waveguide dispersion. It is observed that in case of a single mode operation near zero, dispersion wavelength , the intermodal and material dispersion disappear and waveguide dispersion its name. arises. The propagation constant K is funcfion of where ‘a’ is the radius of core, and is the wavelength of the signal. Hence the amount Of waveguide dispersion depends on the fibre dimension. This effect is only the result of the guiding properties of the fibre and hence its name. 2. Intermodal Dispersion : The intermodal dispersion is a result of the fact that each mode is having different value of the group velocity at a single frequency. The intermodal dispersion gives rise to pulse spreading or pulse dispersion. A dispersion in which the pulse spreading is due to the result of each mode having different value of the group velocity at a single frequency is called intermodal dispersion.

Q 6. An optical fibre has a N.A. of 0.15 and a cladding refractive index is equal to 1.50. Find the N.A. of the fibre in a liquid of refractive index 1 30 . Ans. Given NA. = 0.15

Q 7. Differentiate between Step-index and Graded index fibre Ans.

Q 8. What will be the critical angle for a ray in a step-index fibre for which n = 1.53 and which has a cladding whose refractive index is 2.5% less. Ans. Refractive index of core, n1 = 1.53

Q 9. What is optical fibre cable? Explain the basic theory of propagation of light in fibre. Ans. Optical fibres are used as cables to transmit light signals from one place to another without any appreciable loss in intensity of light signal. It has the following main parts (i) Central core : It is the innermost core of the optical fibre made of thin and fine quality transparent glass, polymer or dielectric. The diameter of this core is , with refractive index (ii) Cladding It is a layer made of glass or plastic which surrounds the central core of optical fibre. Its inner and outer diameters are 100 p.m and 420 jim. Refractive index of cladding is less than that of. centralcore i.e.

(iii) Plastic Jacket Central core and cladding are enclosed in a protective jacket made of plastic, for providing safety and strength to optical fibre.

Q 10 An optical fibre has a numerical aperture of 0 20 and a cladding refractive index of 1.59. Determine the acceptance angle for the fibre in water which has a refractive index of 1.33. Ans. Given N.A. = 0.20 We know

Q 11. Describe briefly the applications of optical fibres. Ans. (i) In computer industry, optical fibres are used to transmit signals from one source to another. (ii) Optical fibres are widely used in communication. (iii) Automobile industry uses optical fibres. (iv) Fibre optics has wide applications in medical field and military for security purpose.

Q 12. Explain the modes of propagation for different types of fibre. Ans. 1. Multi-mode graded index fiber can easily couple light into and out of fiber of single mode step-index fiber but with more difficulty than in multi mode step-index fiber. 2. The distortion of light pulse while passing through multi-mode fiber is more than single-mode step-index fiber. 3. Multi-mode graded index fiber are easier to construct than single mode step-index Material dispersion arises because of variation of the refractive index of the core material as a function of wavelength. This type of dispersion causes a wavelength dependence of the ‘group velocity of any given mode. Pulse dispersion is an optical fibre where the information is coded in the form of discrete ulseS and transmitted through the fibre optic communication system. The number of pulses that can be sent per unit time determines the information capacity of the system. For getting good output various pulses must be resolved in time.

Q 13. Calculate the numerical aperture and acceptance angle of a fiber with a core index of 1.54 and a cladding index of 1.50. Ans.

Q 14. (a) What is numerical aperture? Explain material dispersion and pulse dispersion in optical fibre. Ans. Light gathering ability of a fibre is related to its acceptance angle and is expressed as numerical aperture i.e. Material dispersion arises because of variation of the refractive index of the core material a function of wavelength. This type of dispersion causes a wavelength dependence of the up velocity of any given mode. Pulse dispersion is an optical fibre where the information is coded in the form of discrete pulses and transmitted through the, fibre optic communication system. The number of pulses t can be sent per unit time determines the information capacity of the system. For getting output various pulses must be resolved in time.

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