opt comn 2 mark ques with answers

7/29/2019 Opt Comn 2 Mark Ques With Answers

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UNITIII FIBER OPTICS AND APPLICATIONS

PART-A 2 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 cylindricalform through which light is transmitted by total internal reflection. An optical fiber consists of a

central core glass about 50m diameter surrounded by a cladding about 125m to 200mdiameter which is of slightly lower refractive index than core material.

2. Define fiber optics?

Fiber optics is defined as a branch of physics which deals with the transmission and reception of lightwaves 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 froma 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 isreflected 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 refractiveindex than core. Here the role of cladding is to make the light to suffer total internal reflection inside thefiber, satisfying the condition that the light should travel from denser to rarer medium.

7. Define acceptance angle.Acceptance angle may be defined as the maximum angle at or below which the light can suffertotal Internal Reflection.

8. Define numerical aperture of a fiber.

Numerical Aperture determines the light gathering ability of the fiber. It is a measure of theamount 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 n2 arethe refractive indices of core and cladding respectively. Then

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?Optical fibers are classified into three major categories based on

1. Material2. Number of modes


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3. Refractive index profile.1. Based on the material it can be classified into

a. Glass fiber b. Plastic fiberrefractive indices differences islarge as the core diameter is large3 No dispersion. Dispersion is more.4 Since the information transmission Information can be carried to a

3

capacity is inversely proportional

to dispersion

D .The fiber can

carry informations to longer

distance. shorter distance only.5 Launching of the light and Launching of the light andconnecting to fibers are difficult. connecting to fibers are easy.

6 Initialization or fabrication isdifficult as it is more costly. Initialization or fabrication is easy

as it cost is low.

12. 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 paththen it is called single mode and when many rays propagates through different directions it iscalled 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 indexbetween the core and

thecladding is obtained ina singlestep and hence calledas step

index fiber.

Due to non uniform

refractive indicestheir differences in

refractive indicesbetween the core and thecladdinggradually increases fromthe centre

towards the interface andhence calledgraded index fiber.

2 The light rays

propagate asmeridonial rays and

passesthrough fibre axis.

Light propagation is in

the form ofskew rays and does notcross the fibreaxis.

3 The path of lightpropagation is

in

The path of lightpropagation is Helical

(i.e) spiral manner


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zig-zag manner

4 It has low bandwidth . It has a high bandwidth

High bandwidth 4 It has low bandwidth

Distortion is very less no distortiondue to self focusingeffects

.

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 ormatching gel.

Types : 1. Mechanical Splices2. 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 occurnamely attenuation, distortion and dispersion.Attenuation is mainly caused due to the absorption, scattering and radiation of lightinside the fibers.Distortion and dispersion occurs due to the spreading of light and also due tomanufacturing defects.

16. What is meant by attenuation ?

It is defined as the ratio of the optical power output (Pout) from the fiber of lengthL to the power of input pin i.e attenuation (Pin)

i.e Attenuation dB Km

P

P

L out

in log /

10

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

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(b) Hetero junction laser (c)Injection laser diode.

18. Why is laser much preferred then LED as an optical source?


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Laser light is much preferred than LED because it is more directional , highly coherent, faster rise timeand narrow spectral width than LED.

19. What is meant by LED / give its principlesAn LED is an abbreviation of light emitting diodes. It is a semiconductor p-n junction diode whichconverts electrical energy to light energy under for ward biasing.

20. 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 populationinversion and opposite surfaces should be polished.

21. 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).

22. 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. Itconverts light energy into electrical energy. i.e when the light falls on the photo detector, it produces theelectronhole pairs and thereby generates current in the output . Examples: (i) Pin photo diode (ii)

Avalanche photo diode (APD).

23. 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. 2. A large number of telephone signals

nearly 15000 can be passed through the optical fibers in a particular time without interference. 3. It isused in computer networks , especially in LAN. 4. It is also used in optical sensors.

24. What are the types of sensors used in the fiber optics?

There are two types of sensors used1. Intrinsic sensors- here fiber itself acts as a sensing element2. Extrinsic sensors- separate sensing system collects the light from the fiber.

25. 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 atarget along with its position.

2. 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 asgyroscope.

26. Give any 2 applications of optical fibers in the industry?

1. Fiber optic endoscopes are used to find minor cracks and pores etc in big machinaries.

2. They are used in coagulation in chemical industries and labs.

3. It is also used to connect monitoring stations and remote sensor in industries.


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27. State some of the application of optical fibers in medical field?

1. Fiber optic endoscopes are used in medical diagnosis.

2. 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 .

28. 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

opt comn 2 mark ques with answers

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