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University Question Key- December 2009

Part-A

1. What is the principle used in the working of fibers as light guides?

Total internal reflection

The ray should travel from denser to rarer and the angle of incidence in the

denser medium should be greater than the critical angle of that medium.

2. Among the different fibers which has the least dispersion?

Single mode step index fiber.

Due to single mode propagation there is no multimode dispersion

Due to smaller core diameter and smallest refractive index profile, the value

of total dispersion is very small.

3. Why do we require modulation?

To get high the bit rate or bandwidth length product

To get the long distance communication

To improve S/N ratio at the receiver end

4. What are moire fringes?

Are the relatively thick lines produced when two patterns of thin lines overlap.

5. Define Q-Switching and mode locking.

Q-Switching- maintaining the population inversion to a very high value above

the threshold population inversion and simultaneously bringing down all the

atoms to undergo laser transition.

Mode locking- Due to broadened nature of laser cavity, a laser source can

support oscillations in many modes simultaneously. The number of modes

produced depends on the laser transition line width. If we mode lock the laser,

i.e., if we force all the modes to maintain the same relative phase to oneanother, the total irradiance got will be more.

6. What are the characteristics of laser?

High monochromacity

High intensity

High coherence

High directionality

7. What is meant by laser action? What are the conditions to achieve it?

Laser action the amplification of light by stimulated emission of radiation.

To get laser action There should be population inversion and stimulated

emission takes place.8. Mention the merits and demerits of laser welding.

Heat affected zone is very narrow.

High welding speed or cutting speed can be achieved.

There is minimum residual stress and distortion

The material processing can be done at room temperature

9. State the basic principle of holography.

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Process of image construction by recording and reconstruction of hologram

by means of interference techniques without the aid of lenses.

10. List any four medical applications of laser.

Gynecology

Oncology

Dermatology Thoracic surgery

Urology

Part B

11. a. i) Classify optical fibers based on the modes of propagation and refractive

index profile and discuss their properties.Based on refractive index profile

1. Step index fiber

a. single modeb. Multimode

2. Graded index fiber

ii) Discuss the different scattering losses in optical fibre at the operatingwavelength

b. i) Explain the different types of connectors and splicers.

ii) Give an account of the optical detector response time and its influence ondetector parameters

12. a. i) Explain any two fiber optic sensors.

ii) Give the principle and design of an optical modulatorb. i) Explain the mechanisms involved in interferometric method for the

measurement of length.

ii) How will you measure the pressure and temperature by using an optical

fibre