Fiber Optics: Important part-A 2 marks Short Questions and Answers

ANNA UNIVERSITY PART‒A QUESTIONS & ANSWERS

1. Give four applications of fibre optic sensors.

i. Fibre optic sensors are used as optical displacement sensors, which is used to find the displacement of a target along with its position.

ii. It is used as a fluid level detector.

iii. It is used to sense the pressure, temperature etc., at any environment.

iv. It is also used to measure the number of rotations of the fibre coil using the instrument called as Gyroscope.

2. Explain the basic principle of fibre 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 this phenomenon is called total internal reflection.

3. 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.

4. Give the applications of the fibre optical system. (or) Mention the applications of optical fibres in the engineering field.

i. It can be used for long distance communication in trunk lines.

ii. A large number of telephone signals nearly 15,000 can be passed through the optical fibres in a particular time without any interference.

iii. It is used in computer networks, especially in LAN.

iv. It is also used as optical sensors.

5. Mention any four advantages of fibre optic sensors.

(i) It has no external interference.

(ii) It is used in remote sensing.

(iii) Safety of data transfer.

(iv) It is small in size.

6. Differentiate between active and passive fiber‒optic sensors.

Active sensors

In intrinsic sensors (or) active sensors the physical parameter to be sensed directly acts on the fibre itself to produce the changes in the transmission characteristics.

Examples:

(i) Temperature/Pressure sensor (Phase and polarization sensor) and

(ii) Liquid level sensor.

Passive sensors

In extrinsic sensors (or) passive sensors, separate sensing element will be used and the fibre will act as a guiding media to the sensors.

Examples:

(i) Displacement sensor and

(ii) Laser doppler velocimeter sensor

7. Mention any four advantages of optical fibre.

1. It is light in weight.

2. It is smaller in size and is flexible, so that it can bend to any position.

3. It is non‒conductive, non‒radiative and non‒inductive

4. It has high bandwidth and low loss.

5. There is no short circuiting as in metal wires.

6. It can withstand to any range of temperature and moisture condition.

8. List out the conditions to be satisfied for total internal reflection?

(1) Light should travel from denser medium to rarer medium.

(ii) The angle of incidence (ϕ) on core should be greater than the critical angle (ϕ_c).

i.e. ϕ > ϕ_c.

(iii) The refractive index of the core (n₁) should be greater than the refractive index of the cladding (n₂).

i.e. n₁>n₂

9. Mention the properties of detectors used in fiber‒optic communication.

The detectors should possess the following properties, viz.

(i) Ability to convert optical signal into electrical signal.

(ii) Fast response time.

(iii) Zero dark current and

(iv) Cost effective.

10. Mention the advantages of optical fibre communication over other communication systems?

i) Optical communication can be made even in the absence of electricity.

ii) The optical signals are not affected by any electrical signals or lightning.

iii) Optical fibre communication is free from Electromagnetic interference (EMI).

iv) This type of communication is suitable to any environmental conditions.

v) Easy maintenance, longer life, economical and high quality signal transmission are the additional features of optical fibre communication.

11. What are the main requirements of light sources used in fiber optic communication?

1. The light produced must be as nearly monochromatic as possible.

2. It must modulate the source at high speeds.

3. The light source should have compact size and high efficiency.

4. It should be reliable, durable and inexpensive.

5. It must require very small power for its operation.

6. Spectral line width of the source should be as small as possible.

7. Can operate continuously at room temperature for many years.

8. Should be modulated over a wide range of frequencies.

12. Define total internal reflection

Principle

The principle of optical fiber communication is Total Internal Reflection (TIR).

Total Internal Reflection

The phenomenon of Total Internal Reflection takes place when it satisfies the following two conditions.

Condition 1. Light should travel from denser medium to rarer medium

i.e. n₁ > n₂

Where

n₁ = refractive index of core

n₂ = refractive index of cladding

Condition 2. The angle of incidence on core should be greater than the critical angle.

i.e. ϕ > ϕ_c

Where

ϕ - angle of incidence

ϕ_c - critical angle

ADDITIONAL PART‒A QUESTIONS AND ANSWERS

1. Mention any two fiber optic sources?

(i) Light emitting diode (LED): In LED's we have two types, viz., (1) Planar and (2) Dome shaped LED

(ii) Laser diodes (LD): In laser diodes, we have many types, in which homojunction laser, heterojunction laser, injection laser diode etc. are widely used as fiber optic sources.

2. What is meant by a photo detector? Give any two examples.

Photo‒detector is a device used to detect the light falling over it. It converts the light energy into electrical energy. i.e., when light falls on the photo‒ detector, it produces electron‒hole pairs and thereby generates current in the output.

Examples:

(i) PIN photo diode

(ii) Avalanche Photo Diode (APD)

3. How fibres are used as a sensors?

The fibre optic sensors are used to detect changes in frequency, intensity, temperature, current, polarization of light waves etc. A fibre optic sensor modulates the light passing through it, when it is exposed to change in environment.

4. What are the types of sensors used in the fibre optics?

 There are two types of sensors used viz.,

(i) Intrinsic sensors: Here, fibre itself acts as a sensing element.

(ii) Extrinsic sensors: Separate sensing system collects the light from the fibre.

5. Give any four examples for intrinsic sensors.

i) Pressure sensor.

ii) Liquid level sensor.

iii) Phase and polarization sensor.

iv) Optical fibre flow sensor.

6. Give any four examples for extrinsic sensors.

i) Displacement sensor.

ii) Fluoroptic temperature sensor

ii) Laser Doppler velocimeter sensor.

iv) Current measurement sensor.

7. Give any two applications of optical fibres in industries.

i. Fibre optic endoscopes are used to find minor cracks, pores etc. in big machinaries.

ii. They are used in coagulation in chemical industries and laboratories.

iii. It is also used to connect a monitoring station and a remote sensor in industries.

8. State some of the applications of optical fibres in medical field.

i. Fibre optic endoscopes are used in medical diagnosis.

ii. It is used to visualize the inner organs of the body.

iii. Fibres as endoscopes are used in various medical fields such as cardioscopy, Laproscopy, Cytoscopy etc.