Fibre Optics 2mark

R.M.D ENGINEERING COLLEGE, KAVARAIPETTAI. Unit- III FIBRE OPTICS AND APPLICATIONS Part-A Question and Answers (Two Marks) 1. What is Fibre optics? The branch of physics which deals about the transmission and reception of light waves in optical fibers by the principle of total internal reflection. 2. What is the principle of fibre optical communication?

Total internal reflection is the principle of fibre optical 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 are the conditions to be satisfied for total internal reflection?

The refractive index n1 of the core must always be greater than the refractive index n2 of the cladding. i.e., n1 > n2 . The light must approach the walls of the fibre with an angle of incidence that is greater than the critical angle . i. e., > . 4. Define Acceptance angle. The maximum angle made by a light ray with the fibre axis on the surface of the fibre below which the light is totally internally reflected is called acceptance angle. (θ A)

5. Define Acceptance cone

The imaginary cone within which the light rays enter the optical fiber and totally internally reflected is called acceptance cone.

6. Define: Numerical aperture of a fibre. It can be defined as the sine of the acceptance angle of the fibre. It describes the light gathering capacity of the fibre.

7. How will you classify optical fibres? Optical fibres are classified into 3 types based on (i) Based on material. (a) Glass fiber (b) Plastic fibre (ii) Based on mode of propagation

(a) Single mode fibre (b) multimode fibre (iii) Based on refractive index profile (or) variation in the refractive index. (a) Step – index fibre (b) Graded – index fibre

8. What are optical fibers? Optical fibers are made of transparent dielectric materials which act as waveguides for optical signals in fibre optical communication. 9. What is a single mode fibre?

If only one mode is transmitted through optical fibres, then it is said to be a single mode fibre. 10. What is a multimode fibre?

If more than one mode is transmitted through optical fibres, then it is said to be a multimode fibre. 11.

Differentiate single mode and multimode fibre.

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2

3

4

5

6 7

Single mode fibre In single mode fibre only one mode can propagate through it. The single mode fibre has smaller core diameter 2 to 10 and the difference between the refractive indices of the core and the cladding is very small. In practice there is no dispersion (i.e., no degradation of signal during travelling through the fibre) Launching the light into single mode fibres and joining the two are very difficult. Since the information transmission capacity in optical fibre is inversely proportional to dispersion, the single mode fibres are more suitable for long distance communication. Fabrication is very difficult and so the fibre is expensive. Laser diode can only be used for launching light into the fibre.

Multimode fibre Multimode fibre allows a large number of paths or modes for the light rays travelling through it simultaneously. Generally in multimode fibre the core diameter 50 to 200 and the relative refractive index difference are larger than the single mode fibre. There is signal degradation due to multimode dispersion and material dispersion. Launching the light into fibres and joining of two fibres are easy in these fibres. Due to large dispersion and attenuation of the signal the multimode fibres are less suitable for long distance communication. Fabrication is less difficult and the fibres is not costly. LEDs and Laser diode can be used for launching light into the fibre.

12. Differentiate step index and graded index fibre.

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Step index fibres

Graded index fibres

The refractive index of the core is

The refractive index of the core is

2 3 4

5

11.

made to vary in the parabolic uniform throughout and undergoes manner such that the maximum step change at the cladding refractive index is present at the boundary. centre of the core. The diameter of the core is about The diameter of core is about 50 50 to 200 in the case of in the case of multimode fibre. multimode fibre and 10 in the case of single mode fibre. It has low bandwidth It has high bandwidth. Attenuation is more for multimode step index fibre but Attenuation is less for the single mode step index fibre it is very less. Numerical aperture is more for multimode step index fibre but Numerical aperture is less for the single mode step index fibre it is very less.

What is meant by fractional index change? The fractional index change ∆ is the ratio of refractive index difference between core and cladding to the refractive index of core. It is expressed as, This parameter is always positive because n1 must be large than n2 for the total internal reflection.

12.

Give any three applications of optical fibre in industries.  The fibres are used as a sensor in the measurements of pressure, temperature and displacement of an object.  It is used as a link between sensor and monitoring station.  The endoscope is used to inspect and view the inner structure of the complicated machine parts, nozzles and combustion chambers inside the jet aircraft engine.

13. State some of the applications of optical fibres in engineering field.  It can be used for long distance communication in trunk lines.  A large number of telephone signals nearly 15,000 can be passed through the optical fibres in a particular time without any interference.  It used in computer networks, especially in LAN.  it is also used as optical sensor. 14.What is meant by splicing? To carry information through light for a longer distance, one fibre is to be connected with the other, so that the losses can be minimised. The process of joining two fibres permanently with the help of an elastomer or four rod splice using an adhesive or index matching gel is called splicing. 15.What is meant by attenuation?

The power of the light at the output end is found to be always less than that at the input end. The optical power loss is measured in terms of decibels per kilometer for attenuation losses. It is a logarithmic unit. It is calculated through the formula :

dB =

æ - 10 Pout ö ÷ log ç ç ÷ ÷ ÷ ç L èPin ø

Where, Pout is the power coming out of the fibre. Pin is the power launched into the fibre. L is the length of the fibre tested. 16. What are the basic attenuation mechanisms that affect the performance of the

fiber optical communication system? Attenuation plays a major role in determining the transmission distance. Some of the attenuation mechanisms like absorption loss, scattering loss, bending loss are to be considered while designing optical fibres.

17.What is meant by dispersion? The spreading or broadening of the optical signal during transmission through the fibre is called dispersion. The output pulse is wider than the input pulse. Thus the output pulse is said to be distorted due to the dispersion of light. The two types of dispersion are 1. Intermodal dispersion. and 2. Intramodal or Chromatic dispersion. 18.Mention the major parts of fibre optic communication system. The important parts of fibre optic communication system are. (a) Transmitter, (b) Fibre optic cable and , (c) Receiver. 19.What is a fibre optic sensor? Fibre optic sensor is a transducer which can convert various input variables (Physical quantity) into an electrical signal in a measurable form. 20.Mention any two fibre optic soures?  LEDs  Semi conductor diode lasers. 21. What are the active and passive sensors?

Intrinsic / Active sensors : The physical parameters like pressure or temperature directly acts on the light carrying fibre itself. It produces changes in the transmission properties of light. This type of sensor is called as Active sensor. Eg : Temperature / Pressure sensor. Intensity modulated sensor. Phase modulated sensor, wavelength modulated sensor, Polarisation modulated sensor.

Extrinsic / Passive sensors : In extrinsic / passive sensors, separate sensing element is used, and fibre will act as a guiding media to the sensors. Eg : Fluoroptic temperature sensor, current measurement sensor, laser Doppler velocimeter sensor, displacement sensor. 22.What are the advantages of the fibre optic communication system over the conventional system?  The fibre optic communication system are light in weight and small in size.  There is no possibility of internal noise and cross talk generation.  No hazards of short circuits in metal wires.  Immunity to adverse temperature and moisture.  Since fibre is made up of glass, the raw material of the glass is silica which is available in plenty in the earth. 23.What are the losses that occur during optical fibre communication?  During the transmission of light through the optical fiber, two major losses will occur, viz ., attenuation and dispersion.  Attenuation is mainly caused due to the absorption, scattering of light inside the fibres and bending of fibers.  Dispersion occurs due to spreading of light and also due manufacturing defects. 24. What is the basic principle of fibre optic sensor?

A fibre optic sensor in general, consists of a light source. The light source is coupled to an optical fibre. A light detector which receives the signal – carrying light beam as it emerges from the fibre. The signal from detector is processed electrically for getting the desired output. 25.How fibers are used as a sensor? The fibre optic sensors are used to detect changes in frequency, intensity, temperature, current, polarization of the light waves etc. A fibre optic sensor modulates the light passing through it, when it is exposed to change in environment. 26.Why optical fibres are called as waveguides? A wave guide is tubular structure through which same sort of energy could be guided in the form waves. Since light waves can be guided through a fibre, it is called light guide. it is also known as fibre wave guide.

27.What is Photo detector? Photo detector is a device used to detect the light falling over it. It converts the light energy into electrical energy under reverse bias voltage. 28.What is meant by LED? Give its principle? A LED is an abbreviation of Light Emitting Diode. It is a Semiconductor p-n junction diode which converts electrical energy to light energy under forward biasing. LEDs emit incoherent photons through spontaneous emission process.

29. List out the four main types of photo detectors.  Photoconductors

 Photodiodes  PIN diodes and  Avalanche photodiode. 30.What is meant by endoscope? A medical endoscope is a tubular optical instrument. It is used to view / inspect / photograph the internal parts of human body which are not visible to the naked eye. Short length, large diameter silica fibres are used to design the endoscope. 31.What are Glass fibres? Glass fibres are made up of a mixture of metal oxides with silica glass. They exhibit low losses and are used for long distance communication. 32. What are Plastic fibres? Plastic fibres are made up of plastics. These low cost fibres are flexible, durable and tough, thus they can be handled without any care. 33. Classify the fibres based on the refractive index profile. Based on the refractive index profile, there are two types of optical fibres: 1. Step index fibre and 2. Graded index fibre Step index fibre can further be divided into single mode step index fibre and multi mode step index fibre. 34. What are the advantages of multi mode fibres? The advantages of multi mode fibres are: 1. It is relatively easy to work with. 2. Light coupling can be done easily with less loss due to large core diameter. 3. LEDs and Lasers can be used as sources. 35. What

are the basic requirements of light sources used in fiber optic communication system? The basic requirements of light sources used in fiber optic communication system are: 1. Monochromaticity 2. Narrow band width 3. Capable of being easily modulated. 4. Enough intensity 5. Durable, inexpensive, compact and small. 36. What in meant by injection luminescence?

When the majority carriers are injected from p to n and n to p region, they become excess minority carriers. Then they diffuse away from the junction and recombines with the majority carriers in p and n regions and emit light. This phenomenon is known as injection luminescence.

37.What is Dispersion? The optical signals travel with different velocities along the fibre, then they reach the other end of the fibre at different times. Due to this, spreading or broadening of optical signals, overlapping of data occurs. This type of loss in optical fibres is called as Dispersion. 38.How LED can be converted into Laser diode? In a semi conductor diode, if the emission is not stimulated, the device is called light emitting diode (LED). To convert LED to laser diode, high current density is required to achieve population inversion and the opposite surfaces should be polished.