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MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI - 621213

DEPARTMENT : ECE SUBJECT NAME : OPTICAL COMMUNICATION & NETWORKS SUBJECT CODE : EC 2402 UNIT – I: INTRODUCTION

PART-A (2 MARKS)

1. What is the numerical aperture of an optical fiber? (Nov/Dec 2011)

• Numerical aperture of the fiber is the light collecting efficiency of the fiber and is the measure of the amount of light rays that can be accepted by the fiber. It is equal to the sine of acceptance.

Where n1 and n2 are the refractive indices of core and cladding respectively.

2. What is Snell’s law? (Nov/Dec 2006)

• The relationship at the interface is known as Snell’s law and is given by

3. What is linearly polarized mode? (May/June 2013)

• The electric field intensity profiles for the lowest three LP modes, together with the electric field distribution of their constituent exact modes. It may be observed from the field configuration of the exact modes that the field strength in the transverse direction (Ex or Ey) is identical for the modes which belong to the same LP mode. Hence the origin of the term ‘linearly polarized.

4. What is the principle used in the working of fibers as light guides? (May/June 2012)

• The phenomenon of total internal reflection is used to guide the light in the Optical fiber. To get total internal reflection, the ray should travel from denser

to rarer i.e. from core to clad region of the fiber and the angle of incidence in the denser medium should be greater than the critical angle of that medium.

5. What is the necessity of cladding for an optical fiber? [AUC MAY 2011]

a) To provide proper light guidance inside the core b) To avoid leakage of light from the fiber c) To avoid mechanical strength for the fiber d) To protect the core from scratches and other mechanical damages

EC-2402 OPTICAL COMMUNICATION &NETWORKS - IV/VII ECE – V.SENTHAMIZHSELVAN ASST.PROF/ECE

6. What are the uses of optical fibers? [AUC MAY 2007]

a) To transmit the information which are in the form of coded signals of the Telephone communication, computer data, etc. b) To transmit the optical images (Example: Endoscopy) c) To act as a light source at the inaccessible places. d) To act as sensors to do mechanical, electrical and magnetic measurements.

7. Define acceptance angle? [AUC NOV 2007] The maximum angle “Φmax “with which a ray of light can enter through the entrance end of the fiber and still be totally internal reflected is called acceptance angle of the fiber.

8. Why do we prefer step index single mode fiber for long distance communication? [AUC MAY2010]

Step index single mode fiber has (1) low attenuation due to smaller core diameter, (2) Higher bandwidth, (3)Low dispersion.

9. What are the conditions for total internal reflection? [AUC MAY2010]

a) Light should travel from denser medium to rarer medium. b) The angle of incidence should be greater than the critical angle of the denser medium. 10. What is Intra Modal Dispersion? [AUC NOV 2009]

• Intra Modal dispersion is pulse spreading that occurs within a single mode. The spreading arises from finite spectral emission width of an optical source. This Phenomenon is also called as group velocity dispersion.

11. What is pulse broadening? [AUC APR 2008]

• Dispersion induced signal distortion is that a light pulse will broaden as it travels along the fiber. This pulse broadening causes a pulse to overlap with Neighboring pulses. After a time‘t’ the adjacent pulses can no longer be individually Distinguished at the receiver and error will occur.

12. What is group delay? [AUC June 2006]

• In an optical fiber there are various modes present. Then the optical input, which is propagated along the fiber, will travel in various modes. Because of these modes the velocity of the signal will vary also there may be a delay in the optical signal of these various modes. This is called as the ‘Group Delay’.

13. What is material dispersion? [AUC NOV 2009]

• Material dispersion arises from the variation of the refractive index of the core material as a function of wavelength. Material dispersion is also referred to as chromatic dispersion. This causes a wavelength dependence of group velocity of given mode. So it occurs because the index of refraction varies as a function of optical wavelength. Material dispersion is an intra-modal dispersion effect and is for particular importance for single ode wave-guide.


14. What are step index and graded index fibers? [AUC JUNE 2012]

• In the case of graded index fiber, the refractive index of a core is a constant and is larger than the refractive index of the cladding. The light propagation is mainly by meridional rays. In the case of graded index fiber (GRIN fiber) the refractive index of the core varies parabolically from the center of the core having maximum refractive index to the core-cladding interface having constant minimum refractive index. Here the light propagation is by skew rays.S

15. Define cutoff wavelength of the fiber. [AUC MAY 2010]

• The cutoff wavelength is defined as the minimum value of wavelength that can be transmitted through the fiber. The wavelengths greater than the cutoff wavelength can be transmitted.

PART-B

1. What is numerical aperture of an optical fiber? Deduce an

expression for the same. [AUC NOV 2011]

• The acceptance angle for an optical fiber was defined in the preceding section. However, it is possible to continue the ray theory analysis to obtain a relationship between the acceptance angle and the refractive indices of the three media involved, namely the core, cladding and air.

• This leads to the definition of a more generally used term, the numerical aperture of the fiber. It must be noted that within this analysis, as with the preceding discussion of acceptance angle, we are concerned with meridional rays within the fiber.

Figure 2.5 shows a light ray incident on the fiber core at an angle θ1 to the fiber axis which is less than the acceptance angle for the fiber θa. The ray enters the fiber from a

• Figure 2.5 The ray path for a meridional ray launched into an optical fiber in air at

an input angle less than the acceptance angle for the fiber medium (air) of

refractive index n0, and the fiber core has a refractive index n1, which is slightly

greater than the cladding refractive index n2.


• Assuming the entrance face at the fiber core to be normal to the axis, then

considering the refraction at the air–core interface and using Snell’s law given by


Will only support a discrete no. of guided modes. This becomes critical in small core diameter fibers which only support one or few modes. Hence electromagnetic mode theory is applied for this cases. 2. Distinguish graded index fiber from step index fiber? [AUC NOV 2011] Graded index fiber

Step index fiber

1. The refractive index of the core is non

uniform. the refractive index of the core

decreases parabolically from the axis of the

fiber to its surface.

The refractive index of the core is uniform and

step or abrupt change in refractive index takes

place at the interface of core and cladding in

step index fibers.

2.The light rays propagate in the form of skew

rays or helical rays. They will not cross the

fiber axis.

The light rays propagate in zig-zag manner

inside the core. The rays travel in the fiber as

meridional rays and they cross the fiber axis

for every reflaction.

3. All the light paths end to arrive at one point

simultaneously which results in less modal

dispersion.

Signal distortion is more in case of high-angle

rays in multimode step index fiber. In single

mode step index fiber , ther is no distortion.


3. An optical fiber has a NA of 0.20 and a cladding refractive index of 1.59 Determine (i) The acceptance angle for the fiber in water which has a refractive index of 1.33 (ii) Critical angle at the core cladding interface. [AUC NOV 2011] Ans:


4. Comparison of single mode and multi-mode fiber. [AUC MAY 2010] Single mode fibre

Multi-mode fibre

1.core diameter (8 – 9 microns) Core diameter (50 – 62.5 microns)

2.cladding diameter 125 microns Cladding diameter 125 microns

3.it allows only one light path It allows several light paths

4.long distance transmission Short distance transmission

5. high bandwidth low bandwidth

6. laser is used as source LED is used as source

7.speed up to 100 Tbps. Speed up to 10 Gbps.

5. Define an optical fiber. Explain in detail different types of optical fibers giving neat sketches? [AUC NOV 2009] A dielectric waveguide that operates at optical frequencies is known as optical fiber. It is generally available in cylindrical form. Fiber Types

There are two fiber types

(i) Step index fiber

(ii) Graded index fiber.

(i) Step Index Fiber Step index fiber is further divided in two types,

1. Single mode step index fiber

2. Multi-mode step index fiber.

Single mode step index fiber is shown below,


• The typical dimension of core is 8 to 12 μm and cladding is 125 μm.

In step index fiber, the refractive index of the core is uniform and at the cladding boundary, it

Undergoes a step change. In single mode step index fiber, there is only one mode of

propagation. The multimode step index fiber is shown below,

• In multimode step index fiber, hundreds of modes are present.

The typical dimension of core is 50 to 200 μm and cladding is 125 to 400 μm. Multimode fiber

has several advantages, which includes, the transmitting the light directly in to fiber using LED.

.

Graded Index Fiber Graded index fiber also contains single mode and multimode. The multimode graded index fiber

is shown below,

• In graded index fiber, the refractive index of the core is made to vary as a

function of radial distance taken from the center of the fiber.


• The dimension of its core is 50 to 100 μm and cladding is 125 to 140 μm.

In both cases (step index and graded index) multimode has several advantages. When

compared with single mode, however, multimode has a drawback, that is, it suffers from inter

model dispersion.

6. Distinguish between optical fiber communication system and conventional communication system? And List out the advantageous and disadvantage of optical fiber communication? [AUC NOV 2011]

Advantages Of Optical Fibers Communication: 1. Information bandwidth is more.

2. Optical fibers are small in size and light weighted.

3. Optical fibers are more immune to ambient electrical noise, electromagnetic interference.

4. Cross talk and internal noise are eliminated in optical fibers.

5. There is no risk of short circuit in optical fibers.

6. Optical fibers can be used for wide range of temperature.

7. A single fiber can be used to send many signals of different wavelengths using Wavelengths

Division Multiplexing (WDM).

8. Optical fibers are generally glass which is made up of sand and hence they are cheaper than

copper cables.

9. Optical fibers are having less transmission loss and hence less number of repeaters are used.

10. Optical fibers are more reliable and easy to maintain.


Disadvantages Of Optical Fibers Communication: 1. Attenuation offered by the optical fibers depends upon the material by which it is made.

2. Complex electronic circuitry is required at transmitter and receiver.

3. The coupling of optical fibers is difficult.

4. Skilled labors are required to maintain the optical fiber communication.

5. Separated power supply is required for electronic repeaters at different stages.

7. What are the various elements of an optical communication system? Explain each element in brief? Optical Fiber Communication System: The figure 1.1 shows a block schematic of the different elements in an optical fiber

communication system. The carrier is modulated using analog information signal. The

variation of light emitting from the optical source is a continuous signal. The information

source provides an electrical signal to the transmitter. The transmitter comprises electrical

stage. The electrical stage (circuits) drives an optical source. The optical source output is a

light which is intensity modulated by the information. The optical source converts the

electrical signal into an optical signal. The source may be either semiconductor laser or Light

Emitting Diode (LED). The intensity modulated light signal is coupled to fiber. The fiber

which is made up of a glass acts as a channel between the transmitter and receiver.

At the receiver the optical signal is detected by the optical detectors such as PIN diode and

Avalanche photodiode.

Sometimes photo transistors and photo conductors are used for converting an optical signal into

Electrical signal. The electrical signal is again processed and given to the transducer to get the

original information.


8. A single Mode step index fiber has a core diameter of 7μm and core Refractive index of 1.49.Estimate the shortest wavelength of light which allows single mode operation when the refractive index difference for the fiber is 1%? .


9. Write in detail about ray optics? Ans: Ray optics is used for representing the mechanism of a ray which propagates through an

Ideal multimode step index optical waveguide. There are two types of rays, the skew rays and

meridional rays which propagate through a fiber.

The path of meridional can be tracked very easily as they are confined to a single plane.

Meridional are described in two classes. They are,

(i) Bound rays

(ii) Unbound rays.

Bound rays are those rays which are trapped in a core and they move along the fiber whereas

unbound rays are those rays which get refracted out of the fiber.

Skew rays are those rays which follow helical path but they are not confined to a single plane.

We know that skew rays are not confined to a particular plane so they cannot be tracked easily.

Analyzing the meridional rays is sufficient for the purpose of result, rather than skew rays,

Because skew rays lead to greater power loss.

Now coming to ray theory, we need to consider meridional rays. Representation of meridional

rays is given below.

From the medium of refractive index 'n' which is at an angle ‘θ0’with respect to fiber axis, the

light enters the fiber core. If the light strikes at such an angle then it gets reflected internally and

the meridional ray moves in a zig zag path along the fiber core, passing through the axis of the

guide. Now by using Snell's law the minimum angle ‘фmin’ supports total internal reflection for

meridional ray is given by If the ray strikes the core-cladding interface at an angle less than

фmin then they get refracted out of the core and they will be lost from the cladding.


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