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# Important Short Questions and Answers: Optical Communication and Networking

Optical Communication and Networking - Introduction - Important Short Questions and Answers: Optical Communication and Networking

INTRODUCTION

1.  Write the expression for the refractive index in graded index fibers.

n(r)= n1[1-2∆(r/a)α]1/2 for 0<=r<=a

n1(1-2∆)1/2 ~ n1(1-∆) =n2 for r>=a

n1 refractive index  at the core

n2 refractive index  at the cladding

α       shape of the index profile

index difference

2.     Define Mode-field diameter.

The fundamental parameter of a single mode fiber is the mode-field diameter. This can be determined from the mode field distribution of the fundamental LPo1 mode.

3.  Give the expression for linearly polarized waves.

The electric or magnetic field of a train of plane polarized waves travelling in a direction k can be represented in the general form

A(x,t) = eiAoexp[j(wt-k.x)]

With x=xex+yey+zez representing a general position vector and k=kxex+kyey+kzez representing the wave propagation vector.

4.  What is Snell’s law?

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

n1sinΦ1=n2 sinΦ2

5.     What is the necessity of cladding for an optical fiber?

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

6.     What are the uses of optical fibers?

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

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.

8.  What are step index and graded index fibers?

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.

9.  Why do we prefer step index single mode fiber for long distance communication?

Step index single mode fiber has

a)     Low attenuation due to smaller core diameter

b)    Higher bandwidth and

c)     Very low dispersion.

10.            Define relative refractive index difference.

Thus relative refractive index difference is the ratio between the refractive index difference (of core and cladding) and refractive index of core.

11. What are meridional rays?

Meridional rays are the rays following Zig Zag path when they travel through fiber and for every reflection it will cross the fiber axis.

12. What are skew rays?

Skew rays are the rays following the helical path around the fiber axis when they travel through the fiber and they would not cross the fiber axis at any time.

13. What is V number of fiber or normalized frequency of fiber?

V number of fiber or normalized frequency of fiber is used to find the number of propagating modes through the fiber.

V= 2∏a (N.A) / λ

In step index fiber number of modes propagating through the fiber=V2/2

Taking the two possible polarizations, total number of possible modes propagating through the fiber = [V2/2 ] * 2 = V2

14.            What are the conditions for total internal reflection?

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.

15.            Give the relation between numerical aperture of skew rays and meridional rays.

(N.A)skew = cos γ(N.A)meridional when the fiber is placed in air. Here γ is the half of the angular change in every reflection.

16.            State  Goos-Haenchen effect.

Goos-Haenchen effect states that there is a lateral shift of the reflected ray at the point of incidence at the core-cladding interface. This lateral shift is called the Goos-Haenchen shift.

17. When do you have  phase shift during total internal reflection of light?

When the light ray travels from denser medium to rarer medium, if the angle of incidence is greater than the critical angle of core medium, there is a phase shift for both TE and TM waves.

18. What are hybrid modes? Give two examples.

Hybrid modes are the mixture of TE and TM modes that can be traveled through the optical fiber.

Examples:

1.     HE1m modes in which |Ez|>|Hz|

2.     EH1m modes in which |Hz|>|Ez|

19.            Define cutoff wavelength of the fiber.

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.

λ cutoff = 2∏a (N.A) / V

20. Mention the rule distinguishing ‘mode’ and ‘order’.

The rule states that the smaller the modes propagating angle, the lower the order of the mode. Hus the mode traveling precisely along the fiber’s central axis is zero mode.

21. What is fiber birefringence?

Imperfections in the fiber are common such as asymmetrical lateral stress, non-circular imperfect variations of refractive index profile. These imperfections break the circular symmetry of ideal fiber and mode propagate with different phase velocity and the difference between their refractive index is called fiber birefringence.

B=ko(ny-nx)

22. Give the expression for numerical aperture in graded index fibers.

N.A(r)=N.A.(0) (1-(r/a)α)1/2  for r<=a

where N.A(0) = axial numerical aperture = (n12-n22)1/2 a is core radius and α is the refractive index profile.

Glossary

Mode-field diameter.

The fundamental parameter of a single mode fiber is the mode-field diameter. This can be determined from the mode field distribution of the fundamental LPo1 mode.

2.  Snell’s law.

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

n1sinΦ1=n2 sinΦ2

3.     cladding for an optical fiber

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

4.     Step index and graded index fibers.

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.

5. Relative refractive index difference.

Thus relative refractive index difference is the ratio between the refractive index difference (of core and cladding) and refractive index of core.

6.  Meridional rays.

Meridional rays are the rays following ZigZag path when they travel through fiber and for every reflection it will cross the fiber axis.

skew rays.

Skew rays are the rays following the helical path around the fiber axis when they travel through the fiber and they would not cross the fiber axis at any time.

8.  V number of fiber or normalized frequency of fiber.

V number of fiber or normalized frequency of fiber is used to find the number of propagating modes through the fiber.

V= [ 2∏a/ λ ] (N.A)

9.  Goos-Haenchen effect.

Goos-Haenchen effect states that there is a lateral shift of the reflected ray at the point of incidence at the core-cladding interface. This lateral shift is called the Goos-Haenchen shift.

10. Hybrid modes.

Hybrid modes are the mixture of TE and TM modes that can be traveled through the optical fiber.

Examples:

1.     HE1m modes in which |Ez|>|Hz|

2.     EH1m modes in which |Hz|>|Ez|

11.            Cutoff wavelength of the fiber.

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.

λ cutoff = [2∏a/ V] (N.A)

12. fiber birefringence.

Imperfections in the fiber are common such as asymmetrical lateral stress, non-circular imperfect variations of refractive index profile. These imperfections break the circular symmetry of ideal fiber and mode propagate with different phase velocity and the difference between their refractive index is called fiber birefringence.

B=ko(ny-nx)

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Optical Communication and Networking : Introduction : Important Short Questions and Answers: Optical Communication and Networking |