Important Short Questions and Answers: Transmission Characteristics of Optical Fiber

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

1.  What is Intra Modal Dispersion?

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.

2.  What are the causes of intra modal dispersion?

There is two main causes of intra modal dispersion. They are:

•         Material dispersion

•         Wave guide dispersion

3.     What is material dispersion?

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.

4.  What is waveguide dispersion?

Wave guide dispersion which occurs because of a single mode fiber confines only about 80% of optical power to the core. Dispersion this arises since 20% of light propagates in cladding travels faster than the light confined to the core. Amount of wave-guide dispersion depends on fiber design. Other factor for pulse spreading is inter modal delay

5.  What is group velocity?

If L is the distance traveled by the pulse, β is the propagation constant along axis then the group velocity in the velocity at which energy is a pulse travels along the fiber.

Vg = C. (dβ / dk)

6.  What is group delay?

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’.

7.  What is polarization?

It is a fundamental property of an optical signal .It refers to the electric field orientation of a light signal which can vary significantly along the length of a fibre.

8.  What is pulse Broadening?

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.

9.  What is polarization Mode Dispersion (PMD)?

The difference in propagation times between the two orthogonal polarization modes will result in pulse spreading. This is called as polarization Mode Dispersion.

10. What is Mode Coupling?

It is another type of pulse distortion which is common in optical links. The pulse distortion will increase less rapidly after a certain initial length of fiber due to this mode coupling and differential mode losses. In initial length coupling of energy from one mode to another arises because of structural irregularities, fiber dia. etc.

11. What is Profile Dispersion?

A fiber with a given index profile (alpha) will exhibit different pulse spreading according to the source wavelength used. This is called as Profile Dispersion.

12. What is M-C fiber?

Fibers that have a uniform refractive index throughout the cladding is called as M-C fiber or Matched-cladding fiber.

13. What is D-C fiber?

In depressed cladding fiber the cladding portion next to the core has a lower index than the outer cladding region.

14. Define depresion shifted fiber.

By creating a fiber with large negative waveguide dispersion & assuming the same values for material dispersion as in a standard single mode fiber the addition of waveguide & material dispersion can then shifted to zero dispersion point to long wavelength. The resulting optical fiber are known as dispersion shifted fiber.

15. Define dispersion flattening?

The reduction of fiber dispersion by spreading the dispersion minimum out over a wide range .this approach is known as dispersion flattering.

16. What is effective cut-off  wavelength?

It is defined as the largest wavelength at which the higher order LP11 mode power relative to the fundamental LP01 mode power is reduced to 0.1db.

17. What is intramodal dispersion?

The intramodal dispersion depends on wavelength and its effect on signal distortion increases with the spectral width of the optical source. (It is a band of wavelength over which the source emits light

18. Write a note on scattering losses.

Scattering losses in glass arise from microscopic variation in the material density from compositional fluctuation and from structural in homogeneities or defects occurring during fiber manufacture

19. What is Rayleigh scattering?

The index variation causes a Rayleigh type of scattering of light. Rayleigh scattering in glass in the same phenomenon that scatters light from sun in the atmosphere, giving rise to blue sky. The expression for Rayleigh scattering loss is given by

αscat =(8π³/3λ²)(n²-1)²k_BT_fβ_T

n = refractive index

k_B = boltzman constant

β_T= isothermal compressibility

T_f =fictive temperature,

λ =operative wavelength

20. What is intermodal dispersion?

Intermodal dispersion is a pulse spreading that occurs within a single mode. The spreading arises from finite spectral emission width of an optical source. it is called group velocity dispersion or intermodal dispersion

21. What is intramodal delay?

The other factor giving rise to pulse spreading is intramodal delay which is a result of each mode having a different value of Group velocity at a single frequency.

22. What is the measure of information capacity in optical wave guide?

It is usually specified by bandwidth distance product in MHz For a step index fiber the various distortion effects tend to limit the bandwidth distance product to 20MHz.

23. Mention the losses responsible for attenuation in optical fibers.

Absorption losses, Scattering losses and bending losses

24. .What do you meant by Extrinsic absorption?

Absorption phenomena due to impurity atoms present in the fiber.

25. Define microscopic bending?

Fiber losses occur due to small bending arise while the fiber is inserted into a cable.

26. Define macroscopic bending?

If any bending present in the fiber while cabling, the optical power get radiated

GLOSSARY

1.  Intra Modal Dispersion.

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.

2.  Material dispersion.

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.

3.  Waveguide dispersion.

Wave guide dispersion which occurs because of a single mode fiber confines only about 80% of optical power to the core. Dispersion this arises since 20% of light propagates in cladding travels faster than the light confined to the core. Amount of wave-guide dispersion depends on fiber design. Other factor for pulse spreading is inter modal delay

4.  Group velocity.

If L is the distance traveled by the pulse, β is the propagation constant along axis then the group velocity in the velocity at which energy is a pulse travels along the fiber.

Vg = C. (dβ / dk)

5.  Group delay.

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’.

6.  Polarization

It is a fundamental property of an optical signal .It refers to the electric field orientation of a light signal which can vary significantly along the length of a fibre.

7.  Pulse Broadening.

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.

8.  Polarization Mode Dispersion (PMD).

The difference in propagation times between the two orthogonal polarization modes will result in pulse spreading. This is called as polarization Mode Dispersion.

9.  Mode Coupling.

It is another type of pulse distortion which is common in optical links. The pulse distortion will increase less rapidly after a certain initial length of fiber due to this mode coupling and differential mode losses. In initial length coupling of energy from one mode to another arises because of structural irregularities, fiber dia. etc.

10. Profile Dispersion.

A fiber with a given index profile (alpha) will exhibit different pulse spreading according to the source wavelength used. This is called as Profile Dispersion.

11. M-C fiber.

Fibers that have a uniform refractive index throughout the cladding is called as M-C fiber or Matched-cladding fiber.

12. D-C fiber.

In depressed cladding fiber the cladding portion next to the core has a lower index than the outer cladding region.

13. Dispersion shifted fiber.

By creating a fiber with large negative waveguide dispersion & assuming the same values for material dispersion as in a standard single mode fiber the addition of waveguide & material dispersion can then shifted to zero dispersion point to long wavelength. The resulting optical fiber are known as dispersion shifted fiber.

14. Dispersion flattening.

The reduction of fiber dispersion by spreading the dispersion minimum out over a wide range .this approach is known as dispersion flattering.

15. Effective cut-off wavelength.

It is defined as the largest wavelength at which the higher order LP11 mode power relative to the fundamental LP01 mode power is reduced to 0.1db.

16. Intramodal dispersion?

The intramodal dispersion depends on wavelength and its effect on signal distortion increases with the spectral width of the optical source. (It is a band of wavelength over which the source emits light

17. Scattering losses.

Scattering losses in glass arise from microscopic variation in the material density from compositional fluctuation and from structural in homogeneities or defects occurring during fiber manufacture

18. Rayleigh scattering.

The index variation causes a Rayleigh type of scattering of light. Rayleigh scattering in glass in the same phenomenon that scatters light from sun in the atmosphere, giving rise to blue sky.

19. Intermodal dispersion.

Intermodal dispersion is a pulse spreading that occurs within a single mode. The spreading arises from finite spectral emission width of an optical source. it is called group velocity dispersion or intermodal dispersion

20. Intramodal delay.

The other factor giving rise to pulse spreading is intramodal delay which is a result of each mode having a different value of Group velocity at a single frequency.