Optical Communication question paper with solution for Engineering Students

1. 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 LP01 mode

2. 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 losses

3. Define acceptance angle.

The maximum angle ‘Φmax’ with which a ray of light can enter through the entrance end of the fiber and still be totally internally reflected is called acceptance angle of the fiber

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

Step index single mode fiber has low attenuation due to smaller core diameter & higher bandwidth & very low dispersion.

5. What are meridional rays & skew 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.

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.

6. What is pulse broadening?

Dispertion induced signal distortion is that a light pulse will broaden as it travels along the fibre.This pulse broadening causes a pulse to overlap with neighbouring pulses.After a time ‘t’,the adjacent pulses can no longer be individually distinguished at the receiver and error will occur.

7. 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.

Part – B                                              (3x 12 =36)

(Answer any three questions)

1. What is waveguide dispersion? Derive its expression.

Wave-guide dispersion, which occurs because a single mode fiber confines only 80% of the optical power to the core. Dispersion thus arises since the 20% of the light propagating in the cladding travels faster than the light confined to the core. The amount of wave-guide dispersion depends on the fiber design, since the modal dispersion constant β is a fn of (a / λ)

2. Explain in detail about various attenuation losses.

v  Absorption Loss:

In optical fibers, the electromagnetic waves are absorbed by the material resulting in increase in attenuation.

Absorption is caused by three different mechanisms in detail with relevant diagram.

·         Absorption by atomic defects in glass composition

·         Extrinsic absorption by impurity atoms in the glass material

·         Intrinsic absorption by the basic constituent atoms of the fiber materials.

v  Scattering Loss:

Scattering losses in glass arise from microscopic variations in the materials density, from compositional fluctuations, and from structural in homogeneities or defects occurring during fiber manufacture.

v  Bending Loss:

Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature. Fibers can be subjected to two types of bends

1.      Macroscopic bends

2.      Microscopic bends

v  Explanation with relevant diagrams

3.  Briefly explain about Graded index fiber structure & single-mode fiber.

Graded index fiber:

In the graded index fiber design the core refraction index decreases continuously with increasing radial distance ‘r’ from the center of the fiber.

·         Equation

·         Diagram

Single-mode fiber

•    Dimensions of core diameter

•    Mode-field diameter

Distribution of light in a single-mode fiber above its cutoff wavelength

•    Propagation modes i.  Fiber birefringence ii. Fiber beat length