# Optical Fiber Questions and Answers || Optical Fiber MCQ

Ques.11. In an optical fiber, the refractive index of the cladding material should be

1. Nearly unity
2. Very low
3. Less than that of the core
4. More than that of the core

Answer.3. Less than that of the core

Explanation

The refractive index of the cladding is always less than the refractive index of the core. This is to ensure confinement of optical rays within the core. It is this property of core and cladding which makes light propagate inside the fiber. Thus, In a step-index optical fiber refractive index of the core is higher than the cladding.

Ques.12. Light travels along optical fibers by

1. Refraction
2. Reflection
3. Scattering
4. Total internal reflection

Answer.4. Total internal reflection

Explanation

• The working principle of optical fibers is Total Internal Reflection.
• Optical fiber is mostly used for communication purposes with negligible loss of energy.
• The “Total Internal Reflection” of light is the boundary between transparent media of two different refractive indices.
• At present, Optical fiber cables are used for communication like sending images, voice messages, etc.
• The design of this cable is done with Plastic or glass so that data can be transmitted more effectively and quickly than other modes of communications.

Ques.13. The refractive index of the core is uniform throughout and undergoes an abrupt change at the cladding boundary which is known as ______.

1. Uniform-index fiber
2. Scale-index fiber
4. Step-index fiber

Explanation

Step-index fiber:– A multimode or single-mode optical fiber with a uniform refractive index throughout the core. The step is the shift between the core and the cladding, which has a lower refractive index.

Step index fiber is fiber type cylindrical waveguide core with an inner core that has a uniform refractive index of n1 and the core is surrounded by an outer cladding with a uniform refractive index of n2.

Ques.14. The critical angle θc in an optical fiber is given by ______. Where n1 is the refractive index of medium 1 and n2 is the refractive index of medium 2.

1. sin-1 (n2/n1)
2. sin-1 (n1/n2)
3. sin-1 (n2n1)
4. sin-1 n2

Explanation

Refractive index: The ratio of the speed of light in a vacuum to the speed of light in a medium is called the refractive index of that medium.

It is also called an absolute refractive index.

μ = ( Speed of light in Vacuum)/(Speed of light in Medium)

Critical angle: When a ray of light is going from a denser medium to a rare medium then the angle of incidence at which the refraction angle is 90° is called the critical angle.

The critical angle is given by:

ΘC = critical angle = sin-1(n2/n1)

Where n2 is the refractive index of the second medium in which the light ray is going and n1 is the refractive index of the first medium from which light is going to the second medium.

Ques.15. Which type of optical fiber is used to eliminate modal dispersion during optical communication?

1. Single-mode step-index fiber
2. Multimode step-index fiber
3. Multimode graded-index fiber
4. Does not depend on type of fiber

Answer.1. Single-mode step-index fiber

Explanation

Single-mode step-index fiber is used to eliminate modal dispersion during optical communication.

Advantages of single-mode fiber:

1) Low signal loss

2) No modal dispersion

3) Does not suffer from modal dispersion

4) Can be used for higher bandwidth applications

5) Long-distance applications

6) Cable TV ends

7) High speed local and wide area network

Ques.16. The refractive indices of the core and cladding of an optical fiber are 1.50 and 1.48, respectively. The critical propagation angle, which is defined as the maximum angle that the light beam makes with the axis of the optical fiber to achieve the total internal reflection, (rounded off to two decimal places) is ______ degree.

1. 8.4°
2. 9.4°
3. 10.8°

Explanation

Critical angle: When a ray of light is going from a denser medium to a rare medium then the angle of incidence at which the refraction angle is 90° is called the critical angle.

The critical angle is given by:

θC = critical angle = Sin-1(n2/n1) —(1)

η1 = refractive index of core

η2 = refractive index of cladding

Calculation:

Given:

η1 = 1.50

η2 = 1.48

The critical propagation angle can be calculated using equation (1):

θC = Sin−1(1.48/1.5)

θC = sin-1(0.986)

θC = 80.64°

Here θC is the angle that a light ray can make with the wall of the core.

Whereas from the figure θ is the maximum angle that the light beam makes with the axis of the optical fiber to achieve the total internal reflection.

θ = 90° – θC

θ = 90° – 80.64°

θ = 9.4°

Ques.17. The numerical aperture (NA) in optical fiber transmission is used to describe

1. Light spreading ability
2. Light gathering or light-collecting ability
3. Light output from an external shield
4. Light leakage ability

Answer.2. Light gathering or light-collecting ability

Explanation

The numerical aperture (NA) is closely related to the acceptance angle and is the figure of merit commonly used to measure the magnitude of the acceptance angle. The numerical aperture (NA) in optical fiber transmission is used to describe Light gathering or light-collecting ability  (i.e the ability to couple light into the cable from an external source). The larger the magnitude of the numerical aperture, the greater the amount of external light the fiber will accept.

Ques.18. The main advantage of fiber optic cable over co-axial cable is

1. Easy handling
2. Less weight
3. Easy testing
4. Low loss

Explanation

Optical communication utilizes the principle of total internal reflection by which light signals can be transmitted from one place to another with a negligible loss of energy

Fiber optic cable has many advantages over co-axial cable like high bandwidth, low loss, more security, etc.

The advantages of optical fibers are:

• Optical fibers have greater information-carrying capacity due to large bandwidth
• Optical fibers are free from electromagnetic interference and offer high signal security
• Optical fibers suffer less attenuation than coaxial cable and twisted wire cables.

The main advantage is low loss data can be sent through fiber cable across several miles without needing repeaters while co-axial cables need repeaters after a short distance.

Ques.19. The Dispersion Shifted fiber is normally designed to have zero chromatic dispersion at

1. 850 nm
2. 1300 nm
3. 1480 nm
4. 1550 nm

Explanation

Dispersion shifted fiber: Dispersion Shifted Fiber is a type of single-mode optical fiber with a modified refractive index profile of the core to shift the zero-dispersion wavelength from the standard 1300 nm to 1550 nm.

Ques.20. The modal dispersion in a fiber is 20 ns/km. what will be the overall pulse spread in 10 km long fiber? Assume material dispersion to be zero

1. 50 ns
2. 100 ns
3. 200 ns
4. 300 ns

Explanation

Given that,

Modal dispersion over distance = 20 ns/km

Fiber distance = 10 km

The overall pulse spread in 10 km long fiber will be::

= (20 ns/km) × (10 km)

= 200 ns

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