# Optical Fiber Questions and Answers || Optical Fiber MCQ

Ques.1. Optic fibers are used in:

1. CAT scans
2. X-ray photos
3. Ultrasound scans
4. Endoscopy

Explanation

• The most significant and prevalent applications of fiber optics in medicine are in the imaging and illumination components of endoscopes.
• Flexible and rigid multi fibers composed of step-index fibers and graded-index imaging rods are extensively used for visualization of internal organs and tissue which are accessible through natural openings or transcutaneously.

Ques.2. For a 3-µm-diameter optical fiber with core and cladding indexes of refraction of 1.545 and 1.510, respectively. The cut-off wavelength is.

1. 2.3 μm
2. 1.29 μm
3. 1.5 μm
4. 3.24 μm

Explanation

For single-mode step index fiber normalized frequency number (V) ranges:

0 < V ≤ 2.405

∴ Vmax = 2πa(NA)/λc

For max V, the wavelength is taken as the critical wavelength.

Given:

Core diameter (d) = 3 μm

Core indexes (n1) = 1.545

$\begin{array}{l} N.A = \sqrt {n_1^2 – n_2^2} = 0.326\\ \\ {\lambda _c} = \frac{{2\pi \cdot a \cdot \left( {N.A.} \right)}}{{{V_{max}}}}\\ \\ = \frac{{2\pi \: \times \:1.5\: \times \:0.326}}{{2.405}} \end{array}$

λc = 1.29 μm

Ques.3. Which fiber is preferred for long-distance communication?

1. Step index single-mode fiber
3. Step index multimode fiber

Explanation

• Single-mode step indexed fibers are widely used for wideband communications are preferred for long-distance communication.
• Single-mode step-index fiber is used to eliminate modal dispersion during optical communication.
• In this fiber, a light ray can travel on only one path so minimum refraction takes place hence, no pulse spreading permits high pulse repetition rates.

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.4. The maximum angle to the axis at which light may enter the fiber in order to be propagated is referred to as:

1. Critical angle
2. Refractive index
3. Numerical aperture
4. Acceptance angle

Explanation

• The maximum angle of incidence at the entrance aperture of the fiber for which the light ray suffers total internal reflection at the core-cladding interface and propagates through the fiber is called the acceptance angle.
• Sine of the acceptor angle of the optical fiber measures the light gathering or accepting the power of the fiber.

Ques.5. Graded index fiber is used to

1. Reduce absorption and resulting power loss
2. Reduce dispersion and thereby increase the data rate
3. Establish secure communication
4. Increase the numerical aperture

Answer.2. Reduce dispersion and thereby increase the data rate

Explanation

• Graded-index (GI) fiber reduces multimode dispersion by grading the refractive index of the core so that it smoothly tapers between the core center and the cladding.
• Graded index fiber is used to minimize dispersion. This is because, in the graded-index fiber, the profile of the refractive index is parabolic and due to this refocusing of the signal within the core is increased, which eventually increases the data rate.

Difference between step-index and graded-index fiber.

 Step Index Graded Index The Refractive index profile is uniform within core inside the core Refractive index profile is variable within core Light signals propagate in a zig-zag manner Light signal propagate in the skewed form inside the core It supports single-mode It supports only multi-mode fiber Low bandwidth High bandwidth

Ques.6. Find the maximum possible core diameter which allows single-mode operation at a wavelength of 1.3 μm in a graded-index fiber having a parabolic refractive index profile core with core refractive index at the core axis of 1.5 and relative index difference of 1%.

1. 2.5 μm
2. 3.3 μm
3. 5 μm
4.  6.6 μm

Explanation

For lowest order or single-mode operation, V number is given by

$V = 2.405\sqrt {1 + \frac{2}{\alpha }}$

α = various profile parameter of the fiber

α = 1 for triangular profile

α = 2 for parabolic profile

Given

Operation wavelength (λ) = 1.3 μm

Core refractive index (n1) = 1.5

Relative index difference (Δ) = 0.01

α = 2 for parabolic profile

$\begin{array}{l} V = 2.405\sqrt {1 + \frac{2}{\alpha }} \\ \\ V = 2.405\sqrt {1 + \frac{2}{2}} \end{array}$

V = 2.405 × √2 = 3.401

We know that

$V = \frac{{2\pi a}}{\lambda }{n_1}\sqrt {2{\rm{\Delta }}}$

The above equation can be rearranged as

$a = \frac{{V\lambda }}{{2\pi {n_1}\sqrt {2{\rm{\Delta }}} }}$

Putting all values in the above equation

$a = \frac{{3.401\: \times \:1.3\: \times \:{{10}^{ – 6}}}}{{2\pi \: \times \:1.5\:\sqrt {2\: \times \:0.01} }}$

a = 3.316×10-6  meter

a = 3.316 μm

diameter of core (d) = 2a

d = 2 × 3.316 μm

d = 6.632 μm

Ques.7. Optical fiber works on the phenomenon of _______

1. Diffraction
2. Polarization
3. Total internal reflection
4. Refraction

Explanation

Optical fibers rely on total internal reflection for their operation. An optical fiber is a thin rod of high-quality glass. Light/infrared getting in at one end undergoes repeated total internal reflection and emerges at the other end. Notice that the light refracts towards the normal as it enters the optical fiber.

Ques.8. Out of the following, which light is used for optical fiber communication?

1. Tube light
2. Laser light
3. Bulb light
4. Sunlight

Explanation

Laser light is used for optical fiber communications for the simple reason that it is a single wavelength light source. Sunlight or the light emitted by a light bulb is a mixture of many different wavelengths of light. Because the light waves of such light are all out of phase with one another, they do not produce a very powerful beam. Laser beams, however, have a single wavelength, and so their waves are all in phase, producing very powerful light.

Ques.9. In the structure of the fiber optic cable, the refractive index of the core is always ______ the refractive index of the cladding.

1. Less than
2. Equal to
3. Greater than
4. None of the above

Explanation

The refractive index of the core is always greater than the refractive index of the cladding. 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.10. Light is confined within the core of an optical fiber due to

1. Refraction
2. Scattering
3. Diffraction
4. Total internal reflection at the core, cladding boundary