Optical Fiber Questions and Answers || Optical Fiber MCQ

Answer.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.

Answer.2. 1.29 µm

Explanation

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

0 < V ≤ 2.405

∴ V_max = 2πa(NA)/λ_c

a ⇒ Core radius

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

Given:

Core diameter (d) = 3 μm

Core indexes (n₁) = 1.545

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

λ_c = 1.29 μm

Answer.1. Step index single-mode 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.

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

Answer.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.

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.

Answer.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

Answer.3. Total internal reflection

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.

Answer.2. Laser light

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.

Answer.3. Greater than

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.

Answer.4. Total internal reflection at the core, cladding boundary 

Explanation

In an optical fiber, the information is passed through light, which must not escape outside of it. This phenomenon of confining the light inside the optical fiber is termed Total internal reflection. The refractive index of the core is always greater than the refractive index of the cladding. This is to ensure the 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.