Optical Fiber Waveguides MCQ || Types of Optical fiber Waveguides Questions and Answers

Answer: 3. 1.214μm

Explanation: 

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

Calculation

cut off wavelength is given by

$\lambda _{c}=\frac{2\Pi an_{1}\sqrt{2\Delta }}{V_{c}}$

For cut-off wavelength, Vc = 2.405

$\lambda _{c}=\frac{2\Pi \times 4.5\times 1.46\sqrt{0.005 }}{2.405}$

λc = 1.214 μm

Answer: 3. Hondros and debye

Explanation: 

Given

core radius,= 4.5 μm

Refractive index difference Δ0.25% = 0.0025

Core refractive index n₁ = 1.46

Numerical Aperature = n₁(2Δ)^1/2

= 1.46(2× 0.0025)^1/2

NA =  0.707

Answer: 3. Electromagnetic wave

Explanation: 

• Single-mode step-index fiber allows the propagation of only one transverse electromagnetic mode.
• The core diameter is of the order of 2 to 10 µm.
• As single-mode fibers support only one mode, these types of fibers do not exhibit Intermodal dispersion, whereas with multimode step-index fibers considerable dispersion may take place due to different group velocities of the propagating modes.

Answer: 2. 2 to 10 µm

Explanation: 

• Single-mode step-index fiber allows the propagation of only one transverse electromagnetic mode.
• The core diameter is of the order of 2 to 10 µm.
• As single-mode fibers support only one mode, these types of fibers do not exhibit Intermodal dispersion, whereas with multimode step-index fibers considerable dispersion may take place due to different group velocities of the propagating modes.

Answer: 1. Low in single-mode and considerable in multimode fiber

Explanation: 

• Single-mode step-index fiber allows the propagation of only one transverse electromagnetic mode.
• The core diameter is of the order of 2 to 10 µm.
• As single-mode fibers support only one mode, these types of fibers do not exhibit Intermodal dispersion, whereas with multimode step-index fibers considerable dispersion may take place due to different group velocities of the propagating modes.

Answer: 1. Multimode fiber is advantageous

Explanation:

The single-mode step-index fiber has the distinct advantage of low intermodal dispersion (broadening of transmitted light pulses), as only one mode is transmitted

With multimode step-index fiber, a considerable dispersion may occur due to the different group velocities of the propagating modes. This, in turn, restricts the maximum bandwidth attainable with multimode step-index fibers, especially when compared with single-mode fibers.

In multimode fibers, intermodal dispersion occurs. The group velocities often differ which gradually restricts maximum bandwidth attainability in multimode fibers.

However, for lower bandwidth applications, multimode fibers have several advantages over single-mode fibers.

• The use of spatially incoherent optical sources (e.g., most light-emitting diodes) cannot be efficiently coupled to single-mode fibers.
• Larger numerical apertures, as well as core diameters, facilitate easier coupling to optical sources.
• Lower tolerance requirements on fiber connectors.

Answer: 1. Core, Cladding

Explanation: 

Most of the optical power is carried out in  core region than in cladding.

Cladding in optical fibers is one or more layers of materials of lower refractive index, in intimate contact with a core material of higher refractive index. The cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the two.

The core of a conventional optical fiber is the part of the fiber that guides the light. It is a cylinder of glass or plastic that runs along the fiber’s length. The core is surrounded by a medium with a lower index of refraction, typically a cladding of different glass, or plastic.

In an ideal multimode fiber, there is no mode of coupling. The optical power launched into a particular mode remains in that mode itself. The majority of these modes are mostly confined to fiber core only.

Answer: 3. Curved path along the axis

Explanation: 

Meridional rays pass through the axis of the fiber core. Meridional rays shown appear to follow curved paths through the fiber core. This may be described using the concepts of geometrical optics. The gradual decrease in refractive index from the center of the core creates a large number of refractions of the rays as they are effectively incident on many high to low index interfaces.

This ray indicates the transmission of light in a perfect optical fiber.

Meridional rays can be of two types :

(i) Bound rays

(ii) Unbound rays.

The bound rays follow total internal reflection in the fiber core and do not escape out of the fiber core.

Unbound rays are refracted out of the fiber core and are absorbed in the cladding.

Answer: 4. It is a dimensionless quantity

Explanation: 

The normalized frequency of optical fiber is the frequency that exists at the cut-off condition. There is no propagation and attenuation above the cut-off. It is directly proportional to the numerical aperture which is a dimensionless quantity; hence itself is a dimensionless quantity.

Normalized frequency for a graded-index fiber is given by

V = 2πa(NA)/λ.

where

a is the core radius

λ is the wavelength in vacuum

V is the normalized frequency

NA = numerical aperture