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

Answer: 1. Less light

Explanation: 

Multimode graded-index fibers accept less light than multimode step-index fibers with the same core A. However, graded-index fibers usually outperform the step-index fibers. The core’s parabolic refractive index profile causes multimode graded-index fibers to have less modal dispersion.

Answer: 3. Hondros and debye

Explanation: 

Advantages of skew Rays

1. Skew rays have larger acceptance angles than meridional rays.

2. Light gathering capability of skew rays is higher than meridional rays.

Disadvantage

1. Skew rays propagate in the annular region near outer surface of the core. Hence full fiber core is not utilized.

Answer:4. 6.50μm

Explanation: 

The normalized frequency for the fiber can be adjusted within the range by 0 ≤ V≤ 2.405 reducing core radius and refractive index difference < 1%.

Normalized frequency V≤ 2.405 is the value at which the lowest order Bessel function J=0. Core size(radius).

$a=\frac{v\lambda }{2\pi }\frac{1}{\sqrt{n_{1}^{2}-n_{2}^{2}}}$

$a = \frac{2.405\times 1.55}{2\Pi }\frac{1}{\sqrt{(1.480)^{2}-}(1.478)^{2}}$

a = 6.50

If this fiber also should be single-mode at 1310 nm, then the core radius must be 6.50

Answer: 1. Yes

Explanation: 

The normalized frequency V of cutoff wavelength

V= 2πa.NA/λ

Given

NA = 0.10

core radius = 2μm

λ = 0.80 nm

V = 2π × 2 × 0.10/0.80

V = 2.094

Here, V=2.094 is less than 2.405. Thus, this optical fiber exhibit a single-mode operation.

Answer:2. Plastic Fiber

Explanation: 

Plastic fibers are less widely used because of their substantially higher attenuation than glass fibers. The main use of plastic fibers is in short-distance applications (several hundred meters) and in abusive environments, where the greater mechanical strength of plastic fibers offers an advantage over the use of glass fibers.

The majority of fibers are made of glass consisting of either silica (SiO₂) or a silicate. The variety of available glass fibers ranges from moderate-loss fibers with large cores used for short-transmission distances to very transparent (low-loss) fibers employed in long-haul applications.

Answer: 3. Hondros and debye

Explanation: 

• For single-mode fiber, instead of the core diameter and numerical aperture, the geometric distribution of light in the propagating mode is important in predicting the performance characteristics of these fibers.
• Thus, a fundamental parameter used for characterizing single-mode fiber properties is mode field diameter or MFD.
• MFD takes into account the wavelength-dependent field penetration into the fiber cladding.
• The mode field diameter is analogous to the core diameter in multimode fibers except that in single-mode fibers, not all the light that propagates through the fiber is carried in the core.
• For step-index and graded-index single-mode fibers operating near the cut-off wavelength, the field is well approximated by a Gaussian distribution.

Answer: 3. Peterman equations

Explanation: 

MFD is determined by the numerical aperture (N and cut-off wavelength of the fiber and is related to the diameter of the fiber core. In general, MFD is greater than the physical diameter of the fiber core – which means that some optical power is always guided by the fiber cladding.

The MFD is an important parameter for single-mode fiber because it is used to predict fiber properties such as splice loss, bending loss, cutoff wavelength, and waveguide dispersion.

A variety of models have been proposed for characterizing and measuring the MFD. These include far-field scanning, near-field scanning,

The standard technique is to first measure the far-field intensity distribution and then calculate mode field diameter using Peterman equations.

Answer: 3. Numerical Aperture

Explanation: 

A numerical aperture is a measure of the acceptance angle of a fiber. It also gives the light gathering capacity of the fiber. For single-mode fiber, the core is of constant refractive index. There is no variation with respect to the core. enter or leave the fiber.

The numerical aperture (N is a point function across the end face of the core, depending on the refractive index at the point. In a step-index fiber, the NA does not vary across the end face of the core because the refractive index is constant from point to point. Thus, the Numerical aperture is constant for single-mode fibers.

Answer: 4. 10.1μm

Explanation: 

For single-mode fiber, MFD=2w_o.

$\frac{\omega _{o}}{a}=\frac{1}{\sqrt{InV}}$

Given

V = 2.20

Mode field Diameter ω_o = 10.2

$a=\frac{10.2}{\sqrt{In2.20}}$

a=5.05μm.

Core-diameter = 2a = 10.1μm.

Answer: 1. Better in short distance application

Explanation: 

Plastic fibers are less widely used because of their substantially higher attenuation than glass fibers. The main use of plastic fibers is in short-distance applications (several hundred meters) and in abusive environments, where the greater mechanical strength of plastic fibers offers an advantage over the use of glass fibers.

Answer: 3. Fiber birefringence

Explanation: 

The difference between the modes’ refractive indices is called Fiber birefringence.

Birefringence is the fundamental principle by which polarization-maintaining or HiBi fiber works.

There are two propagation modes in single-mode fibers. These two modes are similar but their polarization planes are orthogonal. In actual fibers, there are imperfections such as variations in refractive index profiles. These modes propagate with different phase velocities and their difference is given by

B_f =n_y – n_x.

Here, n_y and n_x are refractive indices of two modes.

Environmental factors such as bend, twist, and anisotropic stress also produce birefringence in the fiber, the direction, and magnitude which keep changing with time due to changes in the ambient conditions such as temperature.