# Single Mode Fiber MCQ || Single mode Fiber Question and Answers

1.  An optical fiber has core index of 1.480 and a cladding index of 1.478. What should be the core size for single-mode operation at 1310nm?

1. 7.31μm
2. 8.71μm
3. 5.26μm
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

2. Optical fiber has a core radius of 2μm and a numerical aperture of 0.1. Will this fiber operates at single-mode at 600 nm

1. Yes
2. No
3. Can’t say
4. None of the above

Explanation:

The normalized frequency V of cutoff wavelength

V= 2πa.NA/λ

Given

NA = 0.10

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.

3. Which of the following optical fiber is less used?

1. Glass fiber
2. Plastic Fiber
3. Copper fiber
4. None of the above

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.

4. What is needed to predict the performance characteristics of single-mode fibers?

1. The intermodal delay effect
2. Geometric distribution of light in a propagating mode
3. Fractional power flow in the cladding of fiber
4. Normalized frequency

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.

5. Which equation is used to calculate MFD?

1. Maxwell’s equations
2. Allen Cahn equations
3. Peterman equations
4. Boltzmann’s 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.

6. _______ is constant in the case of step-index fiber.

1. Refractive Index
2. Reflective Index
3. Numerical Aperture
4. None of the above

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.

7. Single-mode fiber has a mode field diameter of 10.2μm and V = 2.20. What is the core diameter of this fiber?

1. 11.1μm
2. 13.2μm
3. 7.6μm
4. 10.1μm

Explanation:

For single-mode fiber, MFD=2wo.

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

Given

V = 2.20

Mode field Diameter ωo = 10.2

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

a=5.05μm.

Core-diameter = 2a = 10.1μm.

8.  The main advantage of plastic fiber over glass fiber is

1. Better in short distance application
2. Better in Medium distance application
3. Better in long-distance application
4. Any of the above

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.

9. The difference between the modes’ refractive indices is called as _______

1. Polarization
2. Cutoff
3. Fiber birefringence
4. Fiber splicing

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

Bf =ny – nx.

Here, ny and nx 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.

10. A single-mode fiber has a beat length of 4cm at 1200nm. What is birefringence?

1. 2 × 10-5
2. 1.2 × 10-5
3. 3 × 10-5
4. 2 × 10-6

Explanation:

Birefringence is defined as

Bf = (ny –  nx) = λ/Lp

Here,

λ=wavelength = 12 × 10−1

Lp = beat length = 4cm = 4 × 10-4

Bf = 12 × 10−1/ 4 × 10-4

B= 3 × 10-5

11. How many propagation modes are present in single-mode fibers?

1. One
2. Two
3. Three
4. Five

Explanation:

For a given optical fiber, the number of modes depends on the dimensions of the cable and the variations of the indices of refraction of both core and cladding across the cross-section.

In any ordinary single-mode fiber there are two independent, degenerate propagation modes. These modes are very similar, but their polarization planes are orthogonal. These may be chosen arbitrarily as the horizontal (H) and the vertical (V) polarizations.

Either one of these two polarization modes constitutes the fundamental HE11 mode. In general, the electric field of the light propagating along the fiber is a linear superposition of these two polarization modes and depends on the polarization of the light at the launching point into the fiber.

12. The main advantages of single-mode fiber are?

1. Higher speed
2. No Modal Dispersion
3. Higher Data Rate
4. All of the above

Explanation:

• Single-mode fiber does not face modal dispersion, modal noise, and other effects that arise with the multimode transmission. They are free from intermodal dispersion. The intermodal dispersion is the time difference between the entry of the pulse at one end of optical fiber and the arrival of the pulse at the other end of the fiber.
• Single-mode fiber cable carries signals at much higher speeds than multimode fibers. They are the standard choice for high data rates or long-distance telecommunications (> a couple of Kms) which use diodes as the source.
• Single-mode systems with a 10-µm core are found in long-distance telecommunications applications where ultimate performance is the leading criteria. The most important optical parameters for signal analysis are the operating wavelength, the attenuation in dB/km, and the chromatic dispersion.
• Single-mode fibers do not suffer from modal dispersion (because there’s only one mode of propagation). Likewise, single-mode fiber links do not suffer from differential mode delay, modal noise, or mode partition noise.
• Laser-diode RIN noise is less of an issue for single-mode fiber than for multimode because of the generally higher quality of single-mode connectors, which admit fewer Reflection.

13. The main disadvantage of single-mode fiber over multimode fiber is

1. Difficult to achieve end to end connection
2. Expensive equipements
3. Difficult to launch light
4. All of the above

Explanation:

• Due to the smaller size of the core in single-mode fiber, coupling light requires many tolerances than the coupling light in multimode fiber. Recent developments indicate that these tighter tolerances can be achievable.
• Single-mode fiber components and equipment are very expensive and hence multimode counter parts are widely used.
• It is difficult to launch the light through the fiber.
• It is difficult to achieve end-to-end connections of similar fibers.

The prime disadvantage of single-mode fiber is the difficulty of launching signals into it. A single-mode fiber core spans only 10 µm, much less than a multimode fiber core. LED sources generally do not have a narrow enough beamwidth to couple into such a small core area.

The only choice for single-mode sources is a pinpoint source, like a laser diode. Unfortunately, laser-diodes and their associated output power control circuitry are significantly more expensive than LED sources.

The small core diameter of a single-mode fiber and its attendant requirements for mechanical precision in all coupling components also increase the cost of cable connectors, the cost of optical packages, and the amount of time and energy required in the field to install, test, and maintain the optical connections. Single-mode systems cost big bucks.

Scroll to Top