# Optical Communications Questions and Answers – Fiber Alignment and Joint Loss

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This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Fiber Alignment and Joint Loss”.

1. A measure of amount of optical fiber emitted from source that can be coupled into a fiber is termed as ______________
b) Angular power distribution
c) Coupling efficiency
d) Power-launching

Explanation: Coupling efficiency depends upon the type of fiber attached to the source which should consider the parameters such as numerical aperture, core size, R.I. profile, radiance, core-cladding index difference. All these parameters relate to the performance of the fibers determined by power coupled into the fiber to power emitted by the source. This is called coupling efficiency ηwhich is given by
η = PF/Ps
Where PF = power coupled into the fiber
Ps = power emitted by the source.

2. The ratio r = (n1 – n)/(n1 – n) indicates ____________
a) Fresnel reflection
b) Reflection coefficient
c) Refraction coefficient
d) Angular power distribution coefficient

Explanation: The ratio, r = (n1-n)/(n1-n) is known as Reflection coefficient. It relates the amplitude of the reflected ray to the amplitude of the incident wave.

3. A GaAs optical source having a refractive index of 3.2 is coupled to a silica fiber having a refractive index of 1.42. Determine Fresnel reflection at interface in terms of percentage.
a) 13.4%
b) 17.4%
c) 17.6%
d) 14.8%

Explanation: If the fiber end and the source are in close physical contact, the reflection is given by
r = ((n1-n)/(n1-n))2
Multiplying r by 100, we get the value of r in terms of percentage.

4. A particular GaAs fiber has a Fresnel reflection magnitude of 17.6% i.e. 0.176. Find the power loss between the source and the fiber?
a) 0.86 dB
b) 0.78 dB
c) 0.84 dB
d) 0.83 dB

Explanation: The optical losses in decibels at the joint is given by
Loss = -10log10(1-r)
Where L = loss due to Fresnel reflection
R = magnitude of Fresnel reflection.

5. Two joined step index fibers are perfectly aligned. What is the coupling loss of numerical aperture are NAR= 0.26 for emitting fiber?
a) -0.828 dB
b) -0.010 dB
c) -0.32 dB
d) 0.32 dB

Explanation: Coupling loss for two joined step index fibers is given by
LF(NA) = -10 log (NAR/NAE)2
Where LF = coupling loss
NAR = Numerical aperture of receiving fiber
NAE = Numerical aperture of emitting fiber.

6. Two joined graded index fibers that are perfectly aligned have refractive indices αR = 1.93 for receiving fiber αE = 2.15 for emitting fiber. Calculate the coupling loss.
a) 0.23 dB
b) 0.16 dB
c) 0.82 dB
d) 0.76 dB

Explanation: Coupling loss for two joined and perfectly aligned graded index fiber is given by
LF(α) = -10log10αRE+2)/αER+2)
Where LF(α) = Coupling loss
αR = refractive index of receiving fiber
αE = refractive index of emitting fiber.

7. How many types of misalignments occur when joining compatible fiber?
a) One
b) Two
c) Five
d) Three

Explanation: There are three layers of fiber misalignments and they are: Longitudinal, lateral and angular misalignments.

8. Losses caused by factors such as core-cladding diameter, numerical aperture, relative refractive index differences, different refractive index profiles, fiber faults are known as ____________
a) Intrinsic joint losses
b) Extrinsic losses
c) Insertion losses
d) Coupling losses

Explanation: There are inherent connection problems while joining fibers. These connection problem cause different losses in the fibers and are called as Intrinsic joint losses.

9. A step index fiber has a coupling efficiency of 0.906 with uniform illumination of all propagation modes. Find the insertion loss due to lateral misalignment?
a) 0.95 dB
b) 0.40 dB
c) 0.42 dB
d) 0.62 dB

Explanation: The insertion loss due to lateral misalignment is given by
Loss10t = -10log10t η10t
Where, Loss10t = insertion loss due to lateral misalignment
η10t = Coupling efficiency.

10. A graded index fiber has a parabolic refractive index profile (α=2) and core diameter of 42μm. Estimate an insertion loss due to a 2 μm lateral misalignment when there is index matching and assuming there is uniform illumination of all guided modes only.
a) 0.180
b) 0.106
c) 0.280
d) 0.080

Explanation: The misalignment loss (assuming there is uniform illumination of all guided modes) is given by
Lt = 0.85(y/a)
Where y = lateral misalignment

11. Determine coupling efficiency if the misalignment loss in a graded index fiber is 0.102.
a) 0.136
b) 0.898
c) 0.982
d) 0.684

Explanation: If the misalignment loss is known, the coupling efficiency is defined by
η = 1-Lt
Where η = coupling efficiency
Lt = misalignment loss.

12. In a single mode fiber, the losses due to lateral offset and angular misalignment are given by 0.20 dB and 0.46 dB respectively. Find the total insertion loss.
a) 0.66 dB
b) 0.26 dB
c) 0.38 dB
d) 0.40 dB

Explanation: The total insertion loss in a single mode fiber is given by
TT = TL + Ta
Where, TT = total insertion loss
TL = lateral offset loss
Ta = Angular misalignment loss.

13. The intrinsic loss through a multimode fiber joint is independent of direction of propagation.
a) True
b) False

Explanation: Intrinsic loss is defined as the summation of lateral offset loss and angular misalignment loss. In case of multimode fibers, the intrinsic loss is dependent on the refractive index gradient. The intrinsic loss through a single mode fiber joint is independent of direction of propagation. 