Optical Communications Multiple Choice Questions on “Fiber Numerical Aperture and Fiber Diameter Measurements”.
1. The ____________ affects the light gathering capacity and the normalized frequency of the fiber.
a) Numerical aperture
b) Amplitude modulation
c) Responsivity
d) Quantum efficiency
Answer: a
Explanation: Numerical aperture is an important optical parameter as it dictates the important characteristics of the optical fiber. This in turn dictates the number of propagating modes within the fiber.
2. The numerical aperture for a step index fiber is sine angle of the ____________
a) Efficient angle
b) Aperture
c) Acceptance angle
d) Attenuation
Answer: c
Explanation: The numerical aperture of a step index fiber is given by –
NA = sinθa, where θa is the acceptance angle and NA is the numerical aperture.
3. The calculations of the numerical aperture from a refractive index data are less accurate for the graded index fibers than for step index fibers.
a) False
b) True
Answer: b
Explanation: The refractive indices of the core and cladding are fluctuating, thus causing the data to be less efficient. For graded index fibers, it is usually less accurate than for the step index fibers.
4. Far field pattern measurements with regard to multimode fibers are dependent on the _____________ of the fiber.
a) Amplitude
b) Frequency
c) Diameter
d) Length
Answer: d
Explanation: The accuracy of the measurement technique is dependent upon the visual assessment of the far-field pattern from the fiber. In case of multimode fibers, far field pattern measurements are dependent on the length of the fiber.
5. The screen is positioned 10 cm from the fiber end face. When illuminated from a wide angled visible source the measured output pattern size is 6.2 cm. Calculate the approximate numerical aperture of the fiber.
a) 0.21
b) 0.30
c) 0.9
d) 1.21
Answer: b
Explanation: The numerical aperture can be obtained from a trigonometric relationship given by-
NA = A/(A2+4D2)1/2, where A is constant(38.44) and D is the distance of the fiber end face from the screen in mm.
6. During the fiber drawing process, the fiber outer diameter is maintained constant to within ________
a) 2%
b) 1%
c) 5%
d) 10%
Answer: b
Explanation: During the fiber manufacturing stage, all processes needs to be performed efficiently. Especially, in the drawing process, the outer diameter should be compiled to within 1 % to avoid miscommunication through fibers.
7. What is the minimum value of accuracy in diameter is needed to avoid radiation losses in the fiber?
a) 0.1%
b) 0.2%
c) 0.3%
d) 0.03%
Answer: c
Explanation: Any diameter variations can cause excessive radiation losses and make accurate fiber-fiber connection difficult. Hence, on-line diameter measurement systems are used which provides accuracy of 0.3%.
8. Which of the following is a non-contacting optical method of on-line diameter measurement?
a) Brussels’s method
b) Velocity differentiator method
c) Photo detector method
d) Image projection method
Answer: d
Explanation: On-line diameter measurement technique uses fiber image projection method. It is also known as non-contacting optical method and shadow method.
9. The shadow method is used for measurement of the outer diameter of an optical fiber. The apparatus employs a rotating mirror with an angular velocity of 4 rad/s which is located at 10 cm from the photo detector. Compute the shadow velocity.
a) 0.1 μm μs-1
b) 0.4 μm μs-1
c) 0.87 μm μs-1
d) 1 μm μs-1
Answer: b
Explanation: The shadow velocity is obtained by the below equation:
ds/dt = l. dϕ/dt, where l is the distance of the apparatus from the photodetector and dϕ/dt is the angular velocity.
10. The shadow velocity is given by 0.4 μm μs-1 and shadow pulse of width 300 μs is registered at an instant by the photodetector. Determine the outer diameter of the optical fiber in μm.
a) 100 μm
b) 120 μm
c) 140 μm
d) 90 μm
Answer: b
Explanation: The fiber outer diameter is given by-
d0 = We.Ds/dt, where We = pulse width and ds/dt = shadow velocity.
11. The techniques used to determine the refractive index profile can also be used to determine the core diameter.
a) True
b) False
Answer: a
Explanation: Some of the techniques used to determine the refractive index profile are interferometry, near field scanning and refracted ray technique. The core diameter for step index fibers is defined by the step change in the refractive index profile. Hence, they can be used to measure the core diameter.
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