JNTU
JNTU B.Tech IV OPTICAL COMMUNICATIONS Previous Question Paper (September - 2020 - R16)
Answer any Five Questions. All Questions Carry Equal Marks.
Main
Prove that the total number of modes entering the Step index Fiber is
Answer
In a step-index fiber, the number of guided modes M is related to the normalized frequency V. For a large number of modes, the total number of modes M can be approximated by integrating the area of the phase space in the fiber cross-section. The number of modes is approximately equal to the number of possible values of the propagation constant β, which lies in the range n2k ≤ β ≤ n1k. The total number of modes M is given by the integral of the mode density over the fiber cross-section and the allowed range of the transverse wave vector. For a step-index fiber, this simplifies to the expression:
where V is the normalized frequency defined as V = (2πa/λ) * NA, with a being the core radius, λ the operating wavelength, and NA the numerical aperture. This approximation holds for large V values, where the discrete nature of the modes can be treated as a continuum.
A multimode step index fiber with a core diameter of 80 μm and a relative index difference of 1.5% is operating at a wavelength of 0.85 μm. If the core refractive index is 1.48, estimate i) the normalized frequency for the fiber ii) the number of guided modes.
Answer
Given parameters: Core diameter d = 80 μm (radius a = 40 μm), relative index difference Δ = 0.015, wavelength λ = 0.85 μm, and core refractive index n1 = 1.48.
- First, calculate the Numerical Aperture (NA):NA = n1 * sqrt(2Δ) = 1.48 * sqrt(2 * 0.015) ≈ 1.48 * 0.1732 ≈ 0.2563
- Calculate the normalized frequency (V):V = (2πa / λ) * NA = (2 * π * 40 / 0.85) * 0.2563 ≈ 295.6 * 0.2563 ≈ 75.76
- Calculate the number of guided modes (M):M = V2 / 2 = (75.76)2 / 2 ≈ 5739.57 / 2 ≈ 2870
The fiber supports approximately 2870 guided modes.
Describe with neat diagram different types of optical fiber waveguides. Using ray theory explain the propagation of light inside the fiber.
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Write a short notes on different fiber materials.
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Differentiate between Splicer and Connector. Also, explain about different types of connectors.
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A multimode graded index fiber exhibits total pulse broadening of 0.1 μs over a distance of 15 km. Estimate i) the pulse dispersion per unit length ii) the bandwidth-length product for the fiber.
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Explain different mechanisms which causes absorption.
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Discuss the main causes of intramodal dispersion.
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Describe with aid of suitable diagram, three common technique used for mechanical splicing of optical fibers.
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With the help of a schematic diagram, explain the design of an edge emitting LED.
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Discuss the principle of operation of LASER diodes. What are the effects of temperature on the performance of LASER diode?
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Discuss about the reliability of LED and ILD.
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Explain the working of P-I-N photodiode. Also explain the factors that limit the speed of response of photodiode.
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With the help of a schematic diagram, explain briefly construction and operation of APD.
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Explain link power budget for point–to-point link.
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Discuss about overall fiber dispersion in multi-mode fibers.
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