Opcom Practice Questions 1

University of Malaya, EE Department Practice Questions: Set 1 Subject: KEET4205, Session: 2013-2014 (Sem. 2)

1. Using simple ray theory, describe the mechanism for transmission of light within an optical fiber. Briefly discuss with the aid of a suitable diagram what is meant by the acceptance angle for an optical fiber. Show how this is related to the fiber numerical aperture and the refractive indices for the fiber core and cladding. Mention when and why ray theory is not applicable to optical fibers. 2. A step-index fiber with a large core diameter compared with the wavelength of the transmitted light has an acceptance angle in air of 22° and a relative refractive index difference ∆ of 3%. Estimate the numerical aperture and the critical angle at the core–cladding interface for the fiber. 3. A single-mode step-index fiber has a core diameter of 7µm and a core refractive index of 1.49. Estimate the shortest wavelength of light which allows single-mode operation when the relative refractive index difference ∆ for the fiber is 1%. 4. A multimode step index fiber has a relative refractive index difference ∆ of 1% and a core refractive index of 1.5. The number of modes propagating at a wavelength of 1.3 μm is 1100. Estimate  the diameter of the fiber core. 5. A single-mode step index fiber which is designed for operation at a wavelength of 1.3μm has core and cladding refractive indices of 1.447 and 1.442 respectively. When the core diameter is 7.2μm, confirm that the fiber will permit single-mode transmission and estimate the range of wavelengths over which this will occur. 6. A single-mode step index fiber has core and cladding refractive indices of 1.498 and 1.495 respectively. Determine the core diameter required for the fiber to permit its operation over the wavelength range 1.48 to 1.60 μm. Calculate the new fiber core diameter to enable single -mode transmission at a wavelength of 1.30 μm.

7. An optical fiber has a numerical aperture of 0.20 and a cladding refractive index of 1.59. Determine: (a) acceptance angle for the fiber in water which has a refractive index of 1.33 (b) critical angle at the core–cladding interface. 8.

Describe the various loss mechanisms in optical fibers. Sketch the loss versus wavelength plots for optical fibers of earlier and current generations.

9. The optical power launched into an optical fiber link is 1.5mW and the fiber has an attenuation of 0.5 dB/km. Determine the maximum possible link length l ength without repeaters (assuming lossless connectors) when the minimum optical power level required at the detector is 2μW. 10. Describe the various dispersion mechanisms in optical fibers. Derive the expression for pulse spreading due to multiple modes in multimode fibers. 11. Derive the expression for group velocity of electromagnetic waves. Explain the significance of group velocity using appropriate diagrams. 12. Derive the expression for chromatic dispersion in optical fibers. 13. Derive the expression for material dispersion in optical fibers. When does the material dispersion vanish in optical fibers and how? How to control total chromatic dispersion in optical fibers? 14. Sketch the plots of chromatic dispersion versus wavelength for 1300nm-optimized, 1550-optimized (dispersion-shifted) and dispersion-flattened fibers. Sketch the respective refractive index profiles and justify their patterns. 1

15. Describe polarization mode dispersion using suitable illustrations and indicate when it gets important

for system design. 16. The material dispersion in an optical fiber is governed by |d 2n1/d λ 2| = 4.0 × 10−2 μm−2. Estimate the  pulse broadening per kilometer due to material dispersion within the fiber, when it is illuminated with an LED source with a peak wavelength of 0.9 μm and a spectral width of 45 nm.

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