Question

Attenuation in dB km-1 (e 01 0.5 005 0 I 1.2 Wavelength (um) 90 9 1 The graph above shows the attenuation vs. wavelength characteristics for a typical silica based optical fibre. (3 marks) Describe the physical processes responsible for optical attenuation at short wavelengths, long wavelengths and the peaks at wavelengths of 1.2 and 1.4 um? (2 marks) What are the implications of the graph above for silica fibre long distance communications? b) A fibre has a core refractive index of 1.51, cladding index of 1.50 and core diameter of 8 x 10% m. (2 marks) What is the V number of this fibre at a wavelength of 1300 nm? (2 marks) Approximately how many modes exist in this fibre at 1300nm wavelength? (2 marks) At what wavelength does the fibre become single mode? c) (5 marks) Describe modal dispersion and its impact on data transmission over multimode optical fibre. d) (4 marks) Describe the structure and optical principles associated with a fibre Bragg grating.

Answer 1

(a) (i) The various physical processes that are responsible for the absorptions are:

1. At lower wavelengths the absorptions are due to metal ion impurities in silica, the absorption is due to various metallic ions such as Iron, Copper, Chromium...etc.

2. At longer wavelengths the attenuation is due to the infrared absorption in silica.

3. The peaks in the absorption spectrum are the absorption peaks related to OH ions in the silica.

4. Rayleigh scattering is also involved in attenuation at lower wavelengths.

(ii) The implications from the graph is that for long distance communication using silica based optical fibers we have to use the wavelength which has the least attenuation from the above graph. Hence the most optimum wavelength is around 1.55 micrometers.