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Optical Fibres

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Optical Fibres

  3. 3. Total Internal Reflection<br />When a ray of light travels from a denser to a rarer medium such that the angle of incidence is greater than the critical angle, the ray reflects back into the medium. This is called total internal reflection.<br />
  4. 4. Optical Fibres<br />Consists of a very thin glass core surrounded by a material of slightly lower refractive index called cladding<br />The thin fibre can be bent without breaking and a ray of light can be sent down the fibre’s core<br />Total internal reflection takes place at the boundary of the core and the cladding<br />
  5. 5. Acceptance angle<br />The maximum angle of incidence that a ray can make that will result in total internal reflection is called acceptance angle<br />
  6. 6. PROBLEM<br />The refractive index of the core of an optical fibre is 1.50 and that of the cladding is 1.40.Calculate the acceptance angle of the fibre. Ans:330<br />The refractive index of the core of an optical fibre is 1.50 and the critical angle of the core- cladding boundary is 750.Calculate the refractive index of the cladding. Ans: 1.45<br />
  7. 7. Material Dispersion<br />Light of different wavelengths have different refractive index and hence come out of the fibre at different times. This is called material dispersion<br />
  8. 8. Modal Dispersion<br />
  9. 9. Modal dispersion<br />Rays that undergo many internal reflections are said to follow a high order mode paths<br />Rays undergoing fewer reflections follow low order mode paths<br />Set of rays having same wavelength reach the end at different times due to different paths taken. This is called modal dispersion<br />
  10. 10. Monomode & Multimode fibre<br />In multimode fibres, the core has a diameter of about 100μm and the cladding is about 20μm thick<br />The rays passing through multimode fibres undergo material as well as modal dispersion<br />In monomode fibres, the core has a diameter of about 8 -10μm and the cladding is about 125μm thick.<br />Rays follow just one path eliminating modal dispersion<br />
  11. 11. Step Index fibre<br />The refractive index of core is constant<br />The refractive index of cladding is constant<br />The refractive index of cladding is slightly lower than that of core<br />
  12. 12. Graded index fibre<br />Refractive index of core decreases smoothly from the centre to the outer edge<br />Refractive index of cladding is constant<br />
  13. 13.
  14. 14. ATTENUATION<br />Attenuation in an optic fibre is caused by the impurities of the glass core. The amount of attenuation depends on the wavelength of the light being transmitted.<br />Power loss in decibels is defined as<br />Power loss = 10log (Pfinal / Pinitial) in dB<br />Thus a power loss of 16 decibels means that the initial power of, say,8.0mW has been reduced to 0.2mW<br />
  15. 15. PROBLEM<br />An amplifier amplifies an incoming signal of power 0.34mW to a signal of power 2.2mW.Calculate the power gain of the amplifier in decibels. Ans: 8.1dB<br />A signal of power 12mW is input to a cable of specific attenuation 4.0 dB/km. Calculate the power of the signal after it has travelled 6.0km in the cable. Ans: 0.048mW<br />
  17. 17. Attenuation & Wavelength<br />The specific attenuation ( power loss in dB per unit length ) actually depends on the wavelength of the radiation travelling along the optic fibre<br />The graph shows minima at 1310nm and 1550nm, which implies that these are desirable wavelengths for optimal transmission<br />These are infra red wavelengths<br />
  18. 18. DETECTION<br />The light that enters an optic fibre travels down the length of the fibre and the arrival of light is registered by a photodiode<br />In the absence of any light, falling on the photodiode, the current is zero<br />When light of a specific wavelength falls on the photodiode, a current flows. The magnitude of the current is proportional to the intensity of light<br />
  19. 19. A light detector circuit with a photodiode<br />
  20. 20. NOISE<br />Source of noise in a cable:<br />Random motion of electrons which creates additional electric fields contaminating the signal. This increases with temperature.<br />Lightning<br />Charged particles emitted by the sun during intense solar activity<br />
  21. 21. NOISE IN OPTICAL FIBRES<br />Main source is the dark current of the photodiode. This is the small current that flows even when the photodiode is dark<br />Signal to noise ratio (SNR) is defined as <br />SNR = 10log Psignal / Pnoise<br />
  22. 22. PROBLEM<br />The minimum SNR considered acceptable for a certain signal is 30dB.If the power of the noise is 2.0mW, calculate the least acceptable signal power.<br />The SNR in a certain signal is 10dB.the signal passes through an amplifier of gain 6.0dB.What will be the signal to noise ratio after amplification?<br />