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PHOTONS AND FIBRES

PHOTONS AND FIBRES. Lecturer : Professor Laurie Cahill. PHOTONS AND FIBRES. What is light? Is light a particle or a wave? What is a photon?. HISTORY. Newton - Light is a stream of corpuscles Huygens, Maxwell,Young - Light travels in waves

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PHOTONS AND FIBRES

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  1. PHOTONS AND FIBRES Lecturer : Professor Laurie Cahill

  2. PHOTONS AND FIBRES What is light? Is light a particle or a wave? What is a photon?

  3. HISTORY • Newton - Light is a stream of corpuscles • Huygens, Maxwell,Young - Light travels in waves • Planck - Thermal radiation comprises discrete packets of energy called quanta • Einstein

  4. EINSTEIN (1905) • Light is similarly quantised E is the energy of the light quanta, later called photons h is Planck’s constant f is the frequency of the light This explains the photoelectric effect

  5. THE PHOTOELECTRIC EFFECT The maximum KE of each emitted electron depends on the frequency of the incident light, not the intensity. More photons produce more emitted electrons (Source: D. A. Neamen)

  6. WORK FUNCTION OF A METAL SURFACE (Source : A. Beiser)

  7. De BROGLIE (1924) • Momentum of a photon • Wavelength of a particle • Hence matter waves and the Wave-Particle Duality

  8. DOUBLE SLIT EXPERIMENT Similar experimental result (over time) if we use photons or electrons

  9. WAVES OR PARTICLES? If light only consists of waves, how come we can only generate and detect discrete photons? If light consists only of particles, how does a photon passing through one slit know about the other slit being open? Feynman - Consider all possible paths and assign amplitudes and probabilities to “particles” .

  10. COMMUNICATION USING LIGHT Consider a light ray entering a glass rod

  11. RAYS IN A CIRCULAR FIBRE Exercise: Find an expression for the acceptance angle for rays through the axis (Hint apply Snell’s Law). Answer :

  12. COMPARISON OF MULTIMODE AND SINGLE MODE FIBRE GEOMETRIES Can use ray analysis Not a ray! Can not use ray analysis for SMF - use only modal analysis

  13. CAUSES OF FIBRE LOSS

  14. FIBRE ATTENUATION Loss in db =

  15. OPTICAL FIBRE LINK (Source: D. M. Spirit & M. J. Mahoney)

  16. COMPARISON OF MULTIMODE AND SINGLE MODE FIBRE GEOMETRIES Can use ray analysis Not a ray! Can not use ray analysis for SMF - use only modal analysis

  17. DIFFERENT PATH LENGTHS IN MULTIMODE FIBRES Use ray analysis: • The velocity in the z direction depends on the angle of the ray • A spread of path lengths gives a spread of arrival times • This spreads a sharp pulse and limits the bit-rate

  18. ADVANTAGES OF OPTICAL FIBRES • LOW LOSS • HIGH BANDWIDTH • LOW MATERIAL PRICE • LOW WEIGHT • LOW EMI (INTERFERENCE)

  19. GENERATION AND DETECTION OF PHOTONS Emission Stimulated Emission Detection

  20. ENERGY LEVELS IN A SUITABLE SEMICONDUCTOR Photodiode LED Laser Diode

  21. DETECTION OF PHOTONS

  22. REQUIREMENTS FOR LASING • Population inversion • Optical gain • Mirrors Source: Ghatak

  23. EARLY LASER DIODE Source: Ghatak

  24. LIGHT CONCENTRATED IN ACTIVE REGION OF LASER

  25. LIGHT POWER VS CURRENT

  26. SPECTRA OF LED AND LASER DIODE

  27. SINGLE AND MULTIMODE LASER DIODES

  28. DISPERSION (PULSE SPREADING) IN SMF Since velocity depends on n, a small change in n with wavelength can affect the arrival time of signals of different wavelengths and cause them to overlap.

  29. SINGLE MODE FIBRE DISPERSION (Source: D. M. Spirit & M. J. Mahoney)

  30. INTERSYMBOL INTERFERENCE Dispersion (pulse spreading) can cause pulses to overlap and limit bit-rate (Source: G. Keiser)

  31. PHOTONS AND FIBRES What is a photon? “All these fifty years of conscious brooding have brought me no nearer to the answer to the question, “What are light quanta (photons)?” Albert Einstein

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