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 not use ray analysis for SMF - use only modal analysis Can use ray analysis Not a ray!

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 not use ray analysis for SMF - use only modal analysis Can use ray analysis Not a ray!

17. DIFFERENT PATH LENGTHS IN MULTIMODE FIBRES 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 Use ray analysis:

18. ADVANTAGES OF OPTICAL FIBRES 1. LOW LOSS 2. HIGH BANDWIDTH 3. LOW MATERIAL PRICE 4. LOW WEIGHT 5. 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 Source: Ghatak Population inversion Optical gain Mirrors

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