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Light Waves and Polarization

Light Waves and Polarization. Xavier Fernando Ryerson Communications Lab http://www.ee.ryerson.ca/~fernando. The Nature of Light. Quantum Theory – Light consists of small particles (photons). This theory better explains light detection and generation processes.

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Light Waves and Polarization

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  1. Light Waves and Polarization Xavier Fernando Ryerson Communications Lab http://www.ee.ryerson.ca/~fernando

  2. The Nature of Light • Quantum Theory – Light consists of small particles (photons). This theory better explains light detection and generation processes. • Wave Theory – Light travels as a transverse electromagnetic wave. This theory better explains light propagation. • Ray Theory – Light travels along a straight line and obeys laws of geometrical optics – This theory is usefulwhen the objects are much larger than the wavelength of light (Multi Mode Fiber)

  3. Quantum Theory of Light • Light consists of discrete units called photons. The energy in a photon h= 6.6256 X10(-34)J.sis the Planck’sconstant and νis the frequency. • Ex1: Find the energy of a photon travelling with 200 THz frequency • Ex2: Show

  4. Wave Theory of Light • Electromagnetic light signal has electric and magnetic fields orthogonal to each other. • The frequency of this EM wave is in the order of THz. Therefore, it is convenient to measure it in terms of wavelength. • where, c - speed of light 3 X 108 m/s in air,ν - frequencyandλ- wavelength • Ex: Find the νwhenλ = 1550 nm. • Answer: 193.5 THz

  5. Wavelength Ranges

  6. Plane Waves • Most Light waves are plane waves • A plane wave is a constant-frequency wave whose wave fronts (surfaces of constant phase) are infinite parallel planes. • The electric field vector of a plane wave may be arbitrarily divided into two perpendicular components labeled x and y (with z indicating the direction of travel).

  7. Field distributions in plane E&M waves Electric and magnetic fields are orthogonal to each other and to the direction of propagation Z

  8. Basics about Plane Waves / propagation constant The combined wave s

  9. vp Phase Velocity co: Speed of light in air n: Refractive index Phase velocity:

  10. Phase Velocity • apparentand true depth • Light in fiber core travels slower  ‘waveguide dispersion’

  11. Changing Refractive Index • The refractive index n is not constant • It is a function of the wavelength of light, n = n(λ). • Therefore, different wavelengths will travel at different velocity in glass fiber • The wavelength dependency of n is given by an empirical formula, the Cauchy or Sellmeier equations

  12. Group of Waves Most practical light sources emit group of waves, not just one 2Δω

  13. Carrier and Envelope vp vg

  14. Group Velocity m/s • Group of waves travel at group velocity, slightly different from phase velocity • The group refractive index ng is a function of n,ω and dn/d ω • If ω proportional to k, then the ng = n and vg = vp. • Usually it is not the case; This results in “Group Velocity Dispersion“. • The GVD is important single mode optical fibers.

  15. Sellmeier Equation • Refractive Index n is a nonlinear function of wavelength • The slope of this graph is related to ng

  16. Polarization • Polarization of a plane wave is the orientation of the oscillations of the E field; perpendicular to the direction of propagation • For a simple harmonic wave, the electric vector in orthogonal directions may have: • Different amplitude • Different phase • The resulting wave is • Linearly, elliptically or circularly polarized

  17. When the orthogonal components have different phase and amplitude, resulting wave is Elliptically Polarized (General Case)

  18. When the orthogonal components have 90o phase shift and equal amplitude, the resulting wave is Circularly Polarized(Special Case)

  19. When the orthogonal components have zero phase shift, resulting wave is Linearly Polarized • More useful • Emitted by lasers • Polarization control is possible • Horizontal and vertical polarizations

  20. Linear Polarization

  21. Faraday Effect • When a magnetic field is applied to linearly polarized light, the plane of polarization rotates. • The rotation is proportional to the intensity of the applied magnetic field in the direction of the beam of light This effect is used in Optical Isolators

  22. Optical Isolator output polarizer (allows light at 45o) • Vertically polarized light enters the isolator. • The Faraday rotator rotates it by 45o. • Output polarizer passes the light. • Backward traveling (reflected) light starts with 45o tilt. • It gets horizontal polarization at the rotator and will be extinguished. Faraday rotator Input Polarizer (allows only vertically polarized light) Polarization Controller (creates vertical polarization)

  23. Polarization Mode Dispersion (PMD) Each polarization state has a different velocity  PMD

  24. Polarization Dependent Modulation

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