Light Waves and Polarization

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 useful when the objects are much larger than the wavelength of light (Multi Mode Fiber)

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

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 - frequency and λ- wavelength Ex: Find the ν when λ = 1550 nm. Answer: 193.5 THz

Wavelength Ranges 1

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).

Field distributions in plane E&M waves Electric and magnetic fields are orthogonal to each other and to the direction of propagation Z 2

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

Linear Polarization

Polarization Mode Dispersion (PMD) Each polarization state has a different velocity  PMD 8