Light Waves and Polarization

Transcription

1 Light Waves and Polarization Xavier Fernando Ryerson Communications Lab

2 The Nature of Light There are three theories explain the nature of light: Quantum Theory Light consists of small particles (photons) better explains light detection and generation Wave Theory Light travels as a transverse electromagnetic wave Ray Theory Light travels along a straight line and obeys laws of geometrical optics useful tool when the objects are much larger than the wavelength of light

3 Wavelength Ranges

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. =νλ where, c - speed of light 3 X 10 8 m/s in air, ν - frequency and λ- wavelength Ex: Find the ν when λ = 1550 nm. Answer: THz c

5 Quantum Theory of Light Optical Signal is consists of discrete units called photons. The energy in a photon E g = hν, where h= 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 1.24 E g = λ( µm) ev

6 Plane Waves Most light waves are plane waves For plane waves the transverse condition requires that the electric and magnetic field be perpendicular to the direction of propagation and to each other. 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 E = Eo cos( ωt kz)

9 Phase Velocity v p v p = ω = k c n

10 Group of Waves

11 Carrier and Envelope v p v g

12 Group Velocity v g = ω = k c n g m/s The function ω(k) is known as the dispersion relation. If ω is directly proportional to k, then the group velocity is exactly equal to the phase velocity. Otherwise, the envelope of the wave will become distorted as it propagates. This "group velocity dispersion" is an important effect in the propagation of signals through optical fibers.

13 Polarization Polarization of a plane wave is the orientation of the oscillations of the E field For a simple harmonic wave, amplitude of the electric vector may have Different amplitude Different phase The resulting wave is Linearly, elliptically or circularly polarized

14 Polarization states If the electric field is oscillating along a straight line, it is called a linearly polarized (LP) or plane polarized wave If the E field rotates in a circle (constant magnitude) or in an ellipse then it is called a circular or elliptically polarized wave Natural light has random polarization

15 Adding two linearly polarized waves with a phase shift will produce an elliptically polarized light

16 Elliptical Polarization

17 Adding two linearly polarized waves with zero phase shift will generate another linearly polarized wave E = Eo cos( ωt kz)

18 Linear Polarization

19 Adding two linearly polarized waves with equal amplitude and 90 o phase shift results in circular polarized wave

20 Circular Polarization

21 Unpolarized Light In most natural light sources, the orientation of the electric fields may not be correlated. This light is said to be unpolarized or partially polarized. Partially polarized light can be described as a superposition of a completely unpolarized component, and a completely polarized one.

22 Polarized Light In practice, if the electric field is aligned in one direction, that light is called the polarized light Horizontally polarized light Vertically polarized light

23 Polarization Control Polarisers are used to block reflected light in optical fibres Optical Isolators

24 Polarized Sunglasses These provide better view by blacking reflected (glare) light that has horizontal polarization