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Experiment 5 Polarization and Modulation of Light

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Experiment 5 Polarization and Modulation of Light Experiment 5 Polarization and Modulation of Light 1. Objective Understanding the definition of polarized and un-polarized light. Understanding polarizer and analyzer definition, Maluse’s law. Retarding plates definition and application 2. Theory Polarized light: A propagating light (Electromagnetic wave; EM) has its electric and magnetic fields. These fields are perpendicular to one another as well as to the propagation direction. Polarization of an EM wave (light) describes the behavior of the electric field vector of the light as it propagates through a medium. If light wave has E-field oscillating in random directions (but perpendicular to the propagation direction) is called unpolarized. If the direction of E-field oscillation is well defined, it is called polarized (Figure 1). Figure 1. If the E-field oscillates at all time within a well-defined line, it said that EM is linearly polarized. In a linearly polarized light, the E-field vibrations and propagating direction (z axis here) defines a plane of vibrations shown in figure 2.a. The E-field oscillations in the plane of polarization can be represented by the superposition of two fields Ex and Ey with the right magnitude and phase (Figure 2.b, figure 2.c) E (z,t) E cos(t kz) x xo Ey (z,t) Eyo cos(t kz) Where Ø is the phase difference between Ex and Ey , which defines the delay between Ex and Ey . y Plane of polarization E ^ x y Ey ^x ^ xEx Ex y^E E y z E (a) (b) (c) Figure(a) A 2.linearly a) Electric polarized field in awave linearly has polarized its electric wave field oscillates oscillations along a line defined perpendicular along a to line propagation direction. perpendicular to the direction of propagation,z. The field vector E and z define a plane of b)polarization E field oscillations. (b) The are Ein-field the plane os cillations of polarization. are contained c) E-field represented in the plane by ofEx andpolarization. Ey. (c) A linearly polarized light at any instant can be represented by the superposition of two fieldsEx and Ey with the right magnitude and phase. A©n 1999 EM waveS.O. Kasap, (light) Optoelectronics can be polarized (Prentice by passing Hall) through a polarizer such as Polaroid sheet. This polarizer will only pass E-fields oscillating along a preferred direction, called transmission axis. Malus’s law: Suppose that a linearly polarized light from a polarizer is incident on a second identical polarizer as shown in figure 3. By rotating the transmission axis of the second polarizer the polarization state of the incident light can be analyzed. If the transmission axis of the second polarizer is at angle θ to the E-field of the first polarizer then E cosθ of the field will be allowed to pass through the analyzer as in figure 3. As Irradiance or intensity of the light passing through the analyzer is proportional to square of electric field (E cosθ)2 , the irradiance, I, at any angle is then given by Malus’s Law: 퐼(휃) = 퐼(0) 푐표푠2(휃) Malus’s law relates the intensity of a linearly polarized light passing through a second polarizer to the angle between the transmission axis and E-field vector. Figure 3. Randomly polarized light is incident on polarizer 1 with transmission axis TA1. Light is linearly polarized along TA1 and becomes incident on polarizer 2 (called analyzer) with transmission axis TA2 at an angle θ to TA1. Detector at the end measures the intensity of the light. Experimental procedure. 1. Launch the laser into a power meter detector, bolted on the breadboard at a distance about one meter from the laser. Record the maximum power of the laser. 2. Mount the polarizer (Polaroid sheet) in a rotation stage and bolt on the breadboard very close to the laser. Rotate the sheet to get the maximum power. Record this power. Explain the differences in power values? 3. Mount the second polarizer (called analyzer) in between the first one and detector. 4. Rotate the analyzer to get the maximum power. Record the angle on the rotational stage, call it θ = 0. Explain the direction of the transmission axis of the second polarizer with respect to the first one at this point. 5. Rotate the stage of the analyzer in steps of 10° until θ = 90o. Record the powers at each step. 6. Rotate back the stage to θ = 0, check the power. Now rotate the stage in step of -10° until θ = -90o. 7. Estimate the photosensitive device area. Then calculate the light intensity from the data recorded in steps of 5 and 6. 8. Sketch the calculated light intensities with respect to θ. Explain the results. 9. Sketch calculated light intensities with respect to cos2θ. Explain the results. Retarding Plates In crystalline materials the electronic polarization depends on the crystal direction. As a result the refractive index of a crystal, and the velocity of the light in a crystal depends on the direction of light propagation and the on the state of its polarization. Noncrystalline materials like glasses and cubic crystals have the same refractive index in all directions. These materials are called optically isotropic. The refractive index of most crystals depends on the direction of light propagation and the on the state of its polarization. These materials are called optically anisotropic. These materials are also called birefringent (double refraction) because an incident light beam is doubly refracted. There is a special direction in a birefringent crystal which all waves with different polarization experience the same refractive index (travel with the same phase velocity). This axis is called Optic Axis. Optic axis of a crystal is the direction in which a travelling light experience no birefringence (double refraction) and all the waves have the same phase velocity whatever their polarization is (figure 4). Figure 4. a) A birefringent crystal plate with the optic axis parallel to the plate surface. b) Optic axis perpendicular to plate surface. There is a phase difference between E⊥ and Eǁ in the first case. The phase difference of 180° is a half-wavelength retardation which means that E ǁ is delayed by 180° with respect to E ⊥ and is called a half-wave plate retarder. A quarter wave-plate retarder provides quarter wavelength retardation. (Figure 5) Figure 5. Input and output light polarization through half and quarter wave plate retarders. We can build an intensity modulator by inserting a polarizer and an analyzer before and after a retarding plate. (Figure 6) Retarding plate Analyzer Polarizer Figure 6. Intensity Modulator contains 1) Polarizer 2) Retarding plate 3) Analyzer all mounted on rotating stages. Experimental procedure: 1. Mount the polarizer (Polaroid sheet) in a rotation stage and bolt on the breadboard very close to the laser. Turn on the laser, then rotate the polarizer to get the maximum power. Record this power. 2. Mount the second polarizer (called analyzer) in between the first one and detector. Rotate the analyzer until you get the maximum power, record the angle of the analyzer. 3. Remove the analyzer carefully without any rotation. 4. Mount a retarding plate in another rotating stage and bolt it on the breadboard between the polarizer and detector. 3. Rotate the plate to find the optic axis (you should get the maximum power). Record the power and the angle. 4. Mount the analyzer again (at the same position of step 2) in between the retarding plate and detector. What is the power? Explain. 5. Rotate the analyzer to get the maximum power again. Compare the results with previous experiment. Explain what you have observed. .
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