| | Category | PH | P25 | Light Modulation for Optical Communications |
| | Abstract | This project is a study of the interaction between visible light and |
| | ultrasonic waves in a dense flint glass via the acousto-optic effect. The |
| | practical goal of the project was to assemble a system that uses this |
| | interaction to achieve amplitude modulation of a red Helium-Neon laser |
| | beam and transmit a song from a CD player to a speaker through free |
| | space. This, in turn, has required a detailed study and characterization of |
| | the acousto-optic modulator employed. |
| | A parallel-plate dense flint glass acousto-optic modulator driven by an RF |
| | signal with a central frequency of 80 MHz was employed. The first order |
| | of diffraction produced by the sound-induced grating could be observed |
| | experimentally at an angle of 0.73 degrees. This is within 11% of the |
| | theoretically calculated value of 0.80 degrees, based on a known value of |
| | the speed of ultrasound in dense flint glass of 3630 m/s. Furthermore, a |
| | linear dependence was found between the diffraction angle (in radians) |
| | and the frequency of the RF signal, as predicted by diffraction theory. An |
| | experimental value of 2.0E-10 radians/Hz is found for the slope, in |
| | reasonable agreement with the theoretically calculated value of 1.74E-10 |
| | radians/Hz. |
| | The intensity of the first order diffracted beam varied linearly with the |
| | amplitude of the driving RF signal, allowing undistorted modulation of the |
| | laser beam. The modulated beam was then detected by a photodiode set |
| | to receive the diffracted beam, and the recovered modulated signal was |
| | amplified and played back in a set of speakers, demonstrating the |
| | transmission of sound over a laser beam through free space. |
| | Bibliography | 1. Molecular Expressions - Physics of light and color |
| | (http://micro.magnet.fsu.edu/primer/lightandcolor/index.html) |
| | 2. Hyper Physics - light and vision |
| | (http://hyperphysics.phy/astr.gsu.edu/hbase/hph.html#ahph) |
| | 3. General Physics Java Applets by Surendranath Reddy |
| | (http://surendranath.tripod.com/) |
| | 4. Educypedia - The Educational Encyclopedia |
| | (http://www.educypedia.be/education/physicsjavasound.html) |
| | 5. Wikipedia - The Free Encyclopedia |
| | (http://en.wikipedia.org/wiki/Main_Page) |
| | 6. E. Hecht, Optic, 4th Ed., Addison Wesley 2002, ISBN 0-8053-8566-5 |
| | 7. B.E.A. Sale and M.C. Teich, Fundamentals of Photonics, John Wiley and |
| | Sons 1991, ISBN 0-471-83965-5 |