Electro-Optic Modulator Tutorial

This tutorial examines the working principles of an Electro-optic modulator for use as a demonstrative tool for graduate and undergraduate students. The electro-optic material used is a single-crystal film of DAST approximately 5 microns thick. This film is prepared by a special method (modified shear method, US patent issued). An electric field is applied across the sample such that the direction of the field is along the dipole axis of the material. Electro-optic modulation is observed for a light beam passing through the film when an electric field is applied. The reason for using the DAST film is that its electro-optic coefficient is exceptionally large (770 pm/V at 633 nm) that produces significant electro-optic modulation even for a single-pass through a thin film and for a low electric field.

Experimental setup:
The modulator comprises of a focusing lens, a collecting lens, a single-crystal film of an organic electro-optic material on a micro-positioner and electrical leads attached to the electrodes applied on the electro-optic film. The overall setup for an experimental demonstration involves a laser, a polarizer, an analyzer, an ac power supply, a photodiode, an oscilloscope and the electro-optic modulator. The incident laser beam passes through the polarizer which provides a 45 degree angle of polarization with respect to the vertical axis (dipole axis). The beam is then focused on the DAST sample within the electro-optic modulator. The beam passes through the analyzer and is collected into the photodiode. The modulation signal produced by the electro-optic effect caused by the applied electric field is then recorded using an oscilloscope.

Download