Speckle-Based Optical Sensor for Low Field Faraday Rotation Measurement

Faraday rotation refers to rotation of linearly polarized light by certain materials when exposed to magnetic fields. In this paper, we present a simple and compact method for accurate measurement of low Faraday rotations. The technique measures rotations by determining the changes occurring to a speckle pattern generated by a laser beam passing through the medium. The speckle pattern is generated by a ground glass diffuser placed after the rotation introducing medium. As the polarization state of the beam passing through the medium is changed, the speckle pattern also varies. Change in the speckle pattern is computed by comparing it to a reference speckle pattern. This change is then related to the rotation angle. From the value of the correlation coefficient, we are able to measure rotation changes as low as 0.0066 degrees with less than 8% error. Optical experimental demonstration using a cell phone camera to record speckle pattern is also presented.