Signal averaging with a stabilized dual beam Mach Zehnder interferometer: Monitoring changes in gas density

Abstract

A traditional Mach Zehnder interferometer has been fitted with a feedback loop that maintains a constant and known phase difference between the two optical beams. This stabilization is necessary so that signal averaging will not distort the signal. To increase sensitivity for small phase shifts and determine absolute phase shifts when large phase shifts are involved a dual beam technique, which uses circularly polarized monochromatic light, is implemented. In one arm of the interferometer a phase shift of 90° between the two polarization orientations is introduced via a quarter wave plate. Two photodiodes independently detect the polarized light beams that maintain a 90° phase difference. One of these signals provides the input for the stabilization while the other (with enhanced sensitivity) is used to monitor the density. Unambiguous phase shifts are calculated from the information contained in both signals. The interferometer is being used to monitor gas density changes resulting from vibrational to translational energy transfer.