Lecture demonstrations on the interference of partially coherent light

A system based on the Mach–Zender interferometer is described. It can be used for lecture demonstrations of interference between light waves with adjustable statistical parameters (degree and time of coherence). The light source is a continuously operating gas laser. Partially coherent light can be produced by modulating with one of the mirrors, using a piezoelectric barium titanate cylinder to establish the required amplitude and frequency. Another method used to vary the degree of coherence continuously from 1 to 0 relies on the introduction of a second, similar laser beam. More accurate quantitative data can be obtained by using a scanning mirror in conjunction with a photomultiplier. This transforms the intensity distribution into a “transverse” pattern on the screen of an oscilloscope. Polarization effects can also be demonstrated.