Dual Measurements of Temporal and Spatial Coherence of Light using a Single Experimental Setup*

Temporal coherence is commonly measured with Michelson and Mach-Zehnder interferometers, whereas for spatial coherence measurement, Young’s double slit interferometer is typically used. However, it is not easy to place double slits with variable slit widths and slit separations within a cryostat in close proximity of the sample for spatial coherence measurements at cryogenic temperatures. Here we report an experimental technique to simultaneously measure both temporal and spatial coherences of a light source by altering a Michelson interferometer. Instead of using simple plane mirrors, two retroreflectors and their longitudinal and lateral movements are utilized to incorporate spatial coherence measurement using this modified Michelson interferometer. This modified interferometer can be kept at room temperature outside a cryostat to measure spatio-temporal coherence of a light source placed at cryogenic temperatures. This avoids the added complexities of modulation of interference fringe patterns due to single slit diffraction as well. The process of mixing of spatial-temporal parts of coherences is addressed using a method of ‘temporal filtering’. Similarly, ‘curve overlap’ method is designed to extend the range of the experimental setup without compromising the precision.