A common path interferometer has exceptional stability. The problem is how to introduce significant variable path difference between counter-propagating beams. Two concepts will be presented. A proposed application is to observational astronomy.
The Sagnac common path interferometer is simply constructed and has exceptional opto-mechanical stability. It can therefore be used for light-efficient interferometric spectroscopy in all wavelength regions. The problem is how to introduce a significant variable path difference between the counter-propagating beams, which limits the spectral resolution attainable. We shall discuss two concepts and present some experimental results. One concept is used for hyper-spectral imaging. A second example is shown in the figure below where the interferogram is scanned by rotating a pair of glass plates. A proposed application is to observational astronomy where significant information can be obtained directly from the interferogram.