On automatic parallelism control in a scanning Fabry-Perot interferometer

A method of keeping the optical plates of a scanning Fabry-Perot interferometer parallel when setting on fringe maxima is described. The method needs no additional instrumentation other than the duplication of some of the electronic detection equipment needed for normal use of the instrument. Small oscillatory rotational movements about two axes at right angles are applied to one of the interferometer plates, while the other is moving axially in an oscillatory fashion. The transmitted light flux from the interferometer is detected with the single photodetector, and the electrical signal contains components at different frequencies that can be used to measure and control precisely the plate separation and plate parallelism of the interferometer. Experiment and analysis show that for path differences up to 200 mm using the mercury 198 green line as the light source, the parallelism of the plates can be controlled to better than 10(-8) rad, if parameters in the interferometer system are chosen suitably. For path differences up to 100 mm, the accuracy approached 10(-9) rad. With laser light, the method should be applicable for long path differences. Vibration and medium stability would be the limiting factors.