Precision Displacement Measurement Using a Shearing Interferometer*

The emission spectra from semiconductor quantum dots consists of closely spaced spectral peaks, which cannot be resolved by a conventional grating spectrometer. An alternative solution is to use a scanning Fabry-Perot interferometer (FPI), which functions as a narrow bandwidth tunable filter, to enhance the resolution. This technique demands very precise control of the distance between the two FPI mirrors with a resolution of 1 nm. Direct optical feedback would be ideal, but would require an expensive frequency-tunable laser. We demonstrate an economic way to achieve the required precision by mechanically connecting the FPI cavity to a shearing interferometer where fluctuations in the mirror separation can be measured using the interference pattern formed by a relatively inexpensive single-frequency laser. We are able to stabilize the RMS displacement fluctuation between the two mirrors in the shearing interferometer to 0.11 nm. This precision was sufficient to keep the FPI stable for ~40 minutes.