A high resolution anharmonic λ/2 fringe spacing interferometer

Here we present an interferometer where the fringe spacing is reduced by a factor of two and where the width of a fringe can be extremely narrow, leading to potential applications in very precise displacement measurement. Our interferometer is based on elementary aspects of atomic physics. The laser wavelength is tuned to an atomic transition (i.e. two-fold degenerate two level system). The laser field is initially polarized at 45Aring and sent to a polarizing beam splitter. Vertical and horizontal polarizations experience a relative phase shift and are recombined at the output. We present an experimental demonstration of such an interferometer by exciting a sodium vapour cell on the D₁ line. The problem posed by the hyperfine structure is solved by exciting the whole Doppler-hyperfine transition by means of a dye frequency-shifted feedback laser delivering a 3 GHz broad and continuous spectrum. Both the lambda/2 periodicity and the anharmonic behaviour have been recorded on the fluorescence signal.