A new optical wavelength ratio measurement apparatus: The fringe counting sigmameter

A new, compact and achromatic Michelson-type interferometer with a variable path difference is presented. This ''fringe-counting'' sigmameter allows measurement of optical wavelength ratios between a laser of unknown wavelength and a reference laser of known wavelength, This apparatus, maintained in a vacuum, measures interference order variations in two stages: integer counting of around 400000 and fractional counting (also called ''excess fraction'') with an uncertainty of 10(-3). From these measurements, this ''sigmameter'' can determine laser wavelength from 0.36 mu m to 1.5 mu m with an accuracy of 1.10(-8) using a reference stabilized He-Ne laser.