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/sup -3/. From these measurements, this "sigmameter" can determine laser wavelength from 0.36 /spl mu/m to 1.5 /spl mu/m with an accuracy of 1.10/sup -8/ using a reference stabilized He-Ne laser.