Abstract

High resolution interferometry has been used to determine the wavelength ratio between two molecularly stabilized He–Ne lasers, one locked to a methane absorption at 3.39 μm and the other locked to the k peak of ¹²⁹I₂ at 633 nm. An optical beat frequency technique gave fractional orders while a microwave sideband method yielded the integer parts. Conventional (third derivative) peak seeking servoes stabilized both laser and cavity lengths. Reproducibility of the electronic control system and optics was a few parts in 10¹², while systematic errors associated with curvature of the cavity mirrors limited the accuracy of the wavelength ratio measurement to 2 parts in 10¹⁰. The measured wavelength ratio of the methane stabilized He–Ne laser at 3.39 μm [P(7) line, ν₃ band] to the ¹²⁹I₂ (k peak) stabilized He–Ne laser at 633 nm was 5.359 049 260 6 (0.000 2 ppm). This ratio agrees with that calculated from the (lower accuracy) results of earlier wavelength measurements made relative to the ⁸⁶Kr standard. Its higher accuracy thus permits a provisional extension of the frequency scale based on the cesium oscillator into the visible spectrum.

© 1976 Optical Society of America

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