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Clearing the fog for best in the world air-wavelength



Patrick F. Egan, Jack A. Stone Jr.


Laser interferometry, the basis for modern length metrology, achieves very high accuracies as a consequence of the stable, well-known frequencies of laser sources. However, length measurements in air also require corrections based on precise knowledge of the air refractive index, and this sets the fundamental limitation for practical measurements, preventing us from fully utilizing the inherent high accuracy of the laser. Some years ago we developed a refractometer which used a FP cavity to measure the absolute refractive index of a gas. The device was accurate to 3e-9 (k=2) for dry gas, but showed humidity-related errors of 49e-9 when measuring moist (lab) air. When two cavities were compared side-by-side, one 143 mm in length and the second 329 mm, the error scaled inversely with length, which can be interpreted as a 7 nm shortening in the length of both cavities. We believe the change in length was due to water penetrating into the mirror coatings, which caused their apparent positions to change (even though the coatings were ion-beam sputter), and we present evidence which supports this theory. We hope to be able to quantify humidity errors of the FP cavity and provide a correction mechanism so that the FP cavity can achieve good results for moist gas, more comparable to what is achieved for dry gas. To correct this humidity phenomenon--to clear the fog--we have been developing a wavelength-tracker based on a gas-cell integrated into a heterodyne interferometer. This gas-cell is placed side-by-side with the FP cavity to monitor how each device tracks wavelength as humidity is changed. We aim to correct the humidity error in the FP cavity to better than 3e-9: so with a 25 cm gas-cell, the heterodyne interferometer must be stable to 0.5 nm over the 10 h duration of the measurement (or fractional length stability 1e-11 Hz^{-1/2}). A successful correction at this level would allow the FP cavity to realize air-wavelength a x10 more accurate than state-of-the-art.
Proceedings Title
ASPE 2015 Summer Topical Meeting
Conference Dates
July 8-10, 2015
Conference Location
Golden, CO
Conference Title
Precision Interferometric Metrology


Egan, P. and Stone, J. (2015), Clearing the fog for best in the world air-wavelength, ASPE 2015 Summer Topical Meeting, Golden, CO, [online], (Accessed March 3, 2024)
Created July 1, 2015, Updated February 19, 2017