Wavelength references for interferometry in air

Cavity-mode wavelengths in air are determined by measuring a laserʹs frequency while it is locked to the mode in vacuum during a calibration step and subsequently correcting the mode wavelength for atmospheric pressure compression, temperature difference, and material aging. Using a Zerodur ring cavity, we demonstrate a repeatability of ±2*10^-8 (3(sigma)), with the wavelength accuracy limited to ±4*10^-8 by knowledge of the absolute helium gas temperature during the pressure calibration. Mirror cleaning perturbed the mode frequency by less than (DELTA) v/v~3*10^-9, limited by temperature correction residuals.