Under atmospheric conditions the value of the refractive index of air has to be considered. ĭimensional measurement of real objects has to be mostly done on air, not only for practical reasons, but also due to the influence of atmospheric pressure on their size. The reproducibility of their absolute frequencies is another goal in metrology and is limited to 2.1 × 10 −11 and 9 × 10 −12, respectively and the absolute frequency value is primarily limited by the absorbing medium. Traditional He-Ne lasers stabilized to the active Doppler-broadened line in Ne can operate with relative frequency stabilities at the 10 −8–10 −9 level, He-Ne lasers stabilized through subdoppler spectroscopy in iodine on the 10 −11–10 −12 level and the potential of iodine-stabilized lasers based on frequency doubled Nd:YAG is very close to the 10 −14 level. The stability of the optical frequency of laser sources which has been achieved recently is very high. Under vacuum conditions the conversion to stable wavelength does not mean any loss in uncertainty. Realization of the length standard is thus a highly stable laser source, stable in optical frequency. This principle is consistent with the definition of length where the physical constant-speed of light-can be seen as a conversion from optical frequency into wavelength.
It means counting (and interpolation) of single wavelengths of a coherent light source representing elementary quanta of length.
The principle is demonstrated in an experimental setup.ĭimensional metrology on the fundamental metrology level is a domain of various interferometric techniques. The optical setup consists of three interferometers sharing the same beam path where two measure differentially the displacement while the third evaluates the changes in the measuring range, acting as a tracking refractometer. The technique allows one to track the variations of the refractive index of air on-line directly in the line of the measuring beam and to compensate for the fluctuations. Our proposal is based on the concept of an over-determined interferometric setup where a reference length is derived from a mechanical frame made from a material with a very low thermal coefficient. The key limiting factor in any interferometric dimensional measurement are fluctuations of the refractive index of air representing a dominating source of uncertainty when evaluated indirectly from the physical parameters of the atmosphere. We present an interferometric technique based on a differential interferometry setup for measurement under atmospheric conditions.
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