We have built and characterized a refractometer that utilizes two Fabry-Perot cavities formed on a dimensionally stable spacer. In the typical mode of operation, one cavity is held at vacuum, and the other cavity is filled with nitrogen gas. The differential change in length between the cavities is measured as the difference in frequency between two helium-neon lasers, one locked to the resonance of each cavity. This differential change in optical length is a measure of the gas refractivity. Using the known values for the molar refractivity and virial coefficients of nitrogen, and accounting for cavity length distortions, the device can be used as a high-resolution, multi-decade pressure sensor. We define a reference value for nitrogen refractivity as n-1=(26485.28±0.3)×10(-8) at p=100.0000  kPa, T=302.9190  K, and λ(vac)=632.9908  nm. We compare pressure determinations via the refractometer and the reference value to a mercury manometer.

Resistance thermometry provides a time-tested method for taking temperature measurements. However, fundamental limits to resistance-based approaches has produced considerable interest in developing photonic temperature sensors to leverage advances in frequency metrology and to achieve greater mechanical and environmental stability. Here we show that silicon-based optical ring resonator devices can resolve temperature differences of 1 mK using the traditional wavelength scanning methodology.An even lower noise floor of 80 μK for measuring temperature difference is achieved in the side-of-fringe, constant power mode measurement.Temperature measurements play a central role in modern life ranging from process control in manufacturing 1 , physiological monitoring 2 and tissue ablation 3 in 2 medicine, and environmental control and monitoring in buildings 4 and automobiles 5 .Despite the ubiquity of thermometers, the underlying technology has been slow to advance over the last century. 6 The standard bearer for accurate temperature measurement, the standard platinum resistance thermometer (SPRT) was initially developed over a century ago. 6,7 Furthermore, many modern temperature sensors still rely on resistance measurements of a thin metal film or wire whose resistance varies with temperature. 6 Though resistance thermometers can routinely measure temperature with uncertainties of 10 mK, they are sensitive to mechanical shock which causes the resistance to drift over time requiring frequent off-line, expensive, and time consuming calibrations. 7In recent years there has been considerable interest in developing photonic devices as an alternative to resistance thermometers 8-10 as they have the potential to provide greater temperature sensitivity while being robust against mechanical shock and electromagnetic interference. Furthermore, the low weight, small form factor photonic devices might be multiplexed to provide a low-cost sensing solutions.Photonic temperature sensors exploit temperature dependent changes in a material's properties -typically, a combination of thermo-optic effect and thermal expansion. The temperature dependence of the ring resonator arises from temperatureinduced changes in refractive index (n) and in the physical dimensions of the ring. A qualitative analysis of a ring resonator yields a resonance wavelength for a single ring resonator of:where m is the vacuum wavelength, n eff is the effective refractive index, m is the mode number, L is the ring perimeter, and T is the temperature. Thus, the temperature-induced shift in wavelength is given by:Where the group index is n g = [ ( ) . The variation in the refractive index due to the thermal expansion coefficient for silicon (3.57 x 10 -6 /K) is a factor of 100 smaller 4 than that of the estimated thermo-optic effect (2 x 10 -4 /K) of the silicon waveguide and thus not included in our analysis of the performance of ring resonator devices. wafer with a 220 nm thick layer of silicon on top of a 2 μm thick buried oxide layer that isolates the ...

Since the beginning of measurement of pressure in the 17th century, the unit of pressure has been defined by the relationship of force per unit area. The present state of optical technology now offers the possibility of using a thermodynamic definition-specifically the ideal gas law-for the realization of the pressure unit, in the vacuum regime and slightly above, with an accuracy comparable to or better than the traditional methods of force per area. The changes planned for the SI in 2018 support the application of this thermodynamic definition that is based on the ideal gas law with the necessary corrections for real-gas effects. The paper reviews the theoretical and experimental foundations of those optical methods that are considered to be most promising to realize the unit of pressure at the highest level of metrology.

The properties of the sub-range inconsistency (SRI or Type-I non-uniqueness) of the standard platinum resistance thermometer (SPRT) sub-ranges of the International Temperature Scale of 1990 (ITS-90) are investigated. Mathematically, SRI has the form of an interpolation error with zeros at each of the fixed points shared by the two interpolating equations, and a magnitude dependent on the differences in the ratios (W r − 1)/(W − 1) for each of the fixed points, where W is the resistance ratio R(T )/R(0.01 • C) and W r is the reference resistance ratio defined by ITS-90. The calibration results for a set of 60 SPRTs were analysed to determine the SRI for the water-zinc and water-aluminium sub-ranges. The maximum SRI occurs near 93.15 • C, and has an average value of 0.12 mK and a standard deviation of 0.48 mK. The reciprocal of the ratio (W r − 1)/(W − 1), which is proportional to the sensitivity of the SPRT, was found to be 1.0 ± 0.0004 for all 60 SPRTs, and was within 0.000 05 for all fixed points for any one SPRT. This suggests that a single constraint on the value of the ratio might be a more useful and discerning SPRT-quality constraint than the current three ITS-90 constraints.

An innovative sapphire whispering gallery thermometer (SWGT) is being explored at the National Institute of Standards and Technology (NIST) as a potential replacement for a standard platinum resistance thermometer (SPRT) for industrial applications that require measurement uncertainties of ≤10 mK. The NIST SWGT uses a synthetic sapphire monocrystalline disk configured as a uniaxial, dielectric resonator with whispering gallery modes between 14 GHz and 20 GHz and with Q-factors as large as 90,000. The prototype SWGT stability at the ice melting point (0 • C) is ≤1 mK with a frequency resolution equivalent to 0.05 mK. The prototype SWGT measurement uncertainty (k= 1) is 10mK from 0 • C to 100 • C for all five resonance modes studied. These results for the SWGT approach the capabilities of industrial resistance thermometers. The SWGT promises greatly increased resistance to mechanical shock relative to SPRTs, over the range from −196 • C to 500 • C while retaining the low uncertainties needed by secondary calibration laboratories. The temperature sensitivity of the SWGT depends upon a well-defined property (the refractive index at microwave frequencies) and the thermal expansion of a pure material. Therefore, it is expected that SWGTs can be calibrated over a wide temperature range using a reference function, along with deviations measured at a few fixed points. This article reports the prototype SWGT stability, resolution, repeatability, and the temperature dependence of five whispering gallery resonance frequencies in the range from 0 • C to 100