The absorption spectra of H2O in Ar gas with a mole fraction of 500 nmol/mol were obtained near 7181 cm-1. A reference gas (water vapor in Ar gas) was generated using a multi-gas trace-moisture generator (Multi-TMG), previously developed as a generation system of primary trace-moisture standards for multiple gas species. The values of the line-shape parameters and line intensity were determined with traceability to the International System of Units (SI) and were compared with previously reported values. The ratios of absorption coefficients of H2O in Ar and N2 gases near 7181 cm-1 containing the same amount of water vapor were also evaluated. These results provide important information for trace-moisture measurements in which the moisture concentration is determined only from the absorption coefficient at a specific frequency. Using the evaluated ratios, the measured values obtained in Ar gas can be converted to the correct moisture concentrations, enabling reliable trace-moisture measurements in Ar gas using a moisture analyzer calibrated only in N2 gas.

We recently reported the quantity values of line intensity for H2O near 1.393 mu m in a manner traceable to the International System of Units (SI). This paper briefly describes how to determine the reliable values of the line intensity, mainly focusing on the SI-traceability.

The high-precision measurement of trace water vapor (trace moisture) in gases is important in technologyintensive industries. We have been developing measuring instruments for trace moisture using cavity ring-down spectroscopy (CRDS). In CRDS, an optical cavity consisting of highly reflective mirrors is used as a sample cell to extend the effective optical path length. We measured the ring-down time in dry N-2 gas and acquired the absorption spectra of H2O near 7180 cm(-1). When we acquired the absorption spectra in the dry N-2 gas, periodic structure which was not related to the water absorption line was observed. This structure was caused by fringe noise, which occurred when the reflected light from an optical component surface incident on the cavity. In our measuring system, to stabilize the resonant frequencies of the cavity, the length of the cavity was adjusted using a piezoelectric actuator (PZT) mounted at one end of the cavity. To accurately measure trace moisture, the sample cell was sealed using windows to prevent moisture from entering the cell from the outside, and the PZT was placed outside the sample cell to avoid the effects of water desorption from the element. To adjust the position of the mirror, the mirror and a window were connected and their position was moved using the PZT. In this structure, because the distance between the mirror and the window was small and they were fixed parallel to each other, it was difficult to prevent the reflected light from returning into the cavity, which tended to cause fringe noise. In this study, to eliminate the fringe noise, the mirror and the window were separated so that they could be adjusted independently. In addition, the window was wedged so that the light reflections on both sides were not collinear. The mirror and the PZT were directly connected and placed inside the cell. The gas flow was designed so that the gas was discharged without stagnation to prevent desorbed moisture from entering the measurement space. This improvement has made it possible to measure the trace moisture more precisely.

The absorption spectra of trace water vapor in N2 gas with a mole fraction of 500 nmol/mol in the pressure range of 20 kPa to 140 kPa were obtained near 7181 cm−1 using wavelength-meter-controlled cavity ring-down spectroscopy (WMC-CRDS). The reference gas (water vapor in N2 gas) was produced using a generation system of primary measurement standards for trace moisture in gases, where the amount of water vapor was measured using a gravimetric method. The obtained spectra of H2O were analyzed using a multispectrum fitting technique with a speed-dependent asymmetric Voigt profile (SDAVP). The values of the line intensity were determined with traceability to the International System of Units (SI). We could achieve the expanded uncertainty below 2 % for the line intensities of the strong lines.

We improved the spectral resolution of a miniaturized trace-moisture sensor using cavity ring-down spectroscopy (mini-CRDS sensor). High resolution was achieved using cavity-length modulation by varying the temperature of the cavity using Peltier devices without compromising the sensor's miniaturized design. This technique was sufficiently fast and could be used for real-time measurement of trace moisture in helium (which has a very narrow spectral line width) even at the 10 nmol/mol (10 ppb) level in mole fraction. The measurement accuracy was evaluated by comparison with a primary trace-moisture standard in helium that was metrologically traceable to the International System of Units (SI). Although the relative measurement error at the 500 nmol/mol (500 ppb) level in this study was originally 12 %, it was reduced to 1.2 % using the Peltier devices. The systematic effect because of the use of a Lorentzian profile (LP) was responsible for the remaining 1.2 % error, and this could be further reduced to a negligible level using a Voigt profile (VP). Analysis with the Akaike Information Criterion indicated that the better result could be attributed to use of the VP over the LP rather than overfitting. The LP with a correction factor was also effective for removing the systematic error. The relative expanded uncertainty using the LP with a correction factor in the range of 10 nmol/mol (10 ppb) to 1.5 mu mol/mol (1.5 ppm) was 10-1.0 %. The measurement results with our mini-CRDS are in good agreement with the SI-traceable standard in this range.

The absorption spectra of H 2 O in N 2 gas were measured at atmospheric pressure and room temperature near 7180 cm –1 in the moisture range of 9.7–148.7 nmol mol −1 in mole fraction using cavity ring-down spectroscopy. The line shape profiles used for the spectral analysis for the trace moisture measurement were evaluated based on the Akaike Information Criterion as well as the fitting residuals. The optimal line shape profile in this study was the speed-dependent asymmetric Voigt profile (SDAVP). From the comparison of the fitting results calculated using the Lorentzian profile (LP) with those calculated using the SDAVP, the relative difference in the integrated line area between the two line shape profiles was found to be systematic and constant. The measurement error due to the use of the LP could be corrected using a correction factor of 1.00479 ± 0.00077.

We report three-stage laser cooling of Sr atoms using the $5{s}^{2}\phantom{\rule{0.16em}{0ex}}^{1}S_{0}$ ground state and the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$ metastable state. $^{87}\mathrm{Sr}$ atoms are precooled in a magneto-optical trap simultaneously operated on the $5{s}^{2}\phantom{\rule{0.16em}{0ex}}^{1}S_{0}$--$5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}$ transition at 461 nm and on the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$--$5s5d\phantom{\rule{0.16em}{0ex}}^{3}D_{3}$ transition at 496 nm. Atoms optically pumped to the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}(F=13/2)$ state are magneto-optically trapped and cooled down to 2.5(2) $\ensuremath{\mu}\mathrm{K}$ on the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$--$5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{3}$ transition at $2.9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Using this transition, we investigate one dimensional polarization gradient cooling of $^{88}\mathrm{Sr}$ atoms, which have simple energy levels without hyperfine structure, down to 0.22(18) $\ensuremath{\mu}\mathrm{K}$ or 17 times the recoil temperature. We discuss prospects for continuous generation of ultracold Sr atoms combined with a state transfer technique.

We developed a miniaturized sensor based on cavity ring-down spectroscopy (the mini-CRDS sensor) for the real-time measurement of trace moisture. The dimensions and mass were 24 cm x 9 cm x 13 cm and 2.7 kg, respectively, exclusive of the laser controller and data processing unit. The distance between the two cavity mirrors was 5 cm, and the internal volume and surface area of the cavity between the mirrors were approximately 4 cm3 and 16 cm2, respectively. The Allan-Werle deviation of trace-moisture measurement was approximately 0.46 nmol/mol at an averaging time of 10 s. The minimum detectable absorption coefficient for an averaging time of 10 s was 6.8 x 10-10 cm-1. The limit of detection (3 times the standard deviation) of moisture in nitrogen was 3.1 nmol/mol (3.1 ppb) in the amount-of-substance fraction (mole fraction). The relative standard uncertainty was tentatively evaluated as 10 % to 0.67 % for the range of 12 nmol/mol (12 ppb) to 1.3 mu mol/mol (1.3 ppm). The measurement accuracy of the mini-CRDS in the above range was evaluated by comparison with a primary trace-moisture standard that is metrologically traceable to the International System of Units (SI), and the results demonstrated good agreement between the mini-CRDS and the primary trace-moisture standard. (c) 2021 Elsevier B.V. All rights reserved.

We introduced frequency-control and temperature-control systems in wavelength-meter-controlled cavity ring-down spectroscopy. The frequency-control system shifted the wavelength of the probe laser from 1393 nm to 696.5 nm where no strong absorption lines of water exist, and therefore, it could avoid measurement errors in laser frequency due to residual moisture in the built-in Fizeau interferometer of a wavelength meter. We verified the hypothesis that the nonuniform reflectivity of the mirror surface contributes to fluctuations in the ring-down time observed with multi-transverse-mode CRDS signals. The fluctuations due to this effect were greatly suppressed by the temperature-control system. Using this system, we could improve the minimum detectable absorption coefficient by three times on average and also improve the experimental standard deviations of the averages by nine times compared with those without the system. We measured near-infrared spectra of residual moisture in dry nitrogen at an approximately 1 nmol/mol (1 ppb) level and performed least-squares fitting of the averaged spectrum. The standard deviation of the residuals of the fitting was 6.6 × 10−12 cm−1, corresponding to a mole fraction of water of 6.3 pmol/mol (6.3 ppt).

We report a frequency measurement on the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}\ensuremath{-}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{3}$ transition of $^{88}\mathrm{Sr}$ atoms at $2.9\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$. The excitation spectra are obtained by measuring the photon momenta transferred to ultracold atoms. The transition frequency is determined to be 102 550 196 205(9) kHz using an erbium fiber-based optical frequency comb. We numerically investigate possible frequency corrections by taking an acceleration of atoms and attenuation of the excitation light into account. This measurement technique is particularly useful for transitions where laser-induced fluorescence is difficult to observe but the spatial imaging of atoms is possible.

We report a frequency measurement on the 5s5p P-3(2) - 5s4d D-3(3) transition of Sr-88 atoms at 2.9 mu m. The excitation spectra are obtained by measuring the photon momenta transferred to ultracold atoms. The transition frequency is determined to be 102 550 196 205(9) kHz using an erbium fiber-based optical frequency comb. We numerically investigate possible frequency corrections by taking an acceleration of atoms and attenuation of the excitation light into account. This measurement technique is particularly useful for transitions where laser-induced fluorescence is difficult to observe but the spatial imaging of atoms is possible.