Search

We use optical-optical double-resonance spectroscopy with a continuous wave (CW) pump and a cavity-enhanced frequency comb probe to measure the energy levels of methane in the upper part of the triacontad polyad (P6) with higher rotational quantum numbers than previously assigned. A high-power CW optical parametric oscillator, tunable around 3000 cm-1, is consecutively locked to the P(7, A2), Q(7, A2), R(7, A2), and Q(6, F2) transitions in the ν3 band, and a comb covering the 5800-6100 cm-1 range probes sub-Doppler ladder-type transitions from the pumped levels with J' = 6 to 8, respectively. We report 118 probe transitions in the 3ν3 ← ν3 spectral range with uncertainties down to 300 kHz (1 × 10-5 cm-1), reaching 84 unique final states in the 9070-9370 cm-1 range with rotational quantum numbers J between 5 and 9. We assign these states using combination differences and by comparison with theoretical predictions from a new ab initio-based effective Hamiltonian and dipole moment operator. This is the first line-by-line experimental verification of theoretical predictions for these hot-band transitions, and we find a better agreement of transition wavenumbers with the new calculations compared to the TheoReTS/HITEMP and ExoMol databases. We also compare the relative intensities and find an overall good agreement with all three sets of predictions. Finally, we report the wavenumbers of 27 transitions in the 2ν3 spectral range, observed as V-type transitions from the ground state, and compare them to the new Hamiltonian, HITRAN2020, ExoMol, and the WKMLC line lists., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/).)

Based on a recent experimental determination of the static polarizability and a first-principle calculation of the frequency-dependent dipole polarizability of argon, this work presents, by using a Fabry-Perot refractometer operated at 1550 nm, a realization of the SI unit of pressure, the pascal, for pressures up to 100 kPa, with an uncertainty of [(1.0 mPa) 2  + (5.8 × 10 -6 P) 2  + (26 × 10 -12 P 2 ) 2 ] 1/2 . The work also presents a value of the molar polarizability of N 2 at 1550 nm and 302.9146 K of 4.396572(26) × 10 -6  m 3 /mol, which agrees well with previously determined ones.

A procedure for automated low uncertainty assessment of empty cavity mode frequencies in Fabry-Pérot cavity based refractometry that does not require access to laser frequency measuring instrumentation is presented. It requires a previously well-characterized system regarding mirror phase shifts, Gouy phase, and mode number, and is based on the fact that the assessed refractivity should not change when mode jumps take place. It is demonstrated that the procedure is capable of assessing mode frequencies with an uncertainty of 30 MHz, which, when assessing pressure of nitrogen, corresponds to an uncertainty of 0.3 mPa.

Accurate parameters of molecular hot-band transitions, i.e., those starting from vibrationally excited levels, are needed to accurately model high-temperature spectra in astrophysics and combustion, yet laboratory spectra measured at high temperatures are often unresolved and difficult to assign. Optical-optical double-resonance (OODR) spectroscopy allows the measurement and assignment of individual hot-band transitions from selectively pumped energy levels without the need to heat the sample. However, previous demonstrations lacked either sufficient resolution, spectral coverage, absorption sensitivity, or frequency accuracy. Here we demonstrate OODR spectroscopy using a cavity-enhanced frequency comb probe that combines all these advantages. We detect and assign sub-Doppler transitions in the spectral range of the 3ν 3  ← ν 3 resonance of methane with frequency precision and sensitivity more than an order of magnitude better than before. This technique will provide high-accuracy data about excited states of a wide range of molecules that is urgently needed for theoretical modeling of high-temperature data and cannot be obtained using other methods., (© 2024. The Author(s).)

Fabry-Pérot-based refractometry has demonstrated the ability to assess gas pressure with high accuracy and has been prophesized to be able to realize the SI unit for pressure, the pascal, based on quantum calculations of the molar polarizabilities of gases. So far, the technology has mostly been limited to well-controlled laboratories. However, recently, an easy-to-use transportable refractometer has been constructed. Although its performance has previously been assessed under well-controlled laboratory conditions, to assess its ability to serve as an actually transportable system, a ring-type comparison addressing various well-characterized pressure balances in the 10-90 kPa range at several European national metrology institutes is presented in this work. It was found that the transportable refractometer is capable of being transported and swiftly set up to be operational with retained performance in a variety of environments. The system could also verify that the pressure balances used within the ring-type comparison agree with each other. These results constitute an important step toward broadening the application areas of FP-based refractometry technology and bringing it within reach of various types of stakeholders, not least within industry.

A procedure is presented for in situ determination of the frequency penetration depth of coated mirrors in Fabry-Perot (FP) based refractometers and its influence on the assessment of refractivity and pressure. It is based on assessments of the absolute frequency of the laser and the free spectral range of the cavity. The procedure is demonstrated on an Invar-based FP cavity system with high-reflection mirrors working at 1.55 μm. The influence was assessed with such a low uncertainty that it does not significantly contribute to the uncertainties (k = 2) in the assessment of refractivity (<8 × 10 -13 ) or pressure of nitrogen (<0.3 mPa).

Refractometry is a powerful technique for pressure assessments that, due to the recent redefinition of the SI system, also offers a new route to realizing the SI unit of pressure, the Pascal. Gas modulation refractometry (GAMOR) is a methodology that has demonstrated an outstanding ability to mitigate the influences of drifts and fluctuations, leading to long-term precision in the 10-7 region. However, its short-term performance, which is of importance for a variety of applications, has not yet been scrutinized. To assess this, we investigated the short-term performance (in terms of precision) of two similar, but independent, dual Fabry-Perot cavity refractometers utilizing the GAMOR methodology. Both systems assessed the same pressure produced by a dead weight piston gauge. That way, their short-term responses were assessed without being compromised by any pressure fluctuations produced by the piston gauge or the gas delivery system. We found that the two refractometer systems have a significantly higher degree of concordance (in the 10-8 range at 1 s) than what either of them has with the piston gauge. This shows that the refractometry systems under scrutiny are capable of assessing rapidly varying pressures (with bandwidths up to 2 Hz) with precision in the 10-8 range.

We present a new design of a robust cavity-enhanced frequency comb-based spectrometer operating under the continuous-filtering Vernier principle. The spectrometer is based on a compact femtosecond Er-doped fiber laser, a medium finesse cavity, a diffraction grating, a custom-made moving aperture, and two photodetectors. The new design removes the requirement for high-bandwidth active stabilization present in the previous implementations of the technique, and allows scan rates up to 100 Hz. We demonstrate the spectrometer performance over a wide spectral range by detecting CO 2 around 1575 nm (1.7 THz bandwidth and 6 GHz resolution) and CH 4 around 1650 nm (2.7 THz bandwidth and 13 GHz resolution). We achieve absorption sensitivity of 5 × 10 -9 cm -1 Hz -1/2 at 1575 nm, and 1 × 10 -7 cm -1 Hz -1/2 cm -1 at 1650 nm. We discuss the influence of the scanning speed above the adiabatic limit on the amplitude of the absorption signal.

We report the first measurement of sub-Doppler molecular response using a frequency comb by employing the comb as a probe in optical-optical double-resonance spectroscopy. We use a 3.3  μm continuous wave pump and a 1.67  μm comb probe to detect sub-Doppler transitions to the 2ν&#95;{3} and 3ν&#95;{3} bands of methane with ∼1.7  MHz center frequency accuracy. These measurements provide the first verification of the accuracy of theoretical predictions from highly vibrationally excited states, needed to model the high-temperature spectra of exoplanets. Transition frequencies to the 3ν&#95;{3} band show good agreement with the TheoReTS line list.

Gas modulation refractometry (GAMOR) is a methodology that can mitigate fluctuations and drifts in refractometry. This can open up for the use of non-conventional cavity spacer materials. In this paper, we report a dual-cavity system based on Invar that shows better precision for assessment of pressure than a similar system based on Zerodur. This refractometer shows for empty cavity measurements, up to 10 4 s, a white noise response (for N 2 ) of 3 mPa s 1/2 . At 4303 Pa, the system has a minimum Allan deviation of 0.34 mPa (0.08 ppm) and a long-term stability (24 h) of 0.7 mPa. This shows that the GAMOR methodology allows for the use of alternative cavity materials.

The linewidth of a singly resonant optical parametric oscillator (OPO) has been narrowed with respect to an external cavity by the use of an acousto-optic modulator (AOM). This made possible an improvement of the sensitivity of a previously realized OPO-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry instrument for the 3.2 - 3.9 µm mid-infrared region by one order of magnitude. The resulting system shows a detection sensitivity for methane of 2.4 × 10(-10) cm(-1) Hz(-1∕2) and 1.3 × 10(-10) cm(-1) at 20 s, which allows for detection of both the environmentally important (13)CH(4) and CH(3)D isotopologues in atmospheric samples.

We report on the realization of a Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) instrumentation based on a high-finesse (50,000) cavity with a detection sensitivity of 4×10(-13)  cm(-1) Hz(-1/2). For the P(e)(11) transition targeted at 1.5316 μm, this corresponds to a C(2)H(2) concentration of 240 ppq (parts-per-quadrillion) detected at 100 Torr. The setup was originally affected by recurrent dips in the cavity transmission, which were attributed to excitation of high-order transverse mode by scattering from the mirrors. The effect of these was reduced by insertion of a small pinhole in the cavity.

An optical parametric oscillator based Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) system suitable for addressing fundamental vibrational transitions in the 3.2-3.9 μm mid-infrared (MIR) region has been realized. An Allan-Werle analysis provides a detection sensitivity of methane of 1.5×10(-9)  cm(-1) with a 20 s integration time, which corresponds to 90 ppt of CH4 if detected at the strongest transition addressed at 40 Torr. This supersedes that of previous Db MIR NICE-OHMS demonstrations and suggests that the technique can be suitable for detection of both the environmentally important (13)CH(4) and CH3D isotopologues. It also opens up for detection of many other molecular species at ppt and sub-ppt concentration levels.

To reduce the complexity of fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry, a system incorporating a fiber-coupled optical circulator to deflect the cavity-reflected light for laser stabilization has been realized. Detection near the shot-noise limit has been demonstrated for both Doppler-broadened and sub-Doppler signals, yielding a lowest detectable absorption and optical phase shift of 2.2×10(-12)  cm(-1) and 4.0×10(-12)  cm(-1), respectively, both for a 10 s integration time, where the former corresponds to a detection limit of C2H2 of 5 ppt.