Absolute molecular transition frequencies measured by three cavity-enhanced spectroscopy techniques

Abstract

Absolute frequencies of unperturbed (12)C(16)O transitions from the near-infrared (3-0) band were measured with uncertainties five-fold lower than previously available data. The frequency axis of spectra was linked to the primary frequency standard. Three different cavity enhanced absorption and dispersion spectroscopic methods and various approaches to data analysis were used to estimate potential systematic instrumental errors. Except for a well established frequency-stabilized cavity ring-down spectroscopy, we applied the cavity mode-width spectroscopy and the one-dimensional cavity mode-dispersion spectroscopy for measurement of absorption and dispersion spectra, respectively. We demonstrated the highest quality of the dispersion line shape measured in optical spectroscopy so far. We obtained line positions of the Doppler-broadened R24 and R28 transitions with relative uncertainties at the level of 10(-10). The pressure shifting coefficients were measured and the influence of the line asymmetry on unperturbed line positions was analyzed. Our dispersion spectra are the first demonstration of molecular spectroscopy with both axes of the spectra directly linked to the primary frequency standard, which is particularly desirable for the future reference-grade measurements of molecular spectra.

Absolute frequency determination of molecular transition in the Doppler regime at kHz level of accuracy

“…The relative uncertainty of f would be the same as for the RF reference, that means 10 −12 at 1 s in that case [26]. This is two orders of magnitude worse than in the case of use of the optical atomic clock as a frequency reference.…”

“…This signal serves as a link to the UTC(AOS) and is delivered to our laboratory by a 330 km long, frequency-stabilized fibre link [13,25] from Astro-Geodynamic Observatory (AOS) in Borowiec. All frequency counters and generators used in the PDH-locked FS-CRDS as well as in the OFC are referenced to this signal [26]. The other one is provided by the 88 Sr optical lattice clock in the National Laboratory FAMO in Toruń.…”

Broadband calibration-free cavity-enhanced complex refractive index spectroscopy using a frequency comb

“…Finally, the cavity mode amplitudes, P k , retrieved from the Lorentzian fits are used to obtain a cavity-enhanced transmission spectrum. The comb power envelope, 0 k P , needed for normalization of the spectrum, is found by taking the ratio between the cavity mode amplitudes and the model based on Eqs (5) and (14), smoothening it with a moving average function, and fitting the sum of a third-order polynomial and low frequency sine functions to it. The spectrum is then divided by the fitted comb envelope to yield the cavity-enhanced transmission spectrum, α k A , shown by black markers in Fig.…”

“…Cygan et al developed these concepts into two techniques: cavity mode width spectroscopy (CMWS) [12] and one-dimensional cavity mode-dispersion spectroscopy (1D-CMDS) [13]. Measuring the broadening and shift of the cavity resonances simultaneously allows simultaneous and calibration-free assessment of the imaginary and real parts of the complex refractive index of molecular transitions [14,15]. However, cavityenhanced complex refractive index spectroscopy (CE-CRIS) [16] performed using continuous-wave lasers has a limited spectral coverage and requires precise sequential scanning of a narrow linewidth laser over individual cavity modes, which is time consuming even for FARS.…”

“…Therefore, existing demonstrations of CE-CRIS are limited to 1-2 individual molecular transitions. Moreover, in 1D-CMDS only the relative (rather than the absolute) cavity mode positions are measured to avoid the influence of the cavity length drift, while the absolute frequency calibration is obtained by referencing the probe laser to a frequency comb [14,15].…”