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Comb-locked cavity ring-down spectroscopy of CO2 at 2-micron wavelength

Muhammad Asad Khan, Vittorio D'Agostino, Stefania Gravina, Livio Gianfrani, Antonio Castrillo

Abstract

We report on a comb-locked cavity ring-down spectrometer developed for high-precision, SI-traceable, molecular spectroscopy of air-broadened CO2 gas samples. The experimental setup relies on the use of a singly-resonant optical parametric oscillator that acts as an intermediate link between a 2 micron external-cavity diode laser and an optical frequency comb stabilized against a GPS-disciplined Rb-clock. Absorption spectra of the R(50) ro-vibrational component of the CO2 20012-00001 band have been recorded with high precision and fidelity. As a result of a refined spectral analysis, based on the implementation of the modified Hartmann-Tran profile, line center frequencies, pressure broadening and pressure shifting coefficients have been determined. Finally, we demonstrate the measurement of CO2 mole fractions with a subpromille statistical uncertainty

Comb-locked cavity ring-down spectroscopy of CO2 at 2-micron wavelength

Abstract

We report on a comb-locked cavity ring-down spectrometer developed for high-precision, SI-traceable, molecular spectroscopy of air-broadened CO2 gas samples. The experimental setup relies on the use of a singly-resonant optical parametric oscillator that acts as an intermediate link between a 2 micron external-cavity diode laser and an optical frequency comb stabilized against a GPS-disciplined Rb-clock. Absorption spectra of the R(50) ro-vibrational component of the CO2 20012-00001 band have been recorded with high precision and fidelity. As a result of a refined spectral analysis, based on the implementation of the modified Hartmann-Tran profile, line center frequencies, pressure broadening and pressure shifting coefficients have been determined. Finally, we demonstrate the measurement of CO2 mole fractions with a subpromille statistical uncertainty
Paper Structure (5 sections, 3 equations, 5 figures)

This paper contains 5 sections, 3 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Experimental setup
  3. Spectral analysis
  4. Results and discussion
  5. Conclusions

Figures (5)

  • Figure 1: Schematic of the experimental setup. OFCS stands for optical frequency comb; ECDL, external-cavity diode laser; OPO, optical parametric oscillator; E, etalon; PD, photodiode; AOM, acoustic-optic modulator; BD, laser beam dump; DAQ, acquisition board; HFOR, high-finesse optical resonator; FP, fiber port; PC, personal computer; M, mirror; BS, beam-splitter; LO, local oscillator. Black dashed arrows indicate electrical connections, while continuous black lines stand for optical fiber.
  • Figure 2: CO$_2$ absorption spectra resulting from 10 repeated acquisitions, in coincidence with the R(50) line of the 20012-00001 band (HITRAN notation) at 5007.787078 cm$^{-1}$. The upper inset on the right quantifies the dispersion of the retrieved line-center frequencies. A 1-$\sigma$ fluctuation of 180 kHz was found. As for the vertical scale, the stability is better than 0.1%, without signal averaging, as shown in the lower inset on the right.
  • Figure 3: Upper panel: Averaged CO$_2$ absorption spectra recorded at 9 different pressures of ambient air, from 667 to 4000 Pa. For each spectrum, the observed quality factors ranged from 2750 to 3600. (Quality factor is defined as the ratio of the peak absorption to the rms of the fit residuals). Middle panel: fit residuals at the highest pressure. Lower panel: residuals at the lowest pressure.
  • Figure 4: Dependence on the gas pressure of the Lorentzian component and line center frequency of the investigated CO$_2$ transition.
  • Figure 5: CO$_2$ mole fraction determinations over the pressure range investigated in this work. The dashed-dotted line represents the $x_{CO_2}$ mean value.