Optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and precision beyond the Voigt profile

Abstract

Broadband precision spectroscopy is indispensable for providing high fidelity molecular parameters for spectroscopic databases. We have recently shown that mechanical Fourier transform spectrometers based on optical frequency combs can measure broadband high-resolution molecular spectra undistorted by the instrumental line shape (ILS) and with a highly precise frequency scale provided by the comb. The accurate measurement of the power of the comb modes interacting with the molecular sample was achieved by acquiring single-burst interferograms with nominal resolution precisely matched to the comb mode spacing. Here we give a full theoretical description of this sub-nominal resolution method and describe in detail the experimental and numerical steps needed to retrieve ILS-free molecular spectra, i.e. with ILS-induced distortion below the noise level. We investigate the accuracy of the transition line centers retrieved by fitting to the absorption lines measured using this method. We verify the performance by measuring an ILS-free cavity-enhanced low-pressure spectrum of the 3ν1+ν3 band of CO2 around 1575 nm with line widths narrower than the nominal resolution. We observe and quantify collisional narrowing of absorption line shape, for the first time with a comb-based spectroscopic technique. Thus retrieval of line shape parameters with accuracy not limited by the Voigt profile is now possible for entire absorption bands acquired simultaneously.

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

“…This discrepancy can be caused by inaccuracy of linestrengths reported in the HITRAN database for this absorption band -a similar discrepancy was observed for the R16e line in this band in ref. [18]. This issue will be a subject of future investigation.…”

“…[19]. The resonances of a Fabry-Perot cavity are probed by an Er:fiber frequency comb, and the light transmitted through the cavity is analyzed with a fast-scanning Fourier transform spectrometer (FTS) with sub-nominal resolution [18]. The cavity, which has a finesse of ~1700, consists of two dielectric mirrors glued to a 45-cm-long stainless steel tube, yielding a free spectral range (FSR) of 333 MHz.…”

“…The cavity transmission is measured using a fast-scanning FTS with a nominal resolution of 1 GHz (equivalent to an optical path difference, OPD, of 30 cm). Matching the nominal resolution to the repetition rate of the transmitted comb allows measurements of the comb line intensities without the influence of the instrumental lineshape [18]. The comb interferogram is measured with an auto-balancing InGaAs detector similar to the one described in ref.…”

“…Here we use a frequency comb to perform CE-CRIS over 15 THz of bandwidth by measuring the transmission spectrum of a cavity filled with CO 2 using a Fourier transform spectrometer (FTS) with sub-nominal resolution [17,18]. The narrow lines of a stabilized frequency comb are simultaneously scanned across all cavity modes within the laser bandwidth and the intensities of the individual comb lines are measured by matching the nominal resolution of the spectrometer to the comb repetition rate [17,18]. A Lorentzian function is fitted to each cavity mode to retrieve its center frequency, width, and amplitude.…”

Recent advances in collisional effects on spectra of molecular gases and their practical consequences

“…In the latter case, a high-finesse cavity locked to the comb was used to enhance the path length. This method was used recently to study non-Voigt spectral shapes of CO 2 lines [85]. With a SNR of about 250 the effects of the speed dependence of the broadening were clearly observed.…”