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Direct-comb spectroscopy techniques uses optical frequency combs (OFCs) as spectroscopic light source. They deliver high sensitivity, high frequency resolution and precision in a broad spectral range. Due to these features, the field has burgeoned in recent years. In this work we constructed an OFC-based cavity-enhanced Fourier-transform spectrometer in the near-infrared region and used it for a line-shape study of rovibrational transitions of CO perturbed by Ar. The highly sensitive measurements spanned the wavenumber range from 6270 cm $^{-1}$ to 6410 cm $^{-1}$ , which covered both P and R branch of the second overtone band of CO. The spectrometer delivers high-resolution surpassing the Fourier-transform resolution limit determined by interferogram length, successfully removing ringing and broadening effects caused by instrumental line shape function. The instrumental-line-shape-free method and high signal-to-noise ratio in the measurement allowed us to observe collisional effects beyond those described by the Voigt profile. We retrieved collisional line-shape parameters by fitting the speed-dependent Voigt profile and found good agreement with the values given by precise cavity ring-down spectroscopy measurements that used a continuous-wave laser referenced to a stabilized OFC. The results demonstrate that OFC-based cavity-enhanced Fourier-transform spectroscopy is a strong tool for accurate line-shape studies that will be crucial for future spectral databases.

Recent technological developments are enabling highly sensitive measurements of high-resolution 2D IR gas-phase spectra. In analogy to well-known liquid-phase results, we present polarization sequences that reduce spectral congestion and improve signal separation in gas-phase spectra. We also discuss other unique features of 2DIR gas phase spectra.

We study ultrafast excited-state proton transfer and subsequent internal conversion in salicylideneaniline using broadband cavity-enhanced transient absorption spectroscopy. By comparing gas-phase optical spectra to solution-phase spectra and ab initio theory, competing relaxation pathways are elucidated.

Frequency-based cavity mode-dispersion spectroscopy (CMDS), previously applied for Doppler-limited molecular spectroscopy, is now employed for the first time for saturation spectroscopy. Comparison with two intensity-based, cavity-enhanced absorption spectroscopy techniques, i.e. cavity mode-width spectroscopy (CMWS) and the well-established cavity ring-down spectroscopy (CRDS), shows the predominance of the CMDS. The method enables measurements in broader pressure range and shows high immunity of the Lamb dip position to the incomplete model of saturated cavity mode shape. Frequencies of transitions from the second overtone of CO are determined with standard uncertainty below 500 Hz which corresponds to relative uncertainty below 3 × 10−12. The pressure shift of the Lamb dips, which has not been detected for these transitions in available literature data, is observed.

Broadband ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D spectroscopy, are widely used to study molecular dynamics. However, these techniques are typically restricted to optically thick samples, such as solids and liquid solutions. In this article we discuss a cavity-enhanced ultrafast transient absorption spectrometer covering almost the entire visible range with a detection limit of $\Delta$OD $ < 1 \times 10^{-9}$, extending broadband all-optical ultrafast spectroscopy techniques to dilute beams of gas-phase molecules and clusters. We describe the technical innovations behind the spectrometer and present transient absorption data on two archetypical molecular systems for excited-state intramolecular proton transfer, 1'-hydroxy-2'-acetonapthone and salicylideneaniline, under jet-cooled and Ar cluster conditions.

International audience; We present fully ab initio calculations of second-overtone rovibrational line shapes of carbon monoxide perturbed by argon. The quantum mechanical scattering problem between CO and Ar is solved numerically for two different ab initio interaction potentials. We use the generalized Hess method to determine spectroscopic cross sections which describe the effect of collisions on each spectral line. Using these cross sections, we determine the line-shape parameters that we use to generate the Hartmann-Tran and speed-dependent billiard ball profiles. We compare the generated line shapes with high-quality experimental line profiles of five lines measured at five pressures between 0.01 and 1 atm. A subpercent agreement over the entire pressure range is obtained. Calculations for the P(9) line are used to inspect the effects of the two interaction potentials. The discrepancies for both the considered interaction potentials and the experiment are explained within the described theoretical framework. The presented results are the most accurate collisional line-shape calculations for a system with collision dynamics representative of atmospherically relevant species.

International audience; Temperature dependences of molecular line-shape parameters are important for the spectroscopic studies of the atmospheres of the Earth and other planets. A number of analytical functions have been proposed as candidates that may approximate the actual temperature dependences of the line-shape parameters. In this article, we use our ab initio collisional line-shape calculations for several molecular systems to compare the four temperature ranges (4TR) representation, adopted in the HITRAN database [J. Quant. Spectrosc. Radiat. Transfer 2017;203:3] in 2016, with the double-power-law (DPL) representation. Besides the collisional broadening and shift parameters, we consider also the most important line-shape parameters beyond Voigt, i.e., the speed dependence of broadening and shift parameters, and real and imaginary parts of the complex Dicke parameter. We demonstrate that DPL gives better overall approximation of the temperature dependencies than 4TR. It should be emphasized that DPL requires fewer parameters and its structure is much simpler and more self-consistent than the structure of 4TR. We recommend the usage of DPL representation in HITRAN, and present DPL parametrization for Voigt and beyond-Voigt line profiles that will be adopted in the HITRAN database. We also discuss the problem of the Hartmann-Tran profile parametrization in which the correlation parameter, η, and frequency of the velocity-changing collisions parameter, ν vc , diverges to infinity when collisional shift crosses zero; we recommend a simple solution for this problem.

Optical frequency comb spectrometry has become one of the leading fields devising new ways to investigate molecular structure and characterize optical elements due to its advantage in speed and accuracy. In this work we demonstrate the first dual-comb realization of cavity mode-width and mode-dispersion spectroscopies, achieving single-spectrum acquisition time in millisecond range, maintaining high spectral resolution limited by the comb tooth width. Interleaving of a few tens of spectra allows us to measure shapes of 13 kHz-wide enhancement cavity modes and determine their widths and positions with Hz-level precision. Thanks to this we can simultaneously measure the loss and dispersion of both the cavity and the intracavity medium. As a demonstration, we measure spectra of a gas mixture of methane and nitrogen and compare them with a simulation based on the HITRAN database. This novel method has many possible applications, including gas metrology, high-reflectivity mirror characterization or trace gas sensing.

Absolute positions and spectral line-shape parameters of carbon monoxide 0 → 3 band, P branch transitions are reported. The speed-dependent effects and the influence of velocity-changing collisions were taken into account in the fitted line-shape models. For the first time the values of pressure shift coefficients of CO in argon for this band were determined. The measurements were made with the Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer, with the frequency axis linked through an optical frequency comb to the UTC(AOS) frequency reference based on a hydrogen maser. Achieved uncertainties of line positions are between 70 kHz and 420 kHz.

International audience; We present a procedure for generating the parameters of the Hartmann-Tran profile (HTP) from purely first principles calculations. Starting from an absorber-perturber interaction potential, we calculate S-matrices describing the effect of collisions on the absorbing molecule. We then use the generalized Hess method to calculate speed-dependent pressure shift and broadening parameters, and the complex Dicke parameter, ν opt , which accounts for such effects as the Dicke effect and correlations between dephasing and velocity-changing collisions. Based on these ab initio results, we derive the Hartmann-Tran profile parameters and evaluate the validity of the quadratic approximation of speed dependence and the hard-collision model of velocity-changing collisions adopted in the Hartmann-Tran profile. We also discuss the interpretation and speed dependence of ν opt. Finally, we evaluate the approximation scheme for temperature dependence of HTP line-shape parameters adopted in the 2016 edition of the HITRAN database.

Optical frequency comb spectrometers open up new avenues of investigation into molecular structure and dynamics thanks to their accuracy, sensitivity and broadband, high-speed operation. We combine broadband direct frequency comb spectroscopy with a dispersive spectrometer providing single-spectrum acquisition time of a few tens of milliseconds and high spectral resolution. We interleave a few tens of such comb-resolved spectra to obtain profiles of 14-kHz wide cavity resonances and determine their positions with precision of a few hertz. To the best of our knowledge, these are the most precise and highest resolution spectral measurements performed with a broadband spectrometer, either comb-based or non-comb-based. This result pushes the limits of broadband comb-based spectroscopy to Hz-level regime. As a demonstration of these capabilities, we perform simultaneous cavity-enhanced measurements of molecular absorption and dispersion, deriving the gas spectra from cavity mode widths and positions. Such approach is particularly important for gas metrology and was made possible by the Hz-level resolution of the system. The presented method should be especially applicable to monitoring of chemical kinetics in, for example, plasma discharges or measurements of narrow resonances in cold atoms and molecules.

We report the measurement of the absolute frequencies of the 6s

We present a 100 MHz repetition rate Yb-fiber-laser-pumped synchronously pumped optical parametric oscillator (SPOPO) delivering tunable frequency combs covering almost the entire visible spectral range. By intracavity doubling both the signal and idler combs and using collinear residual pump light, nearly continuous tuning over the range of 420-700 nm is achieved with only small gaps near the OPO degeneracy condition. Output powers range from 10 mW to 200 mW, depending on wavelength, with pulse durations below 150 fs without external compression. Frequency locking of all three collinearly outcoupled combs (pump, doubled signal, and doubled idler) to a femtosecond enhancement cavity facilitates direct comparison of their optical phase noise and phase modulation transfer functions. In the singly-resonant OPO, optical phase modulation of the pump comb is transferred nearly completely to the non-resonant idler comb. This results in a resonant signal comb with reduced optical phase noise and also enables high-bandwidth modulation on the idler comb via phase modulation of the pump.

We present broadband measurements of intracavity loss and dispersion, which enables retrieval of complex refractive index. Results were obtained with a Fourier transform spectrometer achieving Hz-level resolution thanks to frequency comb light.

High resolution spectroscopy enables the detection of atmospheres of exoplanets. To reach the required radial velocity precision of about 1 m/s, calibration with even more precise sources is mandatory. HIRES will employ several calibration sources, the most important ones are an Laser Frequency Comb (LFC) and Fabry-P´erots (FP). The LFC needs to be filtered with a set of FP. One possible solution is to illuminate this set of FP with a broadband light source and use them as calibrators, when they are not used for filtering the LFC. It has been demonstrated that passively-stabilized FP can perform better than 10 cm/s per night. We give an overview of the currently used FP in different surveys and compare their individual features. For the FP which may be used in HIRES we discuss different configuration. We show that the Finesse and FSR of the FP needs to be optimized with regard to the resolution of the spectrograph and we outline how we aim to fulfill the requirements of HIRES.

Astrophysical calibration sources which can be used for high-precision radial-velocity spectroscopy require a calibration with even higher precision and accuracy. Calibration of these sources can be achieved with a high-resolution Fourier-Transform-Spectrograph (FTS). The precision (~ 20 m/s) of the FTS is mainly driven by its reference, often a stabilized HeNe-laser. To reach an acceptable precision, either averaging over a large number of measurements or a better reference is needed. We developed a setup including a Laser-Frequency-Comb (LFC) for referencing a high-resolution FTS. Due to the pulsed source specific evaluation methods have to be used to retrieve the spectrum properly. We extend the sub-nominal method used in absorption spectroscopy by showing an algorithm to determine the interferogram cut points from the interferogram itself, rather than calculating them from the repetition rate and reference laser wavelength. Furthermore, we show that line position errors measured from comb spectra are associated with amplitude variability and phase noise. We give an estimate of the measured line position stability for different evaluation methods (truncation, shifting, apodization, zerofilling) on scales not dominated by these errors.

We performed measurements of overtone band of CO using a frequency comb based cavity-enhanced absorption spectroscopy and Fourier-transform spectrometer with sub-nominal resolution. The technique allows to measure and determine precisely line-shape parameters in wide range.

We report on mid-infrared broadband direct frequency comb spectroscopy as a novel plasma diagnostic applied to spectroscopic investigations of plasma nitrocarburizing processes. We discuss the parameter-dependent behavior of species like HCN, CH4, and NH3.

We present cavity mode width and frequency measurements over 60-cm−1 range at Hz-level precision. We utilize a near-infrared frequency comb and a VIPA spectrometer to retrieve absorption and dispersion of a CO-N2 sample in a high-finesse cavity.

The instrumentation plan for the E-ELT foresees a High Resolution Spectrograph (HIRES). Among its main goals are the detection of atmospheres of exoplanets and the determination of fundamental physical constants. For this, high radial velocity precision and accuracy are required. HIRES will be designed for maximum intrinsic stability. Systematic errors from effects like intrapixel variations or random errors like fiber noise need to be calibrated. Based on the main requirements for the calibration of HIRES, we discuss different potential calibration sources and how they can be applied. We outline the frequency calibration concept for HIRES using these sources.

We report recent developments of comb-based broadband absorption spectroscopy. The comb-line resolving approaches and Fourier transform spectroscopy with sub-nominal resolution overcome the frequency resolution limits of conventional techniques. Advantages for various applications will be discussed.

International audience; We present our experimental spectra of the very weak S(2) transition from the 2–0 band of molecular deuterium, measured with a frequency-stabilized cavity ring-down spectroscopy (FS–CRDS) assisted by the optical frequency comb (OFC). Experimental collisional broadening and shifting are compared with results of ab initio quantum scattering calculations.

A spectroscopic method free from systematic errors is desired for many challenging applications of gas detection. Although existing cavity-enhanced techniques exhibit very high precision, their accuracy strongly depends on propagation of the light amplitude through an optical system and its detection. Here, we demonstrate that the frequency-based molecular dispersion spectroscopy, involving sub-Hz-level precision in frequency measurements of optical cavity resonances, leads to sub-per-mille accuracy and a wide dynamic range, both previously unattainable by any other spectroscopic technique. The method offers great sensitivity of 5×10−11 cm−1, high speed, limited only by the fundamental response time of the cavity, and traceability of both axes of the spectrum to the primary frequency standard. All these features are necessary for convenient realization of comprehensive molecular spectroscopy from Doppler up to collisional regime without changing the spectroscopic method and modification of the experimental setup. Moreover, the presented approach does not require linear, high-bandwidth nor phase-sensitive detectors and can be directly implemented in existing cavity-enhanced spectrometers utilizing either continuous-wave or coherent broadband radiation. We experimentally prove the predominance of frequency-based spectroscopy over intensity-based one. Our results motivate replacement of intensity-based absorption spectroscopy with a pure frequency-based dispersion one in applications where the highest accuracy is required.

We report the measurement of the absolute frequencies of the 6s2 1S0−6s6p 3P1 transition (253.7 nm) and the relevant isotope shifts in five mercury isotopes 198Hg, 199Hg, 200Hg, 202Hg, and 204Hg. The Doppler-free saturated absorption measurements were performed in an atomic vapour cell at room temperature with a four-harmonic generated (FHG) continuous-wave (cw) laser digitally locked to the atomic transition. It was referenced with a femtosecond optical frequency comb synchronized to the frequency of local representation of the International Atomic Time to provide traceability to the SI second by the 330 km-long stabilized fibre optical link. The transition frequencies and isotope shifts have been determined with an accuracy of a few hundred kHz, at least one order of magnitude better than any previous measurement. By making a King plot with the isotope shifts of 6s6p 3P2−6s7s 3S1 transition (546 nm) we determined the accurate value of the ratio of the electronic field-shift parameters E546/E254 and estimated the electronic field-shift term E254.

We present a cavity-enhanced direct optical frequency comb spectroscopy system with a virtually imaged phased array (VIPA) spectrometer and either a dither or a Pound-Drever-Hall (PDH) locking scheme used for stable transmission of the comb through the cavity. A self-referenced scheme for frequency axis calibration is shown along with an analysis of its accuracy. A careful comparison between both locking schemes is performed based on near-IR measurements of the carbon monoxide ν=3←0 band P branch transitions in a gas sample with known composition. The noise-equivalent absorptions (NEA) for the PDH and dither schemes are 9.9×10(-10) cm(-1) and 5.3×10(-9) cm(-1), respectively.

We present a method for broadband measurements of dispersion and loss of an optical cavity. We employ an optical frequency comb directly coupled into a cavity to scan the cavity modes and retrieve their shapes and positions. The measurement data is acquired using instrumental-line-shape-free Fourier transform spectrometry. This method can be developed into a powerful tool for optical loss and dispersion measurements for broadband characterization of optical elements as well as absorption and dispersion spectroscopy. Full Text: PDF References Th. Udem, R. Holzwarth, T.W. Hansch, "Optical Frequency Metrology", Nature 416, 233–237 (2002). CrossRef R.A. McCracken, J.M. Charsley, D.T. Reid "A decade of astrocombs: recent advances in frequency combs for astronomy", Optics Express 25, 13, 15058-15078 (2017). CrossRef M.J. Thorpe, D. Balslev-Clausen, M.S. Kirchner, J. Ye, "Cavity-enhanced optical frequency comb spectroscopy: application to human breath analysis", Optics Express 16, 4, 2387-2397 (2008). CrossRef J. Domyslawska et al., "Cavity ring-down spectroscopy of the oxygen B-band with absolute frequency reference to the optical frequency comb", The Journal of Chemical Physics 136, 024201 (2012). CrossRef P. Maslowski K.C. Cossel, A. Foltynowicz, J. Ye., "Cavity-Enhanced Direct Frequency Comb Spectroscopy", in G. Gagliardi and H.-P. Loock, "Cavity-Enhanced Spectroscopy and Sensing", Springer (2014). CrossRef L. Rutkowski et al., "Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution", Optics Express Vol. 25, Issue 18, pp. 21711-21718 (2017). CrossRef E.D. Black, "An introduction to Pound-Drever-Hall laser frequency stabilization", American Journal of Physics 69, 79 (2001) CrossRef A. Cygan et al., "One-dimensional frequency-based spectroscopy", Optics Express. 23, 14472. 10.1364 ( 2015). CrossRef P. Maslowski et al., "Surpassing the path-limited resolution of Fourier-transform spectrometry with frequency combs", Phys. Rev. A 93, 021802(R) (2016). CrossRef L. Rutkowski, P. Maslowski, A.C. Johansson, A. Khodabakhsh, A. Foltynowicz, "Optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and precision beyond the Voigt profile", Jour. of Quant. Spect. and Rad. Trans., 204, 63-73 (2018). CrossRef S. Schiller, "Spectrometry with frequency combs," Opt. Lett. 27, 766-768 (2002). CrossRef A. Cygan et al., "Absolute molecular transitions frequencies measured by three cavity-enhanced spectroscopy techniques" J. Chem. Phys. 144 214202-1-214202-11 (2016). CrossRef I.E. Gordon et al., "The HITRAN2016 Molecular Spectroscopic Database", J. Quant. Spect. Radiat. Trans. (2017). CrossRef

Fourier transform spectroscopy based on incoherent light sources is a well-established tool in research fields from molecular spectroscopy and atmospheric monitoring to material science and biophysics. It provides broadband molecular spectra and information about the molecular structure and composition of absorptive media. However, the spectral resolution is fundamentally limited by the maximum delay range ({\Delta}$_{max}$) of the interferometer, so acquisition of high-resolution spectra implies long measurement times and large instrument size. We overcome this limit by combining the Fourier transform spectrometer with an optical frequency comb and measuring the intensities of individual comb lines by precisely matching the {\Delta}$_{max}$ to the comb line spacing. This allows measurements of absorption lines narrower than the nominal (optical path-limited) resolution without ringing effects from the instrumental lineshape and reduces the acquisition time and interferometer length by orders of magnitude.

We transmit a frequency comb through an enhancement cavity by PDH locking with a graphene modulator. We comb-resolve the 1940 to 2115 nm spectrum with stable repetition rate and offset frequency via Fourier transform spectrometry.

In direct frequency comb spectroscopy with a VIPA spectrometer, the resolution of the spectrometer is usually insufficient to resolve the comb modes. Thus, one can either filter the modes, reducing the density of spectral elements or cope with the inability to uniquely identify spectral elements with individual comb modes by calibrating the spectrometer itself. Here, we present a way to make use of the inherent frequency accuracy of a stabilized frequency comb to calibrate the spectrometer. We also present a comparison between two commonly used schemes to stabilize the coupling between a frequency comb and cavity resonances: the Pound-Drever-Hall locking scheme and the swept coupling scheme.

Frequency combs can be used directly, for example as a highly precise spectroscopic light source. They can also be used indirectly, as a bridge between devices whose high precision requirements would normally make them incompatible. Here, we demonstrate two ways that a frequency comb enables new technologies by matching optical cavities. One cavity is the laser oscillator. A second cavity is a low-threshold doubly-resonant optical parametric oscillator (OPO). Extending optical referencing to the doubly-resonant OPO turns the otherwise unstable device into an extremely precise midinfrared frequency comb. Another cavity is an optical enhancement cavity for amplifying spectral absorption in a gas. With the high speed of a graphene-modulated frequency comb, we can couple a frequency comb directly into a high-finesse cavity for trace gas detection.

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.

Recent advancements in infrared frequency combs will enable facile recording of coherent two-dimensional infrared spectra of gas-phase molecules with rotational resolution (RR2DIR). Using time-dependent density-matrix perturbation theory and angular momentum algebra techniques, we derive new polarization conditions unique to freely rotating molecules and absent in the condensed phase. These polarization conditions can be used to suppress parts of 2DIR rovibrational response, clarifying complicated RR2DIR spectra. With the polarization control methods described here, RR2DIR spectroscopy can be a powerful tool for studying complex gas mixtures of polyatomic molecules.

We overcome the resolution limit of Fourier transform spectrometry and measure instrumental-lineshape-free high-resolution broadband molecular spectra by matching the spectrometer's optical path difference to the line spacing of an optical frequency comb.

Fourier-transform spectroscopy (FTS) over the years has become the golden standard in broadband spectroscopic measurements in the infrared part of the electromagnetic spectrum, offering measurement range limited only by the detector sensitivity and emission spectrum of the light source. This method however has always had a significant tradeoff: it necessitates a mechanical travel scheme to produce an optical path difference (OPD) between interferometer arms, and the spectral resolution is set by this distance, which made the high-resolution measurements very slow and the instrumentation required for them very spacious.

We perform cavity-enhanced direct frequency comb spectroscopy with a self-referenced VIPA spectrometer. We compare PDH and dither cavity stabilization in near-IR measurements of carbon monoxide and measure absorption sensitivities of 9.9×10−10 and 5.3×10−9 cm−1 respectively.

Conference on Lasers and Electro-Optics : [Proceedings] - OSA Washington, D.C., 2018. - ISBN 978-1-943580-42-2 - doi:10.1364/CLEO_SI.2018.STu3P.5 CLEO: Science and Innovations, CLEO, San Jose, California, 13 May 2018 - 18 May 2018; Piscataway, NJ : OSA 2 pp. (2018). doi:10.1364/CLEO_SI.2018.STu3P.5, We performed measurements of overtone band of CO using cavity-enhanced Fourier-transform spectrometer with sub-nominal resolution. The system’s performance allows to study collisional effects not described by the Voigt profile and precise determination of line parameters., Published by OSA, Piscataway, NJ