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Continuous-filtering Vernier spectroscopy (CF-VS) is a laser-based detection technique that combines the broad spectral coverage of an optical frequency comb (OFC) with the enhanced interaction length provided by an optical cavity. The resonances of the cavity filter the OFC to a small group of comb modes that probe the transitions of the species present in the cavity. Controlling cavity resonances allows for a fast scanning of the selected comb modes across the full bandwidth of the comb. CF-VS delivers high detection sensitivity through its immunity to the frequency-to-amplitude-noise conversion. Previous works have shown the capability of CF-VS to perform sensitive and broadband measurements of multiple species in both the near-infrared (NIR) and the mid-infrared (MIR) regions. Those implementations required high-bandwidth stabilization via feedback to the comb sources, which resulted in bulky setups and complex operations. Moreover, they provided acquisition rates up to 20 Hz, limited by the mechanical design. Besides, the target species were measured under static conditions 一 CF-VS had not yet been employed to monitor any time-dependent processes.

The goal of the thesis was to address these issues. In the first project, we used CF-VS based on an Er:fiber comb to measure consecutive spectra of H2O and OH with 25 ms time resolution in a premixed flame whose fuel/air equivalence ratio was modulated with a square wave to simulate temporal perturbations. The concentrations of both species were retrieved with percent level precision, and their temporal profiles were repeatable in each modulation cycle. The steady-state concentrations were in good agreement with a static flame simulator. This work was the first demonstration of CF-VS and cavity-enhanced comb-based spectroscopy with ms time resolution.

In the second project, we implemented a new design of CF-VS that uses a compact Er:fiber comb and a custom-made moving aperture. This removes the requirement for high-bandwidth stabilization and allows acquisition rates up t0 100 Hz. To verify these capabilities, we measured CO2 and CH4 spectra in two spectral ranges. We developed a simple model to account for the influence of the high scanning speed above the adiabatic limit on the absorption signal.

The last project aimed to implement a robust and compact CF-VS spectrometer in the MIR region. For that, we improved an existing MIR source based on difference frequency generation (DFG) using a low-noise Yb:fiber pump, delay stabilization, and a novel polarization-maintaining silicon crystal fiber. The MIR comb uses a soliton generated in the fiber as the seed for DFG. We characterized the soliton using the pump laser. The wide tuning range of the soliton allows the idler to emit in the 2.7-4.2 μm range with high brightness.  The MIR comb has a simple delay stabilization and a fixed zero-offset frequency and was successfully implemented to measure high-resolution and precision spectra of CH3I using a comb-resolved Fourier transform spectrometer. Finally, we used the source to perform CF-VS by measuring CH4 spectra at around 3.3 μm. We showed that a single-shot spectrum could be successfully retrieved under the robust operation in the fingerprint regime. 

Absorption spectroscopy is a powerful scientific tool for non-invasive and remote sensing applications ranging from atmospheric monitoring to astrophysics. In spectroscopic detection schemes it is necessary to have spectral models for any molecular species to be detected or quantified. Such models are often based on spectroscopic measurements or at the very least require experimental validation. The experimental data need to be accurate in terms of absorption line positions and intensities, but should also cover as many absorption lines as possible, i.e. broadband measurements are highly desirable.

Fourier transform spectroscopy (FTS) based on optical frequency combs (OFCs) can supply laboratory data that meet these requirements. OFCs provide a broad optical bandwidth and high spectral brightness, and also revolutionized our ability to measure optical frequencies, which had a profound impact on the frequency accuracy of spectroscopic measurements. The combination of OFCs and FTS, using the recently developed sub-nominal resolution technique, allows for measuring broadband absorption spectra with very high resolution, and a frequency accuracy provided by the OFCs. The aim of the work in this thesis was to expand the application of sub-nominal OFC-FTS to provide the much needed high-accuracy data for validation and development of spectroscopic databases of molecules relevant for a wide range of sensing application.

We developed a spectrometer to target the strong molecular absorption bands in the mid-infrared using two OFC sources based on difference frequency generation (DFG) emitting in the 3 μm and 8 μm wavelength ranges. We measured the spectra of iodomethane, CH₃I, and dibromomethane, CH₂Br₂, around 3 μm, fitted Hamiltonian models to several bands using the PGOPHER software, and reported molecular constants. For CH₃I we improved on previous models, while for CH₂Br₂ we presented a new interpretation of the spectrum. We also reported the first assessments of line intensities of CH₃I performed using multispectral fitting. At 8 μm, we implemented OFC-FTS based on a fiber-based compact DFG OFC source and measured low pressure spectra of nitrous oxide, N₂O, methane, CH₄, and formaldehyde, H₂CO. After the frequency accuracy was confirmed by excellent agreement with an earlier accurate study of N₂O, we compiled extensive line lists for CH₄ and H₂CO containing hundreds of transition frequencies with a precision improved by one order of magnitude compared to previously available data, and also reported line intensities for most transitions. For CH₄ the new data were used to improve a global Hamiltonian model, while the H₂CO data were incorporated into an algorithm based on spectroscopic networks to yield better precision in predicted energy levels and transition frequencies.

We also further developed a recent implementation of double resonance (DR) spectroscopy where optical pumping by a continuous-wave laser was used to populate selected vibrational energy levels of CH₄ not populated at room temperature, and a near-infrared OFC probed sub-Doppler transitions from the pumped states. Such measurements are necessary to validate theoretical predictions of transitions between excited vibrational levels that are relevant for high-temperature environments such as the atmospheres of hot celestial objects. We reported an improved measurement setup using a new pump laser, new enhancement cavity with an updated OFC-cavity locking scheme, and measured transitions between more highly excited rotational levels than was previously reported. The higher rotational excitations lead to a larger number of DR transitions, which could be readily detected in the broadband high-resolution OFC-FTS spectra. We retrieved parameters of 88 lines of which we could assign 79 to theoretically predicted transitions. We found systematic frequency discrepancies with the predictions, that had not been observed earlier for lower rotational levels.

These implementations of sub-nominal OFC-FTS thus provided highly accurate line lists and improved spectral models of absorption bands of several molecules in the universally important mid-infrared region, as well as the first detection of 88 transitions between excited vibrational states of CH₄ relevant for high-temperature environments. We demonstrated the high potential of these techniques for collecting large amounts of accurate spectroscopic data, that further the scope of applicability of molecular spectroscopy.