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The perfect instrument in spectroscopy measures spectra with high-resolution, high-sensitivity and in a small amount of time over a broad spectral range. Fourier-transform spectrometers (FTS) allow for both high resolution and broadband measurements thanks to their multiplex advantage. By coupling the light source to an optical cavity, the sensitivity of the instrument is considerably increased since the interaction length of the light with the sample is increased by several orders of magnitude (reaching kilometric interaction lengths). The use of frequency combs allows to increase the precision of the measurement and to surpass the maximum resolution of conventional FTS. Finally, when performing spectroscopy of complex molecules in the gas phase at high resolution, spectra are highly congested because many energy levels are populated, allowing a great number of transitions to occur. This results in difficulties to assign each transition to its corresponding energy levels. The advent of the buffer gas cooling technique that permits to cool down atoms and molecules to cryogenic temperatures opens the door to go beyond the state of the art in terms of molecular complexity with the study of large and heavy molecules at relatively high frequencies (NIR,VIS). The objective of this PhD thesis is to present the design, the fabrication and the characterization of an instrument that will ultimately embody all these features, allowing for the study of large molecules in the gaseous phase in the near-infrared and visible regions of the light spectrum. In its final form, it will consist of a frequency comb as a light source, a FTS as a spectrum analyzer and a buffer gas cooling setup in order to perform the spectroscopy of cold molecules, the required sensitivity being achieved by making the measurements within an optical cavity. (SC - Sciences) -- UCL, 2022

We report the measurement of sub-Doppler double-resonance transitions in methane using a 3.3 $\mu$m continuous wave optical parametric oscillator to pump transitions in the fundamental $\nu_3$ band and a 1.67 $\mu$m frequency comb to probe ladder-type transitions in the 3$\nu_3$ band over 200 cm$^{-1}$ of bandwidth. We detected 36 ladder-type resonances for 9 pumped transitions with average center frequency accuracy of 1.7 MHz, limited by the pump frequency stability. The lines are assigned using the TheoReTS line list, and the intensity ratios for different relative pump/probe polarizations. This method provides accurate assignment of highly excited energy levels that cannot be done using high temperature spectra.

Optical frequency comb spectroscopy in the mid-infrared fingerprint region combines broad spectral bandwidth with high detection sensitivity and allows simultaneous detection of trace amounts of many molecular species. We have recently demonstrated a continuous-filtering Vernier spectrometer based on a mid-infrared optical frequency comb and an enhancement cavity for fast and sensitive detection of CH 4 [1]. Here we present an improved, fully automatized and frequency calibrated continuous-filtering Vernier spectrometer, schematically shown in Fig. 1(a). The comb source is a doubly resonant optical parametric oscillator (DROPO) based on an orientation-patterned GaAs crystal synchronously pumped by a Tm:fiber femtosecond laser (125 MHz repetition rate, f rep ). The signal comb (3.1–3.4 μm, 30 mW) is mode matched to a 60-cm long Vernier enhancement cavity with a finesse of ∼350 at 3.25 μm, placed in an enclosure that can be filled with the gas sample. The output mirror is attached to a PZT and mounted on a translation stage. When the cavity free spectral range is perfectly matched to twice the f rep (250 MHz) every other signal comb mode is transmitted through the cavity. By detuning the cavity length from this perfect match position the cavity resonances act as a filter and transmit groups of comb modes called Vernier orders [2]. A diffraction grating mounted on a galvo-scanner separates these orders after the cavity and the chosen order is sent to the detection system. The Vernier order is tuned across the signal comb spectrum by scanning the cavity length (at 20 Hz) and the grating is rotated synchronously to fix the order in space and allow acquisition of the entire spectrum in 25 ms. Any residual mismatch between the cavity length scan and the grating rotation is compensated by a feedback loop acting on the f rep of the pump laser and the PZT of the Vernier cavity [2]. A Fabry-Perot etalon is used for frequency calibration of the spectrometer. Figure 1(b) shows in black the normalized transmission spectrum of a sample containing 5.0 ppm CH 4 and 160 ppm water. The red and blue curves show the corresponding fit of the Vernier spectrum [3] of CH 4 and water, respectively, calculated using Voigt profiles, line parameters from the HITRAN database, and the experimentally determined cavity finesse. The figure of merit of the spectrometer is 1×10−9 cm−1 Hz−1/2 per spectral element and multiline fitting yields minimum detectable concentration of CH 4 of 2 ppb in 25 ms, translating into 400 ppt Hz−1/2 Since the spectrum of the signal comb covers the fundamental C-H stretch transitions we expect low detection limits for other hydrocarbons as well. In conclusion, mid-infrared comb-based continuous-filtering Vernier spectroscopy allows fast and highly sensitive measurement of broadband absorption spectra using a robust and compact detection system.

The mid-infrared (MIR) part of the optical spectrum (3–12 μm) houses the fundamental absorption bands of a multitude of environmentally important molecules, but the abundance of water absorption often causes interference with the target species and makes concentration measurement inaccurate. The broad spectral coverage of optical frequency comb spectroscopy (OFCS) provides access to entire ro-vibrational bands and allows more accurate concentration quantification and retrieval of sample temperature. To further improve detection sensitivity of paramagnetic species in the presence of interfering species, we combine a MIR optical frequency comb with the Faraday rotation spectroscopy (FRS) technique [I], which is insensitive to interferences from diamagnetic molecules, such as H 2 O, CO 2 , and CO. In FRS, the rotation of the polarization induced by an external magnetic field in the vicinity of paramagnetic molecular transitions is translated to an intensity change by the use of a polarization analyzer, which effectively removes the influence of any non-paramagnetic species. In the proof of principle demonstration of OFC-FRS we detect nitric oxide (NO) in the presence of water at 5.3 μm using a Fourier transform spectrometer.

We present a versatile frequency comb spectroscopy system based on a doubly resonant optical parametric oscillator tunable between 3–5.4 μm and two detection methods: a Fourier transform spectrometer and a continuous-filtering Vernier spectrometer.

Using Fourier transform-based cavity-enhanced optical frequency comb spectroscopy around 1.57 µm we measure high precision low pressure spectra of the 3ν1+ ν3 band of CO2 and high temperature H2O and OH spectra in a premixed methane/air flat flame.

Using a doubly-resonant femtosecond optical parametric oscillator in combination with a multipass cell and a Fourier transform spectrometer we measure broadband CH 4 and NO absorption spectra at fundamental transition wavelengths.

We present the recent developments in high-resolution Fourier transform spectroscopy based on optical frequency combs for precision measurements and combustion diagnostics, and the first implementation of continuous-filtering Vernier spectroscopy in the mid-infrared wavelength range for fast multispecies detection.

We measure broadband H 2 O and OH spectra in a flame using near-infrared cavity-enhanced Fourier transform optical frequency comb spectroscopy, we retrieve temperature and OH concentration, and compare water spectra to an improved line list.

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.