arXiv subjects

Lukas Emmenegger

Publications and source records attributed to Lukas Emmenegger.

4 recordsLinked to original sources

arXiv2022

High-Resolution Quantum Cascade Laser Dual-Comb Spectroscopy in the Mid-Infrared with Absolute Frequency Referencing

Quantum cascade laser (QCL) frequency combs have revolutionized mid-infrared (MIR) spectroscopy by their high brightness and fast temporal resolution, and are a promising technology for fully-integrated and cost-effective sensors. As for other integrated comb sources such as micro-combs and interband cascade laser, QCLs have a comb spacing of several GHz, which is adequate for measurements of wide absorbing structures, typically found in liquid or solid samples. However, high-resolution gas-phase spectra require spectral interleaving and frequency calibration. We developed a frequency calibration scheme for fast interleaved measurements with combs featuring multi-GHz spacing. We then demonstrate dual-comb spectroscopy with 600 kHz accuracy in single-shot 54-ms measurements over 40 cm$^{-1}$ using two QCLs at 7.8 $\mathrmμ$m. This work is an important contribution towards fast fingerprinting of complex molecular mixtures in the MIR. Moreover, the calibration scheme could be used with micro-combs for spectroscopy and ranging, both in comb-swept and comb-calibrated setups.

physics.optics↗

arXiv2022

Synchronization of frequency combs by optical injection

Optical frequency combs based on semiconductor lasers are a promising technology for monolithic integration of dual-comb spectrometers. However, the stabilization of the offset frequency fceo remains a challenging feat due the lack of octave-spanning spectra. In a dual-comb configuration, the uncorrelated jitter of the offset frequencies leads to a non-periodic signal resulting in broadened beatnotes with a limited signal-to-noise ratio (SNR). Hence, expensive data acquisition schemes and complex signal processing are currently required. Here, we show that the offset frequencies of two frequency combs can be synchronized by optical injection locking, which allows full phase-stabilization when combined with electrical injection. A single comb line isolated via an optical Vernier filter serves as Master oscillator for injection locking. The resulting dual-comb signal is periodic and stable over thousands of periods. This enables coherent averaging using analog electronics, which increases the SNR and reduces the data size by one and three orders of magnitude, respectively. The presented method will enable fully phase-stabilized dual-comb spectrometers by leveraging on integrated optical filters and provides access for measuring and stabilizing fceo.

physics.optics↗

arXiv2021

Automated fragment identification for electron ionisation mass spectrometry: application to atmospheric measurements of halocarbons

Non-target screening consists in searching a sample for all present substances, suspected or unknown, with very little prior knowledge about the sample. This approach has been introduced more than a decade ago in the field of water analysis, together with dedicated compound identification tools, but is still very scarce for indoor and atmospheric trace gas measurements, despite the clear need for a better understanding of the atmospheric trace gas composition.For a systematic detection of emerging trace gases in the atmosphere, a new and powerful analytical method is gas chromatography (GC) of preconcentrated samples, followed by electron ionisation, high resolution mass spectrometry (EI-HRMS). In this work, we present data analysis tools to enable automated fragment formula annotation for unknown compounds measured by GC-EI-HRMS. Based on co-eluting mass/charge fragments, we developed an innovative data analysis method to reliably reconstruct the chemical formulae of the fragments, using efficient combinatorics and graph theory. The method does not require the presence of the molecular ion, which is absent in ~40% of EI spectra. Our method has been trained and validated on \textgreater50 halocarbons and hydrocarbons, with 3 to 20 atoms and molar masses of 30 to 330 g mol-1, measured with a mass resolution of approx.~3500. For 90% of the compounds, more than 90% of the annotated fragment formulae are correct. Cases of wrong identification can be attributed to the scarcity of detected fragments per compound or the lack of isotopic constraint (no minor isotopocule detected).Our method enables to reconstruct most probable chemical formulae independently from spectral databases. Therefore, it demonstrates the suitability of EI-HRMS data for non-target analysis and paves the way for the identification of substances for which no EI mass spectrum is registered in databases. We illustrate the performances of our method for atmospheric trace gases and suggest that it may be well suited for many other types of samples. The L-GPL licenced Python code is released under the name ALPINAC for ALgorithmic Process for Identification of Non-targeted Atmospheric Compounds.

physics.comp-ph↗

arXiv2019

High-resolution and gapless dual comb spectroscopy with current-tuned quantum cascade lasers

We present gapless, high-resolution absorption and dispersion spectra obtained with quantum cascade laser frequency combs covering 55 cm$^{-1}$. Using phase-sensitive dual comb design, the comb lines are gradually swept over 10 GHz, corresponding to the free spectral range of the laser devices, by applying a current modulation. We show that with interleaving the spectral point spacing is reduced by more than four orders of magnitude over the full spectral span of the frequency comb. The potential of this technique for high-precision gas sensing is illustrated by measuring the low pressure (107 hPa) absorption and dispersion spectra of methane spanning the range of 1170 cm$^{-1}$ - 1225 cm$^{-1}$ with a resolution of 0.001 cm$^{-1}$.

physics.app-ph↗