Iacopo Galli

A widespread use of biofuels is a key recommendation of the Paris Agreement and leading international organizations. It is an important step to mitigate the global warming effects due to greenhouse‐gas emissions from fossil oils. To this aim, an analytical technique sufficiently cheap and compact, to foster its widespread adoption, is necessary. Herein, it is shown that a compact, laser‐based spectrometer is suitable to replace the few established techniques, which have been used to quantify the biofraction in fuel blends, so far. Measurements of the biogenic fraction in different fuel samples are reported, with a precision of 1% in the whole range (0–100%) of possible blends, confirming a performance comparable to the best existing technique. An onsite‐deployable saturated‐absorption cavity ring‐down (SCAR) spectrometer is used. The results demonstrate the potential of laser‐based instrumentation to do the accurate and precise measurements required for the certification of biogenic content of any hydrocarbon‐based material. Worldwide adoption of such laser‐based technology for biofraction certification can significantly boost the market of biofuels and can prove to be a disruptive methodology for assessing the biogenic content in plastics and polymeric materials.

The need for highly performing and stable methods for mid-IR molecular sensing and metrology pushes towards the development of more and more compact and robust systems. Among the innovative solutions aimed at answering the need for stable mid-IR references are crystalline microresonators, which have recently shown excellent capabilities for frequency stabilization and linewidth narrowing of quantum cascade lasers with compact setups. In this work, we report on the first system for mid-IR high-resolution spectroscopy based on a quantum cascade laser locked to a CaF₂ microresonator. Electronic locking narrows the laser linewidth by one order of magnitude and guarantees good stability over long timescales, allowing, at the same time, an easy way for finely tuning the laser frequency over the molecular absorption line. Improvements in terms of resolution and frequency stability of the source are demonstrated by direct sub-Doppler recording of a molecular line.

A prototype analyzer for the detection of hydrogen sulfide (H₂S), based on cavity ring-down spectroscopy, is described. The device exploits, whenever possible, optical fibers, in order to simplify the alignment and to improve the stability. A trade-off between low detection level and simplicity has been pursued. The experimental results obtained during tests on different kinds of H₂S samples are shown.

An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) combined with a high-finesse cavity sensor platform is proposed as a novel method for trace gas sensing. We call this technique Intra-cavity QEPAS (I-QEPAS). In the proposed scheme, a single-mode continuous wave quantum cascade laser (QCL) is coupled into a bow-tie optical cavity. The cavity is locked to the QCL emission frequency by means of a feedback-locking loop that acts directly on a piezoelectric actuator mounted behind one of the cavity mirrors. A power enhancement factor of ∼240 was achieved, corresponding to an intracavity power of ∼0.72 W. CO₂ was selected as the target gas to validate our sensor. For the P(42) CO₂ absorption line, located at 2311.105 cm⁻¹, a minimum detection limit of 300 parts per trillion by volume at a total gas pressure of 50 mbar was achieved with a 20 s integration time. This corresponds to a normalized noise equivalent absorption of 3.2 × 10⁻¹⁰ W cm⁻¹ Hz^−1/2, comparable with the best results reported for the QEPAS technique on much faster relaxing gases. A comparison with standard QEPAS performed under the same experimental conditions confirms that the I-QEPAS sensitivity scales with the intracavity laser power enhancement factor.

We report on the experimental demonstration of the metrological and spectroscopic performances of a mid-infrared comb generated by a nonlinear downconversion process from a Ti:sapphire-based near-infrared comb. A quantum cascade laser at 4330 nm was phase-locked to a single tooth of this mid-infrared comb and its frequency-noise power spectral density was measured. The mid-infrared comb itself was also used as a multifrequency highly coherent source to perform ambient air direct comb spectroscopy with the Vernier technique, by demultiplexing it with a high-finesse Fabry–Perot cavity.

We report on a spectroscopic technique named intracavity quartz-enhanced photoacoustic spectroscopy (I-QEPAS) employed for sensitive trace-gas detection in the mid-infrared spectral region. It is based on a combination of QEPAS with a buildup optical cavity. The sensor includes a distributed feedback quantum cascade laser emitting at 4.33 μm. We achieved a laser optical power buildup factor of ∼500, which corresponds to an intracavity laser power of ∼0.75 W. CO2 has been selected as the target molecule for the I-QEPAS demonstration. We achieved a detection sensitivity of 300 parts per trillion for 4 s integration time, corresponding to a noise equivalent absorption coefficient of 1.4 × 10−8 cm−1 and a normalized noise-equivalent absorption of 3.2 × 10−10 W cm−1 Hz−1/2.

Unprecedented combination of quantum cascade lasers, nonlinear sources and techniques, enhancement cavities and frequency standards is paving the way to new methodologies and deployable instrumentation for metrological-grade environmental monitoring.

We report on the generation of a highly-coherent frequency comb around 4330 nm. A quantum cascade laser has been effectively phase-locked to a comb tooth and its frequency noise has been measured.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) and cavity-ehnanced spectroscopy are merged in a novel gas sensor. Thanks to the intracavity power build up, sensitivity is increased by more than a factor 100 with respect to standard QEPAS.