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Cavity ringdown spectroscopy (CRDS), relying on measuring the decay time of photons inside a high-finesse optical cavity, offers an important analytical tool for chemistry, physics, environmental science, and biology. Through the reflection of a slight amount of phase-coherent light back to the laser source, the resonant optical feedback approach effectively couples the laser beam into the optical cavity and achieves a high signal-to-noise ratio. However, the need for active phase-locking mechanisms complicates the spectroscopic system, limiting its primarily laboratory-based use. Here, we report how passive optical feedback can be implemented in a quantum cascade laser (QCL) based CRDS system to address this issue. Without using any phase-locking loops, we reflect a moderate amount of light (–18.2 dB) to a continuous-wave QCL simply using a fixed flat mirror, narrowing the QCL linewidth from 1.2 MHz to 170 kHz and significantly increasing the laser-cavity coupling efficiency. To validate the method’s feasibility and effectiveness, we measured the absorption line (P(18e), 2207.62 cm−1) of N2O in a Fabry–Perot cavity with a high finesse of ~52000 and an inter-mirror distance of 33 cm. This agile approach paves the way for revolutionizing existing analytical tools by offering compact and high-fidelity mid-infrared CRDS systems.

Rapid and precise detection of hydrogen sulfide (H2S) at trace levels is critical for industrial safety and environmental air quality monitoring, yet existing methods often struggle with cost, speed, or sensitivity. A cost-effective cavity ring-down spectroscopy (CRDS) analyzer is presented, incorporating a novel digital locking circuit for sequential laser-cavity mode matching. This system demonstrates rapid and precise hydrogen sulfide (H2S) detection capability at parts-per-billion (ppb) concentration levels. Compared to traditional wavelength meters, our system delivers a 140-fold improvement in frequency interval precision (0.07 MHz, 0.027% relative uncertainty). Allan variance analysis under vacuum conditions demonstrates a sensitivity limit of 3 × 10−12 cm−1 at a 60-s averaging time. Validated through calibrated gas dilution tests, the analyzer detects a 4 ppb H2S absorption signal with a signal-to-noise ratio (SNR) > 6, establishing a 2 ppb detection limit (3σ criterion). This innovative approach meets stringent industrial and environmental requirements, offering a significant advancement in trace gas-sensing technology.

Low-loss photonic integrated circuits and microresonators have enabled a wide range of applications, such as narrow-linewidth lasers and chip-scale frequency combs. To translate these into a widespread technology, attaining ultralow optical losses with established foundry manufacturing is critical. Recent advances in integrated Si 3 N 4 photonics have shown that ultralow-loss, dispersion-engineered microresonators with quality factors Q  > 10 × 10 6 can be attained at die-level throughput. Yet, current fabrication techniques do not have sufficiently high yield and performance for existing and emerging applications, such as integrated travelling-wave parametric amplifiers that require meter-long photonic circuits. Here we demonstrate a fabrication technology that meets all requirements on wafer-level yield, performance and length scale. Photonic microresonators with a mean Q factor exceeding 30 × 10 6 , corresponding to 1.0 dB m −1 optical loss, are obtained over full 4-inch wafers, as determined from a statistical analysis of tens of thousands of optical resonances, and confirmed via cavity ringdown with 19 ns photon storage time. The process operates over large areas with high yield, enabling 1-meter-long spiral waveguides with 2.4 dB m −1 loss in dies of only 5 × 5 mm 2 size. Using a response measurement self-calibrated via the Kerr nonlinearity, we reveal that the intrinsic absorption-limited Q factor of our Si 3 N 4 microresonators can exceed 2 × 10 8 . This absorption loss is sufficiently low such that the Kerr nonlinearity dominates the microresonator’s response even in the audio frequency band. Transferring this Si 3 N 4 technology to commercial foundries can significantly improve the performance and capabilities of integrated photonics. For widespread technological application of nonlinear photonic integrated circuits, ultralow optical losses and high fabrication throughput are required. Here, the authors present a CMOS fabrication technique that realizes integrate photonic microresonators on waver-level with mean quality factors exceeding 30 million and 1 dB/m optical losses.

We report on the development of a comb-assisted cavity ring-down spectrometer for trace water mole fraction determinations in high purity gases. By tuning the laser light in coincidence with a H 2 O absorption line at 1.3946 µ m, we were able to determine sub-ppm relative concentrations of water vapour in N 2 with a sub-percent statistical uncertainty. The sensitivity of the spectrometer was carefully assessed, yielding a detection limit of about 2 nmol/mol.

For the exploration of gas hydrate resources by measuring the dissolved methane concentration inseawater, a continuous-wave cavity ringdown spectroscopy （CW-CRDS） experimental setup was con-structed for trace methane detection. A current-modulation method, rather than a cavity-modulationmethod using an optical switch and a piezoelectric transducer, was employed to realize the cavityexcitation and shutoff. Such a current-modulation method enabled the improvement of the experi-mental setup construction and stability, and the system size and stability are critical for a sensor tobe deployed underwater. Ringdown data acquisition and processing were performed, followed by anevaluation of the experimental setup stability and sensitivity. The obtained results demonstrate thatgreat errors are introduced when a large fitting window is selected if the analog-to-digital converterhas an insufficient resolution. The ringdown spectrum of methane corresponding to the 2v3 bandR（4） branch was captured, and the methane concentration in lab air was determined to be 2.06 ppm.Further experiments for evaluating the quantitative ability of this CW-CRDS experimental setup areunderway from which a high-sensitivity methane sensor that can be combined with a degassing systemis expected.

Ammonia is a critical indicator in the control of hydrogen impurities for proton exchange membrane fuel cells (PEMFCs). Both international and national standards strictly limit the ammonia content in hydrogen to within 0.1 μmol/mol. This paper summarizes the analytical methods for trace ammonia in hydrogen and conducts experimental comparative analyses of Gas Chromatography with Helium Ionization Detection (GC-HID) and Cavity Ring-Down Spectroscopy (CRDS). The results show that the CRDS method has a detection limit as low as 0.3 ppb, making it an accurate and feasible scheme for detecting trace ammonia in hydrogen used for fuel cell vehicles.

Detection and analysis of volatile compounds in exhaled breath represents an attractive tool for monitoring the metabolic status of a patient and disease diagnosis, since it is non-invasive and fast. Numerous studies have already demonstrated the benefit of breath analysis in clinical settings/applications and encouraged multidisciplinary research to reveal new insights regarding the origins, pathways, and pathophysiological roles of breath components. Many breath analysis methods are currently available to help explore these directions, ranging from mass spectrometry to laser-based spectroscopy and sensor arrays. This review presents an update of the current status of optical methods, using near and mid-infrared sources, for clinical breath gas analysis over the last decade and describes recent technological developments and their applications. The review includes: tunable diode laser absorption spectroscopy, cavity ring-down spectroscopy, integrated cavity output spectroscopy, cavity-enhanced absorption spectroscopy, photoacoustic spectroscopy, quartz-enhanced photoacoustic spectroscopy, and optical frequency comb spectroscopy. A SWOT analysis (strengths, weaknesses, opportunities, and threats) is presented that describes the laser-based techniques within the clinical framework of breath research and their appealing features for clinical use.

We present an original opto-mechanical scheme which, effectively coupling a Lamb-dip saturated-absorption cavity ring-down spectrometer to a buffer-gas-cooling (BGC) source, allows us to determine the absolute frequency of the acetylene ( ν 1 + ν 3 ) R(1)e transition at 6561.0941 cm −1 with an overall (statistical + systematic) uncertainty as low as 1.2 kHz. By improving the previous record with buffer-gas-cooled molecules by one order of magnitude, our achievement opens the door to new kind of ultra-precise low-temperature spectroscopic studies.

For trace gas sensing and precision spectroscopy, optical cavities incorporating low-loss mirrors are indispensable for path length and optical intensity enhancement. Optical interference coatings in the visible and near-infrared (NIR) spectral regions have achieved total optical losses below 2 parts per million (ppm), enabling a cavity finesse in excess of 1 million. However, such advancements have been lacking in the mid-infrared (MIR), despite substantial scientific interest. Here, we demonstrate a significant breakthrough in high-performance MIR mirrors, reporting substrate-transferred single-crystal interference coatings capable of cavity finesse values from 200 000 to 400 000 near 4.5 µm, with excess optical losses (scatter and absorption) below 5 ppm. In a first proof-of-concept demonstration, we achieve the lowest noise-equivalent absorption in a linear cavity ring-down spectrometer normalized by cavity length. This substantial improvement in performance will unlock a rich variety of MIR applications for atmospheric transport and environmental sciences, detection of fugitive emissions, process gas monitoring, breath-gas analysis, and verification of biogenic fuels and plastics. The researchers showcase all-crystalline and hybrid mid-infrared supermirrors with the lowest optical losses ever demonstrated in this wavelength range, representing an unprecedented improvement over any existing mirrors made with any production technology.

Formaldehyde, HCHO, is the highest-volume carbonyl in the atmosphere. It absorbs sunlight at wavelengths shorter than 330 nm and photolyses to form H and HCO radicals, which then react with O2 to form HO2. Here we show HCHO has an additional HO2 formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO2 at low pressures by cavity ring-down spectroscopy and indirectly detect HO2 at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO2 to photophysical oxidation (PPO): photoexcited HCHO relaxes non-radiatively to the ground electronic state where the far-from-equilibrium, vibrationally activated HCHO molecules react with thermal O2. PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O2 pressure.In the atmosphere, photolysis of formaldehyde generates H and HCO radicals, which then react with O2 to form HO2 (important in converting atmospheric carbon to CO2). Now it has been shown that internally excited formaldehyde can also react with atmospheric O2 to make HO2 in a direct, one-step ‘photophysical oxidation’, a mechanism likely to be general in the troposphere.