Absorption Path Length Research Articles

Simultaneous Detection of Carbon Dioxide and Its Isotope (δ13C) Based on Near Infrared Off‐Axis Integrated Cavity Output Spectroscopy

ABSTRACTAccurate monitoring of atmospheric δ13C isotope is critical for quantifying anthropogenic carbon emissions and ecosystem responses within global carbon cycle research. This study used off‐axis integrated cavity output spectroscopy (OA‐ICOS) combined with distributed feedback (DFB) laser and achieved an effective absorption path length of approximately 22 km, for the simultaneous detection of carbon dioxide and its isotope (δ13C) under atmospheric background conditions. Laboratory validation demonstrated detection limits of 104.5 ppb, 3.3 ppb, and 0.72‰ for 12CO2, 13CO2, and δ13C, respectively, with a 2000 s integration time, alongside linear calibration R2 > 0.999. While Kalman filtering reduced concentration variability by 73.5%–74.7% and improved δ13C precision 4.6 times. The sensor's capability to quantify isotopic ratios and molecular concentrations in real‐time addresses critical gaps in atmospheric background monitoring, offering a robust tool for climate research and carbon source–sink analysis, expanding the applicability of laser spectroscopy in environmental sensing.

Robust optical measurement cell for sensitive in-situ characterization of reactive gas flows

We present an optical measurement cell that is capable of measuring in-situ gas-phase mole fractions, temperature, and pressure at high measurement frequencies of up to 10 kHz. The cell is designed for operating under harsh conditions, including high temperatures, reactive flows, soot pollution, and ensures reliable performance in challenging environments. Based on the principle of a White multi-pass cell, it features an inner diameter of just 70 mm and achieves absorption path lengths of up to roughly 5 m. As presented here, this enables in-situ detection of nitrogen monoxide at concentrations as low as single-digit ppm levels under engine exhaust flow conditions. The unique design accommodates both free-beam and fiber-coupled lasers, providing exceptional flexibility. Furthermore, its variable absorption path length enhances robustness, selectivity, and sensitivity, making it highly adaptable to demanding measurement scenarios.

Development of a portable laser-flash photolysis Faraday rotation spectrometer for measuring atmospheric total OH reactivity

Abstract. Quantitative measurements of atmospheric total OH reactivity (kOH′) provide crucial insights into atmospheric photochemistry. However, widespread application of total OH reactivity measurements is challenging due to insufficient equipment and the complexity of existing instrumentation. In this work, we report the development of a portable laser-flash photolysis Faraday rotation spectroscopy (LP-FRS) instrument for real-time and in situ measurement of kOH′. To achieve efficient overlapping between the pump and probe laser and realize a long effective absorption path length, thus enabling high-sensitivity measurement, a specific Herriott-type pump–probe optical multi-pass cell was designed. The instrument's optical box dimensions were 130 cm × 40 cm × 35 cm. The obtained effective absorption path was ∼ 28.5 m in a base length of 77.2 cm. The kOH′ detection precisions of the LP-FRS instrument were 2.3 and 1.0 s−1 with averaging times of 60 and 300 s, respectively. The kOH′ measurement uncertainty was evaluated to be within 2 s−1. Field measurement was performed, and the difference between the measured kOH′ and the model simulated from the measured reactive species was analysed. The developed portable LP-FRS instrument extends the measurement methods of atmospheric total OH reactivity and has certain advantages in terms of cost, operation, and transportation, which will play an increasingly important role in future atmospheric chemistry research.

Multiple optical path length reflections enhancement based on balloon-type photoacoustic cell for trace gas sensing.

Multiple optical path length reflections enhancement based on balloon-type photoacoustic cell for trace gas sensing.

Carbon Dioxide Sensing Based on Off-Axis Integrated Cavity Absorption Spectroscopy Combined with the Informer and Multilayer Perceptron Models.

Off-axis integrated cavity output spectroscopy (OA-ICOS) allows the laser to be reflected multiple times inside the cavity, increasing the effective absorption path length and thus improving sensitivity. However, OA-ICOS systems are affected by various types of noise, and traditional filtering methods offer low processing efficiency and perform limited feature extraction. Deep learning models enable us to extract important features from large-scale, complex spectral data and analyze them efficiently and accurately. We propose a carbon dioxide (CO2) sensor operating in the near-infrared spectral region (1.602 μm) based on OA-ICOS and deep learning models. A radiofrequency (RF) noise source is employed to reduce the cavity-mode noise in OA-ICOS and thus improve the signal-to-noise ratio (SNR). A time-series-based neural network, known as the informer, is employed for filtering CO2 spectral time series. After filtering, spectral features are directly extracted from the filtered spectral data and CO2 concentrations are predicted using a multilayer perceptron (MLP) model. Our results showed that the SNR attained using informer filtering approximately double those obtained using traditional filtering methods (Savitzky-Golay filtering, Kalman filtering, and wavelet threshold). The linear correlation coefficient (R2) between measured concentrations and standard concentrations was increased from 79.74% (obtained by using the absorption-peak-fitting method) to 98.52% (obtained by using the proposed MLP model). Moreover, the detection limit of the CO2 sensor using the MLP model reached 1.38 ppm at 224.4 s, a 3.79-fold improvement compared to that obtained by using the absorption-peak-fitting method. Our results demonstrate the feasibility of integrating deep learning methods in the field of spectroscopy-based sensing and provide a promising approach for spectral data processing.

Sensitive hydrogen detection using off-axis integrated cavity output spectroscopy at 2.1 μm

Reliable hydrogen (H2) detection is essential for advancing hydrogen energy applications. In this study, we report a sensitive H2 detection method using off-axis integrated cavity output spectroscopy. A distributed-feedback laser with fiber output was used to target the H2 absorption transition near 2122 nm. The optical cavity was constructed using two spherical mirrors, each with a reflectivity of 99.985%, providing an effective absorption path length of 2500 m. The Galatry line shape function was selected to accurately fit the narrow absorption feature of H2. Furthermore, we achieved a 1.74-fold improvement in the signal-to-noise ratio by applying radio frequency white noise perturbations. The system's performance was demonstrated by measuring H2 mixtures across a broad concentration range from 1040 ppm to 20.24%. Finally, the Allan–Werle deviation analysis revealed a detection limit of 53 ppm H2 at an average time of 400 s.

Selective response surface methodology for Anti Reflective Coated Nano-film filter thickness optimization in hybrid PVT system

A solar hybrid photovoltaic thermal (PVT) system is a set of combined solar collectors that include a photovoltaic module (PV) and a solar panel in the same frame. When the absorption of solar radiation on the PV cell's surface is low, the system's efficiency decreases. Hence a novel Selective Response Surface Methodology for Hybrid Anti Reflective Coated Nanofilm Filter Thickness Optimization is proposed for increasing the electrical and thermal efficiency of a Hybrid PVT System. Several nano-film filters are used on the surface of the existing hybrid PVT system to capture solar energy, but they degrade over time, and temperature changes reduce the spectrum radiation absorption capabilities and path length of light rays inside the filter. To overcome these issues a novel Hybrid Anti Reflective Coated Nanofilm Filter is proposed in which a Polycarbonate nano-film filter is utilized since its refractive index is unaffected by temperature changes. This nano-film filter is coated with Zirconium oxide (ZrO2), which improves the path length of light rays inside the nano-film filter, and a Cerium Oxide (CeO2) nano-coating is utilized over this coating to lower the quantity of light reflected off the surface of the solar panel. Furthermore, existing approaches for calculating nano-film filter thickness do not consider multi-collinearity , resulting in inaccurate forecasts and disappointing outcomes. To resolve these concerns, the Selective Neuro RSM-Taguchi Optimization method is developed, which employs an MFFNN (Multilayer Feedforward Neural Network) to anticipate the Electrical and Thermal Efficiency of a Hybrid PVT system based on input parameters. Then the Response Surface model is used to choose the parameters for optimization, and Taguchi Optimization is used to selectively identify the Nano-film filter thickness while taking multi-collinearity into account. The ideal thickness obtained by these optimization methods enhances the efficiency and performance of the PVT system.

FDM-assisted OA-CEAS system for simultaneous measurements of temperature, CO2, and CO in flames

FDM-assisted OA-CEAS system for simultaneous measurements of temperature, CO2, and CO in flames

Simultaneous detection of soot, temperature, and C2H2 in laminar premixed flames using a multi-pass diode laser absorption spectrometer.

We report the development of a multi-pass diode laser absorption spectroscopy system for simultaneous measurements of soot volume fraction (SVF), temperature, and C2H2 concentration using a single diode laser near 1.543 µm. A line-shaped beam spot pattern is chosen for the open-path Herriott multi-pass cavity, enabling sensitive detection at various heights above the burner with an effective optical absorption path length of approximately 1.2 m in a 6 cm diameter flame region. The gas parameters (temperature and C2H2 concentration) and the SVF are determined from the absorption spectra of the target C2H2 line pair and the laser extinction of the soot, which can be extracted from the detected signal, respectively. The performance of the system was confirmed in laminar premixed ethylene and air (C2H4/air) sooting flames produced by a standard bronze plug McKenna burner at four representative equivalence ratios. All the measurement results were compared with the two-dimensional (2D) computational fluid dynamics (CFD) simulations using a skeletal mechanism with the Moss-Brookes model. The good quantitative and qualitative agreement between the TDLAS measurements and 2D CFD simulations confirms the powerful capability of the developed system.

Qualitative and Quantitative Analyses of Automotive Exhaust Plumes for Remote Emission Sensing Application Using Gas Schlieren Imaging Sensor System

Remote emission sensing (RES) is a state-of-the-art technique for monitoring thousands of vehicles on the road every day to detect high emitters. Modern commercial RES systems use absorption spectroscopy to measure the ratio of pollutants to CO2 from vehicle exhaust gases. In this work, we present an approach to enable direct concentration measurements by spectroscopic techniques in RES through measurement of the absorption path length. Our gas schlieren imaging sensor (GSIS) system operates on the principle of background-oriented schlieren (BOS) imaging in combination with advanced image processing and deep learning techniques to calculate detected exhaust plume sizes. We performed a qualitative as well as a quantitative analysis of vehicle exhaust and plume dimensions with the GSIS system. We present the system details and results from the GSIS system in the lab in comparison to a BOS model based on flow simulations, the results from characterization measurements in the lab with defined gas mixtures and temperatures, and the results from measurements on the road from different vehicles.

Optimized waveguides for mid-infrared lab-on-chip systems: A rigorous design approach

Optimized waveguides for mid-infrared lab-on-chip systems: A rigorous design approach

Correction of self-absorption effect in laser-induced breakdown spectroscopy based on temperature iteration

Correction of self-absorption effect in laser-induced breakdown spectroscopy based on temperature iteration

Near-infrared dual-gas sensor for simultaneous detection of CO and CH4 using a double spot-ring plane-concave multipass cell and a digital laser frequency stabilization system

A novel double spot-ring plane-concave multipass cell (DSPC-MPC) gas sensor was proposed for simultaneous detection of trace gases, which has lower cost and higher mirror utilization than the traditional multipass cell with 129 m, 107 m, 85 m, 63 m and 40 m effective optical path lengths adjustable. The performance of the DSPC-MPC gas sensor was evaluated by measuring CO and CH4 using two narrow linewidth distributed feedback lasers with center wavelengths of 1567 nm and 1653 nm, respectively. An adjustable digital PID laser frequency stabilization system based on LabVIEW platform was developed to continuously stabilize the laser frequency within ∼±30.3 MHz. The Allan deviation results showed that the minimum detection limits for CO and CH4 were 0.07 ppmv and 0.008 ppmv at integration times of 711 s and 245 s, respectively. The proposed concept of DSPC-MPC provides more ideas for the realization of gas detection under different absorption path lengths and the development of multi-component gas sensing systems.

E-XQR-30: The evolution of Mg ii, C ii, and O i across 2 < z < 6

ABSTRACT Intervening metal absorbers in quasar spectra at z > 6 can be used as probes to study the chemical enrichment of the Universe during the Epoch of Reionization. This work presents the comoving line densities (dn/dX) of low-ionization absorbers, namely, Mg ii (2796 Å), C ii (1334 Å), and O i (1302 Å) across 2 < z < 6 using the E-XQR-30 metal absorber catalogue prepared from 42 XSHOOTER quasar spectra at 5.8 < z < 6.6. Here, we analyse 280 Mg ii (1.9 < z < 6.4), 22 C ii (5.2 < z < 6.4), and 10 O i (5.3 < z < 6.4) intervening absorbers, thereby building up on previous studies with improved sensitivity of 50 per cent completeness at an equivalent width of W > 0.03 Å. For the first time, we present the comoving line densities of 131 weak (W < 0.3 Å) intervening Mg ii absorbers at 1.9 < z < 6.4 which exhibit constant evolution with redshift similar to medium (0.3 < W < 1.0 Å) absorbers. However, the cosmic mass density of Mg ii – dominated by strong Mg ii systems – traces the evolution of global star formation history from redshift 1.9 to 5.5. E-XQR-30 also increases the absorption path-length by a factor of 50 per cent for C ii and O i whose line densities show a rising trend towards z > 5, in agreement with previous works. In the context of a decline in the metal enrichment of the Universe at z > 5, the overall evolution in the incidence rates of absorption systems can be explained by a weak – possibly soft fluctuating – ultraviolet background. Our results, thereby, provide evidence for a late reionization continuing to occur in metal-enriched and therefore, biased regions in the Universe.

High-resolution ro-vibrational spectrum of H[formula omitted]S in highly excited vibrational states: Re-visiting the first decade

High-resolution ro-vibrational spectrum of H[formula omitted]S in highly excited vibrational states: Re-visiting the first decade

Water vapor condensation in optical instruments on Mars

Water vapor condensation in optical instruments on Mars

Fabrication of Low-Cost Miniaturized Gas Cells via SLA 3D-Printing for UV-Based Gas Sensors.

The use of 3D-printing technology for producing optical devices (i.e., mirrors and waveguides) remains challenging, especially in the UV spectral regime. Gas sensors based on absorbance measurements in the UV region are suitable for determining numerous volatile species in a variety of samples and analytical scenarios. The performance of absorbance-based gas sensors is dependent on the ability of the gas cell to propagate radiation across the absorption path length and facilitate interaction between photons and analytes. In this technical note, we present a 3D-printed substrate-integrated hollow waveguide (iHWG) to be used as a miniaturized and ultralightweight gas cell used in UV gas-sensing schemes. The substrates were fabricated via UV stereolithography and polished, and the light-guiding channel was coated with aluminum for UV reflectivity. This procedure resulted in a surface roughness of 11.2 nm for the reflective coating, yielding a radiation attenuation of 2.25 W/cm2. The 3D-printed iHWG was coupled to a UV light source and a portable USB-connected spectrometer. The sensing device was applied for the quantification of isoprene and acetone, serving as a proof-of-concept study. Detection limits of 0.22 and 0.03% in air were obtained for acetone and isoprene, respectively, with a nearly instantaneous sensor response. The development of portable, low-cost, and ultralightweight UV optical sensors enables their use in a wide range of scenarios ranging from environmental monitoring to clinical/medical applications.

Coherent field sensing of nitrogen dioxide.

We introduce a portable dual-comb spectrometer operating in the visible spectral region for atmospheric monitoring of NO2, a pollution gas of major importance. Dual-comb spectroscopy, combining key advantages of fast, broadband and accurate measurements, has been established in the infrared as a method for the investigation of atmospheric gases with kilometer-scale absorption path lengths. With the presented dual-comb spectrometer centered at 517 nm, we make use of the strong absorption cross section of NO2 in this spectral region. In combination with a multi-pass approach through the atmosphere, we achieve an interaction path length of almost a kilometer while achieving both advanced spatial resolution (90 m) and a detection sensitivity of 5 ppb. The demonstrated temporal resolution of one minute outperforms the standard chemiluminescence-based NO2 detector that is commercially available and used in this experiment, by a factor of three.

Towards a Miniaturized Photoacoustic Sensor for Transcutaneous CO2 Monitoring.

A photoacoustic sensor system (PAS) intended for carbon dioxide (CO2) blood gas detection is presented. The development focuses on a photoacoustic (PA) sensor based on the so-called two-chamber principle, i.e., comprising a measuring cell and a detection chamber. The aim is the reliable continuous monitoring of transcutaneous CO2 values, which is very important, for example, in intensive care unit patient monitoring. An infrared light-emitting diode (LED) with an emission peak wavelength at 4.3 µm was used as a light source. A micro-electro-mechanical system (MEMS) microphone and the target gas CO2 are inside a hermetically sealed detection chamber for selective target gas detection. Based on conducted simulations and measurement results in a laboratory setup, a miniaturized PA CO2 sensor with an absorption path length of 2.0 mm and a diameter of 3.0 mm was developed for the investigation of cross-sensitivities, detection limit, and signal stability and was compared to a commercial infrared CO2 sensor with a similar measurement range. The achieved detection limit of the presented PA CO2 sensor during laboratory tests is 1 vol. % CO2. Compared to the commercial sensor, our PA sensor showed less influences of humidity and oxygen on the detected signal and a faster response and recovery time. Finally, the developed sensor system was fixed to the skin of a test person, and an arterialization time of 181 min could be determined.

Quantum cascade laser absorption sensor for in-situ, real-time and sensitive measurement of high-temperature SO2 and SO3

Quantum cascade laser absorption sensor for in-situ, real-time and sensitive measurement of high-temperature SO2 and SO3