Real-time, subsecond, multicomponent breath analysis by Optical Parametric Oscillator based Off-Axis Integrated Cavity Output Spectroscopy

Simultaneous Sensitive Determination of δ13C, δ18O, and δ17O in Human Breath CO2 Based on ICL Direct Absorption Spectroscopy

Previous research revealed that isotopes ¹³C and ¹⁸O of exhaled CO₂ have the potential link with Helicobacter pylori; however, the ¹⁷O isotope has received very little attention. We developed a sensitive spectroscopic sensor for simultaneous δ¹³C, δ¹⁸O, and δ¹⁷O analysis of human breath CO₂ based on mid-infrared laser direct absorption spectroscopy with an interband cascade laser (ICL) at 4.33 μm. There was a gas cell with a small volume of less than 5 mL, and the pressure in the gas cell was precisely controlled with a standard deviation of 0.0035 Torr. Moreover, real-time breath sampling and batch operation were achieved in gas inlets. The theoretical drifts for δ¹³C, δ¹⁸O, and δ¹⁷O measurement caused by temperature were minimized to 0.017‰, 0.024‰, and 0.021‰, respectively, thanks to the precise temperature control with a standard deviation of 0.0013 °C. After absolute temperature correction, the error between the system responded δ-value and the reference is less than 0.3‰. According to Allan variance analysis, the system precisions for δ¹³C, δ¹⁸O, and δ¹⁷O were 0.12‰, 0.18‰, and 0.47‰, respectively, at 1 s integration time, which were close to the real-time measurement errors of six repeated exhalations.

Exhaled breath analysis using cavity-enhanced optical techniques: a review

Cavity-enhanced absorption spectroscopies (CEAS) have gained importance in a wide range of applications in molecular spectroscopy. The development of optical sensors based on the CEAS techniques coupled with the continuous wave or pulsed laser sources operating in the mid-infrared or near-infrared spectral regime uniquely offers molecularly selective and ultra-sensitive detection of trace species in complex matrices including exhaled human breath. In this review, we discussed recent applications of CEAS for analyzing trace constituents within the exhaled breath matrix facilitating the non-invasive assessment of human health status. Next to a brief discussion on the mechanisms of formation of trace components found in the exhaled breath matrix related to particular disease states, existing challenges in CEAS and future development towards non-invasive clinical diagnostics will be discussed.

Real-time measurement of CO2 isotopologue ratios in exhaled breath by a hollow waveguide based mid-infrared gas sensor

A hollow waveguide (HWG) based mid-infrared gas sensor using a 2.73 µm distributed feedback (DFB) laser was developed for simultaneously measuring the concentration changes of the three isotopologues ¹³CO₂, ¹²CO₂, and ¹⁸OC¹⁶O in exhaled breath by direct absorption spectroscopy, and then determining the ¹³CO₂/¹²CO₂ isotope ratio (δ¹³C) and ¹⁸OC¹⁶O/¹²CO₂ isotope ratio (δ¹⁸O). The HWG sensor showed a fast response time of 3 s. Continuous measurement of δ¹³C and δ¹⁸O in the standard CO₂ sample with known isotopic ratios for ∼2 h was performed. Precisions of 2.20‰ and 1.98‰ for δ¹³C and δ¹⁸O respectively at optimal integration time of 734 s were estimated from Allan variance analysis. Accuracy of -0.49‰ and -1.20‰ for δ¹³C and δ¹⁸O, respectively, were obtained with comparison to the values of the reference standard. The Kalman filtering method was employed to improve the precision and accuracy of the HWG sensor while maintaining high time resolution. Precision of 5.45‰ and 4.88‰ and the accuracy of 0.21‰ and -1.13‰ for δ¹³C and δ¹⁸O, respectively, were obtained at the integration time of 0.54 s with the application of Kalman filtering. The concentrations of ¹²CO₂, ¹³CO₂ and ¹⁸OC¹⁶O in breath cycles were measured and processed by Kalman filtering in real time. The measured values of δ¹⁸O and δ¹³C in exhaled breath were estimated to be -21.35‰ and -33.64‰, respectively, with the integration time of 1 s. This study demonstrates the ability of the HWG sensor to obtain δ¹³C and δ¹⁸O values in breath samples and its potential for immediate respiratory monitoring and disease diagnosis.

Mid-infrared supercontinuum-based upconversion detection for trace gas sensing

Recent advancements of mid-infrared (MIR) supercontinuum light sources have opened up new possibilities in laser-based trace gas sensing. While the supercontinuum sources inherently support wide spectral coverage, the detection of broadband absorption signals with high speed and low cost is traditionally limited by the MIR detector arrays. In this work, we demonstrate that this limitation can be circumvented by upconverting the MIR signal into the near-infrared (NIR) region, where cost-effective silicon-based detector arrays can be utilized to measure broadband absorption. We also show that, by combining a MIR supercontinuum source with a MIR-to-NIR upconverter and an astigmatic multipass cell, fast detection (~20 ms) of ethane with sub-ppmv sensitivity can be achieved at room temperature. For multi-species detection, a least-square global fitting method is presented, showing a promising potential for applications such as environmental monitoring and biomedical research.

A near-infrared C2H2/CH4 dual-gas sensor system combining off-axis integrated-cavity output spectroscopy and frequency-division-multiplexing-based wavelength modulation spectroscopy

By combining frequency division multiplexing assisted wavelength modulation spectroscopy (FDM-WMS) and off-axis integrated-cavity output spectroscopy (OA-ICOS), a near-infrared (near-IR) dual-gas sensor system was demonstrated for simultaneous chemical gas-phase detection of acetylene (C2H2) and methane (CH4). Two distributed feedback (DFB) lasers modulated at the frequency of 3 kHz and 4 kHz with an emitting wavelength of 1532 and 1653 nm were used to target two absorption lines, C2H2 at 6523.88 cm-1 and CH4 at 6046.95 cm-1, respectively. A 6 cm-long cavity was fabricated, which reveals an effective path length of 9.28 m (@1532 nm, C2H2) and 8.56 m (@1653 nm, CH4), respectively. Performances of the dual-gas sensor system were experimentally evaluated using C2H2 and CH4 samples generated by an Environics gas mixing system. An Allan deviation of 700 parts-per-billion in volume (ppbv) for C2H2 with an averaging time of 200 s and 850 ppbv for CH4 with an averaging time of 150 s was achieved for these two gas species. Dynamic measurements of a C2H2/CH4 : N2 mixture were performed for monitoring both C2H2 and CH4 simultaneously. This dual-gas sensor has the merits of reduced size and cost compared to two separate OA-ICOS sensors and reveals the minimum detectable column density (DCD) compared to other reported C2H2 and CH4 sensor systems.

Intensity enhancement in off-axis integrated cavity output spectroscopy

In the field of laser-based absorption spectroscopy, off-axis integrated cavity output spectroscopy is considered to be a sensitive and robust method, employing a simple optical design. However, one of the major drawbacks of non-mode-matched cavities combined with highly reflective mirrors (>99.98%) is its low output intensity. Here, we systematically investigate the increase in cavity output intensity, using a third re-injection mirror before the absorption cavity. The presented design not only enables high transmission power but also retains a long effective path length. To investigate the intensity enhancement, we used a CO₂ absorption line in the near-IR wavelength region at 6240.10  cm^-1. In agreement with our simulation model, we achieved an intensity enhancement factor of 38. We achieved a noise equivalent absorption sensitivity to 1.6×10^-8  cm^-1 Hz^-1/2, which is no longer limited by the detectivity of the detector.

Sensitive Spectroscopy of Acetone Using a Widely Tunable External-Cavity Quantum Cascade Laser

We employed a single-mode, widely tunable (~300 cm⁻¹) external-cavity quantum cascade laser operating around 8 µm for broadband direct absorption spectroscopy and wavelength modulation spectroscopy where a modulation frequency of 50 kHz was employed with high modulation amplitudes of up to 10 GHz. Using a compact multipass cell, we measured the entire molecular absorption band of acetone at ~7.4 µm with a spectral resolution of ~1 cm⁻¹. In addition, to demonstrate the high modulation dynamic range of the laser, we performed direct absorption (DAS) and second harmonic wavelength modulation spectroscopy (WMS-2f) of the Q-branch peak of acetone molecular absorption band (HWHM ~10 GHz) near 1365 cm⁻¹. With WMS-2f, a minimum detection limit of 15 ppbv in less than 10 s is achieved, which yields a noise equivalent absorption sensitivity of 1.9 × 10⁻⁸ cm⁻¹ Hz^−1/2.

Laser spectroscopy for breath analysis: towards clinical implementation

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.

A Miniaturized QEPAS Trace Gas Sensor with a 3D-Printed Acoustic Detection Module

A 3D printing technique was introduced to a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and is reported for the first time. The acoustic detection module (ADM) was designed and fabricated using the 3D printing technique and the ADM volume was compressed significantly. Furthermore, a small grin lens was used for laser focusing and facilitated the beam adjustment in the 3D-printed ADM. A quartz tuning fork (QTF) with a low resonance frequency of 30.72 kHz was used as the acoustic wave transducer and acetylene (C₂H₂) was chosen as the analyte. The reported miniaturized QEPAS trace gas sensor is useful in actual sensor applications.

Highly Selective Volatile Organic Compounds Breath Analysis Using a Broadly-Tunable Vertical-External-Cavity Surface-Emitting Laser

A broadly tunable mid-infrared vertical-external-cavity surface-emitting laser (VECSEL) is employed in a direct absorption laser spectroscopic setup to measure breath acetone. The large wavelength coverage of more than 30 cm^-1 at 3.38 μm allows, in addition to acetone, the simultaneous measurement of isoprene, ethanol, methanol, methane, and water. Despite the severe spectral interferences from water and alcohols, an unambiguous determination of acetone is demonstrated with a precision of 13 ppbv that is achieved after 5 min averaging at typical breath mean acetone levels in synthetic gas samples mimicking human breath.

Mid-infrared gas absorption sensor based on a broadband external cavity quantum cascade laser

We developed a laser absorption sensor based on a pulsed, broadband tunable external cavity quantum cascade laser (ECQCL) centered at 1285 cm^-1. Unlike traditional infrared spectroscopy system, a quartz crystal tuning fork (QCTF) as a light detector was used for laser signal detection. Fast Fourier transform was applied to extract vibration intensity information of QCTF. The sensor system is successfully tested on nitrous oxide (N₂O) spectroscopy measurements and compared with a standard infrared detector. The wide wavelength tunability of ECQCL will allow us to access the fundamental vibrational bands of many chemical agents, which are well-suited for trace explosive, chemical warfare agent, and toxic industrial chemical detection and spectroscopic analysis.

Mid-infrared optical parametric oscillators and frequency combs for molecular spectroscopy

Review of mid-infrared optical parametric oscillators and frequency combs for high-resolution spectroscopy, including applications in trace gas detection and fundamental research.

Quartz enhanced photoacoustic spectroscopy based trace gas sensors using different quartz tuning forks

A sensitive trace gas sensor platform based on quartz-enhanced photoacoustic spectroscopy (QEPAS) is reported. A 1.395 μm continuous wave (CW), distributed feedback pigtailed diode laser was used as the excitation source and H2O was selected as the target analyte. Two kinds of quartz tuning forks (QTFs) with a resonant frequency (f0) of 30.72 kHz and 38 kHz were employed for the first time as an acoustic wave transducer, respectively for QEPAS instead of a standard QTF with a f0 of 32.768 kHz. The QEPAS sensor performance using the three different QTFs was experimentally investigated and theoretically analyzed. A minimum detection limit of 5.9 ppmv and 4.3 ppmv was achieved for f0 of 32.768 kHz and 30.72 kHz, respectively.

Pathlength determination for gas in scattering media absorption spectroscopy

Gas in scattering media absorption spectroscopy (GASMAS) has been extensively studied and applied during recent years in, e.g., food packaging, human sinus monitoring, gas diffusion studies, and pharmaceutical tablet characterization. The focus has been on the evaluation of the gas absorption pathlength in porous media, which a priori is unknown due to heavy light scattering. In this paper, three different approaches are summarized. One possibility is to simultaneously monitor another gas with known concentration (e.g., water vapor), the pathlength of which can then be obtained and used for the target gas (e.g., oxygen) to retrieve its concentration. The second approach is to measure the mean optical pathlength or physical pathlength with other methods, including time-of-flight spectroscopy, frequency-modulated light scattering interferometry and the frequency domain photon migration method. By utilizing these methods, an average concentration can be obtained and the porosities of the material are studied. The last method retrieves the gas concentration without knowing its pathlength by analyzing the gas absorption line shape, which depends upon the concentration of buffer gases due to intermolecular collisions. The pathlength enhancement effect due to multiple scattering enables also the use of porous media as multipass gas cells for trace gas monitoring. All these efforts open up a multitude of different applications for the GASMAS technique.

Microsecond fiber laser pumped, single-frequency optical parametric oscillator for trace gas detection

We report on the first microsecond doubly resonant optical parametric oscillator (OPO). It is based on a nested cavity OPO architecture allowing single longitudinal mode operation and low oscillation threshold (few microjoule). The combination with a master oscillator-power amplifier fiber pump laser provides a versatile optical source widely tunable in the 3.3-3.5 μm range with an adjustable pulse repetition rate (from 40 to 100 kHz), high duty cycle (~10(-2)) and mean power (up to 25 mW in the idler beam). The potential for trace gas sensing applications is demonstrated through photoacoustic detection of atmospheric methane.

Power instability of singly resonant optical parametric oscillators: theory and experiment

We present a theoretical model on the effects of mechanical perturbations on the output power instability of singly-resonant optical parametric oscillators (SR-OPOs). Numerical simulations are performed based on real experimental parameters associated with a SR-OPO designed in our laboratory, which uses periodically-poled LiNbO3 (PPLN) as the nonlinear crystal, where the results of the theoretical model are compared with the measurements. The out-coupled power instability is simulated for a wide range of input pump powers the SR-OPO oscillation threshold. From the results, maximum instability is found to occur at an input pump power of ~1.5 times above the OPO threshold. It is also shown theoretically that the idler instability is susceptible to variations in the cavity length caused by vibrations, with longer cavities capable of generating more stable output power. The validity of the theoretical model is verified experimentally by using a mechanical vibrator in order to vary the SR-OPO resonator length over one cavity mode spacing. It is found that at 1.62 times threshold, the out-coupled idler suffers maximum instability. The results of experimental measurements confirm good agreement with the theoretical model. An intracavity etalon is finally used to improve the idler output power by a factor of ~2.2 at an input pump power of 1.79 times oscillation threshold.

Thulium channel waveguide laser in a monoclinic double tungstate with 70% slope efficiency

Laser experiments were performed on buried, ridge-type channel waveguides in an 8 at. % thulium-doped, yttrium-gadolinium-lutetium codoped monoclinic double tungstate. A maximum slope efficiency of 70% and output powers up to 300 mW about 2.0 μm were obtained in a mirrorless laser resonator, by pumping with a Ti:sapphire laser near 800 nm. To the best of our knowledge, this result represents the most efficient 2 μm channel waveguide laser to date. Lasing is obtained at various wavelengths between 1810 nm and 2037 nm.