Rapid, time-division multiplexed, direct absorption- and wavelength modulation-spectroscopy

FDM-assisted opposite two-way OA-CEAS system employing four lasers for simultaneous multi-species detection

An FDM-assisted opposite two-way OA-CEAS system is reported in this paper. Compared with the traditional OA-CEAS system with one-way transmission configuration, the new system has two main advantages. One of the advantages is that four lasers can be employed for simultaneous measurements of multiple species in this system. Another advantage is the combination of the silver-coated concave spherical mirror and the narrow bandpass filter employed to realize the opposite two-way transmission in the optical cavity which can also serve as a re-injection mirror and optical enhancement gotten for free in the system. The performance of the system is demonstrated by simultaneous measurements of CO, CO2, C2H4, and CH4. This work highlights a new strategy for simultaneous detection by using four lasers in a single optical integrated cavity, which can improve the utilization rate of the optical cavity and reduce the cost for multiple gas species sensing.

Real-time, in situ measurement of H2O generated during in situ combustion tests using 1f-normalized wavelength modulation spectroscopy with second harmonic detection

The development and deployment of a real-time, in situ, non-invasive sensor to monitor the concentration of H ₂ O during in situ combustion (ISC) experiments with a heavy-crude oil is described. A real-time sensor to monitor the gas-phase products from ISC can support the study of the kinetics of the complex chemical reactive system in ISC. The mole fraction of H ₂ O was measured using tunable diode laser (TDL) absorption spectroscopy coupled with 1f-normalized wavelength modulation spectroscopy (WMS) and 2f detection. The WMS 2f/1f strategy was used to enhance sensitivity with effective noise rejection, particularly suitable when characterizing the water vapor evolved from oil-water emulsions. H ₂ O was measured at 3934.10c m ^-1 from the fundamental band v ₃. That transition was selected using the HITRAN database to increase the line strength and minimize interference from neighbor compounds. Measurements of H ₂ O concentration were conducted at ambient temperature and pressure using a reference cell (H ₂ O=2% at 98.6 kPa) to validate the sensor architecture under controlled laboratory environments. The TDL sensor was also successfully validated during real ISC experiments involving heavy-crude oil. Validation and combustion experiments showed the potential of the TDL-based sensor for non-invasive, real-time, in situ measurements of gas-phase species in conditions similar to those of laboratory-scale experimental ISC tests.

Review on recent progress in on-line monitoring technology for atmospheric pollution source emissions in China

Emissions from mobile sources and stationary sources contribute to atmospheric pollution in China, and its components, which include ultrafine particles (UFPs), volatile organic compounds (VOCs), and other reactive gases, such as NH₃ and NO_x, are the most harmful to human health. China has released various regulations and standards to address pollution from mobile and stationary sources. Thus, it is urgent to develop online monitoring technology for atmospheric pollution source emissions. This study provides an overview of the main progress in mobile and stationary source monitoring technology in China and describes the comprehensive application of some typical instruments in vital areas in recent years. These instruments have been applied to monitor emissions from motor vehicles, ships, airports, the chemical industry, and electric power generation. Not only has the level of atmospheric environment monitoring technology and equipment been improving, but relevant regulations and standards have also been constantly updated. Meanwhile, the developed instruments can provide scientific assistance for the successful implementation of regulations. According to the potential problem areas in atmospheric pollution in China, some research hotspots and future trends of atmospheric online monitoring technology are summarized. Furthermore, more advanced atmospheric online monitoring technology will contribute to a comprehensive understanding of atmospheric pollution and improve environmental monitoring capacity.

Development and Validation of a Tunable Diode Laser Absorption Spectroscopy System for Hot Gas Flow and Small-Scale Flame Measurement

TDLAS (tunable diode laser absorption spectroscopy) is an important gas analysis method that can be employed to obtain characteristic parameters non-invasively by the infrared absorption spectra of tracer molecules such as CH₄, H₂O and O₂. In this study, a portable H₂O-based TDLAS system with a dual optical path was developed with the aim of assessing the combustion characteristics of flammable gases. Firstly, a calculation method of gas characteristics including temperature and velocity combining absorption spectra and a HITRAN database was provided. Secondly, to calibrate and validate this TDLAS system precisely, a pressure vessel and a shock tube were introduced innovatively to generate static or steady flow fields with preset constant temperatures, pressures, or velocities. Static tests within environment pressures up to 2 MPa and steady flow field tests with temperatures up to 1600 K and flow velocities up to 950 m/s were performed for verification. It was proved that this system can provide an accurate values for high temperature and velocity gas flows. Finally, an experimental investigation of CH₄/air flames was conducted to test the effectiveness of the system when applied to small diffusion flames. This TDLAS system gave satisfactory flame temperature and velocity data owing to the dual optical path design and high frequency scanning, which compensated for scale effects and pulsation of the flame. This work demonstrates a valuable new approach to thermal hazard analysis in specific environments.

Design of a Laser Driver and Its Application in Gas Sensing

A laser driver which features high stability and a graphical user-interface was designed and used in trace gas sensing. The running of the laser driver was managed by an ARM processor which was embedded with a real-time operating system (RTOS). Through clicking on the touch screen that was configured with an emWin graphical user-interface (GUI), the parameters of the driving current can be graphically set and monitored. The circuit model of the distributed feedback laser diode (DFB-LD) was introduced into a TINA-SPICE simulation to evaluate the performance of the current source. Through simulation, the potential self-oscillation can be visually predicted, and the feedback loop can be appropriately compensated. To validate the applicability, the laser driver was used for driving a carefully selected DFB-LD and was employed in wavelength modulation spectroscopy (WMS) for CH4 detection at R(3) absorption line of the 2ν3 overtone. Under the conditions of room temperature, normal pressure and an effective absorption path of 15.4 cm, repetitive experiments were conducted using gas samples, with their concentrations ranging from 400 ppm to 1%, and the detection limit derived from the signal-to-noise ratio (SNR) was 7.2 ppm. The promising result indicates the high potential of this laser driver for use in absorption spectrum-based sensing applications.

Development of a Sub-ppb Resolution Methane Sensor Using a GaSb-Based DFB Diode Laser near 3270 nm for Fugitive Emission Measurement

A challenge for mobile measurement of fugitive methane emissions is the availability of portable sensors that feature high sensitivity and fast response times, simultaneously. A methane gas sensor to measure fugitive emissions was developed using a continuous-wave, thermoelectrically cooled, GaSb-based distributed feedback diode laser emitting at a wavelength of 3.27 μm to probe methane in its strong ν₃ vibrational band. Direct absorption spectra (DAS) as well as wavelength-modulated spectra (WMS) of pressure-broadened R(3) manifold lines of methane were recorded through a custom-developed open-path multipass cell with an effective optical path length of 6.8 m. A novel metrological approach was taken to characterize the sensor response in terms of the linearity of different WMS metrics, namely, the peak-to-peak amplitude of the X_2f component and the peak and/or the integrated area of the background-subtracted quadrature signal (i.e., Q(2f - 2f₀)) and the background-subtracted 1f-normalized quadrature signal (i.e., Q(2f/1f - 2f₀/1f₀)). Comparison with calibration gas concentrations spanning 1.5 to 40 ppm_v indicated that the latter WMS metric showed the most linear response, while fitting DAS provides a traceable reference. In the WMS mode, a sensitivity better than 1 ppb_v was achieved at a 1 s integration time. The sensitivity and response time are well-suited to measure enhancements in ambient methane levels caused by fugitive emissions.

Soil respiration analysis using a mid-infrared quantum cascade laser and calibration-free WMS-based dual-gas sensor

A high response and sensitive dual-gas sensor based on calibration-free wavelength modulation spectroscopy (CF-WMS) has been developed for the simultaneous detection of carbon monoxide (CO) and nitrous oxide (N2O) to eliminate the detection errors caused by light intensity variations. A multi-pass cell (MPC) was employed to lengthen the optical path to improve the precision of the sensing system by combining with a 4.56 μm mid-infrared quantum cascade laser (MIR-QCL). Meanwhile, a LabVIEW-based bi-molecular iterative fitting algorithm was used to infer the respective abundances of each species. The performance of the completed system was accurately evaluated with precisions of 3.4 ppb for CO and 3.8 ppb for N2O at a 1 s averaging time, which could be improved to 0.48 ppb for CO and 0.53 ppb for N2O at an averaging time of 154 s and 278 s, respectively. The grassland soil respiration analysis of CO and N2O was performed under different moisture conditions, which indicated that dried soil samples appeared to be a significant source of CO, while the sinks of CO and the sources of N2O occurred in the moist soil samples. The maximum exchange rates of the two gases were exhibited in moderate moisture soil samples rather than in the over-wet or arid soil samples. Moreover, a possible positive relationship between the sinks of CO and sources of N2O was established to illustrate the correlation of the two species in soil respiration.

Towards an Optical Gas Standard for Traceable Calibration-Free and Direct NO2 Concentration Measurements

We report a direct tunable diode laser absorption spectroscopy (dTDLAS) instrument developed for NO2 concentration measurements without chemical pre-conversion, operated as an Optical Gas Standard (OGS). An OGS is a dTDLAS instrument that can deliver gas species amount fractions (concentrations), without any previous or routine calibration, which are directly traceable to the international system of units (SI). Here, we report NO2 amount fraction quantification in the range of 100–1000 µmol/mol to demonstrate the current capability of the instrument as an OGS for car exhaust gas application. Nitrogen dioxide amount fraction results delivered by the instrument are in good agreement with certified values of reference gas mixtures, validating the capability of the dTDLAS-OGS for calibration-free NO2 measurements. As opposed to the standard reference method (SRM) based on chemiluminescence detection (CLD) where NO2 is indirectly measured after conversion to NO, titration with O3 and the detection of the resulting fluorescence, a dTDLAS-OGS instrument has the benefit of directly measuring NO2 without distorting or delaying conversion processes. Therefore, it complements the SRM and can perform fast and traceable measurements, and side-by-side calibrations of other NO2 gas analyzers operating in the field. The relative standard uncertainty of the NO2 results reported in this paper is 5.1% (k = 1, which is dominated (98%) by the NO2 line strength), the repeatability of the results at 982.6 µmol/mol is 0.1%, the response time of the instrument is 0.5 s, and the detection limit is 825 nmol/mol at a time resolution of 86 s.

Methane leak detection by tunable laser spectroscopy and mid-infrared imaging

Tunable laser spectroscopy (TLS) combined with mid-infrared imaging is a powerful tool for a sensitive and quantitative visualization of gas leaks. This work deals with standoff methane leak detection within 2 m by an interband cascade laser (3270 nm wavelength) and an infrared camera. The concept demonstrates visualization of methane leakage rates down to 2 ml/min by images and sequences at frame rates up to 125 Hz. The gas plume and leak can be localized and quantified within a single image by direct absorption spectroscopy (DAS). The HITRAN database allows a calibration-free, pixelwise determination of the concentration in ppm*m. The active optical imaging concept showed pixelwise sensitivities around 1 ppm*m.

Accurate analysis of HCl in biomethane using laser absorption spectroscopy and ion-exchange chromatography

Biomethane is a renewable energy gas with great potential to contribute to the diversification and greening of the natural gas supply. Ideally, biomethane can directly be injected into the natural gas grid system. For grid injection, specifications such as those in EN 16723-1 shall be met. One of the impurities to be monitored is hydrogen chloride (HCl). To assess conformity with the specification for HCl, accurate and reliable test methods are required. Here, we report the development of three novel test methods, based on a variety of laser absorption spectroscopy techniques (Direct absorption spectroscopy-DAS and wavelength modulation spectroscopy-WMS) and ion-exchange chromatography, for the measurement of HCl in biomethane. Gas mixtures of HCl in biomethane were used to demonstrate the performance of the spectroscopic systems in the nmol mol^-1 to low μmol mol^-1 ranges, achieving uncertainties in the 4% range, k = 2. For ion-exchange chromatography analysis, HCl was first collected on an alkali-impregnated quartz fiber filter. The analysis was performed according to ISO 21438-2 and validated using synthetic biomethane spiked with HCl. The relative expanded uncertainties for the ion exchange chromatography HCl measurements are in the 10-37% range, k = 2. The results presented for the 3 test methods demonstrate that the respective methods can be used for HCl conformity assessment in biomethane.

Advanced Fiber-Coupled Diode Laser Sensor for Calibration-Free 1f-WMS Determination of an Absorption Line Intensity

A new scheme for a calibration-free diode laser absorption spectroscopy (DLAS) sensor for measuring the parameters of harsh zones is proposed. The key element of the scheme is a micro-prism retroreflector (MPRR). The MPRR facilitates an increase in the mechanical stability of the sensor and a decrease in the background thermal radiation in the hot areas of a tested zone. Reduction in the broadband thermal emission allowed the application of a differential logarithmic conversion (LC) technique for elimination of the residual amplitude modulation and other sources of non-selective attenuation of the probing laser beam. LC allows the use of a 1f-wavelength modulation spectroscopy (WMS) detection scheme. Combination of LC and a 1f-WMS algorithm provided a new modification of calibration-free DLAS, which could be particularly useful for probing harsh zones with pronounced strong turbulence and high levels of acoustic and electrical noise. The influence of the experimental parameters and characteristics of the main electronic components of the recording and processing system on the accuracy of the integral line intensity determination is investigated theoretically and experimentally. The proposed optical scheme of a DLAS sensor and algorithm for the data processing allowed the integral intensity of an absorption line to be obtained. The potential for the scheme was exemplified with a single water vapor absorption line at 7185.6 cm^-1. Simultaneous detection of several absorption lines and data processing using the developed algorithm provides the final goal of a DLAS sensor-determination of temperature and partial pressure of a test molecule in a probed gas volume. The developed scheme allows the spatial multiplexing of the radiation of different diode lasers (DLs), which can be used if various test molecules are to be detected, or absorption lines of a test molecule are detected over different wavelength intervals.

Ammonia Gas Sensors: Comparison of Solid-State and Optical Methods

High precision and fast measurement of gas concentrations is important for both understanding and monitoring various phenomena, from industrial and environmental to medical and scientific applications. This article deals with the recent progress in ammonia detection using in-situ solid-state and optical methods. Due to the continuous progress in material engineering and optoelectronic technologies, these methods are among the most perceptive because of their advantages in a specific application. We present the basics of each technique, their performance limits, and the possibility of further development. The practical implementations of representative examples are described in detail. Finally, we present a performance comparison of selected practical application, accumulating data reported over the preceding decade, and conclude from this comparison.

A Wide-Range and Calibration-Free Spectrometer Which Combines Wavelength Modulation and Direct Absorption Spectroscopy with Cavity Ringdown Spectroscopy

A wide-range, calibration-free tunable diode laser spectrometer is established by combining wavelength modulation and direct absorption spectroscopy (WM-DAS) with continuous wave cavity ringdown spectroscopy (CW-CRDS). This spectrometer combines the benefits of absolute concentration measurements, wide range, and high speed, using WM-DAS with enhanced noise reduction in CW-CRDS. The accurate baseline ringdown time, τ0, is calculated by the absorption peak (measured by WM-DAS) and the ringdown time containing gas absorption information (measured by CW-CRDS at the center wavelength of the spectral line). The gas concentration is obtained without measuring τ0 in real time, thus, greatly improving the measuring speed. A WM-DAS/CW-CRDS spectrometer at 1.57 μm for CO detection was assembled for experimental validation of the multiplexing scheme over a concentration ranging from 4 ppm to 1.09% (0.1 MPa, 298 K). The measured concentration of CO at 6374.406 cm-1 shows that the dynamic range of this tunable diode laser absorption spectrometer is extendable up to five orders of magnitude and the corresponding precision is improved. The measurement speed of this spectrometer can extend up to 10 ms, and the detection limit can reach 35 ppb within 25 s.

Hydrogen Sensor Based on Tunable Diode Laser Absorption Spectroscopy

A laser-based hydrogen (H₂) sensor using wavelength modulation spectroscopy (WMS) was developed for the contactless measurement of molecular hydrogen. The sensor uses a distributed feedback (DFB) laser to target the H₂ quadrupole absorption line at 2121.8 nm. The H₂ absorption line exhibited weak collisional broadening and strong collisional narrowing effects. Both effects were investigated by comparing measurements of the absorption linewidth with detailed models using different line profiles including collisional narrowing effects. The collisional broadening and narrowing parameters were determined for pure hydrogen as well as for hydrogen in nitrogen and air. The performance of the sensor was evaluated and the sensor applicability for H₂ measurement in a range of 0–10 %v of H₂ was demonstrated. A precision of 0.02 %v was achieved with 1 m of absorption pathlength (0.02 %v∙m) and 1 s of integration time. For the optimum averaging time of 20 s, precision of 0.005 %v∙m was achieved. A good linear relationship between H₂ concentration and sensor response was observed. A simple and robust transmitter–receiver configuration of the sensor allows in situ installation in harsh industrial environments.

Pressure-dependent sensitivity of a single-pass methane detection system using a continuous-wave distributed feedback laser at 3270 nm

A single-pass, compact, and low-power-consumption methane sensing system is presented, and its sensitivity is examined for a set of reference cells with pressures of 7.4, 74, and 740 Torr and the same methane concentration at laboratory temperature of 23°C. A GaSb-based continuous-wave distributed feedback tunable diode laser at 3270 nm and wavelength modulation spectroscopy were employed to collect 2f spectra in the ν₃, R₃ band of CH₄12. The collected 2f spectra were fitted to a modulated Voigt line profile model. An Allan-Werle variance analysis shows that the best detectivity, 4 ppbm, is obtained for the highest pressure cell. At pressures of 74 and 7.4 Torr the detectivities are 16 and 28 ppbm, respectively. For these low-pressure cells, further sensitivity improvements were achieved using a large modulation depth (m>2.2) at the expense of spectral resolution.

High-Precision 13CO2/12CO2 Isotopic Ratio Measurement Using Tunable Diode Laser Absorption Spectroscopy at 4.3 μm for Deep-Sea Natural Gas Hydrate Exploration

For the detection of deep-sea natural gas hydrates, it is very important to accurately detect the 13CO2/12CO2 isotope ratio of dissolved gas in seawater. In this paper, a 13CO2/12CO2 isotope ratio sensor is investigated, which uses a tunable diode laser absorption spectroscopy (TDLAS) technique at 4.3 μm. The proposed sensor consists of a mid-infrared interband cascade laser (ICL) operating in continuous wave mode, a long optical path multi-pass gas cell (MPGC) of 24 m, and a mid-infrared mercury cadmium telluride (MCT) detector. Aiming at the problem of the strong absorption intensity of the two absorption lines of 13CO2 and 12CO2 being affected by temperature, a high-precision temperature control system for the MPGC was fabricated. Five different concentrations of CO2 gas were configured to calibrate the sensor, and the response linearity could reach 0.9992 for 12CO2 and 0.9996 for 13CO2. The data show that the carbon isotope measurement precision was assessed to be 0.0139‰ when the integration time was 92 s and the optical path length was 24 m. The sensor is combined with a gas–liquid separator to detect the 13CO2/12CO2 isotope ratio of CO2 gas extracted from water. Results validate the reported sensor system’s potential application in deep-sea natural gas hydrate exploration.

Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization

Laser absorption spectroscopy (LAS) is a promising diagnostic method capable of providing high-bandwidth, species-specific sensing, and highly quantitative measurements. This review aims at providing general guidelines from the perspective of LAS sensor system design for realizing quantitative species diagnostics in combustion-related environments. A brief overview of representative detection limits and bandwidths achieved in different measurement scenarios is first provided to understand measurement needs and identify design targets. Different measurement schemes including direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS), and their variations are discussed and compared in terms of advantages and limitations. Based on the analysis of the major sources of noise including electronic, optical, and environmental noises, strategies of noise reduction and design optimization are categorized and compared. This addresses various means of laser control parameter optimization and data processing algorithms such as baseline extraction, in situ laser characterization, and wavelet analysis. There is still a large gap between the current sensor capabilities and the demands of combustion and engine diagnostic research. This calls for a profound understanding of the underlying fundamentals of a LAS sensing system in terms of optics, spectroscopy, and signal processing.

Direct absorption spectroscopy baseline fitting for blended absorption features

A method for determining the gas concentration from blended absorption features using direct absorption spectroscopy is developed. Blended spectra occur when multiple absorption lines are in close proximity in a laser scan and/or when pressures begin to increase. The blended-feature baseline fitting method estimates the baseline by coupling measured data with simulated fractional transmission at the peaks between absorption features. The method is first validated with synthetically created data. The method is then demonstrated using the absorption lines of CO₂(P41e,P42f,P34e,P43e) and H₂O in the ν₁+ν₃ combination band (3683.5  cm^-1 to 3686.5  cm^-1) in a heated static test cell. Results from the fitting method and wavelength modulation spectroscopy measurements agree within 1%. The method is finally applied to measurements of flue gas emissions from a practical combustion system (wood boiler) and compared to measurements from a hand-held gas analyzer. The mean percent difference of the blended-feature baseline fitting and the wavelength modulation spectroscopy is 2.8% and 5.8% for X_CO2 and X_H2O, respectively, indicating that this method of direct absorption data analysis can be used for spectra with moderate feature blending, which is useful for scan ranges with multiple closely spaced absorption lines and moderate pressures.

A Trace C₂H₂ Sensor Based on an Absorption Spectrum Technique Using a Mid-Infrared Interband Cascade Laser

In this study, tunable diode laser absorption spectroscopy (TDLAS) combined with wavelength modulation spectroscopy (WMS) was used to develop a trace C₂H₂ sensor based on the principle of gas absorption spectroscopy. The core of this sensor is an interband cascade laser that releases wavelength locks to the best absorption line of C₂H₂ at 3305 cm⁻¹ (3026 nm) using a driving current and a working temperature control. As the detected result was influenced by 1/f noise caused by the laser or external environmental factors, the TDLAS-WMS technology was used to suppress the 1/f noise effectively, to obtain a better minimum detection limit (MDL) performance. The experimental results using C₂H₂ gas with five different concentrations show a good linear relationship between the peak value of the second harmonic signal and the gas concentration, with a linearity of 0.9987 and detection accuracy of 0.4%. In total, 1 ppmv of C₂H₂ gas sample was used for a 2 h observation experiment. The data show that the MDL is low as 1 ppbv at an integration time of 63 s. In addition, the sensor can be realized by changing the wavelength of the laser to detect a variety of gases, which shows the flexibility and practicability of the proposed sensor.

TDLAS-WMS second harmonic detection based on spectral analysis

A novel method for the detection of the second-harmonic component of tunable laser diode absorption spectroscopy-wavelength-modulated absorption spectroscopy is proposed in this study. The proposed method is based on spectral analysis, and the second-harmonic component is obtained in the frequency domain through a rectangular window. The validity, correctness, and noise immunity of the proposed method are verified through simulations and experiments. Experimental results demonstrate that the relative detection error level of the proposed method is <3%. In contrast to lock-in amplification, the proposed method does not require reference signals and low-pass filtering algorithms. These characteristics simplify data processing in the proposed method.

Data analysis and uncertainty estimation in supercontinuum laser absorption spectroscopy

A set of algorithms is presented that facilitates the evaluation of super continuum laser absorption spectroscopy (SCLAS) measurements with respect to temperature, pressure and species concentration without the need for simultaneous background intensity measurements. For this purpose a non-linear model fitting approach is employed. A detailed discussion of the influences on the instrument function of the spectrometer and a method for the in-situ determination of the instrument function without additional hardware are given. The evaluation procedure is supplemented by a detailed measurement precision assessment by applying an error propagation through the non-linear model fitting approach. While the algorithms are tailored to SCLAS, they can be transferred to other spectroscopic methods, that similarly require an instrument function. The presented methods are validated using gas cell measurements of methane in the near infrared region at pressures up to 8.7 bar.

Wavelength modulation spectroscopy for recovering absolute absorbance

A highly sensitive, calibration-free method, wavelength modulation - direct absorption spectroscopy (WM-DAS), is proposed by extracting the characteristic frequencies of modulated laser intensity. This method combines the advantages of measuring absolute absorbance profile from calibration-free DAS with the enhanced noise rejection and high sensitivity of WMS. The preliminary validation experiment of H₂O transitions shows that the WM-DAS method improves the accuracy of the absorbance profile measurement with a best-fit standard residual of 4.9 × 10^-5. This new method might inspire new approaches to high-precision measurement of spectral line parameters and gas information with weak absorptions or industrial applications.

Active stabilization of alkali-atom vapor density with a solid-state electrochemical alkali-atom source

We report a demonstration of vapor-phase Rubidium (Rb) density stabilization in a vapor cell using a solid-state electrochemical Rb source device. Clear Rb density stabilization is observed. Further demonstrations show that the temperature coefficient for Rb density can be reduced more than 100 times when locked and the device's power consumption is less than 10 mW. Preliminary investigation of the locking dynamic range shows that the Rb density is well stabilized when the initial density is five times higher (33 × 10⁹ /cm³) than the set point density (6 × 10⁹ /cm³). Active stabilization with this device is of high interest for portable cold-atom microsystems where large ambient temperature working ranges and low power consumption are required.

ICL-based TDLAS sensor for real-time breath gas analysis of carbon monoxide isotopes

We present a compact sensor for carbon monoxide (CO) in air and exhaled breath based on a room temperature interband cascade laser (ICL) operating at 4.69 µm, a low-volume circular multipass cell and wavelength modulation absorption spectroscopy. A fringe-limited (1σ) sensitivity of 6.5 × 10^-8 cm^-1Hz^-1/2 and a detection limit of 9 ± 5 ppbv at 0.07 s acquisition time are achieved, which constitutes a 25-fold improvement compared to direct absorption spectroscopy. Integration over 10 s increases the precision to 0.6 ppbv. The setup also allows measuring the stable isotope ¹³CO in breath. We demonstrate quantification of indoor air CO and real-time detection of CO expirograms from healthy non-smokers and a healthy smoker before and after smoking. Isotope ratio analysis indicates depletion of ¹³CO in breath compared to natural abundance.

Intensity-Stabilized Fast-Scanned Direct Absorption Spectroscopy Instrumentation Based on a Distributed Feedback Laser with Detection Sensitivity down to 4 × 10-6

A novel, intensity-stabilized, fast-scanned, direct absorption spectroscopy (IS-FS-DAS) instrumentation, based on a distributed feedback (DFB) diode laser, is developed. A fiber-coupled polarization rotator and a fiber-coupled polarizer are used to stabilize the intensity of the laser, which significantly reduces its relative intensity noise (RIN). The influence of white noise is reduced by fast scanning over the spectral feature (at 1 kHz), followed by averaging. By combining these two noise-reducing techniques, it is demonstrated that direct absorption spectroscopy (DAS) can be swiftly performed down to a limit of detection (LOD) (1σ) of 4 × 10-6, which opens up a number of new applications.

Calibration-free wavelength-modulation spectroscopy based on a swiftly determined wavelength-modulation frequency response function of a DFB laser

A methodology for calibration-free wavelength modulation spectroscopy (CF-WMS) that is based upon an extensive empirical description of the wavelength-modulation frequency response (WMFR) of DFB laser is presented. An assessment of the WMFR of a DFB laser by the use of an etalon confirms that it consists of two parts: a 1st harmonic component with an amplitude that is linear with the sweep and a nonlinear 2nd harmonic component with a constant amplitude. Simulations show that, among the various factors that affect the line shape of a background-subtracted peak-normalized 2f signal, such as concentration, phase shifts between intensity modulation and frequency modulation, and WMFR, only the last factor has a decisive impact. Based on this and to avoid the impractical use of an etalon, a novel method to pre-determine the parameters of the WMFR by fitting to a background-subtracted peak-normalized 2f signal has been developed. The accuracy of the new scheme to determine the WMFR is demonstrated and compared with that of conventional methods in CF-WMS by detection of trace acetylene. The results show that the new method provides a four times smaller fitting error than the conventional methods and retrieves concentration more accurately.

Calibration-free scanned wavelength modulation spectroscopy--application to H(2)O and temperature sensing in flames

A calibration-free scanned wavelength modulation spectroscopy scheme requiring minimal laser characterization is presented. Species concentration and temperature are retrieved simultaneously from a single fit to a group of 2f/1f-WMS lineshapes acquired in one laser scan. The fitting algorithm includes a novel method to obtain the phase shift between laser intensity and wavelength modulation, and allows for a wavelength-dependent modulation amplitude. The scheme is demonstrated by detection of H(2)O concentration and temperature in atmospheric, premixed CH(4)/air flat flames using a sensor operating near 1.4 µm. The detection sensitivity for H(2)O at 2000 K was 4 × 10(-5) cm(-1) Hz(-1/2), and temperature was determined with a precision of 10 K and absolute accuracy of ~50 K. A parametric study of the dependence of H(2)O and temperature on distance to the burner and total fuel mass flow rate shows good agreement with 1D simulations.

Immunity to Laser Power Variation in a DFB Diode Laser Based Optical Gas Sensor Using a Division Process

The division process used in a DFB diode laser-based optical gas sensor was studied to improve the immunity to laser power variation. Residual amplitude modulation (RAM) in wavelength modulation spectroscopy (WMS) detection was eliminated by intensity normalization using a division process. As a result the detected harmonic signals showed a significant improvement in line shape. For the first harmonic (1f) signal, Bias was improved from 38.7% to 1.2%; Baseline Difference was improved from 2.7% to 0.69% and Asymmetry was improved from 15.4% to 0.22%. For the second harmonic (2f) signal, the Asymmetry Coefficient was improved from 103% to 5.1%. Moreover the division process can further suppress the influence of unstable laser power. As a result, for the 1f signal, stable detection with a variation coefficient of 0.59% was obtained over a wide dynamic range (0.38–8.1 mW). For the 2f signal, stable detection with a variation coefficient of 0.53% was obtained from 0.64 mW to 8.27 mW. The test results showed a good agreement with the theoretical analysis and the proposed method has considerable potential application in gas sensing.