Absorption Wavelength Modulation Research Articles

Tomographic measurements of temperature and species concentration via deep learning and wavelength modulated absorption spectroscopy

To achieve the 2-D nonintrusive measurements of temperature and species concentration in the combustion field, a new framework, combining calibration-free wavelength modulation absorption spectroscopy (CF-WMS) with a designed convolutional neural network (CNN), was developed. The principle of the CF-WMS, along with the architecture of the CNN net, the training, and the performance of the network, has been investigated. The region of interest was discretized into 24 × 24 pixels2, and 48 probing beams with six targeted frequencies were used to verify the feasibility of the designed CNN with WMS 2f/1f signal for temperature and species concentration reconstruction. 20 000 samples of temperature and water vapor concentration distributions are randomly fabricated, featuring three randomly positioned Gaussian distributions. Reconstructed images of the phantoms agreed well with the original distributions with the relative error of about 5.0%–9.2% and 8.0%–12.4% using 17 000 training datasets with different beam arrangements for temperature and H2O species, respectively. Several representative beam arrangements with a limited number have been examined and compared. The beam arrangement BA2 and BA3 exhibited better performance than BA1 with average errors of about 5% and 8% for temperature and H2O species, respectively. Such a method can provide an effective way to achieve spatially and temporally resolved, real-time, in situ monitoring in practical combustion environments.

Single-mode GaSb-based laterally coupled distributed-feedback laser for CO2 gas detection

We report on a GaSb-based laterally coupled distributed feedback (LC-DFB) laser with Cr gratings operating at 2004 nm for CO2 detection application. Butterfly packaged with single-mode fiber pigtailed, the laser diode operates in the continuous-wave mode in a temperature range from −10 °C to 60 °C, with a maximum output power of 2 mW and a maximum side-mode suppression ratio over 30 dB. Wavelength-modulated absorption spectroscopy of CO2 demonstrates the applicability of the LC-DFB laser to tunable diode laser absorption spectroscopy. Furthermore, the diode junction temperature, which is measured by using the wavelength shift method, exhibits a maximum value of 17 °C in the single-mode operation range.

Estimation of Staphylococcus aureus growth rates from CO2 with absorption spectroscopy

AbstractIt is very important to monitor and evaluate the growth of Staphylococcus aureus (S. aureus) under various conditions. However, so far there is no technology that can monitor S. aureus in real time. This work proves that the wavelength modulated absorption spectroscopy (WMAS) based on tunable diode lasers can quickly measure the concentration of carbon dioxide produced by microorganisms. We use a variety of fitting models to get the growth curves of S. aureus and calculated the relationship between the growth rate and time through the obtained growth curve. Using the proposed technology, we have studied the growth status of S. aureus at different temperatures (five points in the range of 26–43°C), and the results show that the maximum growth rate have a strong temperature dependence. Therefore, it is proved that WMAS monitoring S. aureus is a user‐friendly, non‐invasive, fast, and high signal‐to‐noise ratio technology, which can be used for rapid and accurate bacterial growth assessment in the industry.

Wide-range and calibration-free H<sub>2</sub>S volume fraction measurement based on combination of wavelength modulation and direct absorption spectroscopy with cavity ringdown spectroscopy

Combining cavity ring down spectroscopy (CRDS) and wavelength modulated direct absorption spectroscopy (WM-DAS), a wide range and calibration-free gas concentration detection method is established, which has the advantages of high signal-to-noise ratio of CRDS and fast speed and measurable absolute concentration of WM-DAS. The baseline ring down time (<i>τ</i><sub>0</sub>) can be calculated by measuring the absorptivity (WM-DAS) and ring down time (CRDS) at the central frequency of the spectral line, without real-time calibration, which greatly improves the speed of CRDS measurement. The measurement results of different H<sub>2</sub>S concentrations at 6336.617 cm<sup>–1</sup> at room temperature and atmospheric pressure show that the dynamic measurement range of this method can be extended to more than 4 orders of magnitude, the measurement accuracy is improved in comparison with WM-DAS, and the detection limit can reach 1 × 10<sup>–9</sup> in 40 s.

Integral absorbance measurement for a non-uniform flow field using wavelength modulation absorption spectroscopy.

Combined with computed tomography (CT), the laser absorption spectroscopy technique is used to measure the two-dimensional distribution information of the flow field. The CT method needs an "integral parameter" as a known quantity. The integrated absorbance satisfies the criterion in the laser absorption spectral measurement. The direct absorption spectroscopy method directly measures the integrated absorbance. However, fitting the absorbance curve is difficult due to the distorted baseline in harsh environments. By contrast, the wavelength modulation spectroscopy (WMS) method has satisfactory noise rejection capability. The difficulty that introduces WMS method to measure the non-uniform flow distribution is the integrated absorbance cannot be written in a mathematical expression. Previous efforts focused on solving the average temperature, concentration, and pressure and recalculating the integrated absorbance. This paper aims to develop an integrated absorbance measurement based on the calibration-free WMS method for non-uniform flow, which is called the calibration-free WMS-A method. First, the relationship between the transmissivity and integrated absorbance was established. Then, integrated absorbance was written into the WMS harmonic signals and solved by comparing the measured and simulated signals. The systematic comparison between the WMS-A and the previous WMS method showed the effectivity of the WMS-A method for non-uniform flow measurement. The reliable integrated absorbance can considerably improve the two-dimensional reconstruction quality.

Band structure properties, phonons, and exciton fine structure in 4H -SiC measured by wavelength-modulated absorption and low-temperature photoluminescence

Owing to its hexagonal symmetry, indirect band gap, and relatively large unit cell, the electronic band structure of $4H$-SiC is comprised of a complicated series of anisotropic valence and conduction band extrema even very near to the uppermost valence band maximum and lowest conduction band minimum. This has presented a difficult challenge to those experiments which have attempted to resolve the small energy separations between these band extrema. To overcome this challenge, we have measured the wavelength-modulated absorption (WMA) spectrum of $4H$-SiC over a broader wavelength range (3500--3800 \AA{}) and at a higher resolution ($l0.1$ \AA{}) than in previous work. By comparing these measurements with the low-temperature photoluminescence spectrum in ultrapure $4H$-SiC, we have identified several features, which we attribute to a $56\ifmmode\pm\else\textpm\fi{}3$ meV crystal-field splitting of the valence band maximum or a $136\ifmmode\pm\else\textpm\fi{}3$ meV separation between the two lowest conduction band minima. We also show that the spin-orbit split-off valence band, which has been observed in previous measurements of $4H$-SiC, contributes to nonparabolic dispersion near the valence band maximum, and this is responsible for several previously misidentified features in the WMA spectrum. Finally, we report the first experimental measurement of fine structure splittings in the free exciton ground state, which manifests as four small ($0.7\ifmmode\pm\else\textpm\fi{}0.1$ meV) splittings in the WMA spectrum due to mass anisotropy and electron-hole exchange interaction.

Real-Time Monitoring of 13C- and 18O-Isotopes of Human Breath CO2 Using a Mid-Infrared Hollow Waveguide Gas Sensor.

Real-time measuring of CO2 isotopes (13CO2, 12CO2, and 18OC16O) in exhaled breath using a mid-infrared hollow waveguide gas sensor incorporating a 2.73 μm distributed feedback laser was proposed and demonstrated for the first time based on calibration-free wavelength modulation spectroscopy. The measurement precisions for δ13C and δ18O were, respectively, 0.26 and 0.57‰ for an integration time of 131 s by Allan variance analysis. These measurement precisions achieved in the present work were at least 3.5 times better than those reported using direct absorption spectroscopy and 1.3 times better than those obtained by calibration-needed wavelength modulation absorption spectroscopy. Continuous measurement of three isotopes in the breathing cycle was performed. Alveolar gas from the expirogram was identified, and the 13C/12C and 18O/16O ratios were found to be almost constant during the alveolar plateau, which enables optimization of breath sampling and provides accurate information on metabolic processes. The 13C/12C and 18O/16O isotope ratios at the alveolar plateau of five breath cycles were averaged, yielding δ13C and δ18O values of (-24.3 ± 3.4) and (-30.7 ± 2.6) ‰, respectively. This study demonstrates the feasibility of real-time analysis of 13C- and 18O-isotopes of human breath CO2 in clinical applications and shows its potential for diagnosing respiratory-related diseases with high sensitivity, selectivity, and specificity.

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.

High Resolution Optical Spectroscopy of Free Exciton and Electronic Band Structure near the Fundamental Gap in 4H SiC

We use thick, relatively high purity 4H SiC boule material to measure the wavelength modulated absorption spectrum with improved wavelength resolution and sensitivity with respect to previous work. We observe several small 0.6 ± 0.1 meV splittings, which we attribute to electron mass anisotropy and electron-hole exchange interaction. In addition, we identify several features in the absorption spectrum as signatures of nonparabolicity in the free exciton dispersion relations, the primary origin of which is likely the nonparabolic energy dispersion of the valence bands, as revealed by published band structure calculations based on density functional theory.

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.

Determination of Hydrogen Sulfide by Wavelength-modulated Diode Laser Absorption Spectroscopy

ABSTRACTRapid, accurate, and sensitive determination of hydrogen sulfide was provided by tunable distributed feedback diode laser absorption spectroscopy. The laser wavelength scanning range covered the strongest absorption band of hydrogen sulfide. The absorption line at 1578 nm was used for analytical measurements. A homemade digital lock-in amplifier was used to demodulate the first and the second harmonic signals. Normalization of the second harmonic by the first harmonic signal provided calibration-free measurements so that interferences from light path, circuitry, and source intensity were minimized. A mass flow controller was used to introduce 0–100 ppm hydrogen sulfide for validating the analytical performance. The results demonstrated the linearity of the normalized harmonic signals with concentration with a correlation coefficient of 0.9995. The response time was approximately 3 s. The detection limit by Allan variance was 45 ppb with an integration time of 35 s. The sensitive hydrogen sulfide sensor is suitable for environmental monitoring with real-time detection.

Detection of OH in an atmospheric flame at 1.5 um using optical frequency comb spectroscopy

We report broadband detection of OH in a premixed CH4/air flat flame at atmospheric pressure using cavity-enhanced absorption spectroscopy based on an Er:fiber femtosecond laserand a Fourier transform spectrometer.By taking ratios of spectra measured at different heights above the burner we separate twenty OH transitions from the largely overlapping water background. Weretrieve from fits to the OH lines the relative variation of the OH concentration and flame temperature with height above the burner and compare them with 1-D simulations of the flamestructure. Full Text: PDF ReferencesG. Meijer, M. G. Boogaarts, R. T. Jongma, D. H. Parker and A. M. Wodtke, "Coherent cavity ring down spectroscopy", Chem. Phys. Lett. 217, 1, 112 (1994). CrossRef S. Cheskis, I. Derzy, V. A. Lozovsky, A. Kachanov and D. Romanini, "Cavity ring-down spectroscopy of OH radicals in low pressure flame", Appl. Phys. B 66, 3, 377 (1998). CrossRef X. Mercier, E. Therssen, J. F. Pauwels and P. Desgroux, "Cavity ring-down measurements of OH radical in atmospheric premixed and diffusion flames.: A comparison with laser-induced fluorescence and direct laser absorption", Chem. Phys. Lett. 299, 1, 75 (1999). CrossRef J. Scherer, D. Voelkel and D. Rakestraw, "Infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) in low pressure flames", Appl. Phys. B 64, 6, 699 (1997). CrossRef R. Peeters, G. Berden and G. Meijer, "Near-infrared cavity enhanced absorption spectroscopy of hot water and OH in an oven and in flames", Appl. Phys. B 73, 1, 65 (2001). CrossRef T. Aizawa, "Diode-laser wavelength-modulation absorption spectroscopy for quantitative in situ measurements of temperature and OH radical concentration in combustion gases", Appl. Opt. 40, 27, 4894 (2001). CrossRef B. Löhden, S. Kuznetsova, K. Sengstock, V. M. Baev, et al., "Fiber laser intracavity absorption spectroscopy for in situ multicomponent gas analysis in the atmosphere and combustion environments", Appl. Phys. B 102, 2, 331 (2011). CrossRef A. Matynia, M. Idir, J. Molet, C. Roche, et al., "Absolute OH concentration profiles measurements in high pressure counterflow flames by coupling LIF, PLIF, and absorption techniques", Appl. Phys. B 108, 2, 393 (2012). CrossRef R. S. Watt, T. Laurila, C. F. Kaminski and J. Hult, "Cavity Enhanced Spectroscopy of High-Temperature H2O in the Near-Infrared Using a Supercontinuum Light Source", Appl. Spectrosc. 63, 12, 1389 (2009). CrossRef C. Abd Alrahman, A. Khodabakhsh, F. M. Schmidt, Z. Qu and A. Foltynowicz, "Cavity-enhanced optical frequency comb spectroscopy of high-temperature H2O in a flame", Opt. Express 22, 11, 13889 (2014). CrossRef A. Foltynowicz, P. Maslowski, A. J. Fleisher, B. J. Bjork and J. Ye, "Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared application to trace detection of hydrogen peroxide", Appl. Phys. B 110, 2, 163 (2013). CrossRef Z. Qu, R. Ghorbani, D. Valiev and F. M. Schmidt, "Calibration-free scanned wavelength modulation spectroscopy ? application to H2O and temperature sensing in flames", Opt. Express 23, 12, 16492 (2015). CrossRef L. Rutkowski, A. Khodabakhsh, A. C. Johansson, D. M. Valiev, et al., "Measurement of H2O and OH in a Flame by Optical Frequency Comb Spectroscopy", CLEO: Science and Innovations SW4H.8 (2016). CrossRef L. S. Rothman, I. E. Gordon, Y. Babikov, A. Barbe, et al., "The HITRAN2012 molecular spectroscopic database", J. Quant. Spectrosc. Radiat. Transf. 130, 4 (2013). CrossRef

Quantification of Supersonic Impulse Flow Conditions via High-Bandwidth Wavelength Modulation Absorption Spectroscopy

A tunable diode laser-based absorption diagnostic has been developed to characterize transient phenomena within the Stanford Expansion Tube Flow Facility. A measurement rate of 25 kHz was achieved to identify and investigate transient flow structures within the facility test time. Key parameters provided by the diagnostic include the temperature, mole fraction, and velocity time histories of the supersonic test gas, as well as the structure and timing of the test gas contact surfaces. The presence of water within the test gas affords well-established rovibrational transitions near that are probed using calibration-free, scanned wavelength-modulation absorption spectroscopy with second harmonic detection. This technique offers the sensitivity and noise resistance necessary for in situ temperature, velocity, and mole fraction measurements within the harsh conditions of the expansion tube. A rigorous scanned wavelength-modulation absorption spectroscopy with second harmonic detection theory has been employed to account for nonideal laser performance associated with high-bandwidth tunable diode laser absorption spectroscopy measurements. The measurements carried out with this sensor provide a quantitative characterization of the test gas conditions produced in the expansion tube and thus lay an essential foundation for supersonic combustion experimentation within the facility.

Self-calibration methodology by normalized intensity for wavelength modulation spectroscopy measurement

A methodology of self-calibration for concentration measurement based on wavelength modulation absorption spectroscopy has been developed by normalized fixed intensity. Experimental results show that the simple self-calibration method not only effectively improves the calibration accuracy, but also greatly improves stabilization and reliability as compared with the popular method called self-calibration by 1f normalized. In addition, the proposed system do not need any additional equipment when comparing with the use of traditional wavelength modulation absorption spectrometer. The standard deviation of a concentration measurement with different optical intensity aimed on the detector has been found to be below 1.0%. The dependence of the concentration assessment on the laser intensity fluctuation has also been investigated, which shows that the method of self-calibration could be applied in the field, specially where the dust is easily splattered on the windows of the detector.

Quantum cascade laser absorption sensor for carbon monoxide in high-pressure gases using wavelength modulation spectroscopy

A tunable quantum cascade laser sensor, based on wavelength modulation absorption spectroscopy near 4.8 μm, was developed to measure CO concentration in harsh, high-pressure combustion gases. The sensor employs a normalized second harmonic detection technique (WMS-2f/1f) at a modulation frequency of 50 kHz. Wavelength selection at 2059.91 cm⁻¹ targets the P(20) transition within the fundamental vibrational band of CO, chosen for absorption strength and relative isolation from infrared water and carbon dioxide absorption. The CO spectral model is defined by the Voigt line-shape function, and key line-strength and line-broadening spectroscopic parameters were taken from the literature or measured. Sensitivity analysis identified the CO-N₂ collisional broadening coefficient as most critical for uncertainty mitigation in hydrocarbon/air combustion exhaust measurements, and this parameter was experimentally derived over a range of combustion temperatures (1100-2600 K) produced in a shock tube. Accuracy of the wavelength-modulation-spectroscopy-based sensor, using the refined spectral model, was validated at pressures greater than 40 atm in nonreactive shock-heated gas mixtures. The laser was then free-space coupled to an indium-fluoride single-mode fiber for remote light delivery. The fiber-coupled sensor was demonstrated on an ethylene/air pulse detonation combustor, providing time-resolved (~20 kHz), in situ measurements of CO concentration in a harsh flow field.

In-situ CO measurement of gas and oil combustion flame using near infrared tunable diode laser with direct and modulated absorption signals

An in-situ optical system consisting of a near infrared single-mode distributed-feedback laser and a range-extended InGaAsSb detector is reported for CO measurement in gas and industrial oil combustion flames. To minimize interference from the other major gas species in the flame, the individual R(30) transition in the first overtone band was selected from the HITRAN/HITEMP database. The average CO volume concentration along the optical path was retrieved using two different signals: the direct absorption signal and the secondary harmonic signal of wavelength modulated absorption spectroscopy. The measurement limit was approximately 300ppm when the direct absorption signal was used and can be improved to 30ppm by using the secondary modulated harmonic signal. An electrically heated tubular gas cell with precisely controlled temperature and CO concentration was used for calibration. The measurement system accuracy was evaluated using a laboratory premixed C3H8/air flat flame. Practical measurements of an industrial oil flame were carried out under different fuel/air equivalence ratios. The results indicate that the system is capable of fast in-situ CO measurement of different flames and that the interference from water vapor is negligible. The maximum relative difference between the measured CO concentration and the theoretically calculated value was less than 200ppm in a stable gas flame. For industrial oil flames, the oscillation behavior of the CO concentration and the effect of the equivalence ratio on CO formation were captured clearly.

Wavelength modulated excitonic spectra of Cu2O thin films sandwiched by MgO plates

We have investigated stress effects on yellow exciton states in a Cu2O thin film sandwiched by MgO plates by measuring wavelength modulated (WM) absorption spectra and their thermal variations. In the WM absorption spectra, dispersive spectral structures owing to 2P~4P states in the yellow excitons are clearly resolved in the thin film sample. A stress due to the lattice mismatch between Cu2O and MgO provides a large red shift of the band gap in the green excitonic system. However, in the yellow excitonic transitions, it is found that a red-shift of the 2P excitonic state is much smaller than that in the green excitonic system. This result suggests that a shallow potential minimum for the yellow excitons is built up in the Cu2O thin film sandwiched by MgO plates.

Wavelength modulation diode laser absorption spectroscopy for high-pressure gas sensing

A general model for 1f-normalized wavelength modulation absorption spectroscopy with nf detection (i.e., WMS-nf) is presented that considers the performance of injection-current-tuned diode lasers and the reflective interference produced by other optical components on the line-of-sight (LOS) transmission intensity. This model explores the optimization of sensitive detection of optical absorption by species with structured spectra at elevated pressures. Predictions have been validated by comparison with measurements of the 1f-normalized WMS-nf (for n = 2–6) lineshape of the R(11) transition in the 1st overtone band of CO near 2.3 μm at four different pressures ranging from 5 to 20 atm, all at room temperature. The CO mole fractions measured by 1f-normalized WMS-2f, 3f, and 4f techniques agree with calibrated mixtures within 2.0 %. At conditions where absorption features are significantly broadened and large modulation depths are required, uncertainties in the WMS background signals due to reflective interference in the optical path can produce significant error in gas mole fraction measurements by 1f-normalized WMS-2f. However, such potential errors can be greatly reduced by using the higher harmonics, i.e., 1f-normalized WMS-nf with n > 2. In addition, less interference from pressure-broadened neighboring transitions has been observed for WMS with higher harmonics than for WMS-2f.

Single photon wavelength modulation absorption spectrum of acetylene for 1.5 m laser wavelength stabilization

We describe a kHz single photon wavelength modulation technology to realize the locking reference signal of the laser frequency. By detecting the single photon acetylene gas absorption spectrum and by amplifying the lock-in the discrete single photon response pulse, we achieve the laser frequency stabilized in real time. The 1.5 m distribution feedback semiconductor laser output frequency is stabilized at acetylene gas absorption peaks, and the typical laser frequency fluctuation is less than 25 MHz within 175 s. This single photon wavelength modulated absorption spectrum technology eliminates the high background noise in low frequency range and can be used in the quantum communication system and optical wavelength division multiplex.

A carbon monoxide and thermometry sensor based on mid-IR quantum-cascade laser wavelength-modulation absorption spectroscopy

A mid-infrared wavelength modulation spectroscopy (WMS) absorption sensor utilizing a quantum cascade laser (QCL) source for carbon monoxide detection at 4.6 μm and high-temperature thermometry has been developed and demonstrated. Measurements are made in a room-temperature gas cell and in high-temperature reflected-shock-heated gases. Room-temperature results demonstrate a minimum CO detectivity of 0.03 ppm per meter of absorption path length at a 1-kHz detection bandwidth, representing an order-of-magnitude increased sensitivity compared to scanned-wavelength direct absorption. High-temperature shock tube measurements demonstrate sensitive thermometry from 850 to 3500 K for pressures near 1 atm at detection bandwidths of 500 Hz and 1 kHz. Sensor determined temperatures agree with the post-reflected-shock conditions within ±1.9% (1σ deviation). To our knowledge, the thermometry measurements presented here represent the first WMS high-temperature thermometry measurements made using a QCL.