Wavelength Modulation Spectroscopy Technique Research Articles

Uncertainty quantification for high-temperature flame sensing using wavelength modulation spectroscopy

In this study, we explore the uncertainty of the wavelength modulation spectroscopy (WMS) technique applied to water profile measurement in a flat flame. The scanned WMS-2f/1f method is used to measure the water profile in a C2H4-O2-Ar flame, using an absorption transition line near 1391.7 nm. The Monte Carlo sampling method is used for the uncertainty quantification (UQ) analysis. The results show a non-negligible uncertainty on the water concentration with an uncertainty range from 10% to 30%, depending on the uncertainty level considered for the input parameters of the spectroscopic model. Further sensitivity analyses show that the lower state energy (E′′), the line strength, and the temperature are the most important uncertainty sources. When multiple transition lines are involved, the line strength value controls the overall influence of the other parameters associated with these specific lines. We also compared the uncertainty obtained when applying the direct absorption spectroscopy (DA) and WMS techniques for a similar flame configuration. Only a slight advantage is observed for the WMS.

High-precision characterization of quantum-cascade laser frequency response using wavelength modulation spectroscopy.

This paper investigates the impact of the quantum cascade laser's frequency modulation response on its tuning rate and tunability. We show a significant disparity in laser tuning rates and tunability between single and dual-frequency modulation schemes frequently used in typical direct absorption and wavelength modulation spectroscopy (WMS) techniques. We show that the DC-characterized tuning rate of a laser can be reduced significantly under a specific set of modulation frequencies of the laser injection current. We characterize these effects by simultaneous measurements of higher harmonic WMS of methane and nitrous oxide in the 7.8 µm spectral regions. We further show that WMS signal modulation broadening mechanisms and spectral structure, i.e., its zero-crossings and turning points, can be used to quantify such laser-modulation effects and validate laser frequency response under dual modulation schemes.

Highly sensitive detection of oxygen based on light-induced thermoelastic spectroscopy with a high power diode laser

Oxygen (O2) is a fundamental element in the atmosphere. It is important to detect it in various fields. In this paper, a sensitive O2 sensor based on light-induced thermoelastic spectroscopy (LITES) with a high power diode laser is reported. A distributed feedback (DFB) diode laser emitting at 760 nm with output power of 16.8 mW was employed as the light source. The O2 absorption line at 13144.54 cm-1 was selected for the investigation. Wavelength modulation spectroscopy (WMS) technique and second harmonic signal demodulation (2f) were adopted to improve the sensing performance. For the concentration dynamic range from 20% to 80%, a linear relationship between the amplitude of 2f signal and O2 concentration was demonstrated. Allan deviation analysis was conducted to evaluate the long-term stability of the sensor system and the minimum detection limit (MDL) was measured to be 583 ppm at the optimum integration time of 100 s. Further improvement of this O2-LITES sensor system was also discussed.

Simultaneous atmospheric CH4, CO2 and O2 detection using fiber-optic switch (FOS)-based time-division multiplexed NIR laser long optical path absorption spectroscopy

A near-infrared (NIR) distributed feedback (DFB) laser sensor based on long optical path absorption spectroscopy technique was developed for simultaneous measurement of atmospheric methane (CH4), carbon dioxide (CO2) and oxygen (O2). A fiber-optic switch (FOS) in conjunction with the time-division multiplexing (TDM) technique was adopted to enable different lasers scan over the target gas absorption lines in turn. The long optical path wavelength modulation spectroscopy (WMS) technique with second harmonic detection as well as a laser power fluctuation correction method was used to improve the sensor’s precision and accuracy. For the selected gas absorption lines, located at 6046.945 cm-1, 6361.245 cm-1 and 7877.647 cm-1, the detection limit (1σ) of 0.034 parts per million (ppm) for CH4, 11.921 ppm for CO2 and 0.14% for O2 was achieved according to the concentration and noise level. Allan deviation analysis indicates that the 1-s measurement precision was 0.030 ppm for CH4, 11.518 ppm for CO2 and 0.14% for O2, which could be improved to 6.5E-4 ppm, 0.255 ppm and 0.005% at an optimal average time of 800 s. A two-day continuous measurement of the CH4, CO2, and O2 concentration from ambient air was carried out, which verifies the stability and robustness of the developed multi-gas sensor.

Fiber-Optic Photoacoustic Gas Sensor with Multiplexed Fabry-Pérot Interferometric Cantilevers.

A fiber-optic photoacoustic (PA) gas sensor with multiplexed Fabry-Pérot (F-P) interferometric cantilevers is demonstrated. A compact cylindrical nonresonant PA tube with a volume of only 0.45 mL is designed. The PA signal is measured by two symmetrically installed fiber-optic interferometric cantilever microphones (FOICMs) to improve the signal-to-noise ratio (SNR). For multiplexing the two cantilevers by a single demodulation system, a dual cavity length synchronous measurement method based on total-phase demodulation algorithm with ultrahigh resolution is developed. The PA signal detection is realized by the second-harmonic wavelength modulation spectroscopy (2f-WMS) technique. The sensor performance is verified by conducting the detection of trace acetylene (C2H2). The normalized noise equivalent absorption (NNEA) coefficient is 2.5 × 10-9 cm-1·W·Hz-1/2, and the minimum detection limit (MDL) downs to about 0.2 ppm with an averaging time of 1 s. The fiber-optic PA gas sensor has characteristics of high resolution and immunity to electromagnetic and vibration interference. Furthermore, the technical scheme of the multiplexed cantilever demodulation shows great potential for remote multipoint monitoring of gases in harsh environments.

Dual-band light-induced thermoelastic spectroscopy utilizing an antiresonant hollow-core fiber-based gas absorption cell

In this paper, dual-band gas detection using a combination of the light-induced thermoelastic spectroscopy (LITES) and an antiresonant hollow-core fiber-based (ARHCF) gas absorption cell is demonstrated. The broad wavelength operation capability of a standard 32 kHz quartz tuning fork and the self-developed fiber-based gas absorption cell was exploited to demonstrate quasi-simultaneous detection of N2O and CO2, at 4570 nm (2188.2 cm−1) and 2006 nm (4985.9 cm−1), respectively. The signal analysis was based on the wavelength modulation spectroscopy technique, allowing to achieve a noise equivalent absorption coefficient (NEA) of 8.6 × 10–7 cm−1 and 1.7 × 10–6 cm−1 for N2O and CO2, respectively. The results indicate that the combination of ARHCFs with the LITES method is well suited for the design of broadband gas detectors and show remarkable potential in the fabrication of miniaturized, versatile and relatively inexpensive gas sensors operating over a wide spectral range, thus allowing multigas detection.

Simultaneous detection of greenhouse gases CH4 and CO2 based on a dual differential photoacoustic spectroscopy system.

In addition to the atmospheric measurement, detection of dissolved carbon oxides and hydrocarbons in a water region is also an important aspect of greenhouse gas monitoring, such as CH4 and CO2. The first step of measuring dissolved gases is the separation process of water and gases. However, slow degassing efficiency is a big challenge which requires the gas detection technology itself with low gas consumption. Photoacoustic spectroscopy (PAS) is a good choice with advantages of high sensitivity, low gas consumption, and zero background, which has been rapidly developed in recent years and is expected to be applied in the field of dissolved gas detection. In this study, a miniaturized differential photoacoustic cell with a volume of 7.9 mL is designed for CH4 and CO2 detection, and a dual differential method with four microphones is proposed to enhance the photoacoustic signal. What we believe to be a new method increases photoacoustic signal by 4 times and improves the signal to noise ratio (SNR) over 10 times compared with the conventional single-microphone mode. Two distributed feedback (DFB) lasers at 1651 nm and 2004nm are employed to construct the PAS system for CH4 and CO2 detection respectively. Wavelength modulation spectroscopy (WMS) and 2nd harmonic demodulation techniques are applied to further improve the SNR. As a result, sensitivity of 0.44 ppm and 7.39 ppm for CH4 and CO2 are achieved respectively with an integration time of 10 s. Allan deviation analysis indicates that the sensitivity can be further improved to 42 ppb (NNEA=4.7×10-10cm-1WHz-1/2) for CH4 and 0.86 ppm (NNEA=5.3×10-10cm-1WHz-1/2) for CO2 when the integration time is extended to 1000 s.

Ppb-level methane detection sensitivity based on a homemade Raman fiber amplifier and differential photoacoustic technology.

A high-power near-infrared wavelength-modulated differential photoacoustic spectroscopy sensor for parts-per-billion (ppb) level methane detection is reported by using a homemade Raman fiber optical amplifier. A commercial 1653.7nm continuous wave distributed feedback laser is employed as a seed source to excite a high light power of ∼550m W, which greatly improves sensor performance. Wavelength modulation spectroscopy and differential techniques are applied to further improve the signal-to-noise ratio of the photoacoustic signal. A 1σ minimum detection limit of ∼10p p b for methane detection is achieved with an integration time of 10s.

WMS-based near-infrared on-chip acetylene sensor using polymeric SU8 Archimedean spiral waveguide with Euler S-bend

SU8 is a cost-effective polymer material that is highly suitable for large-scale fabrication of waveguides. However, it has not been employed for on-chip gas measurement utilizing infrared absorption spectroscopy. In this study, we propose a near-infrared on-chip acetylene (C2H2) sensor using SU8 polymer spiral waveguides for the first time to our knowledge. The performance of the sensor based on wavelength modulation spectroscopy (WMS) was experimentally validated. By incorporating the proposed Euler-S bend and Archimedean spiral SU8 waveguide, we achieved a reduction in the sensor's size by over fifty percent. Leveraging the WMS technique, we evaluated the C2H2 sensing performance at 1532.83 nm for SU8 waveguides of lengths 7.4 cm and 13 cm. The limit of detection (LoD) values were 2197.1 ppm (parts per million) and 425.5 ppm, respectively, with an averaging time of 0.2 s. Furthermore, the experimentally obtained optical power confinement factor (PCF) closely approximated the simulated value, with a value of 0.0172 compared to the simulated value of 0.016. The waveguide loss is measured to be 3 dB/cm. The rise time and fall time were approximately 2.05 s and 3.27 s, respectively. This study concludes that the SU8 waveguide exhibits significant potential for high-performance on-chip gas sensing in the near-infrared wavelength range.

Differential quartz-enhanced photoacoustic spectroscopy

In this paper, a differential quartz-enhanced photoacoustic spectroscopy (D-QEPAS) sensor is reported. The differential photoacoustic cell (PAC) was used to generate the photoacoustic effect. Two quartz tuning forks (QTFs) with a quality factor (Q) up to 10 000 were used as the acoustic wave transducers. The signal of D-QEPAS sensor was doubly enhanced by the differential characteristic of differential PAC and resonant response of QTF. The background noise was suppressed based on the differential principle. With the help of the finite element method, the acoustic field characteristics were simulated and calculated. Wavelength modulation spectroscopy technique and second harmonic (2f) detection technique were applied to detect photoacoustic signal. Trace acetylene (C2H2) gas detection was performed to verify the D-QEPAS sensor performance. The 2f signal amplitude of differential mode was 116.03 μV, which had a 1.65 times improvement compared with the 2f signal amplitudes of QTF1. When the integration time was 334 s, the minimum detection limit of D-QEPAS sensor was about 496.7 ppb. The reported D-QEPAS provides a development and idea for the widely reported QEPAS technique.

Detailed analysis of the R1f/ΔI1 WMS technique and demonstration of significantly higher detection sensitivity compared to 2f WMS for calibration-free trace gas sensing.

Recognizing that wavelength modulation spectroscopy (WMS) is particularly important in the development of high-sensitivity gas sensing systems, this paper presents a detailed analysis of the R 1f /Δ I 1 WMS technique that has recently been successfully demonstrated for calibration-free measurements of the parameters that support detecting multiple gases under challenging conditions. In this approach, the magnitude of the 1f WMS signal (R 1f ) was normalized by using the laser's linear intensity modulation (Δ I 1) to obtain the quantity R 1f /Δ I 1 that is shown to be unaffected by large variations in R 1f itself due to the variations in the intensity of the received light. In this paper, different simulations have been used to explain the approach taken and the advantages that it shows. A 40mW, 1531.52nm near-infrared distributed feedback (DFB) semiconductor laser was used to extract the mole fraction of acetylene in a single-pass configuration. The work has shown a detection sensitivity of 0.32ppm for 28cm (0.089ppm-m) with an optimum integration time of 58s. The detection limit achieved has been shown to be better than the value of 1.53ppm (0.428ppm-m) for R 2f WMS by a factor of 4.7, which is a significant improvement.

Research on the Seed Respiration CO2 Detection System Based on TDLAS Technology

The traditional detection method of CO2 concentration in seed respiration has defects such as low detection accuracy, low detection efficiency, and inability to monitor in real time. In order to solve these problems, we report a seed respiration CO2 detection system based on wavelength modulation spectroscopy (WMS) techniques in tunable diode laser absorption spectroscopy (TDLAS). This system uses a 2004 nm distributed feedback (DFB) laser as the light source, and a double-layer seed respiration device (about 1.5 L) is designed based on Herriott cell with an effective optical path of about 21 meters. Then, the second harmonic (2f) signal is extracted by the wavelength modulation method for CO2 concentration inversion. When the ambient temperature and pressure changes greatly, the corrected 2f signal is used for CO2 concentration inversion to improve the accuracy. A series of verification and comparison experiments have proved that the seed respiration CO2 detection system has the advantages of strong stability, high sampling frequency, and high detection accuracy. Finally, we used the developed system to measure the respiration intensity and respiration rate of 1 g corn seeds. The respiration intensity curves and respiration rate change details show that the seed respiration CO2 detection system is more suitable for a small amount of seeds than nondispersive infrared (NDIR) CO2 sensor and gas chromatography in real-time monitoring of the breathing process.

A sensor based on high-sensitivity multi-pass resonant photoacoustic spectroscopy for detection of hydrogen sulfide

A high-sensitive tunable diode laser sensor based on photoacoustic spectroscopy and multi-pass absorption spectroscopy is developed for hydrogen sulfide (H2S) detection using a target absorption line at 6287.005 cm−1. Two spherical concave mirrors are utilized as the reflectors to make the laser beam pass through the photoacoustic cell for 24 times. The wavelength modulation spectroscopy technique with second harmonic detection (WMS-2f) method is employed to reduce external interference and increase the detection sensitivity. With the enhancement of the effective laser power in the multi-pass PA cell, the photoacoustic 2f signal of the target H2S absorption line has a ∼ 8 times improvement compared to the sensor system without multi-pass cell. Based on the results of Allan variance analysis, the minimum detection concentration of the system could reach 0.68 ppm with a 10-s average time and it can be further lowered down to ∼ 0.1 ppm when a 1-minute averaging time is used. The linear responsivity of the multi-pass PA spectroscopy sensor is estimated to be ∼ 0.00112 mV/ppm by linear fitting for a continuous flow gas measurement results. The excellent performance validates the potential of the sensor for field application.

Mid-infrared fiber-coupled laser absorption sensor for simultaneous NH3 and NO monitoring in flue gases

We report the development of a mid-infrared fiber-coupled laser absorption sensor for in situ and simultaneous detection of NH3 and NO, which is suitable for flue gas monitoring at the SCR (selective catalytic reduction) exhaust. Two quantum cascade lasers (QCLs) are used to detect the optimal absorption lines of NH3 and NO centered at 1103.45 cm−1 and 1929.03 cm−1, respectively. The two QCLs are coupled into a single hollow-core fiber and delivered to an open-path, single-ended optical probe for in situ gas detection. The gas concentration is determined by an improved calibration-free wavelength modulation spectroscopy (CF-WMS) technique with a reduced line model. A series of experiments are conducted to evaluate the sensor performance at varied temperatures from 296 to 573 K and the pressure of 1 atm. Such a dual-species sensor realizes a minimum detection limit of 30 ppb at 100 s average time for NO and 14 ppb at 70 s for NH3, respectively. The uncertainty of NH3 and NO detection is determined to be less than 5 %. By evaluating the WMS spectra of other species, the current sensor shows negligible spectral interferences under typical flue gas conditions.

Standoff sub-ppb level measurement of atmospheric ammonia with calibration-free wavelength modulation spectroscopy

Ammonia (NH3) plays a significant role in the formation of atmospheric particulate matter, and influences on environmental and public health as well as climate change. Thus, it is important to sensitive measurement of atmospheric NH3. In the present work, a sub-ppb level standoff open-path NH3 sensor was developed for on line, sensitive measurement of atmospheric NH3. A 9.06 μm distributed feedback quantum cascade laser was employed to probe the ammonia absorption lines located on fundamental rotational-vibrational absorption band and calibration-free wavelength modulation spectroscopy technique was employed to retrieve NH3 concentration directly. The standoff open-path NH3 sensor performance was investigated in laboratory corridor with 80 m open path length (Hefei, China) and a minimum detection limit of 0.46 ppb (3σ) was obtained. Finally, field campaign measurement was carried out in a winter wheat farmland (Changshu, China). Field measurement shown that the concentration of NH3 varies from 7 ppb to 30 ppb with an average of 14 ppb. The developed standoff sensor has high potential to be a robust tool for monitoring atmospheric NH3 or study of regional ammonia emissions in farmland or feedlot scale.

Highly Sensitive Hydrogen Sensing Based on Tunable Diode Laser Absorption Spectroscopy with a 2.1 μm Diode Laser

As a new form of energy, hydrogen (H2) has clean and green features, and the detection of H2 has been a hot topic in recent years. However, the lack of suitable laser sources and the weak optical absorption of H2 limit the research concerning its detection. In this study, a continuous-wave distributed feedback (CW-DFB) diode laser was employed for sensing H2. Tunable diode laser absorption spectroscopy (TDLAS) was adopted as the detection technique. The strongest H2 absorption line, located at 4712.90 cm−1 (2121.83 nm, line strength: 3.19 × 10−26 cm−1/cm−2 × molec), was selected. We propose a H2-TDLAS sensor based on the wavelength modulation spectroscopy (WMS) technique and a Herriott multipass gas cell (HMPC) with an optical length of 10.13 m to achieve a sensitive detection. The WMS technique and second harmonic (2f) demodulation technique were utilized to suppress system noise and simplify the data processing. The 2f signal of the H2-TDLAS sensor, with respect to different H2 concentrations, was measured when the laser wavelength modulation depth was at the optimal value of 0.016 cm−1. The system’s signal-to-noise ratio (SNR) and minimum detection limit (MDL) were improved from 248.02 and 0.40% to 509.55 and 0.20%, respectively, by applying Daubechies (DB) wavelet denoising, resulting in 10 vanishing moments. The Allan variance was calculated, and the optimum MDL of 522.02 ppm was obtained when the integration time of the system was 36 s.

Spectroscopic trace gas detection in air-based gas mixtures: Some methods and applications for breath analysis and environmental monitoring

Laser absorption spectroscopy as a powerful tool for detecting trace gases has been widely used in the monitoring of atmospheric greenhouse gases, pollutions, and respiration processes, including human breath analysis. The detection is based on the light absorption when it propagates through a medium. Most inorganic and organic molecules have characteristic absorption lines in the mid-infrared (mid-IR), which correspond to fundamental vibrational modes and in the near-IR (first overtones) presenting their absorption fingerprints. Here, we summarize the recent developments of the three techniques, namely, wavelength modulation spectroscopy (WMS), cavity ringdown spectroscopy (CRDS), and frequency comb spectroscopy (FCS), and describe their main features as well as possible applications, illustrated by recent experimental results. Emphasis is made on methane detection as applied to breath analysis and atmospheric monitoring. For the WMS technique, we consider local measurements with a multipass cell and also kilometer long open-path configurations for the near-IR and mid-IR spectral regions. The results of measurements of methane in exhaled breath with the CRDS technique in the near-IR are presented for a group of subjects of different ages. We consider various schemes of the FCS that enable fast broadband detection, including direct spectroscopy, dual FCS, and Vernier FCS, and review numerous applications of this approach that revolutionized the field of absorption spectroscopy. The current trends and possible future developments and applications are also discussed.

High-Sensitivity Silicon Cantilever-Enhanced Photoacoustic Spectroscopy Analyzer with Low Gas Consumption.

A silicon cantilever-enhanced photoacoustic spectroscopy (PAS)-based trace gas analyzer with low gas consumption is presented. A silicon cantilever-based fiber-optic Fabry-Perot (F-P) interferometric acoustic sensor with a compact structure and high sensitivity is designed for photoacoustic signal detection. The non-resonant photoacoustic cell (PAC) is a cylindrical copper tube with a volume of 0.56 mL. A near-infrared laser with a center wavelength of 1532.83 nm amplified using an erbium-doped fiber application amplifier is used as the excitation light. The wavelength modulation spectroscopy (WMS) technique is employed in the present work for second-harmonic photoacoustic signal detection. The experimental results show that the minimum detection limit of C2H2 is 199.8 parts per trillion (ppt) with an average time of 60 s. The normalized noise equivalent absorption coefficient is calculated as 1.72 × 10-9 cm-1 W/Hz1/2. Furthermore, the proposed silicon cantilever-enhanced non-resonant PAS-based gas analyzer can not only analyze the gas concentration in a closed small-capacity PAC with low gas consumption but also detect target gas leakage in real time at a long distance.

Measurement of methane gas with high absorbance based on modified wavelength modulation spectroscopy

In this paper, the wavelength modulation spectroscopy (WMS) technique is modified and used for measuring methane with large absorbance. The WMS has been frequently used for gas measurement and relies on the linear relationship between the second harmonic amplitude and the gas volume concentration. However, the conventional WMS technique is only applicable for the gas whose absorbance is much smaller than 1, which is because the first-order approximation of Lambert-Beer's law is required in the derivation of the traditional WMS theory, and the first-order approximation holds only at low absorbance, hence the linear relationship between the second harmonic and the gas concentration does not hold at large absorbance. In the modified WMS in this work, it is not necessary to make any approximation to Lambert-Beer's law. The measured light is absorbed by the gas to be measured and then collected by the photodetector. The reference light is directly detected by another photodetector without being absorbed. The output signals of the two photodetectors are transmitted to the computer after implementing analog-to-digital conversion. In this way, the demodulated second harmonic signal remains linear with the gas concentration even at large absorbance. In this work, the traditional WMS theory and the modified WMS theory are both introduced, and a series of methane gas with concentration gradients are measured separately. The experimental results of the traditional WMS and the modified WMS are compared with each other. It is confirmed that the linearity in the traditional WMS theory no longer holds under large absorbance, but the improved WMS can still guarantee the linear relationship between the second harmonic and the methane concentration, which verifies the advantages of the modified scheme. Finally, through Allan's standard deviation analysis, the stability of this methane measurement system reaches the optimal value at the average time of 103.6 s, and the corresponding Allan's standard deviation is 1/26.62×10<sup>–9</sup> volume.

Large absorbance methane measurement based on wavelength modulation spectroscopy

In this paper, the wavelength modulation spectroscopy (WMS) technique is modified and used for measuring methane with large absorbance. WMS has been frequently used for gas measurement and relies on the linear relationship between the second harmonic amplitude and the gas volume concentration however, the conventional WMS technique is only applicable for gases whose absorbance is much smaller than 1, which is because of the first-order approximation to Lambert-Beer's law in the derivation of the traditional WMS theory, but the first-order approximation holds only at low absorbance, hence the linear relationship between the second harmonic and the gas concentration does not hold at large absorbance. In the modified WMS in this paper, there is no need to make any approximation to Lambert-Beer's law. The measurement light is absorbed by the gas to be measured and then collected by the photodetector, and the reference light is directly detected by another photodetector without being absorbed, and the output signals of the two photodetectors are transmitted to the computer after analog-to-digital conversion. In this way, the demodulated second harmonic signal remains linear with the gas concentration even at large absorbance. In this paper, the traditional WMS theory and the modified WMS theory are introduced, and a series of methane with concentration gradients are measured separately to compare the experimental results of the traditional WMS and the modified WMS. It is confirmed that the linearity in the traditional WMS theory no longer holds under large absorbance, but the improved WMS can still guarantee the linear relationship between the second harmonic and the methane concentration, which verifies the advantages of the modified scheme. Finally, through Allen's standard deviation analysis, we obtained that the stability of this methane measurement system reaches the optimum at the average time of 103.6s, and the corresponding Allen's standard deviation is 26.62 ppbv.