Wavelength Modulation Spectroscopy Method Research Articles

Infrared dual-gas CH4/C2H2 sensor system based on dual-channel off-beam quartz-enhanced photoacoustic spectroscopy and time-division multiplexing technique

Highly sensitive and stable measurement of methane (CH4) and acetylene (C2H2) based on a novel dual-channel off-beam quartz-enhanced photoacoustic spectroscopy and time-division multiplexing technique was realized by a compact 3D-printed gas cell with a size of 3 × 2 × 1 cm3. Two near-infrared distributed feedback diode lasers were employed to target the CH4 absorption line at 6046.9 cm−1 and the C2H2 absorption line at 6521.2 cm−1, respectively. Second-harmonic wavelength modulation spectroscopy method was used for photoacoustic signal recovery. A minimum detection level of ∼ 7.63 parts-per-million in volume (ppmv) for CH4 and a level of ∼ 17.47 ppmv for C2H2 were achieved with a 1 s lock-in integration time, leading to a normalized noise equivalent absorption (NNEA) coefficient of 7.24 × 10-8 cm−1·W·Hz−1 and 3.73 × 10-8 cm−1·W·Hz−1 for CH4 and C2H2, respectively. Allan-Werle deviation analysis was employed to evaluate the stability and the minimum detection limit (MDL) of the developed photoacoustic CH4/C2H2 dual-gas photoacoustic sensor. Owing to the high stability of the developed sensor system, an MDL of ∼ 0.73 ppmv and an MDL of ∼ 1.60 ppmv with a 100 s averaging time were achieved for CH4 and C2H2, respectively.

Calibration-Free Wavelength Modulation Spectroscopy Method Based on the Height-Width Characteristic of Second Harmonic

Calibration-Free Wavelength Modulation Spectroscopy Method Based on the Height-Width Characteristic of Second Harmonic

Reconstruction and fitting of second-harmonic signals by wavelength modulation spectroscopy method based on fast Fourier transform

Conventional wavelength modulation spectroscopy (WMS) is vulnerable to the influence of low-frequency noise. Accuracy of the method highly depends on the performance of the costly lock-in amplifier. In this article, we report a new and effective method for reconstructing second-harmonic signals through WMS based on fast Fourier transform (FFT). This method is less disturbed by low-frequency noise because it does not use a low-frequency ramp wave. Formulation and detection procedures were presented. The discrete second-harmonic waveform can be obtained by continuously changing the DC signal and FFT analysis in this method. Second-harmonic waveforms acquired by the two means are generally consistent. The experimental study validates the obtained gas concentration from 5% to 30%, showing a good linear relationship by the proposed method. The maximum relative error on concentration extraction is 2.87%; as for conventional WMS, this value is 4.50%. The developed measurement method may have potential in computed tomography.

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.

Dual-beam antiphase method to improve the WMS measurement limit in long-distance methane detection.

Wavelength modulation spectroscopy (WMS) with second harmonic detection is an extremely effective technique to detect gases in site applications. However, the significant levels of nonlinear effects in a system give rise to high background signals that either limit detection sensitivity or distort the harmonic signals. This paper outlines the theory of WMS-involved background signals and focuses on the elimination of undesirable effects in the background. A real-time, long-distance methane sensor using a tunable diode laser near 1653.7 nm is developed to continuously monitor methane by using a variable optical attenuator to suppress the background. Trace methane detection experiments verify that the minimum detection limit of the system can be increased by 47.5 times compared to the traditional WMS method.

Frequency-Division Multiplexing and Main Peak Scanning WMS Method for TDLAS Tomography in Flame Monitoring

The wavelength modulation spectroscopy (WMS) method is typically used in the tunable diode laser absorption spectroscopy (TDLAS) for spectrum extraction in a noisy background. Usually, two entire absorption spectra for target species are scanned in a cascade way in the WMS method, and it limits the temporal response of the TDLAS sensor. In this article, the frequency-division multiplexing and main peak scanning method were introduced to the WMS method and enabled a high frame rate six times that in the classical TDLAS sensor at the same scanning speed. For the first time, two wavelengths modulated at different frequencies were used to extract the absorptions in parallel along various laser paths and implemented successfully on a TDLAS tomography hardware, to reconstruct the distributions of both temperature and species concentrations, at a frame rate up to 10 kHz. A five-view fan-beam tomographic sensor was utilized in the tomographic imaging. In each view, lasers at two wavelengths were selected and modulated at different sinusoidal frequencies on the same sawtooth signal. Numerical simulations varied the high-quality tomographic imaging of both temperature and H2O molar concentration distributions for different phantoms. Experiments were also carried out on dynamic flames, which were generated by exciting a Bunsen burner acoustically. The acoustic excitation frequency varied from 40 to 750 Hz, and the results of the tomographic images agreed well with the flame evolutions.

A high-accurate and universal method to characterize the relative wavelength response (RWR) in wavelength modulation spectroscopy (WMS)

Considering the importance of the laser wavelength response and the difficulty in its real-scenario measurement in WMS, a high-accuracy and universal method was developed to characterize the relative wavelength response (RWR) by analyzing the laser current response. A coupling term that depends on both the current scan and the modulation characteristic was introduced to describe the coupling effect between the wavelength scan and modulation. The accuracy of the proposed method was verified with different laser working conditions and scan waveforms. All fitting residuals of the RWR result from the proposed method are smaller than 0.1% of the total scan range and the fitting residual of the ramp scanned WMS is twice smaller than the minimum value from literature. The better calibration-free 2f/1f fitting and more accurate CO2 concentration results also suggest the high accuracy and superiority of the proposed method. Finally, based on the precise prediction of RWR with small scan and modulation indices, the spectral parameters, including line strength and self-collisional broadening coefficient, of CO2 transition at 6976.2026 cm-1 were successfully measured using WMS.

Artificial absorption creation for more accurate tunable diode laser absorption spectroscopy measurement

A novel strategy for more accurate tunable diode laser absorption spectroscopy (TDLAS) measurement is presented. This method is immune to non-absorption transmission losses, and allows dead zone removal for ultra-low concentration detection, and reference point selection at atmospheric pressure. The method adjusts laser emission and creates artificial absorption peaks according to requirements. By creating an artificial absorption peak next to the real absorption zone, calibration is not necessary. The developed method can be applied to not only wavelength modulation spectroscopy (WMS) but also direct absorption (DA). In WMS, the method does not need two harmonic signals, resulting in higher reliability, better performance, and no electro-optical gain uncertainty. At the same time, non-absorption transmission losses effect is suppressed from 70% to 0.425% with DA and from 70% to 0.225% with WMS method. When the artificial absorption peak coincides with the real one, the dead zone of measurement can be removed to give a lower detection limit, and water vapor still can be detected when concentration is lower than 0.2ppm in our experiment. Reference point selection uncertainty with the DA method, especially when the signal-to-noise ratio is low and absorption line is broad, can also be facilitated. And the uncertainty of reference point selection is improved from 6% to 0.8% by measuring reference point amplitude. The method is demonstrated and validated by WMS and DA measurements of water vapor (1atm, 296K, 1368.597nm). The measurement results obtained using the new method reveal its promise in TDLAS.

Infrared Dual-Gas CH4/C2H6Sensor Using Two Continuous-Wave Interband Cascade Lasers

An infrared dual-gas sensor system for the simultaneous detection and monitoring of methane (CH 4 ) and ethane (C 2 H 6 ) at parts-per-billion by volume (ppbv) concentration levels was developed using two room temperature, distributed feedback interband cascade lasers, and two miniature multipass cells with an effective absorption length of 54.6 m. Laser direct absorption spectroscopy was used to detect CH 4 utilizing the 3038.5-cm -1 absorption line, and second-harmonic wavelength modulation spectroscopy method was used to detect C 2 H 6 using the 2996.88-cm -1 absorption line. The 1σ CH 4 detection limit is ~2.7 ppbv with a 1-s averaging time and exhibits a minimum value of ~1.7 ppbv for a 9-s averaging time; the 1σ C 2 H 6 detection limit is ~2.6 ppbv with a 3.4-s averaging time and shows an optimum averaging time of 65 s corresponding to a stability of ~0.36 ppbv. Using the dualgas sensor system, 24 h monitoring of the two atmospheric gases was performed in the Greater Houston area, TX, USA.

Polymeric fiber sensor for sensitive detection of carbon dioxide based on apodized wavelength modulation spectroscopy

The performance of an optical sensor that employs an unbuffered polydimethylsiloxane (PDMS)-cladding fiber optic is demonstrated for the sensitive detection of CO2 gas in the near-infrared region for around 1.57 µm using the apodized 2f/1f wavelength modulation spectroscopy method. The permeability and diffusion characteristics of the PDMS fiber have been theoretically examined and numerically simulated. The results of the simulation are verified by an experimental setup containing a DFB laser source and 5-m-coiled unbuffered PDMS fiber placed in a pre-vacuumed cell filled with about 980 ± 10 Torr of pure CO2 gas. A minimum detectable absorption of ~0.9 × 10−4 is measured, corresponding to a detection sensitivity of ~4.5 × 10−11 $${\text{cm}}^{ - 1} /{\text{Hz}}^{1/2}$$ . The effect of the scaling k-factor on the apodized signal is subsequently studied, showing close agreement between the simulation and experimental results.

Apodized 2f/1f wavelength modulation spectroscopy method for calibration-free trace detection of carbon monoxide in the near-infrared region: theory and experiment

We introduce the basics of an apodized 2f/1f wavelength modulation method for the spectroscopy of the R(9) transition line in the first overtone band of carbon monoxide (12C16O) in near-infrared (NIR) region around 2.33 μm. Performance of the method is investigated for high gas concentrations beyond the optically thin limit to generalize common 2f/1f wavelength modulation spectroscopy (WMS) reported by Rieker et al. (Appl Opt 48:5546, [28]). Numerical simulations are performed based on real experimental parameters associated with a NIR spectrometer designed in our laboratory. The results primarily show a more linear response and less error than occurred in the common WMS-2f/1f method for an optically thick sample. It is also theoretically shown that the apodized method enables sharpening the spectrum without peak displacement compared to the common WMS-2f/1f method. The validity of the method is verified experimentally by the trace detection of an air-broadened R(9) CO absorption line centered at 4,294.637 cm−1 at atmospheric pressure and room temperature. The effect of a so-called scaling k-factor on the sharpening of WMS-2f/1f signal is investigated through trace simulation and detection of CO and methane (CH4) lines in the scanning range of a distributed feedback laser. The obtained results show very good agreement between simulation and experiment.

Laser offset-frequency locking up to 20 GHz using a low-frequency electrical filter technique

A simple, easy-to-implement, and robust technique is reported to offset lock two semiconductor lasers with a frequency difference easily adjustable up to a couple of tens of gigahertz (10 and 19 GHz experimentally demonstrated). The proposed scheme essentially makes use of low-frequency control electronics and may be implemented with any type of single mode semiconductor laser, without any requirement for the laser linewidth. The technique is shown to be very similar to the wavelength modulation spectroscopy method commonly used for laser stabilization onto molecular absorption lines, as demonstrated by experimental results obtained using 935 nm laser diodes.

Experimental method based on wavelength-modulation spectroscopy for the characterization of semiconductor lasers under direct modulation.

An experimental method is presented for characterization of the combined intensity and frequency modulation produced when the injection current of a laser diode is modulated. The reported technique is based on the analysis of the harmonic signals produced when a modulated laser is used to probe a gas absorption line by the so-called wavelength-modulation spectroscopy method. Based on a theoretical model of this technique, we present two methods that facilitate the determination of (i) the deviation in laser frequency and (ii) the phase shift between intensity and frequency modulation. These methods are illustrated experimentally by measurement of the modulation parameters of a 2-microm distributed-feedback laser by use of a CO2 absorption line. The experimental results have been compared with those obtained with another traditional method and have shown full agreement in the frequency range (400 Hz-30 kHz) considered.