Wavelength modulation spectroscopy for recovering absolute absorbance

Method of adaptive wide dynamic range gas concentration detection based on optimized direct absorption spectroscopy

For wide dynamic range gas concentration detection based on tunable diode laser absorption spectroscopy (TDLAS), direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) are usually used in combination. However, in some application scenarios such as high-speed flow field detection, natural gas leakage, or industrial production, the requirements of wide-range, fast response and calibration-free must be met. Taking applicability and cost of TDALS-based sensor into consideration, a method of optimized direct absorption spectroscopy (ODAS) based on signal correlation and spectral reconstruction is developed in this paper. This method can achieve adaptive selection of the optimal benchmark spectrum for spectral reconstruction. Moreover, methane (CH₄) is taken as an example to carry out the experimental verification. Experimental results proved that the method satisfies wide dynamic range detection of more than 4 orders of magnitude. It is worth noting that when measuring large absorbance with concentration of 75 × 10⁴ppm with DAS and ODAS method, respectively, the maximum value of residual is reduced from 3.43 to 0.07. Furthermore, whether measuring gas of small or large absorbance with different concentrations, which vary from 100 ppm to 75 × 10⁴ppm, the correlation coefficient between standard concentrations and inverted concentrations is 0.997, showing the linear consistency of the method in wide dynamic range. In addition, the absolute error is 1.81 × 10⁴ppm when measuring large absorbance of 75 × 10⁴ppm. It greatly improves the accuracy and reliability with the new method. In summary, the ODAS method can not only fulfill the measurement of gas concentration in wide range, but also further expand the application prospects of TDLAS.

Development of a Stable Oxygen Sensor Using a 761 nm DFB Laser and Multi-Pass Absorption Spectroscopy for Field Measurements

An optical sensor system based on wavelength modulation spectroscopy (WMS) was developed for atmospheric oxygen (O₂) detection. A distributed feedback (DFB) laser with butterfly packaging was used to target the O₂ absorption line at 760.89 nm. A compact multi-pass gas cell was employed to increase the effective absorption length to 3.3 m. To ensure the stability and anti-interference capability of the sensor in field measurements, the optical module was fabricated with isolation of ambient light and vibration design. A 1f normalized 2f WMS (WMS-2f/1f) technique was adopted to reduce the effect of laser power drift. In addition, a LabVIEW-based dual-channel lock-in amplifier was developed for harmonic detection, which significantly reduced the sensor volume and cost. The detailed detection principle was described, and a theoretical model was established to verify the effectiveness of the technique. Experiments were carried out to obtain the device's sensing performances. An Allan deviation analysis yielded a minimum detection limit of 0.054% for 1 s integration time that can be further improved to 0.009% at ~60 s. Finally, the reliability and anti-interference capability of the sensor system were verified by the atmospheric O₂ monitoring.

Fast gas sensing scheme with multi-component gas measurement capacity based on non-dispersive frequency comb spectroscopy (ND-FCS)

A fast gas sensing scheme based on a non-dispersive frequency comb spectroscopy (ND-FCS) is proposed and experimentally demonstrated. Its capacity for multi-component gas measurement is experimentally investigated as well, by using the time-division-multiplexing (TDM) method to realize specific wavelength selection of the fiber laser optical frequency comb (OFC). A dual-channel optical fiber sensing scheme is established with a sensing path consisting of a multi-pass gas cell (MPGC), and a reference path with a calibrated signal to track the repetition frequency drift of the OFC for a real-time lock-in compensation and system stabilization. The long-term stability evaluation and the simultaneous dynamic monitoring are carried out, with the target gases of ammonia (NH₃), carbon monoxide (CO) and carbon dioxide (CO₂). The fast CO₂ detection in human breath is also conducted. The experimental results show that at an integration time of 10 ms, the detection limits of the three species are evaluated to be 0.0048%, 0.1869% and 0.0467%, respectively. A low minimum detectable absorbance (MDA) down to 2.8 × 10^-4 can be achieved and a dynamic response with millisecond time can be realized. Our proposed ND-FCS exhibits excellent gas sensing performance with merits of high sensitivity, fast response and long-term stability. It also shows great potential for multi-component gas monitoring in atmospheric monitoring applications.

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.

SGCS: a signal reconstruction method based on Savitzky-Golaysgz filtering and compressed sensing for wavelength modulation spectroscopy

The residual oxygen concentration in pharmaceutical glass vial variously threatens the aseptic properties of encapsulated agents. The demodulated 2nd harmonic signals in the wavelength modulation spectroscopy (WMS) detection system, the data basis of the inversion of oxygen concentration, are inevitably destroyed by various time-varying industrial noises. In this work, we propose a signal reconstruction method based on self-correcting Savitzky-Golaysgz filter and compressed sensing (namely SGCS) for the urgent signal denoising task, which is a dual-step lightweight denoising scheme. First, in order to avoid the influence of glitch noise on sparse signal reconstruction, Savitzky-Golay (S-G) filter is used to smooth the 2nd harmonic signal while retaining the change information effectively. Then, the well-tuned measurement matrix of compressed-sensing (CS) is applied to aggressively fetch the sparse principal components while bypassing most residual dynamic noises. Finally, the orthogonal matching pursuit (OMP) is used to reconstruct the 2nd harmonic signal according to sparsity constrain and the sparse principal components. Experimental results show that the performance of SGCS method is superior. Compared with other competitive methods the operation time of SGCS is the shortest. When the normalized SNR is 1, the average correct discrimination rate is 98.57%. Even if SNR reduces from 1 to 0.55, the WMS detection system still survives well, with the highest average correct discrimination rate of 89.34%.

Dual-logarithmic demodulation method application in a wide gas optical thickness range

Direct absorption spectroscopy (DAS) is an extremely practical and effective technology to detect gas concentration in site applications. Dual-beam subtraction is one of the most common demodulation methods in DAS, yet this method cannot solve the problem of absolute absorption curve nonlinearization in a wide optical thickness range. A real-time and practical dual-logarithmic demodulation method is proposed and proved to be robust when the optical thickness is much greater than linear region. Moreover, the error of optical thickness peak is only 1.18% between the dual-logarithmic demodulation system and simulation after correcting the dual-beam subtraction demodulation system under a 300 K, 1 atm, and 3 m absorption path. When the range of optical thickness peak of acetylene is from 0.0252 to 2.5335 at 1532.83 nm, the peak voltages always maintain satisfactory linearity (R-square=0.9989).

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.

Exploring the feasibility of wavelength modulated near-infrared spectroscopy

SIGNIFICANCE

The application of near-infrared spectroscopy (NIRS) to determine the concentrations of tissue chromophores has typically relied on three alternative technological approaches: continuous-wave, frequency-domain, and time-domain. It is often the case that uncertain and variable coupling of light into and out of the skin surface renders absolute measurements unreliable, and NIRS methods are mostly used to measure changes of chromophore concentrations and of physiological parameters such as blood volume and oxygenation.

AIM

The aim has been to investigate whether an approach based on a wavelength-modulated source may enable measurements to be acquired, which are independent of surface coupling and may facilitate derivation of absolute values of tissue parameters.

APPROACH

An analysis is performed using the modified Beer-Lambert law.

RESULTS

It is shown that the relative modulation in detected intensity resulting from a wavelength-modulated source could be used to estimate absolute concentrations of chromophores if unknown surface coupling losses and geometrical factors are insensitive to small changes in wavelength.

CONCLUSIONS

Wavelength modulated NIRS could be an effective tool for quantitative in vivo analysis of tissues, although it may be technically challenging.

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.

Highly Sensitive, Calibration-Free WM-DAS Method for Recovering Absorbance-Part I: Theoretical Analysis

The absorbance is of great importance in the tunable diode laser absorption spectroscopy (TDLAS) as it contains information of both gas properties and spectroscopic parameters. A novel, calibration-free wavelength modulation-direct absorption spectroscopy (WM-DAS) is proposed and experimentally verified in this two-part paper. This method combines the capability of absorbance measurement from DAS and the advantages of enhanced noise rejection and high sensitivity from WMS. In this Part I, we focus on the full theoretical basis and procedures of this method from the following three aspects: the high-accuracy characterizations of laser frequency and intensity, noise rejection ability by extracting the characteristic spectra through the fast Fourier transform (FFT) of the light intensity, and the simultaneous fitting strategy for both baseline and absorbance. The preliminary validation experiment of CO transition at 4300.6999 cm-1 in a static gas cell shows the high accuracy of the proposed method.

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.

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.