Optimal Integration Time Research Articles

Demonstration of record sensitivity for water vapor detection by means of comb-locked cavity ring-down spectroscopy

We report on the development, characterization, and test of a comb-locked cavity ring-down spectrometer (CL-CRDS) operating in the spectral region around 1.39 µm. The system is based on the use of a hemispherical optical resonator with a finesse as high as ∼507000, which gives an empty-cavity ring-down time of about 285 µs. An Allan-Werle analysis on repeated acquisitions of the ring-down time at a fixed laser frequency suggests a minimum detectable absorption coefficient of 2×10−12cm−1 for the optimum integration time of 45 s. This limit can be exceeded by adopting the strategy of long-term spectral averaging. Taking advantage of the frequency stability guaranteed by the optical frequency comb, the CL-CRDS spectra were averaged over more than two days, thus removing efficiently the effect of mechanical, acoustic, and thermal noises. As a result, we could achieve a minimum detectable absorption coefficient as low as 3.7×10−13cm−1, which corresponds to a limit of detection for H2O in N2 of nine parts per trillion and a H2O partial pressure of 2×10−8 Pa (or 2×10−10 mbar). The potentialities of our approach are demonstrated by recording the absorption features of HD16O and HD18O in flows of ultra-high-purity N2 and ambient air, respectively.

EC-QCL-based photoacoustic spectroscopy for simultaneous detection of SF6 and SO2F2

Abstract In recent years, sulfur hexafluoride (SF6) and sulfuryl fluoride (SO2F2) have been widely used as gas insulating medium and fumigant, respectively, leading to an increase in their emissions. Additionally, SF6 will decompose into SO2F2 in gas-insulated equipment due to overheating and discharge faults. Considering the hazards of SF6 and SO2F2 to the atmosphere and human health, it is essential to develop an accurate measurement system capable of simultaneously monitoring trace amounts of SF6 and SO2F2 in the ambient air at the application site. In the present work, a photoacoustic (PA) system combining an external-cavity quantum cascade laser (EC-QCL) and a differential Helmholtz resonator has been used to realize the detection of SF6 and SO2F2. When the characteristic absorption wavenumbers of SF6 and SO2F2 were 1256.9 cm−1 and 1270.9 cm−1, respectively, the minimum detection limits of 2.3 ppm and 6.3 ppb were achieved. Based on the Allan deviation analysis, the stability of the PA system was evaluated and the detection sensitivity of both SF6 and SO2F2 was improved under the optimal integration time. An analysis of four different mixtures of SF6 and SO2F2 in N2 demonstrated the ability of the PAS apparatus to measure both gases simultaneously.

Cubic nonlinear scanning for improved TDLAS-based methane concentration detection

In this study, we developed and validated an improved tuning method for a nonlinear scanning laser diode in methane gas sensing, using wavelength modulation spectroscopy - tunable diode laser absorption spectroscopy (WMS-TDLAS). This approach combines cubic nonlinear scanning with high-frequency modulation to precisely target the methane absorption peak at 6046.96 cm−1. The method enhances absorption time, thus increasing system sensitivity and stability. Our results show that this nonlinear scanning, optimized for peak absorption, significantly improves the slope of the methane concentration relationship curve by 76.9%. It also reduces the standard deviation by 61.1% and the limit of detection by 60% compared to linear scanning. The limit of detection the system is 4.2 ppm, and the optimal integration time is 48s. This novel modulation strategy significantly enhances the WMS-TDLAS system's efficiency, offering significant benefits for various applications.

Ultra-low concentration detection of dissolved C2H2 in oil using fixed-frequency RF-assisted calibration-free wavelength modulation spectroscopy

The monitoring sensitivity of dissolved C2H2 in transformer oil is crucial to ensuring the safe operation of electric power systems. A highly sensitive sensor system based on fixed-frequency radio frequency assisted calibration-free wavelength modulation spectroscopy (FF-RF-WMS-2f/1f) for dissolved C2H2 in transformer oil was developed and evaluated. Lowering the headspace sampling pressure to 0.1 atm effectively increased the outgassing ratio and detection signal of dissolved C2H2, while also reducing background interference. By employing a novel fixed-frequency RF-assisted modulation technique, we effectively suppressed the amplitude and influence of optical fringes and improved the system’s linear fitting R-squared value for gas-phase concentration from 0.9997 to 0.9999. Furthermore, the minimum detection limit (MDL), as calculated by Allan deviation, was reduced from 8.58 ppb to 4.36 ppb, while the optimal integration time increased slightly from 62 s to 79 s. The equivalent MDL for dissolved C2H2 reached 0.9 ppb. The comparative experiments showed that the error was less than 0.2 ppm or 8 %, the relative standard deviations (RSDs) of continuous measurement were less than 3 %, and the RSD at ultra-low concentration was less than 8 %. The experiments verified the high sensitivity and accuracy of the sensor system, making it suitable for ultra-low concentration dissolved C2H2 detection in transformer insulating oil.

Simultaneous measurement of 13C-,18O-, and 17O- isotopes of CO2 using a compact mid-infrared hollow waveguide gas sensor

A compact gas sensor has been developed for the first time employing a quantum cascade laser (QCL) with a central wavelength of 4.32 μm, combined with direct absorption spectroscopy technology and a hollow waveguide. The sensor could measure the concentrations of four isotopic molecules (16O12C16O, 18O12C16O, 16O13C16O, and 17O12C16O) in CO2 gas simultaneously, thereby obtaining the relative isotope ratios of δ13C, δ18O, and δ17O. By adjusting the incident light path, the anomalous absorption peaks in the system have been effectively suppressed, and a compact feedback function generator has been integrated to ensure control of the laser output wavelength to eliminate the loss of measurement precision caused by wavelength fluctuations. Under the two conditions of free-running and wavelength-controlled, a standard CO2 sample with a concentration of 5 % was measured continuously. The results showed that under wavelength-controlled conditions, the precision of the four isotopic molecule concentration measurements was improved by a minimum of 1.8 times, and the measurement precision could be further improved after filtering. According to the Allan variance analysis, the detection precision for δ13C, δ18O, and δ17O were 0.7 ‰, 0.62 ‰, and 1.58 ‰, respectively, at the optimal integration time of 98 seconds. This study has the potential for respiratory diagnosis to detect simultaneously the values of δ13C, δ18O, and δ17O in exhaled samples.

Chicken Meat and Beef Identification in UV-Vis Absorbance Spectrum by Applying Savitzky-Golay Method

Spectroscopy is one of the techniques suitable for validation of the quality, safety, and hygiene of meat due to some advantages, such as rapid detection capability, ease of use, and non-destructive measurement. Utilizing an ultraviolet-visible (UV-Vis) spectrometer, with a wavelength range of 200 nm to 1100 nm, this paper proposes a method for processing spectrum data profiling of chicken meat and beef. A 360-time reflectance spectrum was acquired from each chicken breast and beef loin at the optimum integration time of 60 ms. Subsequently, the spectrum was converted into an absorbance spectrum, and the apparent noise was excluded by applying the Savitzky-Golay (SG) method. The result shows the difference in spectrum between chicken meat and beef in which the chicken meat has two peaks at the wavelength of 284.79 nm and 422.69 nm, meanwhile, the beef shows three peaks at wavelengths of 282.46 nm, 419.03 nm, and 577.97 nm. It was found that the SG method enhanced the absorbance spectrum, showing the difference in spectrum behaviors in the wavelength range of 450∼600 nm. Consequently, to shorten the computational time, building a light, cheap, and compact prototype reading device within this specific wavelength range will be the next target in the near future.

Anti-disturbance self-alignment method for dual-axis rotational inertial navigation system under marine mooring conditions

Under the marine mooring conditions, the inertial navigation system (INS) is often disturbed by angular and linear motions, and it is difficult to perform self-alignment with high accuracy. To solve this problem, an anti-disturbance self-alignment method for dual-axis rotational INS is proposed. The method transforms the self-alignment problem into an attitude matrix solving problem, so as to deal with the disturbance of angular motion. By analyzing the relationship between the amount of measurement data and the accuracy of self-alignment, the optimal integration time is designed and a partial integral algorithm is proposed. Furthermore, the influence of initial velocity error on self-alignment under linear motion disturbance conditions is analyzed, and an initial velocity compensation method is proposed. Experimental results show that high-accuracy self-alignment can be achieved using the proposed method under the joint disturbance of angular and linear motions.

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.

Temporal integration of sound motion: Motion-onset response and perception

The purpose of our study was to estimate the time interval required for integrating the acoustical changes related to sound motion using both psychophysical and EEG measures. Healthy listeners performed direction identification tasks under dichotic conditions in the delayed-motion paradigm. Minimal audible movement angle (MAMA) has been measured over the range of velocities from 60 to 360 deg/s. We also measured minimal duration of motion, at which the listeners could identify its direction. EEG was recorded in the same group of subjects during passive listening. Motion onset responses (MOR) were analyzed. MAMA increased linearly with motion velocity. Minimum audible angle (MAA) calculated from this linear function was about 2 deg. For higher velocities of the delayed motion, we found 2- to 3-fold better spatial resolution than the one previously reported for motion starting at the sound onset. The time required for optimal discrimination of motion direction was about 34 ms.The main finding of our study was that both direction identification time obtained in the behavioral task and cN1 latency behaved like hyperbolic functions of the sound's velocity. Direction identification time decreased asymptotically to 8 ms, which was considered minimal integration time for the instantaneous shift detection. Peak latency of cN1 also decreased with increasing velocity and asymptotically approached 137 ms. This limit corresponded to the latency of response to the instantaneous sound shift and was 37 ms later than the latency of the sound-onset response. The direction discrimination time (34 ms) was of the same magnitude as the additional time required for motion processing to be reflected in the MOR potential. Thus, MOR latency can be viewed as a neurophysiological index of temporal integration. Based on the findings obtained, we may assume that no measurable MOR would be evoked by slowly moving stimuli as they would reach their MAMAs in a time longer than the optimal integration time.

Miniaturized and Portable Laser Gas Sensor for Standoff Methane Detection With Non-Cooperative Targets.

A standoff methane (CH4) sensor with actual hard topographic targets (usually called non-cooperative targets) is essential for natural gas pipeline leakage inspection and many other practical applications. To address this requirement, a miniaturized and low-power-consumption gas sensor was developed based on tunable diode laser absorption spectroscopy for standoff CH4 detection with a non-cooperative target. Wavelength modulation spectroscopy with a 1f normalized 2f detection method was employed for calibration-free CH4 measurement. A Kalman filter algorithm was used to improve the precision of the detection. The performance of the standoff CH4 sensor was evaluated comprehensively under various conditions, including different incident angles, different hard topographic targets, and different standoff distances. The results show that the measurement precision is 0.107% and the sensitivity is 4.08 parts per million per meter (ppm·m) with a time resolution of 1 s and a standoff distance of 40 m. The detection limit can achieve 1.24 ppm·m at an optimal integration time of 70 s. This sensor can be easily integrated into mobile platforms, which lays the foundation for intelligent leak inspection.

A new method for evaluating the penetration ability of near infrared diffuse reflectance light to fruit peel with chemometrics

In this paper, the penetration ability of near infrared (NIR) diffuse reflectance light to peel was studied with grating NIR spectrometer. 15 kinds of fruit, including 3 kinds of thin-peel non-citrus fruit, 9 kinds of medium-peel citrus and 3 kinds of thick-peel citrus were investigated. A new strategy to evaluate the NIR penetration ability was developed with a designed penetration experiment and chemometric methods. The penetration abilities of different integration time, peel thickness and wavelength bands were compared. The results show that NIR light can easily penetrate the peel of thin-peel fruit, while it is not easy to penetrate the peel of citrus due to the thick peel. Both the thickness and composition of the peel have relationship with the penetration ability. Increase of integration time can improve the penetration ability to some extent. However, as the integration time lengthens, noise interference and spectral distortion may appear in long wave near infrared (LWNIR) region. The optimal integration time is 65 ms for citrus. Besides, the penetration ability of short wave near infrared (SWNIR) light is better than that of LWNIR light, and the absorption peaks of peel are mainly located in LWNIR region, indicating that SWNIR is more suitable for fruit internal quality detection.

Simultaneous detection of CH4 and CO2 through dual modulation off-axis integrated cavity output spectroscopy

This study established a novel method for the simultaneous detection of two-component gases. Radio frequency (RF) white noise disturbance laser current and wavelength modulation were simultaneously used to improve the off-axis integrated cavity output spectroscopy technique, and a high-precision dual modulation OA-ICOS (RF-WM-OA-ICOS) system was established. The two laser beams were coupled into one laser beam that was applied incident to the cavity of RF-WM-OA-ICOS system. The second harmonic signals of CH4 and CO2 gas simultaneously appeared in the rising or falling edge of a triangular wave. This method was used to measure CH4 and CO2 with different concentrations. The results indicated that the proposed system has high stability and can accurately and simultaneously measure the concentrations of CH4 and CO2, with an optimal integration time of 220 s. The minimum detection limit was 10 ppb for CH4 and 1.5 ppm for CO2. The corresponding noise equivalent absorption sensitivity values were calculated as 2.67 × 10−13 cm−1⋅Hz−1/2 and 5.18 × 10−11 cm−1⋅Hz−1/2, respectively. The proposed dual-component gas simultaneous detection method can also be used for high-precision simultaneous detection of other gases. Therefore, this study may serve as a reference for developing portable multicomponent gas analyzers.

Ppb-level mid-IR quartz-enhanced photoacoustic sensor for sarin simulant detection using a T-shaped tuning fork

A ppb-level mid-IR quartz-enhanced photoacoustic sensor for the highly sensitive detection of dimethyl methyl phosphonate (DMMP) —the sarin simulant—is developed using a T-shaped tuning fork. The sensor employs a narrow-linewidth mid-IR quantum cascade laser (QCL) to target a strong, interference-free DMMP absorption band. A custom quartz tuning fork with 800-μm prong spacing is employed to avoid significant photothermal noise, thus removing the requirement for an optical spatial filter. The performance of the photoacoustic sensor was optimized in terms of exciting power, modulation frequency, and gas pressure. A minimum detection limit of 6 ppb was achieved at an integration time of 300 ms, which can be further improved to 0.45 ppb at the optimal integration time of 20 s

Micro-Quartz Crystal Tuning Fork-Based Photodetector Array for Trace Gas Detection.

In this paper, a micro-quartz crystal tuning fork (M-QCTF) was first demonstrated for developing a low-cost, highly sensitive quartz tuning fork photodetector array for spectroscopic applications. A gas sensing system based on the M-QCTF photodetector and highly sensitive wavelength modulation spectroscopy was developed. Typically, an atmospheric greenhouse gas methane (CH4) molecule was selected as the target analyte for evaluating the M-QCTF and standard commercial QCTF detectivity. The results indicate that the M-QCTF photodetector exhibits ∼3.3 times sensitivity enhancement compared to the standard commercial QCTF. The long-term stability was evaluated by using the Allan deviation analysis method; a minimum detection limit of 1.2 ppm was achieved with an optimal integration time of 85 s, and the corresponding normalized noise equivalent absorption coefficient was calculated to be 4.45 × 10-10 cm-1 W/√Hz. Finally, a two-M-QCTF array detection scheme was experimentally demonstrated, and a signal-to-noise ratio enhancement factor of more than 1.7 times compared to that achieved using a single M-QCTF photodetector was realized, which proves a great potential for developing ultra-sensitive quartz tuning fork photodetector arrays for various applications.

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.

Noninvasive Skin Respiration (CO2) Measurement Based on Quartz-Enhanced Photoacoustic Spectroscopy.

A noninvasive method for disease diagnosis that does not require complex specialized laboratory facilities and chemical reagents is particularly attractive in the current medical environment. Here, we develop a noninvasive skin respiration sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) that can monitor the skin elimination rate of carbon dioxide (CO2). A 3.8 mW distributed feedback laser emitting at 2.0 μm is used as an excitation source, and a three-dimensional (3D)-printed acoustic detection module is designed to apply to the skin as a sensor head. The performance of the noninvasive skin respiration sensor is assessed in terms of detection sensitivity, linearity, long-term stability, and water effect. A minimum detection limit of 35 ppb is achieved at the optimal integration time of 670 s. The skin respiration measurements from eight healthy volunteers are recorded, and the real-time results are analyzed.

Model Parameter Self-Correcting Fuzzy Proportional–Integral–Derivative Adaptive Control of the Integration Time of Raman Spectrometers

The control of the integration time of Raman spectrometer is a crucial factor for the acquisition quality of Raman spectrum. However, due to the coupling effect of linear and nonlinear factors such as the intensity of Raman scattering effect, acquisition noise, fluorescence effect, and spectrometer optical path, it is rather difficult to fast control the integration time, which influences the acquisition efficiency. In this article, aiming at the nonlinear automatic optimal control of the integration time, a model parameter self-correcting fuzzy proportional–integral–derivative adaptive (MPSC-FPIDA) control method is proposed. By constructing an initial parameter prediction model and the fuzzy PID controller, two processing stages, namely, Raman spectrum intensity detection and fuzzy PID iterative control, are carried out, with which the automatic calculation and optimal control of the integration time of the Raman spectrometer are achieved according to the set point of spectrum intensity or signal-to-noise ratio (SNR). By introducing a parameter self-correcting factor of the initial model, the adaptive correction of the parameters of the initial prediction model of Raman spectrometers with different optical laser wavelengths is realized, which ensures the universality of this method to spectrometers of different types. The experiment demonstrates that when it comes to acquisition of Raman spectrum for various samples, the method proposed in this article can accurately calculate the optimal integration time within 2–3 iterations and automatically adapt to the performance parameters of different Raman spectrometers, of which the control performance is as good as that of modeling spectrometers. This shows that the method features fast convergence speed, strong robustness, and versatility. In addition, compared with commonly used linear control methods, it can effectively suppress the coupling effect of linear and nonlinear factors, which improves the efficiency of spectrum acquisition and enhances the control performance by over 50% as well.

EC-QCL based photoacoustic spectroscopy for detection of SF6 decomposition components

The detection of multiple SF6 decomposition gases based on photoacoustic spectroscopy (PAS) has been a research hotspot in recent years. A highly sensitive sensor for the detection of sulfuryl fluoride (SO2F2), carbon tetrafluoride (CF4), and sulfur dioxide (SO2) in sulfur hexafluoride (SF6) was demonstrated by using PAS. The PA sensor is composed of an external cavity quantum cascade laser (EC-QCL) emitting between 1278 and 1360 cm−1, a Helmholtz differential resonant PA cell isolating the target gas from the surrounding environment, and an acousto-optic modulator (AOM) modulating the laser intensity. When the noise level was evaluated based on the PA signal noise of pure SF6 gas, minimum detection limits of 510 ppb, 9 ppb, and 69 ppb with 1 s integration time for SO2F2, CF4, and SO2 in SF6 were achieved at the characteristic absorption wavenumbers of 1282.8 cm−1, 1283.0 cm−1, and 1348.4 cm−1, respectively. Further, if the integration time was increased to the optimal integration time determined via Allan-Werle deviation analysis, the minimum detection limits were further improved to 70 ppb, 1.5 ppb, and 7 ppb for SO2F2, CF4, and SO2, respectively. The multilinear regression (MLR) statistical algorithm based on the least square method was utilized for discrimination of three-gas spectra and retrieving of three-gas concentrations from a three-gas mixture in SF6 despite the significant overlap between their absorption features.

The research on high‐performance CO2 gas measurement using the Kalman‐wavelet algorithm

AbstractTo measure the concentration of trace CO2 with high performance, tunable diode laser absorption spectroscopy technology was adopted in this paper. Kalman‐wavelet algorithm was used to depress the detector noise and optical interference fringe noise. Experimental results show that the second harmonic signal‐to‐noise ratio of the measuring system at 50ppmv CO2 gas was improved by 2.06 times compared with no filtering. Under different CO2 concentrations, the measuring errors are from 2.57% to 2.66%. According to Allan variance analysis, the minimum detection limit was reduced to 5.2 ppmv with the optimal integration time of 61 s. And, the standard deviation value was decreased from 33 to 19 ppmv during long‐term measurement, which can guarantee the high‐performance measurement of CO2 gas.

Multi-Integration Time Adaptive Selection Method for Superframe High-Dynamic-Range Infrared Imaging Based on Grayscale Information.

With the development of superframe high-dynamic-range infrared imaging technology that extends the dynamic range of thermal imaging systems, a key issue that has arisen is how to choose different integration times to obtain an HDR fusion image that contains more information. This paper proposes a multi-integration time adaptive method, in order to address the lack of objective evaluation methods for the selection of superframe infrared images, consisting of the following steps: image evaluation indicators are used to obtain the best global exposure image (the optimal integration time); images are segmented by region-growing point to obtain the ambient/high-temperature regions, selecting the local optimum images with grayscale closest to the medium grayscale of the IR imaging system for the two respective regions (lowest and highest integration time); finally, the three images above are fused and enhanced to achieve HDR infrared imaging. By comparing this method with some existing integration time selection methods and applying the proposed method to some typical fusion methods, via subjective and objective evaluation, the proposed method is shown to have obvious advantages over existing algorithms, and it can optimally select the images from different integration time series images to form the best combination that contains full image information, expanding the dynamic range of the IR imaging system.