Jiachen Sun

This study presents an innovative calibration-free artificial peak-laser absorption spectroscopy (AP-LAS) technique that fundamentally overcomes the inherent dynamic range-sensitivity trade-off and eliminates reliance on specific absorption line pairs for multiparameter measurements in laser absorption spectroscopy. By implementing a unique signal-driven and wavelength-tuning mechanism, the proposed approach generates tailored artificial absorption peaks within a single natural spectral line, enabling simultaneous temperature and concentration quantification. This method is validated through CO absorption experiments in a high-temperature, high-pressure shock tube environment. Experimental results demonstrate that the AP-LAS technique significantly extends the detectable concentration range by 4 times compared to conventional methods while maintaining excellent linearity. The technique achieves enhanced dynamic range and multiparameter quantification performance without requiring matched absorption line pairs under extreme conditions. These advancements highlight AP-LAS technique as a promising solution for real-time gas monitoring in demanding applications such as combustion diagnostics.

A novel three-color high-sensitivity thermometry method is proposed, providing more accurate temperature measurements for nitrogen-containing fuel reactive flows. This method determines temperature by simultaneously sweeping three absorption peaks of NO. A relationship between absorbance and temperature was established with parameters determined from shock tube experiments from 1100 to 2200 K. To evaluate its performance, six sets of NO absorption spectra data at various temperatures were collected. The results demonstrate that the three-color thermometry achieves an average absolute error of 7 K, significantly lower than the two-color thermometry’s average absolute errors of 16 and 20 K. This three-color thermometry methods offers higher sensitivity and accuracy, making it a practical solution for measuring temperatures during the combustion of nitrogen-containing fuels.

Fixed wavelength modulation spectroscopy (FWMS) featured as that wavelength is modulated around a constant value instead of a dynamic scanning ramp in the standard wavelength modulation spectroscopy (WMS). To eliminate the influence of center wavelength drift on absorption measurement in the FWMS system, a software-based proportional integral derivative (PID) controller is developed to lock the laser wavelength at methane line 1653.72 nm for CH ₄ detection in this study. Moreover, two crucial parameters of PID, gain and integral time are optimized to improve the laser wavelength locking performance. As a result, the laser wavelength is locked to the target CH ₄ line with a 1σ fluctuation less than 406 kHz (better than 3.7×10 ^-3 pm) with a robust 100% locking success rate, which is the best result reported so far to our knowledge. Based on the wavelength locked FWMS CH ₄ sensor, linearity is tested to be R ² = 0.998 in low concentration range of 0-1000 ppm. Detection sensitivity is 52 ppb at 0.1 s and could be improved to 36 ppb if the integration time is extended to 1.4 s. Compared with standard WMS, CH ₄ detection stability is improved by 5 times.

The line width of different line shapes is a very important parameter in absorption spectroscopy sensing techniques. Based on the high sensitivity and low noise properties of wavelength modulation spectroscopy, we report a novel line width measurement method. After theoretically proving the relationship between line width, modulation amplitude and the amplitude of the second harmonic at the center frequency, the absorption lines of CH₄ near 6046.96 cm⁻¹ and CO₂ 4989.97 cm⁻¹ were chosen for simulation, and the relative errors of the line width between our method and theoretical data were kept at about 1%. A distributed feedback laser diode operating near 1653 nm with three different concentrations of CH₄ was used for experimental validation, and the results were consistent with the numerical simulation. Additionally, since only the peaks of second harmonic need to be measured, the advantages of wavelength modulation can be utilized while reducing the difficulty of data acquisition.

The performance of Raman Distributed Temperature Sensing (RDTS) is greatly limited by the effect of optical and other random noise on the backscattered Anti-Stokes signal. Therefore, it is necessary to study the signal denoising method of the system. Existing methods to improve system performance mainly include improving the system structure and using denoising methods like cumulative averaging, wavelet transform, etc., which still have room for improvement. In this paper, a Fully-Connected-RDTS model based on fully connected neural network is designed to reduce temperature errors and improve spatial positioning accuracy. Compared with traditional denoising algorithms, the maximum temperature error is reduced to 0.53 °C (±0.25 °C) using only 500 averages of the temperature data, which is better than 2.61 °C of 10,000 cumulative averages, and 2.25 °C of wavelet transforms. The predictions for the boundary of the temperature curve are highly consistent with the original data, and the error of spatial positioning accuracy is less than 1 m without blurring the spatial information like cumulative averaging and wavelet transform algorithms. Compared with the results of 10,000 cumulative averages, the detection response time is reduced from 8 s to 1-s while the noise is greatly filtered. The measurement performance of the RDTS system is greatly improved by the Fully-Connected-RDTS model without sacrificing spatial accuracy. And once the model is successfully trained, there is no need to change the parameters of the model, so it could be used for large-scale real-time detection.

A new defined parameter, tuning efficiency (TE), is proposed to evaluate the tuning performance of distributed feedback laser diode (DFB-LD). The TE is defined as the ratio of output wavelength response to power response of DFB-LDs when working in a current driving mode. Simulations and experiments are performed to study its influence on harmonic signal, signal-to-noise ratio (SNR), and residual amplitude modulation (RAM) effect in wavelength modulation spectroscopy (WMS). It is the first time to be found that the parameter TE would change as driving frequency of DFB-LD getting changed; in addition, different DFB-LDs result in changes in TE. A WMS-based water vapor sensing system is constructed to discuss the TE's influence on gas detection applications, and the experimental results show highly consistent with the simulations. In conclusion, the TE is a robust parameter guiding us to choose favorable DFB-LDs for gas sensing applications.

The temperature fluctuations inside commercially distributed feedback laser diode were experimentally measured and analyzed in detail in this study. In tunable diode laser absorption spectroscopy (TDLAS) based gas sensing systems, the temperature of the laser source is usually fixed and a current signal is used to drive the laser for wavelength scanning or modulation. Even if the laser is controlled by a robust temperature controller, the internal temperature still fluctuates due to the delayed response of the temperature controller when rapidly changing the drive current. These temperature fluctuations result in distortions in the measured absorption signals which can be effectively suppressed by limiting the drive current modulation to the resonance frequency of the temperature control proportional-integral-derivative loop. This study demonstrates a reduction in the temperature dependent absorption distortions and improves the detection uncertainty using this technique.