KEYWORDS: Signal intensity, Signal detection, Laser spectroscopy, Signal to noise ratio, Signal processing, Modulation, Gas lasers, Modulation frequency
In-situ laser absorption spectroscopy is commonly used for environmental monitoring and industrial process control, but fluctuations in field environmental conditions can affect measurement accuracy and stability. In the wavelength modulated spectroscopy (WMS) technique, the first harmonic or DC signal is often used for light intensity normalization, which to a certain extent weakens the influence of light intensity fluctuations caused by vibration, turbulence, etc. in the measurement optical path. However, the simultaneous extraction of different harmonic signals from the same absorbed spectrum using a lock-in amplifier requires at least two channels, and the inconsistency between the channels increases the complexity and uncertainty of the system. Therefore, a harmonic extraction method based on the short-time Fourier transform (STFT) is proposed, in which the discrete Fourier transform (DFT) is performed on the signal segment by segment by shifting the window function, and the DC component and the harmonic signals of each order can be extracted simultaneously according to the multiples of the modulation frequency. The effectiveness of this method is verified in the experimental system of CO2 in-situ measurement of laser absorption spectroscopy, and the results show that the relative errors of the harmonic signals extracted by the method in this paper are always kept within 0.6%, and the average time saved is about 34.62%.
Vertical Radial Plume Mapping (VRPM) technique is often used in the measurement of gas emission flux in open space. It is necessary to use optical remote sensing equipment (ORS) to scan multiple measurement points to reconstruct the gas concentration field, but the fluctuation of field environmental conditions and the mechanical error of the system will lead to the optical path deviation. Although the optical path calibration can be completed by researching and positioning the central position of the measurement point according to the signal strength, the search range needs to be preset, which can not balance the time cost and positioning accuracy, reducing the time resolution of the concentration data, and resulting in flux calculation error. To solve this problem, this paper proposes a Q-learning multi-optical path localization method based on detection signal quality. This method uses the change of signal strength when the optical path moves as a reward to learn the environment, affects the selection of the next calibration direction, and makes the optical path preferentially choose the direction with enhanced signal strength. The effectiveness of this method is verified on the 25 * 25 map established of simulating the optical path offset. The results show that this method can get the optimal path to the center point, the minimum number of steps is 14, the running time is less than 2 seconds, and the success rate can reach 100% after many episodes of learning, which proves the effectiveness of Q-learning method in multi-optical path scanning.
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