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Bao W.-Y.,Chongqing University | Bao W.-Y.,China Aerodynamics Research And Development Center | Zhu Y.,Chongqing University | Chen J.,Chongqing University | And 3 more authors.
Guang Pu Xue Yu Guang Pu Fen Xi/Spectroscopy and Spectral Analysis

In the present paper, the signal of a tunable diode laser absorption spectroscopy (TDLAS) trace gas sensing system, which has a wavelength modulation with a wide range of modulation amplitudes, is studied based on Fourier analysis method. Theory explanation of spectrum distortion induced by laser intensity amplitude modulation is given. In order to rectify the spectrum distortion, a method of synchronous amplitude modulation suppression by a variable optical attenuator is proposed. To validate the method, an experimental setup is designed. Absorption spectrum measurement experiments on CO2 gas were carried out. The results show that the residual laser intensity modulation amplitude of the experimental system is reduced to ~0.1% of its original value and the spectrum distortion improvement is 92% with the synchronous amplitude modulation suppression. The modulation amplitude of laser intensity can be effectively reduced and the spectrum distortion can be well corrected by using the given correction method and system. By using a variable optical attenuator in the TDLAS (tunable diode laser absorption spectroscopy) system, the dynamic range requirements of photoelectric detector, digital to analog converter, filters and other aspects of the TDLAS system are reduced. This spectrum distortion correction method can be used for online trace gas analyzing in process industry. Source

Zhang J.,Chongqing University | Zhang J.,Chongqing Sichuan Instrument Complex Co. | Zhu Y.,Chongqing University | Chen J.-Q.,Chongqing Sichuan Instrument Complex Co. | Liang B.,Chongqing University
Guang Pu Xue Yu Guang Pu Fen Xi/Spectroscopy and Spectral Analysis

After decades of development, the tunable diode laser absorption spectroscopy (TDLAS) became one of the most promising techniques for online trace gas analyzing in process industry. Limited by its principle, the measurement result of TDLAS system is seriously affected by temperature and gas pressure variation. For this reason, most TDLAS systems employed temperature and pressure sensors, which can provide information for partly correcting the error. Theoretically, the gas absorption theory itself is not perfect enough to give an analytical relation between the measurement error and the temperature & pressure variation. Practically, temperature and pressure sensors are not available in some harsh working condition. To address these problems, an online self-calibration technique with a reference gas cell is proposed to compensate temperature and pressure variation induced measurement error in a TDLAS system. More specifically, a reference gas cell filled with known proportion target gas is placed on site, surrounded by working gas to be measured. The main body of the gas cell is made from a stainless tube, one end is a silica glass window and the other end is a reflector. A pressure bellows is connected to the middle of the stainless tube by a branch conduit. The pressure bellows can adaptively deform to keep the pressure balance between the inside and outside gas. Thereby, the temperature and pressure inside the reference cell are equal to that of the gas outside. To ensure the similarity between the reference gas cell and working gas cell, they share the same laser diode source and signal processing circuit. In one working cycle, the TDLAS system obtains the absorption spectrum of both gas cells synchronously. Then the concentration of the trace gas can be easily obtained by calculating the absorption intensity proportion of both absorption spectra without considering the affection of temperature and pressure. The principle, design, and experiments of this technique were presented in this paper. Source

Liang B.,Chongqing University | Zhu Y.,Chongqing University | Bao W.,Chongqing University | Chen J.,Chongqing Sichuan Instrument Complex Co. | And 2 more authors.
Chinese Journal of Sensors and Actuators

The influence of the remaining intensity modulation on second harmonic detection and digital signal processing in tunable diode laser absorption spectroscopy system was analyzed. A synchronous compensation method was put forward and an experimental setup based on the variable optical attenuator (MEMS VOA) was put up. The experimental result shows that with the synchronous compensation of the VOA, the remaining intensity modulation level was dropped down to 1/20 of its original level, and about 60 % of the second harmonic distortion was removed. The feasibility of this synchronous compensation method was tentatively demonstrated by our experiments. Source

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