Bian Wu

Simultaneous detection of multiple gas concentrations with multi-frequency wavelength modulation spectroscopy

A multi-frequency wavelength modulation spectroscopy (MF-WMS) method for simultaneous monitoring multiple gas concentrations is reported. Two wavelength lasers are selected and scanned at the same sawtooth waveform and different sinusoidal modulation frequencies. Also, a 2-in-1 coupler is used to collect laser beams into a single-mode fiber to deliver the combined beams to a same optical path. The outputs from the same InGaAs detector are demodulated by a set of homemade digital lock-in-amplifiers to obtain the harmonic signals, accordingly. The new multi-pass cell was designed and filled with five different concentrations of H2S and HCl gas mixtures for further analysis. In addition, an digital filter and a data running means method were used to further improve the stability. The results show that the signal-to-noise ratio and the anti-interference ability had been improved and the MF-WMS system gives a good performance in terms of reproducibility and feasibility.

Reconstruction of combustion temperature and gas concentration distributions using line-of-sight tunable diode laser absorption spectroscopy

Abstract. Spatial temperature and gas concentration distributions are crucial for combustion studies to characterize the combustion position and to evaluate the combustion regime and the released heat quantity. Optical computer tomography (CT) enables the reconstruction of temperature and gas concentration fields in a flame on the basis of line-of-sight tunable diode laser absorption spectroscopy (LOS-TDLAS). A pair of H2O absorption lines at wavelengths 1395.51 and 1395.69 nm is selected. Temperature and H2O concentration distributions for a flat flame furnace are calculated by superimposing two absorption peaks with a discrete algebraic iterative algorithm and a mathematical fitting algorithm. By comparison, direct absorption spectroscopy measurements agree well with the thermocouple measurements and yield a good correlation. The CT reconstruction data of different air-to-fuel ratio combustion conditions (incomplete combustion and full combustion) and three different types of burners (one, two, and three flat flame furnaces) demonstrate that TDLAS has the potential of short response time and enables real-time temperature and gas concentration distribution measurements for combustion diagnosis.

Analysis of Random Noise and Long-Term Drift for Tunable Diode Laser Absorption Spectroscopy System at Atmospheric Pressure

The random noise and the slow drift, which respectively limit the signal-to-noise ratio and the long-term stability, of a wavelength modulated tunable diode laser absorption spectroscopy system, are critically analyzed and evaluated experimentally. The noise sources considered in detail include the noise contributions from the lock-in amplifier, detector-preamplifier combination noise, excess noise from the DFB laser, and detection cell instabilities, especially in multiple reflection long-path cells. We also analyze contributions to long-term drift and find that the drift may be significantly compensated by the use of an optical reference path. We find experimentally that a white cell with a path length of 17.5 m introduces excess noise resulting in its having a performance equivalent to 6-m single pass absorption path. The experimental system optimized using the results of this analysis and using the 17.5-m white cell is found to operate at a sensitivity of 1.5-ppmv carbon monoxide in a bandwidth of 4 Hz. The long-term zero offset drift levels measured over 8 h are found to be ~120 ppmv without the reference optical path and are reduced to 20 ppmv using the balance path technique.

Signal analytical processing based on wavelet transform for tunable diode laser absorption spectroscopy

Gas analysis based on tunable diode laser absorption spectroscopy (TDLAS) provides features of high sensitivity, fast response and high selectivity. When target gas concentration is below a few parts-per-million spectrometers become more and more sensitive towards noise, interference, drift effects and background changes associated with low level signals. It is purpose of this paper to address some of the problems which are encountered at this low signal levels and to describe a signal processing strategy for gas monitoring with wavelet transform. Different parameters of wavelet by taking gas detection of H2S as an illustration are studied and an improved wavelet-based signal enhancement process is proposed based on the feature of TDLAS second-harmonic signal. The algorithm uses bior3.9 wavelet basic function and multi-resolution decomposition. The results show a plausible improvement in performance of TDLAS system and enhancement of detection limit from 10ppm to hundreds of ppb level under various noise conditions.

Development of a tunable diode laser absorption sensor for online monitoring of industrial gas total emissions based on optical scintillation cross-correlation technique.

We report the first application of gas total emission using a DFB diode laser for gas concentration measurements combined with two LEDs for gas velocity measurements. In situ gas total emissions and particle density measurements in an industrial pipeline using simultaneous tunable diode laser absorption spectroscopy (TDLAS) and optical scintillation cross-correlation technique (OSCC) are presented. Velocity mean values obtained are 7.59 m/s (OSCC, standard deviation is 1.37 m/s) and 8.20 m/s (Pitot tube, standard deviation is 1.47 m/s) in a steel plant pipeline for comparison. Our experiments demonstrate that the combined system of TDLAS and OSCC provides a new versatile tool for accurate measurements of total gas emissions.

Online monitoring of industrial flue gases using tunable diode laser with a digital-control module

Increased demands on air quality have created incentives for new methods to monitor gas pollution. In this paper we will present an online gas analyzer for industrial flue pollution monitoring based on tunable diode laser absorption spectroscopy (TDLAS) technology. Signal measurements with a sensitive device inevitably suffer from the predictable or unpredictable sources such as intensity fluctuations and the laser output wavelength dithers. In order to eliminate or at least reduce the measurement uncertainty and gain high reliability, a close-circle digital-control module with functions of digital signal generator, digital lock-in-amplifier (D-LIA), data acquisition and data processing has been developed to substitute the previous independent signal generator board, analog lock-in-amplifier and data acquisition card in our TDLAS system. With the help of the digital-control module and the advanced digital signal processing techniques the performance of the TDLAS system has demonstrated great improvement in long term field tests.

High-sensitive monitoring of carbon monoxide in industry flue gases using tunable diode lasers

Carbon monoxide (CO) is a very important gas generated in the industrial process; therefore to implement CO concentration on-line monitoring is a key factor for industrial process control. Tunable diode laser absorption spectroscopy (TDLAS) is a high sensitive, high selective and fast trace gas detection technique. With the features of tunability and narrow linewidth of distributed feedback (DFB) laser and by precisely tuning its wavelength to a single isolated absorption line of the gas, TDLAS technique can be utilized to accurately perform online gas concentration monitoring with very high sensitivity. In this paper, a system for online monitoring of CO concentration is developed by our group employing TDLAS technique. The experimental results are present and discussed in this report. The characteristics of the system are: the sensitivity, 10ppm; detection accuracy, 0.02%; long term stability, 1%.

Detection of gas temperature using a distributed feedback laser at O 2 absorption wavelength 760 nm

Direct absorption spectroscopy for gas temperature monitoring in a tube furnace is proposed over the temperature range 300–900 K with intervals of 100 K. This detecting technique is based on the relationship between two lines’ absorption strength and temperature. The gas temperature can be inferred from the ratio of the integrated spectral area of the oxygen absorption features measured with a distributed feedback diode laser near 760 nm. Compared with the thermocouple measured results, the direct absorption spectroscopy approach also provides a temporal resolution. The results show that the accuracy is better at low temperatures than at high temperatures. In the future, we hope to improve the detection accuracy and demonstrate the utility of the diode laser absorption sensors operating for active combustion diagnostics and optimizations, simultaneously.

Research on reduction of long-term distortions and suppression of light intensity fluctuations in a TDLAS system

For any tunable diode laser absorption spectroscopy (TDLAS) system, its performance is often degraded by long-term distortion and fluctuations of light intensity. The source of long-term distortion and the corresponding proposal are discussed at first in this paper. It is proved that the long-term distortion of the system is mainly induced by the change of wavelength dependent transfer function. A balanced optical path can be used to reduce it effectively. In order to decrease the disturbance of intensity fluctuation, a novel method for online correction is presented. It is developed according to the linear relation between the peaks of harmonic power spectra and the incident light intensity. It is demonstrated by the experiments and explained as residual sum frequency and difference frequency power of signal and reference after the lock-in amplifier. This method could achieve real-time light intensity correction with only little calculation. By using a 17.5m multi-pass cell, the experiments show that the system can achieve about 20ppmv stability for long-term continual monitoring. Allan variance indicates that the detection limit for short-term measurement is between 0.3ppmv and 1.5ppmv depending on the response time allowed by the instrument.