Study on the Method of Voigt Profiles Two Wings Fitting Non-Uniform Flow Field Absorbance.

In the field of the absorption spectrum, especially for direct tunable diode laser absorption spectroscopy (dTDLAS) technology, the integrated area of the absorption spectrum is needed to be measured accurately for calculating the temperature and the component concentration of the flow field. Doing single optical path absorption spectroscopic measurement in the non-uniform flow field, spectral lineshape broadening is varied with the flow changes, in previous research reports, researchers mainly use single Voigt or Lorentz profile to fit absorbance curve or use directly integral to obtain the integrated area of the absorption spectrum. There are some shortcomings in these methods, resulting in certain error between the fitting result and the actual area, which is not conducive to the accurate measurement of flow field parameters. Firstly, the error is analyzed theoretically, and then, we adopt the simulation method to obtain the error size of the method. Finally, we proposed the Voigt wings fitting absorbance method to reduce the fitting error. The operation of Voigt wings fitting method is to Select the wings of the spectral line, and then use Voigt profile fitting, The difference between the two wings was used the numerical integral method to calculate area, the integrated area is sum of Voigt profile fitting area and numerical integral area. We have used water vapor as the target gas, with eight absorption lines which have different low-level states energy from HITRAN 2012 database being selected-, building two kinds of non-uniform flow field model base on the flat flame furnace, and through the method of segmentation to equivalent processing the no uniformity of flow field. Using Voigt profile fitting method, numerical integral method and Voigt profile wings fitting method to obtain the integral area of models, the error size is obtained by comparing with the theoretical value. As the result of contrast, the fitting error of Voigt profile fitting method is large and related to the different absorption line, the error of numerical integral method is biggest but it is nothing to do with absorption line, the fitting error of Voigt profile wings fitting method is least and stable. By force of contrast, we determined the appropriate method to obtain integral area in the different non-uniform flow field, which is beneficial to obtain accurate integrated area and flow field parameters.

[Laser Tuning Performance Testing and Optimization in TDLAS Oxygen Measuring Systems].

TDLAS (tunable diode laser absorption spectroscopy) technology, with its unmatched advantages such as high selectivity molecular spectra, fast response, high sensitivity, non-contact measuring, become the preferred scheme for combustion process diagnosis, and can be effectively used for oxygen measuring. DFB (distributed feedback) laser diode with its small size, low power consumption, long service life, narrow linewidth, tunable wavelength has become the main choice of the TDLAS system. Performance of laser tuning characteristics is a key factor restricting TDLAS's measuring performance. According to TDLAS oxygen measuring system's working requirements, a simple experimental method was used to test and analyze tuning characteristics such as wavelength current, power current and wavelength temperature of a 764 nm DFB laser diode in the system. Nonlinear distortion of tuning curves was obvious, which affects oxygen measuring accuracy. The laser spectra's characteristics such as narrow linewidth, high side mode suppression ratio and wide wavelength tuning range are obvious, while its wavelength-current tuning curve with a tuning rate of about 0.023 nm x mA(-1) is not strictly linear. The higher the temperature the greater the threshold current, the PI curve is not strictly linear either. Temperature tuning curve is of good linearity, temperature-wave-length tuning rate keeps constant of about 0.056 nm/DEG C. Temperature tuning nonlinearity can be improved by high temperature control accuracy, and current power nonlinearity can be improved by setting the reference light path. In order to solve the wavelength current tuning nonlinear problems, the method of DA controlling injection current was considered to compensate for non-linear wavelength current tuning according to DFB laser diode tuning mechanism and polynomial fitting of test results. In view of different type of lasers, this method needs only one polynomial fitting process before the system's initial work. The compensation scheme is reasonable and the realization is simple, what's more, it does not affect the measuring process. The experiments prove that XI curve's linear fit residuals are less than 1 µm after compensation, far less than those of before compensation 22 pm, the compensation effect is obvious, which provided a basis for various oxygen parameters' TDLAS measuring and inversion.

[Reconstruction Research for Gas Concentration and Temperature of Flame Based on Algebraic Reconstruction Technique].

We specify water vapor among combustion products as the target gas based on tunable diode absorption spectroscopy in this paper. The direct absorption signals of water vapor after being processed can be used to calculate the gas concentration distributions and temperature distributions of the combustion region of methane and air flat flame furnace via algebraic reconstruction technique (ART). In the numerical simulation, reconstruction region is a grid of five by five, we assume a temperature and water vapor concentration distribution of 25 grid, then simulate different direction laser rays which cross the combustion region, generating projection of each ray, by ART reconstruction algorithm, it turns out that the temperature and water vapor distribution reconstruction error is less than 1%. In the experiment, we chose a distributed-feedback laser to scan the target gas H2O7 153.722, 7 153.748 and 7 154.354 cm(-1) as absorbtion line pair to measure temperature of the flame, we consider the former two line as one absorbtion line. By Stages multi-directional scanning, the authors abtain 16 different regions distributin of temperature and gas concentration of furnace when we collecte 30 different angle data by spectral data processing, reconstruction algorithm, two absorbtion line ratio method for temperature sensing, finding the temperature and water concentration are higher in the center than in the edge, it turns out that the reconstruction algorithm is good enough to achieve the distributions of gas concentration and temperature of the combustion region.

[Measurements of CO2 Concentration Profile in Troposphere Based on Balloon-Borne TDLAS System].

The main source and sink of CO2 in the atmosphere are concentrated in the troposphere. It is of great significance to the study of CO2 flux and global climate change to obtain the accurate tropospheric CO2 concentration profile. For the characteristics of high resolution, high sensitivity and fast response of tunable diode laser absorption spectroscopy (TDLAS), a compact balloon-borne system based on direct absorption was developed to detect the CO2 concentration profiles by use of the 2 004. 02 nm, R(16), v1+v3 line without the interfere of H2O absorption and the CO2 density of the number of molecules below 10 km in the troposphere was obtained. Due to the balloon-borne environment, a compact design of one single board integrated with laser driver, signal conditioning, spectra acquiring and concentration retrieving was developed. Limited by the working capability and hardware resources of embedded micro-processor, the spectra processing algorithm was optimized to reduce the time-cost. Compared with the traditional TDLAS sensors with WMS technique, this system was designed based on the direct absorption technique by means of an open-path Herriott cell with 20 m optical-path, which avoided the process of standardization and enhanced the environmental adaptation. The universal design of hardware and software platform achieved diverse gas measuring by changing the laser and adjusting some key parameters in algorithm. The concept of compact design helped to reduce the system's power and volume and balanced the response speed and measure precision. The power consumes below 1.5 W in room temperature and the volume of the single board is 120 mm x 100 mm x 25 mm, and the measurement accuracy is ± 0.6 x 10(-6) at 1.5 s response time. It has been proved that the system can realize high precision detection of CO2 profile at 15 m vertical resolution in troposphere and TDLAS is an available method for balloon-borne detection.

Research on remote sensing of broadband absorbers by using near-infrared diode lasers.

This paper reports the development of an experimental technique for optical remote sensing of broadband absorbers in ambient air. Broadband absorbers have been difficult to detect due to a lack of narrow absorption features, which makes it hard to separate them from interference with other absorbing species and background. In combination with a multidimensional linear regression procedure, we have developed a further step to correct for water vapor and background influences. Various physical processes limiting the detection sensitivity were studied and solutions were developed to reduce their influences. Although the received optical signal from backscatter was very weak as no corner cube reflector was used, we have demonstrated the technique on remote sensing of broadband absorption of ethanol vapor in ambient air with a moderate detection limit of 200 ppm · m. This portable handheld system is particularly suitable for quick "point-and-measure" applications. The developed technique is also applicable for detection of other broadband absorbers.

[Study of high temperature water vapor concentration measurement method based on absorption spectroscopy].

Tunable diode laser absorption spectroscopy (TDLAS) has been developed to realize the real-time and dynamic measurement of the combustion temperature, gas component concentration, velocity and other flow parameters, owing to its high sensitivity, fast time response, non-invasive character and robust nature. In order to obtain accurate water vapor concentration at high temperature, several absorption spectra of water vapor near 1.39 µm from 773 to 1273 K under ordinary pressure were recorded in a high temperature experiment setup using a narrow band diode laser. The absorbance of high temperature absorption spectra was calculated by combined multi-line nonlinear least squares fitting method. Two water vapor absorption lines near 7154.35 and 7157.73 cm(-1) were selected for measurement of water vapor at high temperature. A model method for high temperature water vapor concentration was first proposed. Water vapor concentration from the model method at high temperature is in accordance with theoretical reasoning, concentration measurement standard error is less than 0.2%, and the relative error is less than 6%. The feasibility of this measuring method is verified by experiment.

[Simulation and analysis of second-harmonic signal based on tunable diode laser absorption spectroscopy].

Tunable diode laser absorption spectroscopy (TDLAS) is a new gas detection technique developed recently with high spectral resolution, high sensitivity and fast time response. The second-harmonic signal of wavelength modulation spectroscopy (WMS) is often used as the detection signal for gas concentration inversion. Using Simulink, a visual modeling and simulation platform, the authors simulated the WMS signal based on TDLAS, and got the second-harmonic signal by using lock-in amplifier algorithm. Digital orthogonal algorithm was studied in this paper. The relationship between second-harmonic signals and the modulation indexes was analyzed by comparing changes of second-harmonic under different modulation indexes, in order to find out the optimized parameters for second-harmonic detection.

[A mobile sensor for remote detection of natural gas leakage].

The detection of natural gas pipeline leak becomes a significant issue for body security, environmental protection and security of state property. However, the leak detection is difficult, because of the pipeline's covering many areas, operating conditions and complicated environment. A mobile sensor for remote detection of natural gas leakage based on scanning wavelength differential absorption spectroscopy (SWDAS) is introduced. The improved soft threshold wavelet denoising was proposed by analyzing the characteristics of reflection spectrum. And the results showed that the signal to noise ratio (SNR) was increased three times. When light intensity is 530 nA, the minimum remote sensitivity will be 80 ppm x m. A widely used SWDAS can make quantitative remote sensing of natural gas leak and locate the leak source precisely in a faster, safer and more intelligent way.

[Measurement of OH radicals in flame with XeCl excimer laser].

The present paper describes how to measure OH radicals in the flame of alcohol burner flame by using XeCl excimer laser. The instrument consists of a XeCl excimer laser as light source, a multiple-reflection cell with whole path length of light achieving 2 560 meters, and a double pass high resolution echelle spectrometer with resolution 3.3 pm. This paper describes the reason for using the XeCl excimer laser without spectral etaloning and how to get rid of the water in reactor chamber. The results of the experiment get some absorption peak of OH radicals from 308.145 to 308.175 nm. By choosing the appropriate fit area and fitting, the results contrast with the measure data by using Xe lamp as light source.

[Measurement of OH radicals in flame with high resolution differential optical absorption spectroscopy].

The present paper describes a new developed high resolution differential optical absorption spectroscopy instrument used for the measurement of OH radicals in flame. The instrument consists of a Xenon lamp for light source; a double pass high resolution echelle spectrometer with a resolution of 3.3 pm; a multiple-reflection cell of 20 meter base length, in which the light reflects in the cell for 176 times, so the whole path length of light can achieve 3 520 meters. The OH radicals'6 absorption lines around 308 nm were simultaneously observed in the experiment. By using high resolution DOAS technology, the OH radicals in candles, kerosene lamp, and alcohol burner flames were monitored, and their concentrations were also inverted.

[Study on the arithmetic of absorbance inversion based on tunable diode-laser absorption spectroscopy].

Tunable diode-laser absorption spectroscopy techniques have been widely used in many regions, such as environmental monitoring and detection of industrial process, due to their high spectral resolution, high sensitivity, fast time response and nonintrusive character. It's important to obtain absorbance curve and integrated absorbance for gas concentration inversion and line strength calibration in direct-absorption spectroscopy techniques. An approach was taken to process the laser direct absorption spectrum. First a low-order polynomial baseline fitting was carried on sub-regions of the transmitted signal to eliminate the effect of diode-laser output fluctuation and obtain the absorbance curve. Then integrated absorbance can be inverted through line-shape fitting using Levenberg-Marquardt non-linear least-squares fitting method. This approach was verified by experimental processing of absorption lines of water vapor.

[An investigation of temperature compensation of HCL gas online monitoring based on TDLAS method].

HCL, with the character of strong erosion and toxicity, is a kind of chemical material of vital importance. So measuring the HCL in-situ can not only optimize its production process, but also be necessary to reduce the environment pollution. TDLAS (tunable diode laser absorption spectroscopy) technology, and owning the advantage of the tunability and narrow line width of the diode laser, this method can relatively easily select the absorption line of the detected gas without the interference from other gas, thus making the rapid and accurate HCL measurement possible. In the present paper, the HCL measurement system and the implemented experiment are introduced. The impact of the temperature on the measurement as well as the temperature compensation method is emphasized. The final experimental results validated the rationality of the empirical equation and therefore the improvement of the accuracy and feasibility of the TDLAS technology. The system, whose detection limitation reaches 2 ppm, can satisfy the needs of industrial in-sit measurement.

[Monitoring of oxygen concentration based on tunable diode laser absorption spectroscopy].

Oxygen is a widely used important gas in the industrial process. It is very meaningful to on-line monitor the oxygen concentration for the enhancement of combustion efficiency and reduction in environmental pollution. Tunable diode laser absorption spectroscopy (TDLAS) is a highly sensitive, highly selective and fast time response trace gas detection technique. With the features of tunability and narrow linewidth of distributed feedback (DFB) diode laser and by precisely tuning the laser output wavelength to a single isolated absorption line of the gas, TDLAS technique can be utilized to accurately implement gas concentration measurement with very high sensitivity. In the present paper, the authors used a DFB laser was used as the light source, and by employing wavelength modulation method and measuring the second harmonic signal of one absorption line near 760 nm of oxygen molecule, the authors built a system for online monitoring of oxygen concentration. The characteristics of the system are as follows: the scope of detection is 0.01%-20%; detection accuracy is 0.1%, long term stability is 1%.

[System design of open-path natural gas leakage detection based on Fresnel lens].

Based on the technology of tunable diode laser absorption spectroscopy (TDLAS) in conjunction with second harmonic wave detection, a long open-path TDLAS system using a 1.65 microm InGaAsP distributed feedback laser was developed, which is used for detecting pipeline leakage. In this system, a high cost performance Fresnel lens is used as the receiving optical system, which receives the laser-beam reflected by a solid corner cube reflector, and focuses the receiving laser-beam to the InGaAs detector. At the same time, the influences of the concentration to the fluctuation of light intensity were taken into account in the process of measurement, and were eliminated by the method of normalized light intensity. As a result, the measurement error caused by the fluctuation of light intensity was made less than 1%. The experiment of natural gas leakage detection was simulated, and the detection sensitivity is 0.1 x 10(-6) (ratio by volume) with a total path of 320 m. According to the receiving light efficiency of the optical system and the detectable minimum light intensity of the detector, the detectable maximal optical path of the system was counted to be 2 000 m. The results of experiment show that it is a feasible design to use the Fresnel lens as the receiving optical system and can satisfy the demand of the leakage detection of natural gas.

[Open-path online monitoring of ambient atmospheric CO2 based on laser absorption spectrum].

With the conjunction of tunable diode laser absorption spectroscopy technology (TDLAS) and the open long optical path technology, the system designing scheme of CO2 on-line monitoring based on near infrared tunable diode laser absorption spectroscopy technology was discussed in detail, and the instrument for large-range measurement was set up. By choosing the infrared absorption line of CO2 at 1.57 microm whose line strength is strong and suitable for measurement, the ambient atmospheric CO2 was measured continuously with a 30 s temporal resolution at an suburb site in the autumn of 2007. The diurnal atmospheric variations of CO2 and continuous monitoring results were presented. The results show that the variation in CO2 concentration has an obvious diurnal periodicity in suburb where the air is free of interference and contamination. The general characteristic of diurnal variation is that the concentration is low in the daytime and high at night, so it matches the photosynthesis trend. The instrument can detect gas concentration online with high resolution, high sensitivity, high precision, short response time and many other advantages, the monitoring requires no gas sampling, the calibration is easy, and the detection limit is about 4.2 x 10(-7). It has been proved that the system and measurement project are feasible, so it is an effective method for gas flux continuous online monitoring of large range in ecosystem based on TDLAS technology.

[Intra-pulse spectroscopy based on room-temperature pulsed quantum-cascade laser for N2O detection].

Mid-infrared lasers are very suitable for high-sensitive trace-gases detection in that their wavelengths cover the fundamental absorption lines of most gases. Quantum-cascade lasers have been demonstrated to be ideal light sources with their especially high power, wide range of tuning capability and favorable operating condition on room-temperature. The intra-pulse spectroscopy based on a room-temperature distributed-feedback pulsed QC laser is a simple and effective trace gas detective method to detect trace-gas qualitatively or quantificationally. When a long excitation pulse is applied to a QC laser, the laser frequency tunes almost linearly to lower wave number (lower frequency) as a function of time so all absorption spectral elements are recorded during a single laser pulse. In the present paper, the method was introduced, and identification of N2O spectral fingerprint using this spectroscopy was demonstrated experimentally. The thermal chirp from a 500 ns long excitation pulse was applied to a quantum-cascade laser to get a fast wavelength scanning, thus a wave number tuning of about 1 cm(-1) was produced. The N2O absorption spectrum centered at 1 273.7 cm(-1) was also obtained. The measured absorption spectrum is consistent with HITRAN data precisely.

[Monitoring the change in CO concentration in combustion with tunable diode laser absorption spectroscopy].

In the present paper, the technology of tunable diode laser absorption spectroscopy (TDLAS) in conjunction with the open path multi-pass Herriot cell and the new-style detection method of auto-balanced detection combined with wavelength modulation technology were used, and the concentration of CO produced in combustion of alcohol blowtorch was measured. It was found in the measured result that the change in CO concentration in the flame of alcohol blowtorch presented a stated periodicity in the process of combustion and the average concentration of CO was calculated to be 49.4 (10(-6) ratio by volume). The experiment is showed that with the conjunction of auto-balanced detection and the second harmonics detection method, adopting the open path multi-pass Herrriot cell to detect the concentration of CO in the combustion of alcohol blowtorch is accurate and contents the detection requirement. It was proved that the system made for measuring the concentration of CO in the flame of alcohol blowtorch in combustion establishes foundation well for developing on-line combustion monitoring based on TDLAS.

[The development of acetylene on-line monitoring technology based on laser absorption spectrum].

As one of the materials in organic chemical industry, acetylene has been used in many aspects of chemical industry. But acetylene is a very dangerous inflammable and explosive gas, so it needs in-situ monitoring during industrial storage and production. Tunable diode laser absorption spectroscopy (TDLAS) technology has been widely used in atmospheric trace gases detection, because it has a lot of advantageous characteristics, such as high sensitivity, good selectivity, and rapid time response. The distribution characteristics of absorption lines of acetylene in near infrared band were studied, and then the system designing scheme of acetylene on-line monitoring based on near infrared tunable diode laser absorption spectroscopy technology was discussed in detail. Moreover, the system of experiment measurement was set up and the method of signal detection and the algorithm of concentration inversion were studied. In addition, the sample cell with a path length of 10 cm, and the acetylene of different known concentrations were measured. As a result, the detection limit obtained reached 1.46 cm3 x m(-3). Finally the dynamic detection experiment was carried out, and the measurement result is stable and reliable, so the design of the system is practicable through experiment analysis. On-line acetylene leakage monitoring system was developed based on the experiment, and it is suitable for giving a leakage alarm of acetylene during its storage, transportation and use.

[Concentration calibration method of ambient trace-gas monitoring with tunable diode laser absorption spectroscopy].

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser used to obtain the absorption spectroscopy in the characteristic absorption region It needs to be combined with a long absorption path in the ambient trace-gas measurements. TDLAS is a new trace gas detective method developed with the combination of a tunable diode laser source and a long absorption path; it has significant advantages not only in the sensitivity but also in rapidity of response. It has been widely used in many atmospheric trace-gases detection, ground trace-gas detection and, gas leakage detection. On-line calibrating is necessary to most trace gas monitor, and in the present paper the authors introduced a simple and accurate method, analyzed it in the theory, and proved it's feasibility in the experiment.

[Spectral study of N2O under pulsed DC discharge].

A new equipment of pulsed DC discharge supersonic molecular beam was set up for producing transient species. Thepulsed DC discharge can be used stably under 10 kV. In data collection using software, it can be utilized as gated integrators and averagers. Using this equipment, the processes of N2O discharge were studied. The emission spectra of N2O+ (A2sigma+ - X2IIi) were obtained. From the intensity of vibrational spectra, the vibrational populations of the N2O+ (A2sigma+ ) states were calculated. Compared with other results, A states of N2O+ depart even more from a Boltzmann distribution.