ABSTRACT
Tunable diode laser absorption spectroscopy (TDLAS) is a widely used technique for high sensitivity, good selectivity and fast response. It is widely used in environment monitoring, industrial process control and biomedical sensing. In order to overcome the drawbacks of TDLAS including high cost, poor stability and center wavelength shift problem. A multi-mode diode laser system based on correlation spectroscopy and wavelength modulation spectroscopy (TMDL-COSPEC-WMS) was used to measure O2 concentration near 760nm at the 1%~30% range of near room temperature. During the experiment, the light is splitter into two beams, respectively through the sample and measuring cell, two receiving optical signal collection containing gas concentration information sent back stage treatment, invert the oxygen concentration through correlation and ratio between measured signal and reference signal, the correlation spectroscopy harmonic detection technique is used to improve the stability of the system and the signal to noise ratio. The result showed that, there was a good linear relationship between the measured oxygen concentration and the actual concentration value. A detection limit of 280 pmm. m in the 1 atmospheric which approved of the same sample. A continuous measurement for oxygen with the standard deviation of 0. 056% in ambient air during approximately 30 minutes confirms the stability and the capability of the system. The design of the system includes soft and hardware can meet the needs of oxygen online monitoring. The experimental device is simple and easy to use, easy to complex environment application.
ABSTRACT
The present research was planned to develop a method for species concentration measurements under high temperature and pressure environments. The characteristics of CO2 spectrum at high temperature and pressure were studied at first. Based on the research above, tunable diode-laser absorption of CO2 near 2.0 microm incorporating fixed-wavelength modulation spectroscopy with second-harmonic detection was used to provide a method for sensitive and accurate measurements of gas temperature and CO2 concentration at high temperature and pressure. Measurements were performed in a well-controlled high temperature and pressure static cell. The results show that the average error of the CO2 concentration measurements at 5 atm, 500 K and 10 atm, 1000 K is 4. 49%. All measurements show the accuracy and potential utility of the method for high temperature and pressure diagnostics.
ABSTRACT
The spectrum of carbon monoxide was obtained around 1. 573 microm using a tunable distributed feedback semiconductor laser with a high-finesse cavity at room temperature via off-axis cavity enhanced absorption (CEA) spectroscopic technique. The absorption line of carbon monoxide at 6 357. 311 6 cm(-1) was chosen for trace detection. Meanwhile, in order to get more accurate measurements, absorption path length of the cavity calibration methods was studied, and a simple and practical calibration method was given. The results show that, the equivalent absorption path length of high-precision optical resonator was -1 195.73 m. At last, we got the concentration of carbon monoxide in the real atmosphere to be -388.346 ppm (S/N = 22), and the detection limit of carbon monoxide was 17.65 ppm. By combination of wavelength modulation technology and OA-CEAS technology, a minimum detectable concentration of 0.36 ppm (S/N = 1 064) was achieved eventually.
ABSTRACT
Measurements strategies based on absorption spectroscopy techniques, especially the measurements in high temperature, require accurate values of important spectroscopic parameters of the probed species. Sometimes the parameters listed in widely used HITRAN and HITEMP2004 database are uncertain to some extent. In order to validate the spectroscopic parameters of 9 selected CO2 lines which should be used in combustion diagnosis, spectra of those lines were recorded in a high temperature experiment setup as a function of temperature (in the range of 300-800 K) and pressure (in the range of 9-450 torr) using a distributed feed-back (DFB) diode laser. The recorded absorption spectra were fitted to Voigt profile. Line intensity, air-broadening coefficient and temperature exponent of each line were deduced from those data. Through comparison of experimental results and those listed in HITRAN and HITEMP2004 database, the discrepancies of most line intensities, air-broadening coefficients and their temperature exponents are less than 3%, 5% and 2% respectively. Those results show good consistency between the experimental data and that in HITRAN and HITEMP2004 database. The discrepancy in line intensities may be caused by the fitting of absorption spectra, the reading of thermocouple and pressure gage, uniformity of temperature in the heated cell, and uncertainty of the optical path. Those factors also cause the discrepancy in air-broadening coefficients and their temperature exponent. CO2 contained in air also introduces error in air-broadening coefficients and their temperature exponent beside those factors. Though we have deducted them in data-processing, the little change of CO2 in partial region also exists. Those results will be helpful to the measurement of CO2 concentration in combustion diagnosis in the future.
ABSTRACT
A compact instrument based on the off-axis integrated-cavity output spectroscopy (ICOS) technology was developed for sensitive measurements of gas mixing ratios (ammonia in air) at room temperature by using fiber-coupled distributed feedback (DFB) diode laser operating at 1.531 microm. The absorption line of ammonia at 6 528.764 cm(-1) was chosen for trace detection. The mirrors' effective reflectivity R2 of 0.996 9 was first calibrated by carbon dioxide under this condition, and the cavity 35.8 cm in length as an absorption cell could yield an optical path of presumably 115.46 m. As a result, a minimum detectable concentration of approximately 2.66 ppmv (S/N-3) at the total pressure of 100 torr was obtained. Then the lock-in amplifier was added in the system to acquire the second harmonic signal by combination of wavelength modulation technology, which could better suppress background noise and improve the signal-to-noise ratio, and a detection limit of 0.293 ppmv (S/N-3) was achieved eventually. This work demonstrated the potential of the system for a range of atmospheric species sensing in the future.
