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Applications of cw Quantum Cascade Laser near 8 m in Gas Sensing Research M.B Sajid, A. Farooq* Clean Combustion Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia *Corresponding author email:
[email protected]
Abstract: Quantum cascade laser based sensors operating near 8 m to detect H2O2, C2H2, CH4, N2O and H2O are discussed and demonstrated for applications in chemical kinetics, combustion and spectroscopic measurements. OCIS codes: (120.1740) (140.5965)
1.
Introduction
Recent advancements in quantum cascade laser (QCL) technology have resulted in the availability of roomtemperature operated, continuous wave (cw) lasers covering the mid-infrared wavelength region [1]. These lasers typically provide large tuning range, high output power and highly collimated light with narrow line width, thus making them ideal choice for high-resolution measurements in spectroscopy, combustion and other gas sensing applications like industrial control, environmental research and breath analysis. Mid-infrared (Mid-IR) i.e. 5-25 m region covers rovibrational bands of various important species including H2O, CO2, CO, H2O2, CH4, C2H2,N2O, NO, C2H4, HCN, and NO2. The fundamental and combination bands of these species are generally stronger than overtone bands in the near-infrared (NIR) and hence provide enhanced sensitivity. The present work is focused on the applications of cw-QCL near 8 m for spectroscopy, combustion and gas sensing research. The particular laser used here can be tuned from 1218-1328 cm-1 and covers the rovibrational bands of various molecules important for variety of applications. These molecules include H2O2, CH4, C2H2, H2O, and N2O. Acetylene (C2H2) is an important species in combustion, human breath, environmental studies and astrophysics research. In combustion, acetylene is produced as an intermediate product and has been identified as a soot precursor. It can also be used as an indicator for monitoring air quality and has been recognized as smoking biomarker in human breath. Its presence has also been confirmed in various celestial body atmospheres. Hydrogen peroxide (H2O2) is produced in larger quantities before the onset of hydrocarbon ignition at intermediate temperature conditions. The thermal decomposition of H2O2 results in two highly reactive OH radicals and hence H2O2 decomposition controls the overall reactivity of the system under these conditions. Furthermore, it plays a vital role in atmospheric chemistry. Methane (CH4) is an important greenhouse gas and hence accurate quantification in atmosphere can provide valuable information for atmospheric research. It is also produced during the oxidation and pyrolysis of large hydrocarbons. Nitrous oxide (N2O) is also an important greenhouse gas present in earth’s atmosphere and combustion processes emit nitrous oxide as a pollutant. Water (H2O) is a major combustion product and hence its measurement in combustion systems can be used follow the overall progress of the ignition process and deduce gas temperature using two-line thermometry. 2. Interference-free line selections Accurate quantification of a species in a certain environment requires the selection of a frequency which exhibits large absorption cross-section and has minimum interference from other major species present in the system. Spectral simulations using HITRAN 2012 database [2] have been carried out to identify suitable frequencies for measuring C2H2, CH4, H2O and H2O2 in combustion environments over the 7.5 – 8.2 m wavelength region. The results from these analyses are plotted in Fig. 1 (a)-(d) which show that optimum frequencies for measuring C2H2, CH4, H2O and H2O2 are 1275.51 cm-1, 1303.56 cm-1, 1296.50 cm-1 and 1302.00 cm-1 respectively. 3. Mid-infrared laser source A cw external cavity (EC) QCL manufactured by Daylight solutions (model: 21077 MHF) has been used in this study as the tunable mid-IR light source. The laser head is connected to a control box which displays centerline frequency and allows operating the laser under different settings. The laser can be tuned to different wavelengths by adjusting the laser frequency, temperature or injection current. The highly collimated laser light can reach an energy output of 120 mW. The laser is expected to have a line-width of approximately 0.001 cm-1. The laser chip was maintained at 16 0C with the help of thermoelectric cooling and a recirculating chiller. Depending on the nature of
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the application, the laser can be operated in three different tuning modes. These include fixed wavelength, coarse modulation and fast modulation modes. Fast modulation can provide a fine tuning range of approximately 0.1 cm-1 with a sinusoidal repetition rate of 10 kHz – 2 MHz, while coarse modulation provides wider tuning range of nearly 1 cm-1 at a maximum sinusoidal repetition rate of 100 Hz. Coarse modulation is achieved by using a piezoelectric transducer (PZT) which mechanically modulates the external cavity grating. The laser PZT is driven by an external piezo-driver (Thorlabs MDT694A) which receives sinusoidal wave (100 Hz and 0 – 10VDC) from a standard function generator (Stanford Research Systems DS 345).
Fig. 1 Spectral Simulations based on HITARN 2012 to identify optimum line for measurement of each species under combustion relevant conditions.
Figure 2 Shock tube experimental setup
LW1D.3.pdf
Imaging and Applied Optics © 2014 OSA
4. Shock tube setup Fig. 2 shows a schematic of the optical setup used for measurements behind reflected shock waves. The IR light from cw-QCL was transmitted through GaAs windows located at an axial location 2 cm from the shock tube endwall. In order to control the beam steering, an iris was placed upstream the shock tube to reduce the beam size. A second iris was placed downstream of the shock tube to minimize thermal emissions originating from the shock tube. Common-mode-rejection scheme was used to minimize the effect of laser noise on the absorption. The laser intensity was measured with thermoelectrically-cooled, optically immersed photovoltaic detectors (Vigo PVI 3TE10.6). A Germanium etalon, with a free spectral range (FSR) of 0.0163 cm-1, was used to measure the wavelength tuning in the coarse modulation mode. The laser intensity and Kistler pressure transducer signals were recorded through National Instruments data acquisition system (NI PCI- 6133 DAQ) with a sampling rate of 2.5 MS/sec. 5. Results and Discussions A number of studied are carried out recently in our laboratory; some of these are briefly mentioned here. (i) H2O2 decomposition reaction rate was measured behind reflected shock waves. Loading of H2O2 vapors in shock tube was carried out by bubbling argon through highly concentrated aqueous H2O2 solution. Initial mole faction of H2O2 was determined by scanning absorption line near 1302.59 cm-1. Fig 3 (a) shows absorbance profiles of H2O2 at different reflected-shock temperatures measured during thermal decomposition of H2O2. The absorbance profiles were converted into mole fractions which could be used to determine the 2nd order reaction rate for H2O2 decomposition [3]. Our measurements were carried out in the temperature range of 930- 1230 K and at 1, 2 and 10 atm pressures. The measured rate constants show that reaction is in low-pressure limit at 1 and 2 atm while it is in fall-off region at 10 atm. (ii) Spectroscopic measurements were performed for P branch of 4+5 band of acetylene in the 1250-1310 cm-1 region by operating the laser in coarse modulation mode. Room-temperature linestrengths, self, N2, Ar and He broadening coefficients were determined for a total of 25 lines. Temperature dependence of N2 and Ar broadening coefficients were also measured for 5 lines. Fig. 3 (b) shows room temperature Ar- and N2 broadening coefficients of acetylene as function of rotational quantum number. (iii) - (iv) Differential absorbance based interference-free diagnostics for measuring CH4 and C2H2 were developed for shock tube combustion experiments. Fig. 3 (c) and Fig. 3 (d) show two sample traces for measuring C2H2 and CH4 during n-pentane pyrolysis in shock tube. The measured profiles were also compared with mechanism predictions based on C8-C16 n-alkane kinetic mechanism [4]. To our knowledge, the measurements described above are the first of its kind for the absolute quantification of these important species using a mid-IR laser source.
Figure 3 (a) Absorption signals for H2O2 decay at different temperatures for measurements near 1 atm. (b) Broadening coefficients 2 C2H2-Ar/N2 for4+5 band of acetylene, (c) and (d) acetylene and methane mole fractions during n-pentane pyrolysis
6.
References
[1] Wysocki G, Curl RF, Tittel FK, Maulini R, Bulliard J-M, Faist J. Widely tunable mode-hop free external cavity quantum cascade laser for high resolution spectroscopic applications. Applied Physics B. 2005;81:769-77. [2] Rothman L, Gordon I, Babikov Y, Barbe A, Chris Benner D, Bernath P, et al. The HITRAN2012 molecular spectroscopic database. Journal of Quantitative Spectroscopy and Radiative Transfer. 2013;130:4-50. [3] Sajid M, Es‐sebbar E, Javed T, Fittschen C, Farooq A. Measurement of the Rate of Hydrogen Peroxide Thermal Decomposition in a Shock Tube Using Quantum Cascade Laser Absorption Near 7.7 μm. International Journal of Chemical Kinetics. 2013. [4] Westbrook CK, Pitz WJ, Herbinet O, Curran HJ, Silke EJ. A comprehensive detailed chemical kinetic reaction mechanism for combustion of n-alkane hydrocarbons fromn-octane to n-hexadecane. Combustion and Flame. 2009;156:181-99.
