Subcategory: Environmental Engineering
May Hlaing - Delaware State Universtiy
Co-Author(s): Caio Azevedo, Zaki J. Harris, Tarik Zerrad, and Amir Khan Delaware State University
Atmospheric methane (CH4), nitrous-oxide (N2O) and water-vapor (H2O) are the most important greenhouse gases with significant contributions to global warming and impact on global climate change. In the study of climate change, accurate, sensitive, and reliable field-deployable sensors are in demand for monitoring these gases, as well as understanding their sources and sinks. As these gases have a relatively strong molecular absorption and rotational-vibrational transitions in the mid-infrared region, tunable mid-IR Quantum cascade lasers (QCLs) are commonly used in laser-based trace gas sensors. In this paper we present a theoretical and experimental methodologies using mid-IR QCL laser-based trace gas sensor to simultaneously detect atmospheric CH4, N2O and H2O along with analysis of modulation characteristics of a continuous-wave mid-IR QCL at 7.8 μm. The sensor consists of typical components of a diode laser based system including a continuous-wave QCL operating at 7.83 μm, a long pathlength optical multi-pass cell and room-temperature Mercury Cadmium Telluride (MCT) detector. To resolve overlapping and interfering line transition, higher harmonic wavelength modulation spectroscopy (WMS) is used as a powerful technique for detection of low concentration species with an increased signal-to-noise ratio (SNR) and sensitivity by shifting the detected high-harmonic signals away from laser excessive noise (1/f noise). The optimal high harmonic signals are obtained by varying modulation frequencies and amplitudes. Moreover, for an accurate measurement, understanding QCL tuning characteristics and responses of WMS signals to modulation parameters is highly required. Our detailed characterizations of QCL show that the wavelength tuning range of QCL is highly related to the amplitude and frequency modulation characteristics. In our analysis we show such optimal modulation characteristics for simultaneous and optimal detection of N2O and CH4 in open path configuration.  Mohammad Jahjah, Wei Ren,Przemysław Stefański, Rafał Lewicki, Jiawei Zhang, Wenzhe Jiang, Jan Tarka and Frank K. Tittel “A compact QCL based methane and nitrous oxide sensor for environmental and medical applications” Analyst, 139 (9), 2065-2069 (2014).  Lei Tao, Kang Sun, David J. Miller, M. Amir Khan, and Mark A. Zondlo “Current and frequency modulation characteristics for continuous-wave quantum cascade lasers at 9.06 μm” Optics Letters / Vol. 37, No. 8 / April 15, 2012.  Kuijun Wu ,Faquan Li, Xuewu Cheng, Yong Yang, Xin Lin,Yuan Xia, “Sensitive detection of CO2 concentration and temperature for hotgases using quantum-cascade laser absorption spectroscopy” Appl. Phys. B DOI 10.1007/s00340-014-5880-4 (2014).May_Hlaing ERN Abstract.docx
Funder Acknowledgement(s): Acknowledgements: This work was sponsored in part by the National Science Foundation's CREST (Grant no. HRD-1242067), NASA MIRO (Grant no. NNX15AP84A) and NIH DE-INBRE (Grant no. NIGMS (5P20 GM103446-16)).
Faculty Advisor: Dr. Mohammad A. Khan, email@example.com
Role: Literature reviews, work on the experimental equipments and optics setups and processing and analyzing the experimental data