External Cavity Quantum Cascade Lasers for Spectroscopic Applications
Speaker: Tracy Tsai
Series: Final Public Orals
Location: Engineering Quadrangle J401
Date/Time: Friday, September 28, 2012, 10:00 a.m. - 12:00 p.m.
Mid-infrared spectroscopy is a power tool in monitoring and quantifying trace gases for industrial process, homeland security, and environmental applications. The ideal trace gas sensor or spectrometer is like a Star Trek tricoder: capable of sensitive measurements, multi-functional, and portable. In other words, the spectrometer must have a large spectral range and high spectral resolution. However, although current mid-infrared spectrometers (i.e. Fourier Transform Spectrometers or FTS) have a wide spectral range, they are too large with slow scan rates for practical use in high resolution spectroscopic applications. Quantum cascade lasers (QCLs) are compact, powerful, and efficient mid-infrared sources that can be quantum engineered with broadband gain profiles. Placed inside a diffraction grating based external cavity arrangement, they can provide frequency range >10% of the laser wavelength with a spectral resolution limited by the very narrow laser linewidth. Therefore, the external cavity quantum cascade laser (EC-QCL) provides both high spectral resolution and a wide frequency range. This dissertation describes the study and development of EC-QCLs for spectroscopic applications.
A new active wavelength method is presented to simplify the spectrometer system by allowing for reliable operation of the EC-QCL without additional wavelength diagnostic equipment. Typically, such equipment must be added to the spectrometer, because the grating equation is inaccurate in describing the EC-QCL output wavelength due to spectral misalignment of other wavelength-selective resonances in the EC-QCL. The active wavelength locking method automatically controls the EC-QCL wavelength, which significantly improves the accuracy of the grating equation and offers a precision suitable for atmospheric gas analysis.
For industrial spectroscopic sensing applications in which scan rates must be on the order of kilohertz so that the turbulent gas system can be approximated as a quasi-stable one, a fast-wavelength-scanning folded EC-QCL design capable of 1 kHz scan rate is presented. Two modes of operation have been studied: 1) low resolution pulsed mode and 2) high resolution continuous-wave (cw) mode.
Lastly, a custom EC-QCL is used as part of a laser heterodyne radiometer (EC-QC-LHR) to perform atmospheric sounding measurements in conjunction with a high resolution FTS. The EC-QC-LHR spectral range covers absorption features of five different trace gases of importance to atmospheric chemistry: ozone, nitrous oxide, methane, Freon-12, and water vapor. With one system that offers 60 MHz spectral resolution, the vertical profiles of the five target molecules are successfully retrieved, and the results agree with computer simulations, literature, and profiles retrieved from the FTS data.