External cavity quantum cascade lasers for spectroscopic applications

Abstract

Mid-infrared spectroscopy is a powerful tool in monitoring trace gases for applications in atmospheric science, industrial processes, and homeland security. However, although current mid-infrared spectrometers (i.e. Fourier Transform Spectrometers or FTS) have a wide spectral range for multi-species and/or broadband molecular detection, 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 easily provide >100 cm<super>-1</super> frequency range with a spectral resolution limited by the laser linewidth (~10<super>-3</super> cm<super>-1</super>). Therefore, the external cavity quantum cascade laser (EC-QCL) provides both high spectral resolution and a wide frequency range. This thesis 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 improves the accuracy of the grating equation to 0.06 cm<super>-1</super> and offers an ultimate 3&sigma precision of 0.042 cm<super>-1</super>.

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 system provides 0.002 cm<super>-1</super> spectral resolution over 1120 - 1238 cm<super>-1</super> frequency range, which is sufficient to resolve the absorption features of ozone, nitrous oxide, methane, dichlorodifluoromethane, and water vapor. 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.