Mid-infrared laser spectroscopy for trace gas detection

Thesis

Laser spectroscopy offers a powerful and adaptable toolkit of techniques for sensitive and selective gas detection. New semiconductor laser sources that can emit high power midinfrared radiation at room temperature have reawakened interest in exploiting the spectrally separated, strongly absorbing ro-vibrational transitions in this region. The aim of this thesis is the demonstration of absorption spectroscopy methods with interband cascade lasers (ICLs) and quantum cascade lasers (QCLs) to detect gases relevant to physiology and atmospheric science.

Modulation of laser frequency via the injection current can increase instrument performance and prevent long-term drifts in laser output. An 8.2 μm QCL was locked to a CH4 rovibrational line using a 3f signal, limiting the laser frequency deviation to 3 MHz over 120 s. Frequency modulation spectroscopy (FMS) at 20 MHz demonstrated a twofold improvement for NO measured with a 5.3 μm QCL despite residual amplitude modulation. Broadband RF current perturbation of a 5.3 μm QCL was shown to broaden the laser linewidth and enhance the sensitivity of off-axis cavity-enhanced absorption spectroscopy (CEAS) tenfold.

Optical-feedback CEAS (OF-CEAS), which achieves high intracavity power and elongated periods of resonant cavity excitation, was performed using V-shaped and linear optical cavities. ICL and EC-QCL sources were utilized for the first time for sensitive OF-CEAS detection of VOCs and N2O. OF-CEAS using a V-shaped cavity and 3.2 μm ICL achieved an α_min of 2:5x10^-8 cm^-1 for N₂O measurements. While the 5 μm EC-QCL achieved similar single-scan precision, mode-hopping prevented consistent measurements. The novel use of a linear optical cavity for OF-CEAS was demonstrated theoretically and experimentally with a 5.3 μm QCL to achieve excellent sensitivities (4 x 10^-9 cm^-1 Hz^-1/2). The successful demonstration of these techniques paves the way for new QCL and ICL technology to be incorporated in future laser absorption spectrometers for the laboratory, field, and clinic.

Authors: Manfred, K

• DOI: 10.5287/ora-dqnddqdmd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: infrared spectroscopy; trace gas detection; modulation spectroscopy
• Subjects: Optics; Chemistry, Physical and theoretical