Advanced laser based spectroscopic techniques for trace gas detection based on optical cavity enhancement and multipass absorption cells

In this thesis, three advanced experiments based on laser spectroscopy are introduced for the first time which address several experimental topics for trace gas analysis. The implementation of a diode laser based gas phase Raman detector is introduced, capable of parts per million (ppm) detection limits. The spectrometer features a low power laser diode (10 mW) which is enhanced by power build up in an optical cavity. This new technique is characterised by recording spectra of N2, O2, H2, CH4 and benzene.
A second advanced laser based spectroscopy technique for trace gas detection, mid infrared cavity enhanced resonant photoacoustic spectroscopy (mid-IR CERPAS) is set up and characterised. This scheme uses optical cavity power build-up, optical feedback stabilisation and resonant photoacoustics. A single-mode continuous wave quantum cascade laser is coupled to a three mirror V-shape optical cavity. Gas phase species absorbing in the mid-IR are detected using the photoacoustic (PA) technique. Mid-IR CERPAS was characterised by measuring acetylene (limit of detection 50 ppt) and nitromethane (0.8 ppb). The mid-IR CERPAS equipment was also used to detect explosives’ vapours; TNT (1.2 and 5.5 ppb), 2, 4-DNT (7 ppb), TATP (4 ppb) and explosives’ taggants such as DMNB (11 ppb). Significant interferences from ambient water in lab air are observed and are overcome. Normalized noise-equivalent absorption coefficients are determined as » 6 x 10-10 cm-1 s1/2 (1 s integration time) and 6 x 10-11 cm-1 s1/2 W (1 s integration time and 1 W laser power).
Finally, a near infrared Herriott cell enhanced resonant photoacoustic spectroscopy spectrometer is set up and characterised. This scheme uses enhancement from the absorption pathlength by a multipass Herriott cell and detection of the gas phase species by resonant photoacoustics, Herriott cell enhanced resonant photoacoustics, HERPAS. A single-mode continuous wave near infrared external cavity diode laser is coupled to a Herriott cell. Absorbing gas phase species are detected using the photoacoustic (PA) technique which was characterised by measuring acetylene (150 ppb detection limit at 100 ms integration time). HERPAS was extended to measure several toxic industrial gases including hydrogen sulfide, ammonia and carbon monoxide. Normalized noise-equivalent absorption coefficients are determined for H2S as » 5.3 x 10-9 cm-1 s1/2 (1 s integration time) and 1.6 x 10-10 cm-1 s1/2 W (1 s integration time and 1 W laser power).
These three novel advanced spectroscopic techniques allow the detection of IR-inactive and IR- active gas phased species with great sensitivities and selectivity and improve significantly current capabilities for trace gas phase detection.