Development and application of spectroscopic techniques in the mid-infrared
Keywords: optical cavities
breath analysis
mid-infrared
difference frequency generation
spectroscopy

Applications of laser absorption spectroscopy for trace gas detection are many and diverse, ranging from the environmental and atmospheric to the medical and industrial. The aim of creating a spectrometer which combines high sensitivities and selectivities (in order to measure small amounts of absorbers or species that are only weakly absorbing, in a complex background matrix) with a wide spectral coverage (to allow broadband absorbers or multi-component samples to be studied) can be realised by implementing three separate concepts: the exploitation of the strong, fundamental transitions of the mid-infrared; the use of sensitive spectroscopic techniques; and the selection of a widely tunable laser source. In this thesis, these ideas are investigated individually and in combination in order to achieve such a goal. Laser spectroscopic techniques based on optical cavities are used to build a high resolution spectrometer covering a large spectral range capable of selectively detecting low levels of gaseous compounds of interest, especially those of medical or environmental significance. Work in both the near- and mid-infrared is presented, including much of the initial, developmental work which was conducted in the former region.
The thesis begins with an overview of both narrowband and broadband near-infrared radiation sources, with a particular emphasis on commonly available diode lasers (DLs). A novel laser source, the digital supermode distributed Bragg reector (DS-DBR) laser, is introduced as a useful laser source for spectroscopy, combining the usual benefits of telecom DLs with a wide tunability (1563 – 1613 nm). The laser can be operated in an internal or external ramping mode, allowing the output wavelength to be scanned or stepped across a desired region. The observation of mode-hopping during the application of the scanning methodology is examined and rationalised. The ability of the DS-DBR laser to perform high resolution spectroscopy over its entire spectral coverage is demonstrated by recording spectra of carbon dioxide (CO2) over this range, covering transitions from two of the four Fermi resonance components of the 3ν1 + ν3 combination band. The results of conducting wavelength modulation spectroscopy on CO2 are also reported.
A system developed for performing cavity ring-down spectroscopy (CRDS), capable of the real-time retrieval of ring-down times (RDTs), is presented and discussed. The outcomes of initial tests performed with a conventional DL at 1557 nm, to study a calibrated mixture of CO2 in air at various pressures, are given. In addition, the results of combining this system with the DS-DBR laser are discussed. The bandwidth of the DS-DBR laser was found to be larger than that of a standard DFB DL, resulting in the presence of noisy cavity modes. Despite this, the acquisition of reproducible RDTs is demonstrated, with single wavelength studies of an evacuated cavity at 1605.5 nm yielding a RDT of 24.54 ± 0.04 µs and Allan variance calculations signalling an attainable minimum detectable absorption coefficient, αmin, of 2.8 x 10-10 cm-1 over 20 s. The ability to perform CRDS across the whole DSDBR laser wavelength range without the need for cavity re-alignment is illustrated, and studies conducted on CO2 in air, calibrated mixtures and breath are reported. Investigations are also described into the accurate determination of the 13C/12C ratio in exhaled CO2 undertaken using CRDS and cavity enhanced absorption spectroscopy (CEAS) on CO2 isotopologues, an approach which can be utilised as a diagnostic aid in determining Helicobacter pylori infection.
The focus of the thesis then moves to the mid-infrared, to describe quasi phase matching difference frequency generation (QPM-DFG) and its use to generate laser light at 3 µm by optically mixing near-infrared DLs. The theory behind this non-linear optical interaction is outlined, and the construction of a free-space QPM-DFG system using periodically poled lithium niobate is detailed and characterised.
This DL-based QPM-DFG arrangement has been coupled with the CRDS system developed to create a mid-infrared CRD spectrometer. The results of single wavelength studies indicate RDTs of ~ 6 µs and an achievable αmin of 2.9 x 10-9 cm-1 over 44 s for an evacuated cavity. Spectroscopic investigations carried out on methane (CH4), acetone and deuterium are documented; for the latter species, Dicke narrowing of the electric quadrupole ν(1←0) Q(2) transition at 2987.29 cm-1 is observed and the integrated absorption cross-section for the same transition measured as 2.29 ± 0.03 x 10-27 cm2cm-1molec-1. The results of modifications made to the system, namely the use of a more powerful Nd:YAG laser as the pump radiation source, as well as a faster detector combined with a variable amplifier, are presented; these include the observation of an improved optimal αmin of 6.4 x 10-10 cm-1 over 151 s for an empty cavity.
Finally, work utilising the DS-DBR laser as one of the near-infrared sources for the QPM-DFG set-up is presented. This configuration generates radiation covering a wide mid-infrared range (3130 – 3330 nm) and has been used to perform direct absorption and wavelength modulation spectroscopy on ro-vibrational transitions within the fundamental ν3 (F2) band of CH4. The spectrum of methanethiol (CH3SH) over this region has also been investigated, with preliminary studies identifying a feature at 3040 cm-1 as a potential indicator for monitoring this biomarker in breath. The results of coupling this mid-infrared radiation with an optical cavity to perform CEAS combined with phase sensitive detection are subsequently reported. Studies were conducted on calibrated CH4 mixtures and ambient air to examine two transitions of the fundamental ν3 (F2) band of CH4 in order to characterise the system: effective path lengths of ~ 700 m and αmin of 6.2 x 10-8 cm-1 over 8 s were found. The RQ4 CH3SH absorption feature at 3040 cm-1 was also further studied with this system using prepared samples of CH3SH in N2 at different concentrations, yielding a CH3SH detection limit of 2.4 ppm at 19 Torr. The potential of such a cavity-based, DS-DBR sourced, QPM-DFG mid-infrared spectrometer for trace gas sensing having thus been demonstrated, possible improvements that could be implemented to increase the sensitivity of the system are then discussed.