Coherent sources and techniques for chemical sensing using broadband mid-infrared light
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Motivated by the requirements of vibrational spectroscopy of liquid-phase chemicals – which are distinguished by their broad and smooth absorption features – this thesis describes the development and applications of coherent mid-infrared sources in the 2.1 to 12-µm wavelength band. Two ultrafast optical parametric oscillator (OPO) midinfrared sources were constructed using periodically poled lithium niobate (PPLN) and orientation patterned gallium phosphide (OPGaP). The PPLN OPO produced spectrally broad output pulses tunable from 2.1 - 4.1 µm with hundreds of milliwatts of average power. The OPGaP OPO tuned from 5 - 12 µm producing tens of milliwatts of average power, covering a large fraction of the molecular ﬁngerprint region with comparable spectral brightness to state-of-the-art sources. Aerosols of harmful chemicals have an impact on health and the environment and using mid-infrared light scattering offers a route to standoff aerosol detection. The spectra of scattered light from aerosols of liquid chemicals were measured at wavelengths from 3.2 - 3.55 µm using the PPLN OPO as an active illumination source. Different chemicals were distinguished by clearly different spectral behaviour, closely following the mid-infrared transmission spectrum of each. To understand the factors inﬂuencing the spectroscopy of aerosols, a Mie-scattering model was developed which predicted a stronger inﬂuence of particle size scattering effects at longer wavelengths, introducing signiﬁcant differences between scattered spectra and liquid transmission spectra. Using the OPGaP OPO as illumination, scattered spectra from a range of liquid chemicals were measured between 7.2 - 11.2 µm, with differences between the scattering and transmission observed as predicted by the model. These results highlighted that understanding the particle size distribution is critical when using scattered light to identify aerosols. The extension of standoff detection to identifying liquid and solid material deposited on a surface was explored using active spectral imaging at wavelengths of 3 - 4 µm. Liquid chemicals were distinguished by their absorption using narrow band illumination tuned with a monochromator, and initial experiments using a high speed camera and Fourier transform spectrometer with broad band illumination demonstrated the ability to differentiate between a number of different powder samples. Finally, a novel implementation of chemical spectroscopy using compressive sensing was investigated, showing potential for spectral imaging using a single-pixel detector.