Development of continuous-wave pump-enhanced optical parametric oscillators and their application to photo-thermal spectroscopy

Abstract

The development of both a spectrally tunable radiation source and a sensitive detection transducer for spectroscopic applications are described in this thesis. The emission source is an optical parametric oscillator operating in the continuous-wave regime with a pump-enhanced singly-resonant architecture and involves the separate development of two constituent parts: A continuous-wave pump laser operating at 1064 nm based on neodymium-doped crystals focused on achieving high power whilst maintaining single-frequency operation; and a split-ring pump-enhanced optical parametric oscillator operating from this pump source focused on fine-tuning in mode-hops. This source was capable of coarsely tuning in the mid-infrared over 3 – 4 μm with an average of 140 mW of power with the ability of automated mode-hop tuning continuously over 90 cm⁻¹ in 0.07 cm⁻¹ steps. The detection transducer is based upon photo-thermal interferometric spectroscopy which employs a phase-sensitive method to detect the heating of a gas sample under radiation from the appropriately tuned source. Utilisation of a spectrally-independent probe beam permits samples with strong absorption features in the mid/deep-infrared to be examined whilst utilising the low-cost, high-sensitivity photodetectors in the visible/near-infrared region. This work implements both a Mach-Zehnder and a Sagnac interferometer, where the latter holds the potential to minimise the effects of the environment and simplify the associated experimental steps by removing stabilising electronic circuitry. Combined utilisation of this transducer alongside the excitation source has demonstrated an ability to detect the presence of ethane down to 100 parts-per-billion. Further development considered the interferometer inherent in photo-thermal spectroscopy, where this provides an avenue for employing optical squeezing techniques to increase the ultimate sensitivity of this technique. Construction of a squeezed light generator to use as a probe beam based on second-order nonlinearity has additionally been attempted in this work and has been shown to exhibit anti-squeezing characteristic behaviour.