Techniques in molecular spectroscopy: from broad bandwidth to high resolution

<p>This thesis presents a range of different experiments all seeking to extend the capabilities\&nbsp;<span style="font-size: 13px; line-height: 1.6em;">of molecular spectroscopy and enable new applications. The new technique of cavity-enhanced direct\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">frequency comb spectroscopy (CE-DFCS) provides a unique combination of broad bandwidth,\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">high resolution, and high sensitivity that can be useful for a wide range of applications. Previous\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">demonstrations of CE-DFCS were confined to the visible or near-infrared and operated over a limited\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">bandwidth: for many applications it is desirable to increase the spectral coverage and to extend\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">to the mid-infrared where strong, fundamental vibrational modes of molecules occur. There are several\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">key requirements for CE-DFCS: a frequency comb source that provides broad bandwidth and\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">high resolution, an optical cavity for high sensitivity, and a detection system capable of multiplex\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">detection of the comb spectrum transmitted through the cavity. We first discuss comb sources with\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">emphasis on the coherence properties of spectral broadening in nonlinear fiber and the development\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">of a high-power frequency comb source in the mid-infrared based on an optical-parametric oscillator\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">(OPO). To take advantage of this new mid-infrared comb source for spectroscopy, we also discuss\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">the development of a rapid-scan Fourier-transform spectrometer (FTS). We then discuss the first\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">demonstration of CE-DFCS with spectrally broadened light from a highly nonlinear fiber with the\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">application to measurements of impurities in semiconductor manufacturing gases. We also cover\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">our efforts towards extending CE-DFCS to the mid-infrared using the mid-infrared OPO and FTS\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">to measure ppb levels of various gases important for breath analysis and atmospheric chemistry\&nbsp;</span><span style="font-size: 13px; line-height: 1.6em;">and highlight some future applications of this system.</span></p>