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Optics Express








The performance of sensitive spectroscopic methods in the mid-IR is often limited by fringing due to parasitic etalons and the background noise in mid-infrared detectors. In particular, the technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS), which is capable of determining the frequencies of strong rovibrational transitions of molecular ions with sub-MHz uncertainty, needs improved sensitivity in order to probe weaker transitions. In this work, we have implemented up-conversion detection with NICE-OHVMS in the 3.2 - 3.9 μm region to enable the use of faster and more sensitive detectors which cover visible wavelengths. The higher bandwidth enabled detection at optimized heterodyne frequencies, which increased the overall signal from the H43 cation by a factor of three and was able to resolve sub-Doppler features which had previously overlapped. Also, we demonstrate the effectiveness of Brewster-plate spoilers to remove fringes due to parasitic etalons in a cavity enhanced technique. Together, these improvements reduced the instrument's noise equivalent absorption to 5.9 X 10-11 cm-1 Hz-1/2, which represents a factor of 34 improvement in sensitivity compared to previous implementations of NICE-OHVMS. This work will enable extended high-precision spectroscopic surveys of H43 and other important molecular ions.


"Optics Express, Optical Materials Express, Biomedical Optics Express, Optica, and OSA Continuum are all open access journals, which means that all content is freely available without charge to the user or his/her institution. Beginning in September 2017, users are allowed to read, download, copy, distribute, print, search, or link to the full texts of the articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This is in accordance with the BOAI definition of open access. Authors may also opt to make their articles freely available in our hybrid journals, Journal of the Optical Society A, Journal of the Optical Society B, Applied Optics, and Optics Letters."

Original Publication Citation

Markus, C. R., Perry, A. J., Hodges, J. N., & McCall, B. J. (2017). Improving cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and brewster-plate spoilers. Optics Express, 25(4), 3709-3721. doi:10.1364/OE.25.003709