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The mid-infrared spectral range (wavelength ~ micron to 20 micron) is known as the "molecular fingerprint" region as many molecules have their highly characteristic, fundamental ro-vibrational bands in this part of the electromagnetic spectrum. Broadband mid-infrared spectroscopy therefore constitutes a powerful and ubiquitous tool for optical analysis of chemical components that is used in biochemistry, astronomy, pharmaceutical monitoring and material science. Optical frequency combs, i.e. broad spectral bandwidth coherent light sources consisting of equally spaced sharp lines, have revolutionized optical frequency metrology one decade ago. They now demonstrate dramatically improved acquisition rates, resolution and sensitivity for molecular spectroscopy mostly in the visible and near-infrared ranges. Mid-infrared frequency combs have therefore become highly desirable and recent progress in generating such combs by nonlinear frequency conversion has opened access to this spectral region. Within this project we plan to use an mid IR CW laser (in the form of the requested OPO of this R'Equip) and demonstrate a promising alternative to mid-infrared frequency comb generation with a continuous-wave pumped ultra-high Q crystalline microresonator made of magnesium fluoride. This would constitute the first mid IR comb generated using the novel Kerr frequency comb technique. Its distinguishing features are compactness, efficient conversion, large mode spacing and high power per comb line. This work therefore opens the path to a versatile mid-infrared spectrometer, and holds promise to facilitate dual-comb spectroscopy. Equally important, combining the broad transparency window (up to 7 micron) of crystalline magnesium fluoride microresonators with high power quantum cascade lasers, a compact frequency comb source that extends deep into the mid-infrared can be envisioned. Specifically this proposal seeks to extend microresonator based optical frequency comb technique into the mid-infrared region. The objective of this proposal is to make use of a newly developed technology of the laboratory of ultra-high Q crystalline resonators. These resonators – with transparency window in the mid IR - have not only the highest optical Q factor to date (>10 billion) but moreover are also capable of generating optical frequency combs via parametric frequency conversion. These resonators have so far only been demonstrated by one group at the JPL in Pasadena. Recent work in the applicants group has reproduced these results and made these resonators now also available for studies regarding optical frequency comb generation. Crystalline resonators from the Fluorides offer a unique playground in this context, since they are transparent into the mid IR (i.e. up to ca.8 micron a Q-factor of >100 million can be maintained). Moreover, by tailoring their geometric properties such as diameter, the zero dispersion region can be moved to 2.5 micron wavelength; this opens directly the intriguing opportunity to generate octave spanning comb spectra in the mid-IR from a CW laser source. Having a comb in the mid-IR would allow using techniques such as multi-heterodyne spectroscopy; in particular since the technique could be readily used to demonstrate the required pair of frequency combs. So far, combs in the mid IR have only been generated using difference frequency generation. This technique however cannot produce octave spanning combs and it suffers moreover from low power per comb component. A promising, but to date never verified new approach, could therefore be taken by pumping crystalline resonators with a high power continuous wave laser source, afforded by a CW optical parametric oscillator (Mid IR OPO). It is precisely this OPO that is part of this R'Equip request, which can both be used to pump the crystaline resonators as well as to serve as a narrowband source with which to verify the phase noise. References: [1] T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, Science 332, 555 (2011).
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