12 March 2013 High resolution mid-infrared spectroscopy based on frequency upconversion
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We present high resolution upconversion of incoherent infrared radiation by means of sum-frequency mixing with a laser followed by simple CCD Si-camera detection. Noise associated with upconversion is, in strong contrast to room temperature direct mid-IR detection, extremely small, thus very faint signals can be analyzed. The obtainable frequency resolution is usually in the nm range where sub nm resolution is preferred in many applications, like gas spectroscopy. In this work we demonstrate how to obtain sub nm resolution when using upconversion. In the presented realization one object point is imaged through the upconverter. Assuming homogeneous spherical emission from the object point, the upconverted radiation will carry the spectral information as con-centric rings. From the optical path length and dispersion properties of the nonlinear material, the acceptance bandwidth of the upconversion process is calculated. It is then straightforward to deduce the spectral information of the light emitted from the object point by a simple analysis of the upconverted radiation. In order to increase resolution, a scanning Fabry-Perot etalon is inserted in a collimated geometry of the upconverted light generated by the crystal. The etalon is designed with a free-spectral range larger than the bandwidth of the upconversion process. Hence, the spectral resolution is now set by the finesse of the etalon. Based on this approach a spectral resolution of 0.2 nm has been reached around 2.9 μm. We demonstrate high resolution spectral performance by observing emission from hot water vapor in a butane gas burner.
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Jeppe Seidelin Dam, Jeppe Seidelin Dam, Qi Hu, Qi Hu, Peter Tidemand-Lichtenberg, Peter Tidemand-Lichtenberg, Christian Pedersen, Christian Pedersen, } "High resolution mid-infrared spectroscopy based on frequency upconversion", Proc. SPIE 8604, Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XII, 86040S (12 March 2013); doi: 10.1117/12.2004012; https://doi.org/10.1117/12.2004012

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