Jeppe Seidelin Dam

Two dimensional mid-infrared upconversion imaging provides unique spectral and spatial information showing good potential for mid-infrared spectroscopy and hyperspectral imaging. However, to extract spectral or spatial information from the upconverted images an elaborate model is needed, which includes non-collinear interaction. We derive here a general theory providing the far field of the upconverted light when two arbitrary fields interact inside a nonlinear crystal. Theoretical predictions are experimentally verified for incoherent radiation and subsequently applied to previously published data with good agreement.

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.

Upconversion of images is a generic method for shifting the spectral content of entire images. A comprehensive theory for upconversion of incoherent light images is presented and compared against experiments. In particular we consider the important case for upconversion of infinity corrected light. We show that the spatial resolution for upconversion of incoherent light images is better than for the corresponding coherent image upconversion case. The fundamental differences between upconversion of coherent and incoherent images are investigated theoretically and experimentally. The theory includes the general case of upconversion using TEM_nm modes.