摘要： State-of-the-art mid-infrared (MIR: 2 – 15 μm) direct detectors (e.g. semiconductor based HgCdTe, PbS, PbSe, and microbolometer) suffer from high background noise when operating at room temperature. Low noise detection therefore requires multi-stage or cryogenic cooling (-195°C) and perfect shielding to avoid temperature fluctuations [1]. Such systems easily become sophisticated and bulky, non-suitable for widespread applications. As the MIR spectral range, especially around 6 μm, is very relevant for spectroscopic-imaging of bio-molecules (protein, lipids), tissues (cancer, tumour), or for sensing of environmental pollutants (CH4, NO, NO2, SO2), high resolution spectroscopic systems in the 6 μm range is much in need. The numbers of pixels in typical HgCdTe or microbolometer based array detectors are limited to few 100s, making them less than ideal for fast high resolution broad-band spectroscopy. This work aims to solve that problem, by using frequency upconversion detection (UCD) [2]. In the presence of a strong near-infrared (NIR) LASER pump (at 1.03 μm), the MIR signal (6 μm) is translated to the NIR wavelength range (below 1 μm) using parametric frequency conversion in a nonlinear crystal (AgGaS2), without losing the spectral information encoded in the MIR signal. After upconversion, a standard silicon-CCD based spectrometer (pixel number = 2064) is used to detect the upconverted signal. In this way, high resolution, sensitive spectroscopic measurements can be performed, without the need for sophisticated cooling. The proposed upconversion system is based on a diode (940 nm) pumped Yb:YAG based solid state LASER operating at 1.03 μm, where the nonlinear crystal AgGaS2 (bulk, 5×5×10 mm3) is placed inside the LASER cavity to access the high intracavity power (Fig. 1(a)). This arrangement essentially gives higher upconversion efficiency. A Globar (heat source at ~ 800°C) is used as MIR illumination in the 6 μm range, giving an upconverted signal in the 880 nm range (grey area plot in Fig. 1(b)). In comparison to the previous demonstrations [3], the primary novelties of this system are (i) long-wavelength pumping of the laser, meaning that the pump diode wavelength (940 nm) is longer than the upconverted wavelength (< 900 nm), simplifying the spectral filtering of the upconverted signal, (ii) the LASER (at 1.03 μm) cavity is only 4 cm long, which makes the footprint of the upconversion module < 10 cm2, and (iii) no moving parts are needed in the upconversion spectrometer system (Fig. 1(a)). Using this system, we have successfully upconverted the 6 – 6.8 μm range (800 nm wide) within a single acquisition (50 ms) using Type-II birefringent phase matching in an AgGaS2 crystal. The wide bandwidth of the upconversion detection is achieved by exploiting non-collinear interaction between the pump LASER and the MIR signal inside the crystal. As an experimental verification we measured the MIR absorption spectrum of a polystyrene film placed at the MIR input side (Fig. 1(b)). Further details and results will be presented. We believe this is a promising route towards a small footprint, low-noise, efficient upconversion detector for high resolution spectroscopic application, directly in the molecular fingerprint wavelength range.