Abstract
Low noise detection with state-of-the-art mid-infrared (MIR) detectors (e.g., PbS, PbSe, InSb, HgCdTe) is a primary challenge owing to the intrinsic thermal background radiation of the low bandgap detector material itself. However, researchers have employed frequency upconversion based detectors (UCD), operable at room temperature, as a promising alternative to traditional direct detection schemes. UCD allows for the use of a low noise silicon-CCD/camera to improve the SNR. Using UCD, the noise contributions from the nonlinear material itself should be evaluated in order to estimate the limits of the noise-equivalent power of an UCD system. In this article, we rigorously analyze the optical power generated by frequency upconversion of the intrinsic black-body radiation in the nonlinear material itself due to the crystals residual emissivity, i.e. absorption. The thermal radiation is particularly prominent at the optical absorption edge of the nonlinear material even at room temperature. We consider a conventional periodically poled lithium niobate (PPLN) based MIR-UCD for the investigation. The UCD is designed to cover a broad spectral range, overlapping with the entire absorption edge of the PPLN (3.5 - 5 µm). Finally, an upconverted thermal radiation power of ~30 pW at room temperature (~30°C) and a maximum of ~70 pW at 120°C of the PPLN crystal are measured for a CW mixing beam of power ~60 W, supporting a good quantitative agreement with the theory. The analysis can easily be extended to other popular nonlinear conversion processes including OPO, DFG, and SHG.
Highlights
Introduction and backgroundApplications in the mid-infrared (MIR: 2 ~15 μm) wavelength range are numerous, ranging from precise molecular spectroscopy, food and pharmaceutical product analysis, soil contaminants and environmental monitoring, semiconductor processing, military applications, to astronomy [1,2,3,4,5]
We have developed a numerical approach to quantify the contribution of internal thermal radiation in a broadband frequency upconversion detector (UCD)
Thermal light, originating from black-body radiation of the semi-transparent nonlinear material itself, dominates the noise in a highly sensitive upconversion based detectors (UCD) when operating at the optical absorption edge of the nonlinear material
Summary
Applications in the mid-infrared (MIR: 2 ~15 μm) wavelength range are numerous, ranging from precise molecular spectroscopy, food and pharmaceutical product analysis, soil contaminants and environmental monitoring, semiconductor processing, military applications, to astronomy [1,2,3,4,5]. Few investigations of thermal noise contribution to the upconverted signal have been reported in the 1970’s; one using a proustite crystal at room temperature [21] and a second using a LiNbO3 crystal at 600 K [22] In both cases, narrowband noise emission is considered in a relatively transparent region of the crystals (e.g. absorption coefficient = 0.22 cm−1 [21]). Narrowband noise emission is considered in a relatively transparent region of the crystals (e.g. absorption coefficient = 0.22 cm−1 [21]) Comparing these initial results of the thermal noise contribution to a state-of-the-art UCD module, where the efficiency, η is 5 to 6 orders of magnitude higher and phase matches over a broad wavelength range, covering the absorption edge of the material, it is very important to include this thermal noise component in the analysis of the noise in the system. To the best of our knowledge, this is the first analysis and experimental demonstration of the internal thermal contribution in a highly efficient, broadband upconversion system with a quantification of the upconverted thermal radiation power when operated at room temperature or above
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