Thermal camera based on frequency upconversion and its noise-equivalent temperature difference characterization

We present a scheme for estimating the noise-equivalent temperature difference (NETD) of frequency upconversion detectors (UCDs) that detect mid-infrared (MIR) light. In particular, this letter investigates the frequency upconversion of a periodically polarized crystal based on lithium niobate, where a mid-infrared conversion bandwidth of 220 nm can be achieved in a single poled period by a special design. Experimentally for a temperature target with a central wavelength of 7.89 {\mu}m in mid-infrared radiation, we estimated the NETD of the device to be 56 mK. Meanwhile, a direct measurement of the NETD was performed utilizing conventional methods, which resulted in 48 mK. We also compared the NETD of our UCD with commercially available direct mid-infrared detectors. Here, we showed that the limiting factor for further NETD reduction of our device is not primarily from the upconversion process and camera noise, but from the limitations of the heat source performance. Our detectors have good temperature measurement performance and can be used for a variety of applications involving temperature object identification and material structure detection.