Angular acceptance of compact-size infrared-to-visible image upconverters with a temperature gradient

Summary form only given. Nonlinear image up-conversion is an alternative for the acquisition of real-time and full unscanned 2D images at different spectral ranges of the infrared [1, 2] where sensors are not fully developed, as compared to existing imaging sensors in the visible in terms of noise, speed, resolution, or uncooled operation. Wavelength shift is accomplished through a sum-frequency mixing (SFM) process in a periodically-poled lithium niobate crystal (PPLN). For a good resolution of the upconverted image, the IR image needs to be focused to a smaller spot than that of the pump wave, and thus the image formation relies on the upconversion of a set of infrared incoming angles in a non-collinear quasi-phase matching (NCQPM) process. The upconverted area is, typically, a trade-off between the degree of focusing of the incoming infrared image and the nonlinear (NL) crystal angular acceptance. Therefore, under tight focusing conditions, the PPLN crystal angular acceptance may avoid the upconversion of some incoming angles. Different techniques for increasing the upconverted area rely on multiple-wavelength infrared illumination sources [3]. In contrast, we have recently shown that an upconverter with a thermal gradient applied to the nonlinear crystal can increase the upconverted area while illuminating with a single-wavelength infrared source [4]. In this work we give further insight in the formation of the temperature gradient and how the exact function describing the temperature change with the crystal length may affect the upconverter angular response. Our work focuses on compact upconverters, in which both a short PPLN crystal length and high numerical aperture lenses are employed. We performed 3D simulation (fig.1.a) and experimentally measured the PPLN crystal temperature distribution and found the temperature gradient was nonlinear along the PPLN crystal. This impacts the upconverter angular acceptance for different illumination wavelengths, which is sensitively modified if compared to an applied linear temperature gradient (fig.1.c). Exact modelling of the temperature through the PPLN crystal enables accurate wavelength selection in order to optimize the upconverter area or, inversely, optimize the temperature distribution for a given illumination wavelength.