Short-wave infrared broadband up-conversion imaging by using a noncritical phase-matched bulk KTiOPO₄ crystal

Compared with direct short-wave infrared (SWIR) detection, up-conversion imaging enables room-temperature SWIR imaging using mature visible-band detectors. However, poled crystals are limited by their small apertures, while large-aperture critical phase-matched (CPM) bulk crystals usually suffer from walk-off-induced image degradation. Here we demonstrate broadband SWIR up-conversion imaging using a noncritical phase-matched KTiOPO₄ crystal with a 6 mm × 7 mm aperture and a length of 0.5 mm. The system operates over a spectral range of 1.3-2.2 μm and achieves finest resolvable linewidths of 8.8 μm and 12.4 μm at illumination wavelengths of 1.334 μm and 1.792 μm, respectively. These values correspond to about 5-6× and 2× improvements in two orthogonal directions over the aperture-limited resolution of a representative 1 mm × 3 mm poled crystal. The absence of walk-off improves image fidelity, and the measured performance approaches the theoretical resolution limit. This study presents the first, to the best of our knowledge, comprehensive characterization of NCPM-based broadband up-conversion imaging and establishes a fixed-crystal route to high-resolution broadband SWIR up-conversion imaging.