Short-wave infrared broadband up-conversion imaging by using a noncritical phase-matched bulk KTiOPO 4 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 KTiOPO 4 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.