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

Area of Science:

• Photonics and Optical Imaging
• Materials Science
• Spectroscopy

Background:

• Up-conversion imaging allows room-temperature short-wave infrared (SWIR) imaging with visible-band detectors, surpassing direct SWIR detection limitations.
• Existing methods using poled crystals have small apertures, and critical phase-matched (CPM) bulk crystals suffer from walk-off-induced image degradation.

Purpose of the Study:

• To demonstrate broadband SWIR up-conversion imaging with high resolution and fidelity.
• To characterize the performance of a noncritical phase-matched (NCPM) KTiOPO4 crystal for SWIR up-conversion imaging.

Main Methods:

• Utilized a large-aperture (6 mm × 7 mm) KTiOPO4 crystal with noncritical phase-matching (NCPM).
• Operated the system over a broad spectral range of 1.3-2.2 μm.
• Measured finest resolvable linewidths at specific illumination wavelengths (1.334 μm and 1.792 μm).

Main Results:

• Achieved finest resolvable linewidths of 8.8 μm and 12.4 μm at 1.334 μm and 1.792 μm, respectively.
• Demonstrated 5-6× and 2× improvements in resolution compared to a representative poled crystal.
• Observed improved image fidelity due to the absence of walk-off, approaching theoretical resolution limits.

Conclusions:

• Established a fixed-crystal route for high-resolution broadband SWIR up-conversion imaging.
• Presented the first comprehensive characterization of NCPM-based broadband up-conversion imaging.
• The developed system offers a promising solution for advanced SWIR imaging applications.