A broadband SWIR upconversion spectrometer based on noncritically phase-matched KTP

We report a broadband scanning short-wave infrared (SWIR) upconversion spectrometer based on noncritically phase-matched (NCPM) bulk KTP. The SWIR signal is dispersed and selected before upconversion, allowing the dispersive element to resolve the larger wavelength intervals at the original SWIR wavelengths rather than the compressed intervals after upconversion. For the pump wavelength and type-II interaction used here, the phase mismatch vanishes at two signal wavelengths in the SWIR region, and a sufficiently short crystal broadens the corresponding acceptance bands and allows them to overlap, producing a nearly continuous broadband response. After calibrating the wavelength axis, we measured the upconversion spectral responses of 2.0 and 0.5 mm crystals over 1.35–2.14 μm. The 2.0 mm crystal exhibited two separated response peaks, whereas the 0.5 mm crystal produced a broad, nearly continuous response, consistent with the phase-matching model. A narrowband measurement at 1550 nm yielded a spectral resolution of 0.14 nm. These results demonstrate the operating principle of broadband scanning SWIR upconversion spectroscopy with bulk NCPM KTP and provide the wavelength-dependent system-response characterization needed for broadband spectral measurements.