Low-coherence interferometry with undetected mid-infrared photons in the high-gain regime

We develop a high-parametric-gain SU(1,1) interferometer based on an aperiodically poled potassium titanyl phosphate (apKTP) crystal, enabling frequency-domain low-coherence interferometry with undetected mid-infrared photons. The system achieves a signal-to-noise ratio as high as 40 dB and axial resolution of 30 μm, with a 3 μm-centered idler beam. By increasing the poling-period range, we also improve the axial resolution to 17 μm, demonstrating a straightforward route to enhance the performance by working on the crystal design.