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

TL;DR

This work demonstrates MIR low-coherence interferometry with undetected photons in the high-gain SU(1,1) regime using apKTP crystals with aperiodic poling to generate broadband PDC. The MIR idler probes the sample while the visible signal is detected, achieving an SNR up to $40$ dB and an axial resolution of $30$ μm (improved to $17$ μm with broader poling), with a depth range of ~270 μm and a probing/detected-power ratio > $200$. The results are enabled by a two-pass, high-gain amplification that yields strong signal enhancement (up to ~208×) while maintaining noninvasive probing; crystal design is shown to be a practical route to further improve axial resolution. The approach promises noninvasive MIR imaging of scattering materials and devices, with potential for time-gated measurements and OCT-like focusing by idler manipulation, and highlights the role of poling-profile engineering in optimizing bandwidth and spectral flatness. Overall, the work provides a scalable path to high-resolution, high-sensitivity MIR LCI with undetected photons using accessible detectors.

Abstract

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 $μ$m, with a 3 $μ$m-centered idler beam. By increasing the poling-period range, we also improve the axial resolution to 17 $μ$m, demonstrating a straightforward route to enhance the performance by working on the crystal design

Figures (11)

• Figure 1: Schematic of LCI with undetected MIR photons. HWP: Half-wave plate; DM: Dichroic mirror; LPF: Long-pass filter; MMF: Multimode fiber; OSA: Optical spectrum analyzer
• Figure 2: (a): Signal spectrum at the output of the interferometer, for a fixed position of the mirror along the sample arm. The interference pattern is related to the optical path difference between pump, signal, and idler beams; (b): Resulting depth profile, with the position of the peak corresponding to the one of the sample; (c): Reconstructed roll-off curve, measured by scanning several axial positions of the mirror.
• Figure 3: (a): 3D schematic of Al-stepped sample; (b): lateral scan of the Al-stepped sample; (c): Reconstructed depth profile of the thin Si layer, placed behind a Ge window.
• Figure 4: Comparison between the signal spectrum after the first (blue-green) and the second (blue) passage through the nonlinear crystal, in log scale.
• Figure 5: (a): Comparison between the output signal spectra for the crystal design with $\Lambda_1=12.3-14.0$$\mu$m (dotted sky-blue line) and the one with $\Lambda_2=10.9-14.0$$\mu$m (solid orange line), obtained for similar positions of the mirror; (b): Comparison between the corresponding LCI spectra, showing the improvement in the axial resolution.