Upconversion-based mid-infrared spectrometer using intra-cavity LiNbO₃ crystals with chirped poling structure

Upconversion from mid-infrared (MIR) to near-visible wavelengths using sum-frequency generation enables sensitive and low-noise MIR light detection using near-visible-light detectors. MIR spectroscopy is important for a wide spectrum of compound characterization applications but challenged by the thermal noise inherent in conventional MIR detectors, giving low detectivity. In this Letter, we show an upconversion-based spectroscopy scheme with an unprecedented combination of large bandwidth and short sampling time. Using LiNbO₃ crystals with a chirped poling structure placed inside a laser cavity, we demonstrate simultaneous upconversion from 2.15 μm to 5.3 μm with sampling times as low as 10.8 μs and an average signal-to-noise ratio of >6000 at 1 s. We conduct numerical studies to show the inverse relationship between efficiency and bandwidth of the upconversion process in the chirped crystal structure and find good agreement with our experimental results.