Nonlinear interferometers allow for mid-infrared spectroscopy with near-infrared detection using correlated photons. Previous implementations have demonstrated a spectral resolution limited by spectrally selective detection. In our work, we demonstrate mid-infrared transmission spectroscopy in a nonlinear interferometer using single-pixel near-infrared detection and Fourier-transform analysis. A sub-wavenumber spectral resolution allows for rotational-lineresolving spectroscopy of gaseous samples in a spectral bandwidth of over 700 cm −1 . We use methane transmission spectra around 3.3 µm wavelength to characterize the spectral resolution, noise limitations and transmission accuracy of our device. The combination of nonlinear interferometry and Fourier-transform analysis paves the way towards performant and efficient mid-infrared spectroscopy with near-infrared detection.

Nonlinear interferometers allow spectroscopy in the mid-infrared range by detecting correlated visible light, for which non-cooled detectors with higher specific detectivity and lower dark count rates are available. We present a new approach for the registration of spectral information, which combines a nonlinear interferometer using non-degenerate spontaneous parametric down-conversion (SPDC) with a Fourier-transform spectroscopy concept. In order to increase the spectral coverage, we use broadband non-collinear SPDC in periodically poled LiNbO 3 . Without the need for spectrally selective detection, continuous spectra with a spectral bandwidth of more than 100 cm −1 are achieved. We demonstrate transmission spectra of a polypropylene sample measured with 6 cm −1 resolution in the spectral range between 3.2 µm to 3.9 µm.

We present dispersive Fourier-transform spectroscopy with undetected photons using a nonlinear interferometer based on spontaneous parametric down-conversion. The interferometer transfers mid-infrared information of sample interaction to the near-infrared, which allows using low-noise silicon-based detectors, enabling accurate measurements with extremely low light exposure. We demonstrate a dispersion measurement of gaseous methane with an accuracy greater than 1 part in 106, which is limited by random phase noise. The Fourier-transform approach allows high resolution, rotational-line resolving spectroscopy of both, the absorptive and dispersive properties of a sample in a simple and robust measurement concept.

Nonlinear interferometers based on non-degenerate spontaneous parametric down-conversion (SPDC) create a link between separate spectral ranges. This allows for measurements in remote spectral regions while detecting light in easily accessible wavelengths. In our work, we use periodically poled lithium niobate to create correlated signal (visible or near-infrared) and idler (mid-infrared) photon pairs. Using a nonlinear interferometer in Michelson geometry, we obtain broadband mid-infrared spectra from light detected with a silicon avalanche photodiode. Combining the nonlinear interferometer with a measurement scheme in close analogy to classical Fourier-transform infrared spectroscopy allows for sub-wavenumber spectral resolution, which opens up possibilities for applications such as precise spectroscopic gas analysis.

Mid-infrared spectroscopy is one of the most important techniques in chemical analysis. However, the detectors for the mid-infrared range suffer from lower specific detectivities in comparison to their visible counterparts, cost more and often require cryogenic cooling. Nonlinear interferometers allow measuring mid-infrared spectra by detecting only visible light using the induced coherence effect. In our work, we realize a nonlinear interferometer designed for broadband mid-infrared spectra with high resolution, which is easily tunable, and in analogy to classical Fourier transform infrared (FTIR) spectrometers requires no additional spectral selection.