The potential for improving the penetration depth of optical coherence tomography systems by using increasingly longer wavelength light sources has been known since the inception of the technique in the early 1990s. Nevertheless, the development of mid-infrared optical coherence tomography has long been challenged by the maturity and fidelity of optical components in this spectral region, resulting in slow acquisition, low sensitivity, and poor axial resolution. In this work, a mid-infrared spectral-domain optical coherence tomography system operating at 4 micron central wavelength with an axial resolution of 8.6 microns is demonstrated. The system produces 2D cross-sectional images in real-time enabled by a high-brightness 0.9-4.7 micron mid-infrared supercontinuum source with 1 MHz pulse repetition rate for illumination and broadband upconversion of more than 1 micron bandwidth from 3.58-4.63 microns to 820-865 nm, where a standard 800 nm spectrometer can be used for fast detection. Images produced by the mid-infrared system are compared with those delivered by a state-of-the-art ultra-high-resolution near-infrared optical coherence tomography system operating at 1.3 {mu}m, and the potential applications and samples suited for this technology are discussed. In doing so, the first practical mid-infrared optical coherence tomography system is demonstrated, with immediate applications in real-time non-destructive testing for the inspection of defects and thickness measurements in samples that are too highly scattering at shorter wavelengths.
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