  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.
