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Non-destructive testing (NDT) is crucial for ensuring product quality andsafety across various industries. Conventional methods such as ultrasonic,terahertz, and X-ray imaging have limitations in terms of probe-contactrequirement, depth resolution, or radiation risks. Optical coherence tomography(OCT) is a promising alternative to solve these limitations, but it suffersfrom strong scattering, limiting its penetration depth. Recently, OCT in themid-infrared (MIR) spectral region has attracted attention with a significantlylower scattering rate than in the near-infrared region. However, the highestreported A-scan rate of MIR-OCT has been 3 kHz, which requires long dataacquisition time to take an image, unsatisfying industrial demands forreal-time diagnosis. Here, we present a high-speed MIR-OCT system operating inthe 3-4 um region that employs the swept-source OCT technique based ontime-stretch infrared spectroscopy. By integrating a broadband femtosecond MIRpulsed laser operating at a repetition rate of 50 MHz, we achieved an A-scanrate of 1 MHz with an axial resolution of 11.6 um and a sensitivity of 55 dB.As a proof-of-concept demonstration, we imaged the surface of substratescovered by highly scattering paint coatings. The demonstrated A-scan ratesurpasses previous state-of-the-art by more than two orders of magnitude,paving the way for real-time NDT of industrial products, cultural assets, andstructures.

Mid-infrared optical coherence tomography with MHz axial line rate for real-time non-destructive testing