Multi-wavelength holography with flexible synthetic wavelengths

Interferometric shape measurement is inherently limited by its unambiguity range, which is directly determined by the wavelength. Multi-wavelength interferometry extends the unambiguity range, allowing it to better address individual samples. However, deploying such systems outside the laboratory remains difficult. Tunable or individually stabilized lasers are prone to long-term wavelength drift, which makes additional calibration equipment necessary. In this work, we demonstrate a robust light source that requires no wavelength calibration and enables flexible generation of synthetic wavelengths-from meter-scale down to 15 mm using a single non-stabilized laser in combination with single-sideband modulation. Importantly, the entire setup is built exclusively from commercially available, fiber-coupled components, many of which are field-proven in the telecommunications industry. This ensures high reliability, robustness, and ease of integration into industrial environments. As a proof of concept, we reconstruct the surface profiles of two samples made of entirely different materials and geometries, featuring height variations from 1 to 120 mm. For each sample, the synthetic wavelength is individually tailored between 375 and 15 mm. Our results show that all features are accurately resolved with the expected height-resolution. Furthermore, this resolution can be improved by an order of magnitude through the use of commercially available thin-film lithium niobate modulators. This makes the proposed scheme a highly practical solution for robust inline inspection systems across a wide range of industrial parts.