Multi-wavelength phase retrieval-based optical metrology platform: high-resolution characterization of plane-parallel surfaces

Substrates with flat, parallel surfaces are generally challenging to characterize optically due to signal artifacts affecting methods like Fizeau interferometry. In addition, spectral properties of the material or eventual coatings of the part, often increase complexity by imposing constraints on the testing wavelength(s). Alternatives, such as contact-based techniques or temporary coatings are unsuitable for fragile, coated parts and hinder quality control. This work presents a one-stop optical platform that combines flexibility and robustness for characterizing plane-parallel optics. Its wavefront sensing approach allows the use of incoherent light, and opens up the choice of test wavelengths matching the requirements of the sample to be tested, while avoiding destructive interference due to unwanted surface reflections. The phase retrieval approach brings significant phase points increase, enabling large optical optics to be tested, while being achromatic on a wide spectral range. We will provide experimental results demonstrating these claims and present transmitted wavefront error, surface shape and material homogeneity measurements performed at multiple wavelengths.