Compensating for Laser Wavelength Instability in Multiwavelength Digital Holography: The Residual Method for Accurate Heightmap Generation

Accurate surface measurements are crucial for various industrial applications. Multiwavelength digital holography (MDH) is a cutting-edge technology that extends the range of holographic measurement to the mesoscopic scale while maintaining sub-micron axial accuracy. MDH achieves this by digitally “beating” precisely measured phasers at two wavelengths to create phase maps at longer wavelength scales. However, laser mode instability and wavelength uncertainty can cause errors in the scaling factor. This leads to misestimations and unwanted phase jumps in the heightmap. Therefore, MDH typically requires highly stable lasers. Our algorithm addresses this problem by detecting wavelength shifts during post-processing. It uses the residuals in the spatial frequency of the phasers that arises due to the misestimation of wavelength to compensate for the scaling factors and synthetic phase maps. This ensures an accurate, phase-jump-free heightmap. We find this technique essential for making MDH viable with environmental wavelength shifts and more affordable laser sources while maintaining accuracy and precision.