Characterisation of laser marks using digital holographic microscopy

The inspection and characterisation of laser marks using digital holographic microscopy (DHM) is presented in this paper. A DHM system in transmission mode was designed and the reconstruction algorithm for this configuration was investigated. The software was developed to provide live reconstruction of holograms for real time numerical evaluation of amplitude and phase contrast images. A CO2 laser-based marking system was employed to create marks on glass substrates. By analysing the quality, 3D profile measurement, and material distribution of the marked area, the parameters of the laser system could be optimised to achieve the desired mark. The phase contrast images provide quantitative refractive index analysis and 3D profile studies. The results were compared with those obtained using white light confocal microscopy. The capabilities and advantages of the DHM system for the analysis of laser marks are also presented.