Multi-contrast imaging of femtosecond-laser-induced modifications in glass by variable illumination with a projector-based microscope

An active illumination microscope provides simultaneous acquisition of multi-contrast images such as bright-field, dark-field, and differential phase-contrast using various illumination patterns. We experimentally demonstrate the multi-contrast images of structural modifications produced in glass by a femtosecond laser. The multi-contrast images of structural modifications were obtained by changing illumination patterns with a projector for microscopic contrast enhancement. The refractive index change and scattering damage produced in BK7 glass were identified by differential phase-contrast images and dark-field images. We can enhance the direction of contrast in the imaging of modifications by changing the rectangular illumination pattern. Multi-contrast microscopy by different illumination pattern is a technique for in situ monitoring of modifications by femtosecond laser pulses in transparent materials.