Time-resolved White-light Interferometry for Ultrafast Metrology.

The material modification in the volume of transparent dielectrics using tightly focused fs-laser radiation is an important topic for many research groups all over the world. A wide range of applications like the writing of waveguides, micro-structuring by material modification and subsequent etching, or the micro-welding of glass is based on the localized melting and quenching in a different state. Time-resolved white-light interferometry is adopted for the measurement of the optical phase changes in processed materials. A modified Mach-Zehnder interferometer setup combined with microscope objectives is used. The white light is generated by focusing ultrafast laser radiation (t_p = 80 fs) in a sapphire crystal using a micro-lens array to minimize temporal and spatial fluctuations in the white-light continuum. Lateral and coaxial pump-probe measurements of the phase changes during material processing are performed using one or two coupled ultrafast laser sources at different repetition rates (f_rep = 1 kHz-1 MHz) or by adopting single pulses. The temporal delay between the pump and the probe can be adjusted in the range τ<=1.8 μs in dependence on the repetition rate of the pump radiation. The optical phase shift and therefore the refractive index of the material is calculated from the interference images. The knowledge of the refractive index during the modification process with a temporal resolution in the ps-range and a spatial resolution of several microns leads to a better understanding of the initial processes for the permanent material modifications. [ABSTRACT FROM AUTHOR]