Frequency dependence of nonlinear absorption in fused silica measured with picosecond pulses at various focal lengths.

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Nonlinear absorption is the key process to generate laser-induced in-volume modifications in transparent dielectrics such as waveguides or three-dimensional data matrix codes. We present a comprehensive parameter study about nonlinear absorption in fused silica using a picosecond laser at various focal lengths. Beginning at a focal length of 100 mm, we measure a strong frequency dependence of the saturation absorption. Reducing the focal length results in a decrease of the saturation absorption. After passing a threshold focal length, the saturation absorption increases drastically and the frequency dependence starts to decrease. At the final focal length of 6 mm we measure almost no frequency dependence. In order to explain our measurements, we used the theory of optical breakdown and filamentation. Nonlinear absorption measurement can become a promising tool for better process control during the generation of in-volume modifications in transparent dielectrics.