11 - Thermal writing of photonic devices in glass and polymers by femtosecond lasers

Abstract

Femtosecond laser microprocessing is a direct, maskless technique capable of inducing a permanent refractive index increase buried beneath the surface of transparent glasses and polymers, enabling photonic circuit fabrication in 3D geometries. We describe how the repetition rate influences the heat accumulation between laser pulses, which determines the regime of modification and the resulting morphological change in glasses in polymers. In most silicate and phosphate glasses, higher repetition rates are shown to be beneficial for driving increased heat accumulation, leading to rapid fabrication of low-loss optical waveguides. In pure silica which has low absorption due to its high bandgap, heat accumulation effects are reduced and higher fluences provided by the second harmonic visible wavelength from Yb-based femtosecond lasers are required to form highly confining optical waveguides. In polymers, femtosecond laser waveguide writing generally leads to depressed refractive index changes and a time decaying behavior.