Quantitative photochemical scaling model for femtosecond laser micromachining of ophthalmic hydrogel polymers: effect of repetition rate and laser power in the four photon absorption limit

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We report on the effect of laser repetition rate on the refractive index (RI) change or phase change induced by the femtosecond micromachining hydrogel-based ophthalmic materials. A repetition rate tunable femtosecond ytterbium (Yb) doped fiber laser was used to study the relation between the laser-induced phase change and the pulse repetition rate in the range of 1 MHz to 60 MHz. We present both the qualitative and quantitative results of the laser-induced phase change obtained in hydrogel-based contact lenses at different repetition rates and discuss the effect of repetition rate on the magnitude of the phase shift, the optical damage threshold and the maximum achievable phase change just below the optically-induced damage threshold. A photochemical model derived in the four photon absorption limit with pulse overlapping is employed to fit the experimental data obtained at four different repetition rates, 5 MHz, 10 MHz, 15 MHz and 60 MHz. This work contributes to the current knowledge of the response of hydrogel polymers to various laser irradiation parameters and the optimization of laser repetition rates enables femtosecond micromachining of ophthalmic materials at a lower average power and a faster writing speed.