Femtosecond laser written (FLW) reconfigurable photonic integrated circuits are attracting large attention from many applications thanks to their unique features. However, in these devices the frequency response of thermo-optic phase shifters is currently limited to the Hz range, hindering the diffusion of FLW circuits in different fields. Here, we present a thermally-reconfigurable Mach-Zehnder interferometer inscribed just a few micrometers below the surface of a fused silica substrate. The interferometer design, purposely optimized by finite element simulations, allows for switching time lower than 200 μs, phase modulation of the optical signal at kHz frequency and limited impact on the power dissipation.
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