Low-power reconfigurable photonic integrated circuits fabricated by femtosecond laser micromachining

Femtosecond laser micromachining (FLM) is a powerful technique that allows for rapid and cost-effective fabrication of photonic integrated circuits (PICs), even when a complex 3D waveguide geometry is required. Among the features of these devices, it is worth mentioning the possibility to dynamically reconfigure the circuit by thermal phase shifting. However, an integrated microheater dissipates more than 500 mW to induce a 2π phase shift in FLM devices operating at telecom wavelength (i.e. 1550 nm) and induces significant thermal crosstalk to adjacent devices. These issues prevent the integration of more than a few microheaters on the same chip. In order to cope with this, we exploited a new water-immersion FLM process to integrate high-quality single-mode waveguides (0.29 dB/cm propagation losses and 0.27 dB/facet coupling losses at 1550 nm) with two different types of thermally insulating microstructures: trenches on the sides of the heated photon path and a bridge waveguide, a structure in which the ablation is performed also under the optical path. Both the strategies are employed for the fabrication of compact reconfigurable Mach-Zehnder interferometers having inter-waveguide pitch down to 80 μm. Interferometers featuring insulating trenches show a reconfiguration period down to 57 mW, whilst bridge waveguides result in a further improvement, with a 2π phase shift that can be induced with an electrical power as low as 37 mW. Both structures reduce thermal crosstalk from more than 50% down to 3:5% on the nearest device.