Femtosecond laser writing is a powerful technique that allows rapid and cost-effective fabrication of photonic integrated circuits with unique 3D geometries. In particular, the possibility to reconfigure such devices by thermo-optic phase shifters represents a paramount feature, exploited to produce adaptive and programmable circuits. However, the scalability is strongly limited by the flaws of current thermal phase shifters, which require hundreds of milliwatts to operate and exhibit large thermal crosstalk. In this work, thermally-insulating 3D microstructures are exploited to decrease the power needed to induce a 2 phase shift down to 37 mW and to reduce the crosstalk to a few percent. Further improvement is demonstrated when operating in vacuum, with sub-milliwatt power dissipation and negligible crosstalk. These results pave the way toward the demonstration of complex programmable integrated photonic circuits fabricated by femtosecond laser writing, thus opening exciting perspectives in integrated quantum photonics.

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Photonic integrated circuits (PICs) are steadily becoming an established technology with a wide range of applications in communications, analog signal processing and sensing. Considerable research interest is currently focused particularly on universal photonic processors (UPPs), i.e. reconfigurable PICs which are able to implement any arbitrary unitary transformation on a given input photonic state. The basic building block of a UPP is a (often thermo-optically) reconfigurable Mach-Zehnder interferometer (MZI). UPPs with various topologies and number of modes have been reported in multiple photonic platforms such as silicon nitride, silica on silicon and glass-based femtosecond laser writing (FLW). FLW is a versatile technology that allows for rapid and cost-effective fabrication of three-dimensional waveguide geometries with low propagation losses (<0.3 dB cm −1 ) from the infrared to the whole visible range. Recently, an efficient implementation of reconfigurable MZIs in this platform was developed featuring thermal isolation structures (i.e. deep trenches and bridge waveguides) and thermal phase shifters, allowing for a dramatic reduction in dissipated power (down to 25 mW for full reconfiguration in air at 785 nm wavelength) and in thermal crosstalk (down to 10 % of the induced phase). Performance of these interferometers is especially advantageous in vacuum, with 0.9 mW dissipation and 0.5 % crosstalk at 2.5 × 10 −3 mbar. To demonstrate the potential of this technology we fabricated and characterized a 6-mode FLW-UPP with thermal isolation trenches in a rectangular MZI mesh layout with a total of 30 thermal phase shifters.