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.
Programmability in femtosecond-laser-written integrated circuits is commonly achieved with the implementation of thermal phase shifters. Recent work has shown how such phase shifters display significantly reduced power dissipation and thermal crosstalk with the implementation of thermal isolation structures. However, the aforementioned phase shifter technology is based on a single gold film, which poses severe limitations on integration density and circuit complexity due to intrinsic geometrical constraints. To increase the compactness, we propose two improvements to this technology. Firstly, we fabricated thermal phase shifters with a photolithography process based on two different metal films, namely (1) chromium for microheaters and (2) copper for contact pads and interconnections. Secondly, we developed a novel curved isolation trench design that, along with a state-of-the-art curvature radius, allows for a significant reduction in the optical length of integrated circuits. As a result, curved Cr-Cu phase shifters provide a compact footprint with low parasitic series resistance and no significant increase in power dissipation (∼38 mW) and thermal crosstalk (∼20%). These results pave the way toward the fabrication of femtosecond-laser-written photonic circuits with a steep increase in terms of layout complexity.
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