  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 chromium for microheaters and 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.
