Toward Higher Integration Density in Femtosecond-Laser-Written Programmable Photonic Circuits

Abstract

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.

Universal photonic processors fabricated by direct femtosecond-laser writing

“…The evaluation of thermal cross-talk between different shifters revealed a phase induction of approximately 20 % on the first neighbour, which is comparable to the results shown in Ref. 8 Finally, to evaluate the stability of the fabricated phase shifters, we conducted extensive testing over a continuous operation period of several hours at a dissipated power of 60 mW (approximately 3πphase shift). Remarkably, the resistive elements exhibited minimal variations, with the resistance value deviating by less than 0.1 %.…”

“…For their realization, we adopted a two-metal approach as presented in Ref. 8 Chromium was selected for the resistive heaters for its relatively high resistivity and lower temperature coefficient of resistance, while copper, with its lower resistivity, was chosen for interconnections and contact pads. For the interconnections, a 10 nm thin film of titanium was used as an adhesion layer before the deposition of copper.…”

Integration of Plasmonics Structures in Photonics Waveguides: Enabling Novel Electromagnetic Functionalities in Photonic Circuits

“…The key features of the all-optical solution include the ability to parallelize optical carriers through spectral multiplexing and specific information encodings. Photonic integrated circuits (PICs) [1][2][3] have been developed to generate complete optical systems and to simplify the use of different optical functions from source to modulation and signal detection. Leveraging manufacturing techniques inspired by microelectronics, planar waveguide circuits have achieved two complementary objectives: (1) to control the propagation of the optical signal in a 2D system with very low losses and (2) to facilitate the serial or parallel association of several functions without the use of spatial optical elements.…”