Universal photonic processors fabricated by femtosecond laser writing

Abstract

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.

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

“…Single-mode operation has been optimized for a wavelength of 940 nm, achieving propagation losses down to 0.14 dB/cm and negligible bending losses at curvature radii as low as R = 15 mm. This value shows an improvement of a factor of 2 or higher with respect to the one employed in previous laser-written UPPs [15,16], and it is in line with the state of the art for the FLM platform [25], where advanced techniques for the reduction in bending losses were introduced.…”

“…Circuit topologies for the realization of UPPs are well assessed in the literature, with two notable examples being triangular [11] and rectangular [12] meshes of reconfigurable Mach-Zehnder interferometers (MZIs). UPPs have been already demonstrated in several integrated photonics platforms such as silicon nitride [9,10,13], silica-on-silicon [14] and femtosecond laser micromachining (FLM) [15,16]. In particular, FLM is a versatile and cost-effective fabrication platform for implementing integrated optical devices.…”

Universal photonic processors fabricated by direct femtosecond-laser writing

“…These results are remarkable since we were able to increase the complexity of the processor while keeping comparable performances, in terms of optical losses, to previous works realized with the same fabrication platform. 5 In particular, the curvature radius has been halved, and the null inner-arm approach actively contributes to reducing the footprint of the UPP, with an MZI-based cell size reduced by about 25%.…”

“…Until now, multiple works have successfully demonstrated UPPs across various photonic platforms, including silicon nitride, 3 silica-on-silicon, 4 and femtosecond laser writing (FLW). 5 FLW of glass-based materials provides a platform for rapid and cost-effective prototyping of waveguides with low insertion losses (down to 0.1 dB cm −1 propagation and 0.2 dB per facet coupling losses) and low birefringence over a wide wavelength range. Furthermore, the ability to micro-structure the substrate using the same fabrication setup, enhances the versatility of FLW with the possibility of integrating thermal isolation trenches.…”