Universal photonic processors (UPPs) are reconfigurable photonic integrated circuits able to implement arbitrary unitary transformations on an input photonic state. Femtosecond laser writing (FLW) allows for rapid and cost-effective fabrication of circuits with low propagation losses. A FLW process featuring thermal isolation allows for a dramatic reduction in dissipated power and crosstalk in integrated thermally-reconfigurable Mach-Zehnder interferometers (MZIs), especially when operated in vacuum, with 0.9 mW dissipation for full reconfiguration and 0.5% crosstalk at 785 nm wavelength. To demonstrate the potential of this technology we fabricated and characterized a 6-mode FLW-UPP in a rectangular MZI mesh with 30 thermal shifters.
Femtosecond laser micromachining (FLM) is considered today a key technology for the fabrication of high-quality photonic integrated circuits, especially when a 3D geometry is required. However, when a thermal phase shifter is exploited to reconfigure an FLM device, its operation requires many hundreds of milliwatts. This issue strongly limits the scalability of these circuits. With this work, we present a new FLM fabrication process that takes advantage of thermally insulating microstructures (i.e. trenches and bridge waveguides) to demonstrate low propagation losses (0.29 dB/cm at 1550 nm), along with a power dissipation for a 2π phase shift down to 37 mW.
Femtosecond laser micromachining (FLM) is a powerful technique that allows for rapid and cost-effective fabrication of photonic integrated circuits (PICs), even when a complex 3D waveguide geometry is required. Among the features of these devices, it is worth mentioning the possibility to dynamically reconfigure the circuit by thermal phase shifting. However, an integrated microheater dissipates more than 500 mW to induce a 2π phase shift in FLM devices operating at telecom wavelength (i.e. 1550 nm) and induces significant thermal crosstalk to adjacent devices. These issues prevent the integration of more than a few microheaters on the same chip. In order to cope with this, we exploited a new water-immersion FLM process to integrate high-quality single-mode waveguides (0.29 dB/cm propagation losses and 0.27 dB/facet coupling losses at 1550 nm) with two different types of thermally insulating microstructures: trenches on the sides of the heated photon path and a bridge waveguide, a structure in which the ablation is performed also under the optical path. Both the strategies are employed for the fabrication of compact reconfigurable Mach-Zehnder interferometers having inter-waveguide pitch down to 80 μm. Interferometers featuring insulating trenches show a reconfiguration period down to 57 mW, whilst bridge waveguides result in a further improvement, with a 2π phase shift that can be induced with an electrical power as low as 37 mW. Both structures reduce thermal crosstalk from more than 50% down to 3:5% on the nearest device.
Entangled photons generation is an interesting field of research, since progress in this area will directly affect the development of photonic quantum technologies, including quantum computing, simulation and sensing. Several methods have been sifted to increase the performances of entangled photon sources and the integrated optics approach represents a promising strategy. In particular, integrated waveguide sources represent a robust tool, thanks to their stability and the enhancement of nonlinear light-crystal interaction provided by waveguide field confinement.
Here, we show the versatility of a hybrid approach, realizing an integrated optical source for the generation of entangled photon-pairs at telecom wavelength. The nonlinear active medium used is lithium niobate, while the routing and manipulation of the generated signal is performed in aluminum-borosilicate glass photonic circuits. The system is composed of three cascaded devices. First, a balanced directional coupler at the fundamental wavelength equally splits the pump in the lithium niobate waveguides, which generate single-photon pairs through type 0 spontaneous parametric down-conversion process. A third chip, encompassing directional couplers and waveplates, closes the interferometer and recombines the generated photons, thus giving access to different quantum states of light: path-entangled or polarization-entangled states. A thermal phase shifter, which controls the relative phase between the interferometer arms, gives an additional degree of freedom for engineering the output state of the presented photon pairs source. All these components are entirely fabricated by femtosecond laser micromachining, a direct and very versatile technique that allows to process different kind of materials and realize high quality optical circuits.
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