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.
Integrated modulators of optical phase or intensity are essential elements to reconfigure dynamically the operation of a complex waveguide circuit, or to achieve convenient optical switching within a fiber network. Thermo-optic effects are commonly exploited to achieve dynamic phase modulation in glass-based devices, since nonlinear optical effects are weak in such substrates. Thermo-optic modulators rely on electric resistive heaters patterned on top of the waveguides: they are reliable and easy to fabricate, but they suffer from slow response, dictated by the thermal diffusion dynamics. On the other hand, optically-coupled microstructures in glass, driven at their mechanical resonances, may provide interesting possibilities to achieve modulation of the optical signals in the kilohertz range and higher. In this work, we demonstrate integrated-optics intensity modulators based on micro-cantilevers with resonant oscillation frequencies in the tens-of-kilohertz range. The mechanical structures are realized in alumino-borosilicate glass substrate by water-assisted femtosecond-laser ablation. With the same femtosecond laser an optical waveguide is inscribed within the oscillating beam; a waveguide also continues in the substrate beyond the cantilever's tip. Since the entire device, with all its optical and mechanical parts, is realized in a single fabrication process, relative alignment is guaranteed. If the cantilever is at rest, light propagating in the internal waveguide yields maximum coupling to the remaining part of the waveguide. When the device is excited at resonance by means of a piezo-electric actuator, the cantilever oscillation produces periodical variations of the coupling efficiency, with an observed contrast higher than 10 dB.
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.