Design and fabrication of reconfigurable laser-written waveguide circuits

Abstract

Reconfigurability is an important requirement for implementing quantum photonic processing using waveguide circuits in which both high fidelity and the ability to change the optical transformation dynamically are necessary. This work aims to address the issue of scalability in reconfigurable waveguide circuits fabricated using the femtosecond laser direct-write (FLDW) technique. A set of reconfigurable waveguide Mach-Zehnder interferometers were designed and fabricated using a combination of femtosecond laser waveguide inscription and picosecond laser ablation. Thermal cross-talk between adjacent phase-shifters was managed by machining microchannels into the chip surface to isolate individual waveguides from the rest of the substrate. The tuning efficiency as defined by the dissipated power per unit of induced phase shift was improved by a factor of two in this way while maintaining a smaller device footprint than previous demonstrations of phase tuning in laser-written waveguides. The phase response of the waveguide interferometers to the heaters was thoroughly characterised and was well predicted by simulation. A characterization of the time-dependent response of the reconfigurable interferometers was also performed. A rise time measurement revealed that the circuit can be reconfigured within seconds. No long-term phase drifts away from a set point were observed over a period of more than 12 hours, and the short term phase stability was better than 6 mrad.