General‐Purpose Hexagonal Programmable Photonic Integrated Circuit with a >30 GHz Free Spectral Range

ABSTRACT A programmable photonic circuit is demonstrated based on a 7‐cell hexagonal waveguide mesh of Mach–Zehnder interferometer gates. Benefiting from undercut heaters, the optical gates have a small insertion loss of 0.28 dB/gate and a short optical delay of 5.5 ps, which results in a compact circuit with a large free spectral range (FSR) of 31 GHz for a 6‐gate ring resonator. We demonstrate the advantages of this circuit through the applications of beam splitters, wavelength multiplexers, and wavelength filters. To enable a larger FSR, Vernier rings and a dual‐injected ring configuration can be constructed, showing an improved FSR of 93 GHz.