Mode-hop-free tuning of a chip-based InP-Si3N4 hybrid laser

Combining a semiconductor optical amplifier with a long and low-loss external laser cavity made of dielectric integrated circuits has the advantage of obtaining extremely narrow laser linewidths. However, extending the cavity length also reduces the cavity mode spacing, and this spectral interval forms the fundamental limit for wavelength tuning without perturbing mode hops. The InP-Si3N4 hybrid integrated laser we consider here has a mode spacing of 5 GHz. The external cavity consists of a phase section and a wavelength tunable filter, which is formed by two microring resonators. Via the phase section alone, mode-hop-free tuning was measured to be limited to 5 GHz as expected. We demonstrate that this range can be significantly extended, here to over 28 GHz, by tuning the phase section simultaneously and properly with the microring resonators. The results indicate that both a narrow linewidth and a broad mode-hop-free tuning range can be achieved with a chip-based laser.