Silicon nitride integrated mode-locked laser with widely tunable line spacing

For microwave photonics (MWP), integrated mode-locked lasers can find applications as a local oscillator. Using hybrid intregrated lasers, it is possible to directly integrate such a light source with a signal processing circuit. In this paper we show the mode-locking of a hybrid integrated laser designed for continuous wave operation. Due to the filter present in the laser cavity, the laser outputs narrow combs preserving high power per line, which is a key requirement in many MWP-related applications. The filter also serves as a tunable delay, which can be used to tune the line spacing of the optical frequency comb. We show a 573 MHz tuning range of the fundamental beat note of the optical frequency comb, between 1.9 and 2.5 GHz. This wide tuning range, adds flexibility to the modelocked laser without needing a driving radio frequency source to modulate the optics as local oscillator.