  Integrated semiconductor mode-locked lasers have shown promise in many
applications and are readily fabricated using generic InP photonic integration
platforms. However, the passive waveguides offered in such platforms have
relatively high linear and nonlinear losses that limit the performance of these
lasers. By extending such lasers with, for example, an external cavity the
performance can be increased considerably. In this paper, we demonstrate for
the first time that a high-performance mode-locked laser can be achieved with a
butt-coupling integration technique using chip scale silicon nitride
waveguides. A platform-independent SiN/SU8 coupler design is used to couple
between the silicon nitride external cavity and the III/V active chip.
Mode-locked lasers at 2.18 GHz and 15.5 GHz repetition rates are demonstrated
with Lorentzian RF linewidths several orders of magnitude smaller than what has
been demonstrated on monolithic InP platforms. The RF linewidth was 31 Hz for
the 2.18 GHz laser.
