  Semiconductor mode-locked lasers can be used in a variety of applications
ranging from multi-carrier sources for WDM communication systems to time base
references for metrology. Their packaging in compact chip- or module-level
systems remains however burdened by their strong sensitivity to
back-reflections, quickly destroying the coherence of the mode-locking. Here,
we investigate the stabilization of mode-locked lasers directly edge coupled to
a silicon photonic integrated circuit, with the objective of moving isolators
downstream to the output of the photonic circuit. A 2.77 kHz 3 dB RF linewidth,
substantially improved compared to the 15.01 kHz of the free running laser, is
obtained in the best case. Even in presence of detrimental reflections from the
photonic circuit, substantial linewidth reductions from 20 kHz to 8.82 kHz,
from 572 kHz to 14.8 kHz, and from 1.5 MHz to 40 kHz are realized.
