Compact, power-efficient, mass-producible low noise semiconductor passively mode-locked lasers (MLLs) are
important because they can generate ultra-stable high frequency pulse trains and coherent frequency combs that can
be employed in dense wavelength-division multiplexing (DWDM), high speed optical time-division multiplexing
(OTDM), interchip/intrachip clock distribution, arbitrary waveform generation, millimeter wave signal generation,
gas sensing, and spectroscopy [1-6]. Low noise passive mode-locking operation is usually obtained by lowering the
optical confinement factor Γ to reduce the amount of amplified spontaneous emission (ASE) noise coupled to the
oscillating mode, which is believed to be the main noise contributor [2]. An alternative method is to deploy an
external feedback cavity to store the coherent photon energy and feed a small portion of light back into the cavity to
suppress the ASE noise, which can also help to enhance the mode-locking quality [4-6]. In this paper, we
demonstrate a fully integrated colliding pulse semiconductor MLL with tunable on-chip feedback by leveraging
CMOS compatible heterogeneous silicon platform that provides low loss SOI waveguides and efficient light
generation via bonding high-quality III/V material [7]. The length of the on-chip external cavity is an integer
multiple of the laser cavity length, and it comprises of two phase tuners for controlling the optical feedback signal’s
strength and phase. Compared to its InP counterparts with cleaved facets, this fully integrated low noise
heterogeneous MLL has all the components on chip, enabling integration within a more complex photonic integrated
circuit.