  Combining optical gain in direct-bandgap III-V materials with tunable optical
feedback offered by advanced photonic integrated circuits is key to chip-scale
external-cavity lasers (ECL), offering wideband tunability along with low
optical linewidths. External feedback circuits can be efficiently implemented
using low-loss silicon nitride (Si$_{3}$N$_{4}$) waveguides, which do not
suffer from two-photon absorption and can thus handle much higher power levels
than conventional silicon photonics. However, co-integrating III-V-based gain
elements with tunable external feedback circuits in chip-scale modules still
represents a challenge, requiring either technologically demanding
heterogeneous integration techniques or costly high-precision multi-chip
assembly. In this work, we demonstrate Si$_{3}$N$_{4}$-based hybrid integrated
ECL that exploit 3D-printed structures such as intra-cavity photonic wire bonds
and facet-attached microlenses for low-loss optical coupling with relaxed
alignment tolerances, thereby overcoming the need for active alignment while
maintaining the full flexibility of multi-chip integration techniques. In a
proof-of-concept experiment, we demonstrate an ECL offering a 90 nm tuning
range (1480 nm - 1570 nm) with on-chip output powers above 12 dBm and side-mode
suppression ratios of up to 59 dB. We achieve an intrinsic linewidth of 979 Hz,
which is among the lowest values reported for comparable feedback
architectures. The optical loss of the intra-cavity photonic wire bond between
the III-V gain element and the Si$_{3}$N$_{4}$-based tunable feedback circuit
amounts to approximately (1.6 $\pm$ 0.2) dB. We use the ECL as a tunable pump
laser to generate a dissipative Kerr soliton frequency comb. To the best of our
knowledge, our experiments represent the first demonstration of a
single-soliton Kerr comb generated with a pump that is derived from a hybrid
ECL.
