  In the last decade, remarkable advances in integrated photonic technologies
have enabled table-top experiments and instrumentation to be scaled down to
compact chips with significant reduction in size, weight, power consumption,
and cost. Here, we demonstrate an integrated continuously tunable laser in a
heterogeneous gallium arsenide-on-silicon nitride (GaAs-on-SiN) platform that
emits in the far-red radiation spectrum near 780 nm, with 20 nm tuning range,
<6 kHz intrinsic linewidth, and a >40 dB side-mode suppression ratio. The GaAs
optical gain regions are heterogeneously integrated with low-loss SiN
waveguides. The narrow linewidth lasing is achieved with an extended cavity
consisting of a resonator-based Vernier mirror and a phase shifter. Utilizing
synchronous tuning of the integrated heaters, we show mode-hop-free wavelength
tuning over a range larger than 100 GHz (200 pm). To demonstrate the potential
of the device, we investigate two illustrative applications: (i) the linear
characterization of a silicon nitride microresonator designed for
entangled-photon pair generation, and (ii) the absorption spectroscopy and
locking to the D1 and D2 transition lines of 87-Rb. The performance of the
proposed integrated laser holds promise for a broader spectrum of both
classical and quantum applications in the visible range, encompassing
communication, control, sensing, and computing.
