  Demand for low-noise, continuous-wave, frequency-tunable lasers based on
semiconductor integrated photonics has been advancing in support of numerous
applications. In particular, an important goal is to achieve narrow spectral
linewidth, commensurate with bulk-optic or fiber-optic laser platforms. Here,
we report on laser-frequency-stabilization experiments with a heterogeneously
integrated III/V-Si widely tunable laser and a high-finesse,
thermal-noise-limited photonic resonator. This hybrid architecture offers a
chip-scale optical-frequency reference with an integrated linewidth of 60 Hz
and a fractional frequency stability of 2.5e-13 at 1-second integration time.
We explore the potential for stabilization with respect to a resonator with
lower thermal noise by characterizing laser-noise contributions such as
residual amplitude modulation and photodetection noise. Widely tunable, compact
and integrated, cost effective, stable and narrow linewidth lasers are
envisioned for use in various fields, including communication, spectroscopy,
and metrology.
