  Precision frequency and phase synchronization between distinct fiber
interconnected nodes is critical for a wide range of applications, including
atomic timekeeping, quantum networking, database synchronization,
ultra-high-capacity coherent optical communications and hyper-scale data
centers. Today, many of these applications utilize precision, tabletop laser
systems, and would benefit from integration in terms of reduced size, power,
cost, and reliability. In this paper we report a record low 3x10^-4 rad^2
residual phase error variance for synchronization based on independent,
spectrally pure, ultra-high mutual coherence, photonic integrated lasers. This
performance is achieved with stimulated Brillouin scattering lasers that are
stabilized to independent microcavity references, realizing sources with 30 Hz
integral linewidth and a fractional frequency instability less than or equal to
2x10^-13 at 50 ms. This level of low phase noise and carrier stability enables
a new type of optical-frequency-stabilized phase-locked loop (OFS-PLL) that
operates with a less than 800 kHz loop bandwidth, eliminating traditional power
consuming high bandwidth electronics and digital signal processors used to
phase lock optical carriers. Additionally, we measure the residual phase error
down to a received carrier power of -34 dBm, removing the need to transmit
in-band or out-of-band synchronized carriers. These results highlight the
promise for a path to spectrally pure, ultra-stable, integrated lasers for
network synchronization, precision time distribution protocols, quantum-clock
networks, and multiple-Terabit per second coherent DSP-free fiber-optic
interconnects.
