Optically synchronized fiber links with spectrally pure integrated lasers

TL;DR

This paper demonstrates ultra-stable, spectrally pure integrated lasers achieving record low phase error for fiber link synchronization, enabling more efficient, compact, and reliable optical networks and quantum applications.

Contribution

It introduces a new synchronization method using integrated lasers with unprecedented low phase noise and stability, reducing reliance on high-bandwidth electronics.

Findings

Achieved residual phase error variance of 3x10^-4 rad^2.

Realized lasers with 30 Hz linewidth and fractional frequency instability ≤ 2x10^-13.

Enabled OFS-PLL with bandwidth less than 800 kHz, reducing power consumption.

Abstract

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.