Optically synchronized fibre links using spectrally pure chip-scale lasers

Abstract

1Department of Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, CA, USA. 2Department of Applied Physics and Materials Science, Northern Arizona University, Flagstaff, AZ, USA. 3Time and Frequency Division 688, National Institute of Standards and Technology, Boulder, CO, USA. 4Morton Photonics, West Friendship, MD, USA. 5Center for Materials Interfaces in Research and Applications (MIRA), Northern Arizona University, Flagstaff, AZ, USA. 6Department of Physics, University of Colorado Boulder, Boulder, CO, USA. 7Present address: Jet Propulsion Laboratory, Pasadena, CA, USA. 8Present address: Department of Applied Physics, Hebrew University of Jerusalem, Jerusalem, Israel. ✉e-mail: [email protected] The distribution of precision optical frequency and phase references over fibre-optic links1–4 enables applications such as high-precision metrology, investigation of variations in fundamental physical constants5, interferometry for radio astronomy6, optical clock comparison7–10 and high-capacity coherent communications11. To achieve high precision, scientific applications employ ultralow-linewidth, frequency-stabilized lasers12,13, requiring sophisticated laser systems at the table-top scale. At the same time, applications like coherent fibre communications use power-intensive digital signal processors (DSPs)14–16 to achieve phase synchronization with lower-cost and less stable lasers. Miniaturization of stable laser technologies would broaden the use of optically synchronized fibre links to a wider range of applications, including distributed network synchronization17, precision time protocols18,19 and deployable optical clock networks10, as well as enabling new approaches to realizing energy-efficient, DSP-free, high-capacity coherent fibre-optic interconnects20,21. Optical frequency and phase synchronization can be accomplished using electronic phase-locked loops (EPLLs)22,23, optical phase-locked loops (OPLLs)24–28 and DSPs14–16. Several factors drive how well these techniques scale in terms of performance, complexity and power consumption at optical carrier frequencies, including laser and fibre phase noise and stability, modulation bandwidth, feedback loop noise, bandwidth requirements and high-frequency analogue and digital electronics. Compact, ultralow-linewidth, stabilized lasers with ultralow phase noise can enable circuitry normally associated with coherent radiofrequency (RF) and wireless communications to support links at optical frequencies. Examples of low-bandwidth feedback techniques used in precision scientific experiments include the Pound–Drever–Hall (PDH)29 method coupled with thermally engineered glass or single-crystal high-Q optical reference cavities12,13 housed in sophisticated environmental isolation systems. These state-of-the-art, laboratory-scale lasers are capable of linewidths below 10 mHz with a carrier instability of 4 × 10−17 at timescales between one and a few tens of seconds13. Chip-scale stabilized lasers with exceptional phase noise and frequency stability have been realized by locking semiconductor or stimulated Brillouin scattering (SBS) lasers to whispering-gallery-mode resonators30,31, photonic integrated spiral waveguides32 and compact Fabry–Pérot resonators33. However, so far, the use of independent, ultralow-linewidth, stable sources in a precision fibre frequency link and the demonstration of ultralow residual phase error using low-bandwidth synchronization methods have not been reported. In this Article, we demonstrate precision phase-locked optical sources over a fibre link using independent, mutually coherent lasers that are frequency-stabilized at the chip scale. The fibre-connected lasers are phase-synchronized with a low residual phase error variance25 of 3 × 10−4 rad2 for a homodyne lock, orders of magnitude lower than achieved with OPLLs employing large-linewidth integrated lasers24–28. This performance is achieved by consideration of frequency noise and drift contributions from the integrated lasers, a compact reference cavity and optical fibre, as well as feedback loop and lock dynamics. We combine these stabilized, spectrally pure, independent chip-scale lasers with an optical-frequency-stabilized phase-locked loop (OFS-PLL) to provide precise synchronization using only low-bandwidth feedback loops (<800 kHz), without the need for the high-bandwidth electronics of EPLL or OPLL circuits or power-intensive DSPs. The lasers have a cavity-stabilized SBS laser (CS-SBS) design, consisting of a photonic integrated silicon nitride (SiN) SBS laser34 locked to a compact silica Fabry–Pérot optical reference cavity33. We measure the optical frequency noise (FN) of the transmit (Tx) and receive (Rx) lasers, as well as the heterodyne beat note spectrum between these lasers, resulting in an integral linewidth of ~30 Hz for each CS-SBS laser and a fractional Optically synchronized fibre links using spectrally pure chip-scale lasers