Source-linked AI summary
Simultaneous remote transfer of accurate timing and optical frequency over a public fiber network
Olivier Lopez, Amale Kanj, Paul-Eric Pottie, Giovanni D. Rovera, Joseph Achkar, Christian Chardonnet, Anne Amy-Klein, Giorgio Santarelli
TL;DR
Accurate simultaneous transfer of optical frequency and timing over public fiber networks remains challenging at continental scale. This work demonstrates a method using a shared ultra-stable optical carrier and achieves 20 ps long-term timing stability over a 540 km link carrying Internet data.
Problem
Simultaneous dissemination of ultra-stable optical frequency and accurate timing over public telecommunication networks remains an open research challenge.
Method
The method uses an ultra-stable 1.55 µm laser and optical phase modulation to carry frequency information and timing over a public fiber link.
Results
20 ps timing noise and 2x10-18 frequency resolution at 30 000 s demonstrate simultaneous transfer over the 540 km link.
Takeaways & Limitations
The approach enables precise synchronization of distant experiments using public fiber networks carrying Internet data.
Takeaways & Limitations
The cause of measurement jumps over several days is not yet well understood.
Abstract
from arXiv · showhide
In this work we demonstrate for the first time that it is possible to transfer simultaneously an ultra-stable optical frequency and a precise and accurate timing over 540 km using a public telecommunication optical fiber networks with Internet data. The optical phase is used to carry both the frequency information and the timestamps by modulating a very narrow optical carrier at 1.55 $μ$m with spread spectrum signals using two-way satellite time transfer modems. The results in term of absolute time accuracy (250 ps) and long-term timing stability (20 ps) well outperform the conventional Global Navigation Satellite System or geostationary transfer methods.
1. Introduction
The introduction motivates extending ultra-stable optical frequency transfer from dedicated links to public fiber networks carrying Internet data, while simultaneously disseminating accurate timing. This approach targets remote clock comparisons and precision applications beyond the roughly 1 ns timing accuracy of satellite-based methods.
- Related work: 920 km is the record distance reported for ground-breaking frequency transfer on dedicated fiber.The work extends this transfer technique to public fiber networks with simultaneous data traffic for scalable continental-level extension.
- Applications: Accurate timing enables precise synchronization of distant experiments and supports advanced time-frequency metrology and fundamental-physics tests.The introduction highlights neutrino-speed measurement from CERN to Gran Sasso as a salient application.
- Timing motivation: 1 ns is the timing accuracy of GNSS or geostationary telecommunication satellite methods in the best case.Dedicated fiber-optical two-way time transfer has demonstrated accuracy of one hundred ps or better.
- Contribution: 540 km optical link simultaneously disseminates an ultra-stable optical frequency and accurate timing over a public telecommunication network carrying Internet data.The experiment uses a dedicated “dark” channel.
2. Experimental set –up description.
The setup uses an ultra-stable 1.55 µm laser and optical phase modulation to carry frequency and timing over a 540 km fiber link. The link uses RENATER fibers, long-haul spans carrying Internet data traffic, and bidirectional amplification.
- Experimental setup: An ultra-stable laser at 1.55 µm carries frequency information and timing through optical phase modulation.The timing signal is transmitted through optical phase modulation alongside the frequency information.
- Experimental setup: The 540 km LPL-Reims-LPL link starts and ends at the LPL laboratory to compare signals at both ends and evaluate distribution performance.LPL denotes the Laboratoire de Physique des Lasers, Université Paris 13.
- Experimental setup: Five fiber spans of the French NREN RENATER compose the link, with the third through fifth spans carrying Internet data on long-haul intercity connections.Each span contains two identical parallel fibers.
- Experimental setup: Optical Add-Drop Multiplexers extract and insert the science signal into the telecommunication fibers.The multiplexers support simultaneous use of the public telecommunication network and the science signal.
- Experimental setup: In excess of 165 dB total end-to-end attenuation is reduced with six bidirectional EDFAs and about 100 dB total amplification, leaving net optical losses exceeding 65 dB.The stated attenuation and amplification characterize the 540 km link.
13 DCF
The DCF section describes a hybrid system for simultaneous ultra-stable optical-frequency distribution and time comparison over an optical fiber link. It combines round-trip optical phase stabilization with spread-spectrum time-signal modulation and recovery.
- System layout: The experimental setup combines a long-distance optical fiber link with a hybrid time-comparison and ultra-stable optical-frequency distribution system.The layout includes OADM, DWDM, DCF, Faraday-mirror, photodiode, AOM, EOM, OPSU, and PLL elements.
- Optical frequency transfer: The ultra-stable frequency signal from a cavity-stabilised laser is stabilized by detecting round-trip fiber propagation noise and compensating the optical phase.An optical interferometer compares the input phase with the round-trip phase, with corrections applied through an AOM and OPSU.
- Time transfer: The time-transfer signals come from two-way satellite time-transfer modems referenced to common-clock 1 pps and 5 MHz signals.The modems use pseudorandom-noise modulation, orthogonal codes, signal correlation, and differential time-of-arrival measurements.
- Optical encoding: The time signal is encoded on the optical carrier using frequency-shifted replicas at 400 MHz and 700 MHz, with approximately 1% modulation depth.The shifts avoid interference and enable filtering, while the spread-spectrum signals preserve a pure optical spectrum for frequency transfer.
- Signal recovery and limitation: ~20 MHz code bandwidth and parasitic reflections 40 dB above the useful signals make time-signal processing and detection challenging.The recovered signals undergo optical heterodyne detection followed by successive frequency mixing and filtering before modem processing.
3. Results and discussion.
The 540-km public-fiber link simultaneously achieved 2x10-18 optical-frequency stability and accuracy with timing noise below 20 ps. Timing accuracy was 250 ps, while robustness tests and compatibility with installed Internet-carrying fibers supported broader deployment.
- Transfer performance: 2x10-18 frequency stability was reached at 30 000 s averaging time, with frequency-transfer accuracy of about 2x10-18.The measured stability was almost identical to that reported in [3].
- Timing stability: Less than 20 ps timing noise was measured over the whole measurement period, while differential time-delay variation versus distance was at most 50 ps.The overall 540-km chromatic-dispersion effect was below 25 ps, and PMD-related timing fluctuations were well below 20 ps.
- Accuracy and limitations: 250 ps preliminary conservative accuracy was mainly dominated by phase jumps of about 50-80 ps, whose origin was related to the long-haul Internet fiber link but remained poorly understood.Additional fiber-spool tests found no phase jumps over several days, indicating that the scarce jumps were specific to the long-haul link conditions.
- Robustness: Below 200 ps peak-to-peak time fluctuations were observed in several week-long runs, while one-way fluctuations exceeded 10 ns.The results in timing stability and accuracy clearly outperformed satellite methods.
- Deployment potential: The method was applicable to non-dedicated installed long-haul fibers carrying Internet data and could support frequency and time dissemination on a continental scale.The link could be extended beyond a thousand kilometres under lower fiber losses and improved optical-amplification distribution, and segmented links could use intermediate regeneration stations.