Source-linked AI summary
High-resolution microwave frequency dissemination on an 86-km urban optical link
O. Lopez, A. Amy-Klein, M. Lours, Ch. Chardonnet, G. Santarelli
TL;DR
The paper demonstrates an 86-km compensated optical link using an urban telecom network. It achieves frequency instability of 1.3×10^-15 at 1 s and below 10^-18 at one day, while the ultimate link stability remains open.
Problem
Ultra-stable frequency transfer between remotely located laboratories is needed, but the reported frequency instability is insufficient for transferring ultra-stable oscillator properties for distant clock comparisons.
Method
The authors use a compensated optical link with fiber chromatic-dispersion compensation based on two slightly different modulation frequencies.
Results
1.3×10^-15 frequency instability at 1 s integration time and below 10^-18 at one day are obtained for the link.
Takeaways & Limitations
The demonstrated 86-km compensated optical link significantly improves frequency stability and provides a basis for long-distance transfer through an urban telecom network.
Takeaways & Limitations
The ultimate link stability remains an open question, requiring analysis of different limiting factors.
Abstract
from arXiv · showhide
We report the first demonstration of a long-distance ultra stable frequency dissemination in the microwave range. A 9.15 GHz signal is transferred through a 86-km urban optical link with a fractional frequency stability of 1.3x10-15 at 1 s integration time and below 10-18 at one day. The optical link phase noise compensation is performed with a round-trip method. To achieve such a result we implement light polarisation scrambling and dispersion compensation. This link outperforms all the previous radiofrequency links and compares well with recently demonstrated full optical links.
1. Introduction
The paper addresses the need for ultra-stable frequency transfer between distant laboratories, where satellite-based methods reach only 10^-15 at one day. It introduces a 9.15 GHz microwave reference transmitted over an 86-km urban optical link using round-trip phase-noise correction.
- Motivation: 10^-15 fractional frequency instability at one day limits satellite-based frequency comparisons.Modern cold-atom microwave standards reach a few 10^-16 at one day, motivating improved links.
- Motivation: Optical fibers provide low attenuation, high reliability, and continuous availability for long-distance frequency transfer.
- Related work: 5×10^-15 at 1 s and 2×10^-18 at one day were previously demonstrated for 1 GHz RF transmission over 86 km.
- Approach: Round-trip phase-noise correction compensates propagation-delay fluctuations caused by mechanical perturbations and temperature variation.Microwave frequencies near 10 GHz are relevant to particle accelerators, astronomy, and the caesium transition.
- Contribution: The study transfers a 9.15 GHz microwave reference through an 86-km urban fiber link connecting LPL and LNE-SYRTE.The work extends earlier 100 MHz and 1 GHz demonstrations between the laboratories.
2. Experimental set-up
The experimental system disseminates a microwave signal over two cascaded 43-km fiber spans and compensates link perturbations with round-trip feedback. Polarization scrambling and negative-dispersion fiber address propagation asymmetries and chromatic-dispersion effects.
- Link architecture: The 86-km link uses two cascaded 43-km twin fibers connecting LPL and LNE-SYRTE.
- Compensation: 9.15 GHz forward and 9.25 GHz backward signals enable round-trip phase sensing while avoiding interference from reflections and backscattering.
- Dispersion compensation: Less than 5% of the original 86-km fiber dispersion remains after compensation, preventing periodic signal fading and extinction.Without compensation, the first complete extinction occurs at around 22 km.
3. Results and discussion
Round-trip compensation strongly suppresses propagation-delay fluctuations and enables high stability over the 86-km link. The link reaches 1.3×10^-15 at 1 s and better than 10^-18 at one day, while short- and long-term limits arise from distinct noise sources.
- Propagation-delay correction: 2 ns free-running propagation-delay span is reduced to well below 200 fs peak-to-peak in closed loop.The correction system therefore provides a rejection factor close to 10^4.
- Frequency stability: 1.3×10^-15 frequency instability at 1 s and better than 10^-18 at one day are obtained for the compensated link.
- Comparison: The compensated link significantly improves frequency stability relative to the earlier 1 GHz result by using higher microwave frequency and minimizing fiber effects.
- Frequency bias: The measured frequency bias is about 2×10^-19 and is compatible with zero within the error bars.
- Stability mechanisms: Removing polarization scrambling degrades long-term stability to a flicker floor around 10^-17, while replacing dispersion fiber with attenuation affects both short- and long-term stability.
- Limitations: Optical-detection signal-to-noise ratio limits the short-term noise floor, whereas residual PMD or AM/PM conversion may limit long-term stability.
4. Conclusion.
The paper demonstrates an 86-km compensated urban optical link for microwave frequency transfer, achieving high stability through polarization scrambling, dispersion compensation, and related design choices. The system enables comparison of leading microwave and optical frequency standards over distances up to 100 km, while attenuation limits longer links.
- An 86-km compensated optical link was demonstrated using an urban telecom network.
- 1.3×10^-15 at 1 s and below 10^-18 after one day were achieved for frequency instability.
- Different modulation frequencies, polarization scrambling, and fiber chromatic-dispersion compensation supported the reported performance.
- The compensator itself reached an ultimate limitation that cannot easily be overcome.
- The system enables comparison of the best microwave and optical frequency standards over distances up to 100 km.
- For longer distances, fiber attenuation is a crucial limitation, while optical amplifiers limit instability slightly below 10^-14 at 1 GHz.
- The signal-to-noise ratio is sufficient for distances up to about 200 km, and dispersion-shifted fibers could potentially extend the link beyond 200 km.
Figure captions
The figure materials identify the microwave-transfer scheme, phase-noise analysis, and stability comparisons, including compensated and uncompensated configurations.
- The phase-analysis data include end-to-end phase measurements sampled every 100 s and a 15 Hz measurement bandwidth for the compensated 9.15 GHz link.
- The stability comparison includes uncompensated, polarization-scrambled, compensated, and shortened-link system-noise configurations.
- Figure 1 presents the full microwave frequency-transfer scheme and identifies the SAW oscillator.
- Figure 2 measures phase-noise spectral density at 9.25 GHz for the compensated link and related noise references.
LNE SYRTE
The LNE SYRTE passage identifies a 2×43-km link with 25 dB loss.
- The LNE SYRTE link consists of 2×43 km with 25 dB loss.
REMOTE
The REMOTE materials identify closed-loop propagation delay and fractional frequency stability as reported quantities for the link.
- Closed-loop propagation delay is reported in femtoseconds for the remote link.
- Fractional frequency stability is the reported stability metric.