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Cascaded multiplexed optical link on a telecommunication network for frequency dissemination
Olivier Lopez, Adil Haboucha, Fabien Kéfélian, Haifeng Jiang, Bruno Chanteau, Vincent Roncin, Christian Chardonnet, Anne Amy-Klein, Georgio Santarelli
TL;DR
Longer-distance optical frequency dissemination is needed, but Internet fibers bring additional attenuation and propagation noise. This work demonstrates a two-segment Internet-fiber link with an autonomous repeater, achieving instability from 3×10^-15 at 1 s to around 5×10^-20 after about 20 hours.
Problem
Extending optical-frequency transfer beyond dedicated links of about 200 km is a challenge, while Internet fibers add attenuation and propagation noise.
Source: §1 · pp. 2–3Method
The paper demonstrates a cascaded frequency-transfer link with two Internet-fiber segments and a repeater station, using OADMs to insert and extract the optical signal.
Source: §1 · p. 3, §3.2 · p. 6Results
3×10^-15 at 1 s averaged to around 5×10^-20 after about 20 hours for the 300 km optical transfer over installed telecommunication fibers carrying Internet data.
Source: §4 · p. 9Takeaways & Limitations
Non-dedicated fiber links are a valuable alternative to dark fibers for ultra-stable frequency transfer.
Source: §4 · p. 9The repeater lacks automatic polarization adjustment, an issue for links with multiple stations.
Source: §4 · p. 9
Abstract
from arXiv · showhide
We demonstrate a cascaded optical link for ultrastable frequency dissemination comprised of two compensated links of 150 km and a repeater station. Each link includes 114 km of Internet fiber simultaneously carrying data traffic through a dense wavelength division multiplexing technology, and passes through two routing centers of the telecommunication network. The optical reference signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station operates autonomously ensuring noise compensation on the two links and the ultra-stable signal optical regeneration. The compensated link shows a fractional frequency instability of 3 \times 10-15 at one second measurement time and 5 \times 10-20 at 20 hours. This work paves the way to a wide dissemination of ultra-stable optical clock signals between distant laboratories via the Internet network.
1. Introduction
Optical-frequency transfer is needed for distant laboratories, but extending it beyond dedicated links of about 200 km remains a challenge. Existing research networks offer data-carrying Internet fibers, though their added equipment and connectors increase attenuation and propagation noise, motivating a segmented approach with repeater stations.
1. Introduction
- Extending optical-frequency dissemination beyond dedicated links of about 200 km is a challenge for connecting laboratories over longer distances.The paper situates this need in applications including time and frequency metrology, fundamental physics, particle accelerators, and astrophysics.Source: §1 · p. 2
- NRENs already connect research facilities and universities using Internet fibers that carry digital data alongside the ultra-stable signal via DWDM.The paper describes this as an approach developed by the authors.Source: §1 · pp. 2–3
- Internet fibers have more connectors and WDM equipment than dedicated fibers, increasing attenuation and propagation noise.Source: §1 · p. 3
- Segmenting a link can increase correction bandwidth and attenuation robustness, but requires a repeater station between segments.Source: §1 · p. 3
2. Repeater station for multi-segment link
The repeater station stabilizes the incoming segment, regenerates the optical signal, and compensates noise on the next segment. Its optical and electronic systems support phase locking and autonomous operation, while propagation asymmetry and polarization adjustment remain relevant limits.
2.1 Principle of operation
- Each repeater stabilizes the incoming link, regenerates its signal with a phase-locked cleanup oscillator, and compensates noise on the following link.Source: §2.1 · p. 3
- Propagation asymmetry, including the Sagnac effect, ultimately limits frequency resolution; automatic polarization control remains an issue for links with multiple stations.Source: §2.1 · p. 4
2.2 Optical set-up
- The station’s laser and three AOMs generate beat notes for locking to the incoming signal and compensating round-trip fluctuations on the next link.The optical module minimizes and matches non-common fiber pigtails; it is housed with thermal and acoustic shielding and active temperature stabilization.Source: §2.2 · p. 4
2.3 Electronic set-up
- A tracking oscillator filters the next-link beat note before PLL2 applies correction through AOM1; frequency division increases compensation control dynamic range.Source: §2.3 · p. 4
2.4 The station automatic operation
- Microcontrollers acquire the laser lock by scanning its frequency, then close the tracking-oscillator loop and PLL2; opposite 75 MHz shifts cancel local-oscillator drift in transfer.Source: §2.4 · p. 5
3. Cascaded Link
The cascaded link was tested on spools and then demonstrated over installed RENATER fibers, with Internet traffic sharing fiber spans with the ultra-stable signal. The installed-link design uses OADMs and bidirectional amplifiers to manage signal insertion and round-trip losses.
3.1 Station testbed
- The testbed cascaded two 150 km segments through a repeater and evaluated end-to-end beat-note frequency with simultaneous 100 kHz and 10 Hz measurement channels.Source: §3.1 · p. 5
- 3×10^-16 at 1 s, averaging to the mid 10^-20 range at 30000 s, was measured for the 2×150 km compensated testbed link with a 10 Hz bandwidth.The authors attribute its stability to lower spool noise than in installed fiber; the zero-length system floor reaches 10^-20 at 10^4 s.Source: §3.1 · p. 5
3.2 Cascaded link using the telecommunication network
- The authors demonstrated a 300 km cascaded optical link on RENATER, with 114 km of Internet fiber carrying traffic in each uplink and downlink path.The route uses three fiber spans and processes the signal at the intermediate Nogent l’Artaud repeater station.Source: §3.2 · p. 6
- Eight OADMs insert and extract the ultra-stable signal, while two bidirectional EDFAs per 150 km link address round-trip attenuation exceeding 100 dB.The OADMs provide better than 25 dB isolation for an adjacent channel and better than 40 dB for other channels.Source: §3.2 · p. 6
4. Results and discussions
The 300-km cascaded link suppresses propagation delay and phase noise while delivering very low fractional frequency instability. Its performance is sustained over several days despite nonstationary environmental noise, although polarization control remains a practical limitation.
4. Results and discussions
- The cascaded closed-loop delay stays below 10 fs peak-to-peak, while the simultaneously recorded open-loop up-link fluctuates more than the free-running fiber.The correction system's rejection factor is close to 10^5.Source: §4 · p. 7, Figure 4 · p. 7
- The Interxion1–Telehouse2 segment is the noisiest of the measured link segments.Phase-noise spectral density was evaluated from the end-to-end beatnote of a very narrow-linewidth laser.Source: §4 · p. 7, Figure 5 · p. 7
- 50 dB phase-noise reduction at 1 Hz is close to the theoretical limit of 51 dB.Below 5 Hz, both spectra have peaks likely due to seismic noise along the buried railway-track fibers.Source: §4 · pp. 7–8, Figure 6 · p. 8
- 3×10^-15 fractional frequency instability at 1 s, falling to 5×10^-20 at 70,000 s with a 10 Hz filter; full-bandwidth instability is six times larger.Fig. 7 compares the compensated link with the free-running up-link and shows the filtered and full-bandwidth measurements.Source: §4 · p. 8, Figure 7 · p. 8
- Despite nonstationary noise, the link remains locked for several days; rare human-activity phase jumps cause cycle slips, and polarization still requires manual adjustment.After a few days, polarization must be adjusted to optimize the local laser phase-lock beatnote; automatic polarization control is not implemented.Source: §4 · pp. 8–9
4. Conclusion
The paper demonstrates ultrastable optical-frequency transfer over 300 km of installed telecommunication fiber carrying Internet data. It reports instability reaching around 5×10^-20 after about 20 hours and presents non-dedicated links as a potential alternative for broad dissemination.
4. Conclusion
- 3×10^-15 at 1 s, averaging down to around 5×10^-20 after about 20 hours, was obtained for the optical-frequency transfer.The transfer spans 300 km of installed telecommunication-network optical fiber simultaneously carrying Internet data.Source: §4 · p. 9
- Installed telecommunication fiber carrying Internet data over 300 km demonstrates that non-dedicated links are a valuable alternative to dark fiber.Source: §4 · p. 9
- Combined with the proposed PTB technique for serving several users from one optical link, Internet links are attractive for widescale frequency dissemination.Source: §4 · p. 9
- Such links could support high-sensitivity measurements, including searches for fundamental-constant variation and gravitational mapping.Source: §4 · p. 9