A Low Cost Picoseconds Precision Timing and Synchronization Over A Hundred Kilometer

This paper demonstrates that an enhanced CERN White Rabbit protocol can achieve picosecond-precision timing synchronization over distances exceeding one hundred kilometers without environmental corrections, offering a low-cost solution for large-scale accelerator systems.

The Gist

The Big Picture: Keeping a Giant Orchestra in Sync

Imagine a massive orchestra playing inside a stadium that is 100 kilometers wide (about the size of a small city). In this stadium, the "musicians" are actually high-tech particle accelerators and giant detectors used to study the building blocks of the universe.

For the music to sound right, every single musician needs to play their note at the exact same micro-second. If the violinist in the north end of the stadium is even a tiny fraction of a second off from the drummer in the south end, the whole performance falls apart.

In the world of particle physics, this "music" is the timing of laser beams and particle bunches. If they aren't perfectly synchronized, the experiments fail.

The Problem: The "Expensive Conductor"

Until now, keeping this 100-kilometer-wide orchestra in sync required a very expensive, complex, and fragile "conductor" system. These systems are like high-end, custom-built orchestral conductors that cost a fortune and are difficult to set up. They are so precise that they can keep time down to the femtosecond (a quadrillionth of a second).

But here's the catch: most of the instruments in the orchestra (like laser diagnostics or beam monitors) don't actually need that level of perfection. They only need picosecond precision (a trillionth of a second). Using the ultra-expensive system for these tasks is like hiring a world-famous symphony conductor just to tell a marching band when to step left. It's overkill and too costly.

The Solution: The "White Rabbit" and the "Smart Watch"

The authors of this paper wanted to build a low-cost, simple, and cheap way to keep time over 100 kilometers.

They used a protocol called White Rabbit. Think of White Rabbit as a super-accurate digital watch that can talk to other watches over a fiber-optic cable (like a super-fast internet cable made of light).

They built a custom electronic board called Idrogen.

• The Metaphor: Imagine the Idrogen board as a "Smart Watch" that doesn't just tell time; it can also generate a specific rhythm (frequency) with incredible accuracy.
• The Trick: They connected a "Master Watch" (in a lab) to a "Slave Watch" (near the laser) using a 100-kilometer-long fiber optic cable. The Master Watch sends a signal, and the Slave Watch adjusts its own internal rhythm to match perfectly, even though the signal takes time to travel through the long cable.

The Experiment: The Laser Dance

To test this, they set up a real-world scenario:

1. The Master: A high-precision clock in a lab.
2. The Connection: A fiber optic cable stretched to simulate 100 kilometers (they actually used 10m, 5km, 50km, and 100km cables to test).
3. The Slave: Their new Idrogen board connected to a commercial laser.

The goal was to make the laser "dance" in perfect step with the Master clock, even though they were separated by a distance as long as a drive from Paris to Lyon.

The Results: It Worked!

The results were impressive:

• Precision: The system kept the laser and the clock synchronized with picosecond precision.
• Stability: Even without fancy temperature controls or expensive enclosures (the boards were just sitting on a table!), the system only drifted by a few picoseconds over many hours.
• Cost: This solution is a fraction of the cost of the traditional systems.

The "But..." (The Real World Factor)

There was one small hiccup. When the boards were just sitting on a table, the temperature in the room changed slightly (like when the air conditioning kicks on). This caused the timing to wiggle a tiny bit over the course of an hour.

• The Analogy: It's like trying to keep a tightrope walker balanced. If the wind (temperature) blows, they wobble. If you put them in a windless room (a temperature-controlled box), they stand perfectly still.
• The Fix: The authors say that if they put these boards in proper cases with temperature control (which is standard for real machines), the wobble will disappear.

Why This Matters

This paper proves that we don't need to spend millions of dollars to get picosecond precision for particle accelerators.

• For Scientists: They can now add precise timing to their experiments without breaking the bank.
• For the Future: This technology could be used in giant particle colliders, medical physics (like cancer treatment with particle beams), and even detecting cosmic rays.

In a nutshell: The authors built a cheap, smart "timekeeper" that can keep a laser perfectly in sync with a master clock over a distance of 100 kilometers. It's a bit wobbly if left out in the cold, but once you give it a proper home, it's a game-changer for big science.

Technical Summary

1. Problem Statement

Large-scale particle accelerators (e.g., LHC, future circular colliders) require precise timing and synchronization over distances of tens to hundreds of kilometers.

• Current Limitations: Existing high-precision solutions (e.g., all-optical systems or complex RF distributions) achieve femtosecond precision but are costly, complex, and difficult to integrate. They are often "overspecified" for applications that only require picosecond precision, such as accelerator diagnostics, beam polarimetry, and large-scale detector synchronization.
• The Gap: There is a need for a low-cost, easily integrable solution that can synchronize laser systems and diagnostic tools with picosecond precision over long distances (up to 100 km) without requiring extensive environmental control or expensive integration adjustments.

2. Methodology

The authors propose and test a system based on the White Rabbit (WR) protocol, enhanced by a custom electronic system called Idrogen.

• Hardware Platform (Idrogen Board):
  • A $\mu$TCA board based on an Altera Arria 10 FPGA.
  • Features an ultra-low-noise clock tree centered on a Texas Instruments LMK04828 jitter cleaner (91 fs RMS jitter).
  • Implements the WR protocol in slave mode to lock to a master reference.
  • Includes a Digital Dual Mixer Time Difference (DDMTD) architecture for high-precision phase measurement.
  • Equipped with SFP+ (1 Gb/s WR link) and QSFP (40 Gb/s data) ports.
• Test Setup:
  • Master: An Idrogen board (MIB) disciplined by a 10 MHz reference from a high-quality signal generator (R&S SMB100A with OCXO).
  • Slave: A second Idrogen board (SIB) connected to the MIB via optical fiber links of varying lengths (10 m, 5 km, 50 km, and 100 km).
  • Frequency Synthesis: The SIB drives a Skyworks SI5362 clock generator, which produces arbitrary frequencies with Hertz precision.
  • Target Device: A MENHIR-1030 passively mode-locked laser (nominal frequency ~216.66 MHz), representative of lasers used in Compton polarimeters.
  • Feedback Loop: The laser's output is detected by a photodiode, mixed with the SI5362 output (at the 4th harmonic), and fed into a commercial PID controller (LaseLock) to lock the laser's repetition rate to the WR-synchronized clock.
• Measurement:
  • Phase noise power spectral density (PSD) measured using a NoiseXT DNA-400M analyzer.
  • Allan deviation and RMS delay calculated over long durations (up to 19 hours).
  • Tests conducted in a standard lab environment without active thermal enclosure or rack integration to prove the concept's robustness under minimal conditions.

3. Key Contributions

• Low-Cost Architecture: Demonstrated that picosecond synchronization can be achieved using a modular, FPGA-based system (Idrogen) rather than expensive, custom optical fiber links.
• Long-Distance Validation: Successfully validated the system over 100 km of optical fiber, a distance relevant for future large-scale colliders.
• Arbitrary Frequency Generation: Showed the ability to generate and lock arbitrary frequencies (specifically for laser repetition rates) with Hertz-level precision using the SI5362 chip driven by the WR-synchronized FPGA.
• Proof of Concept for Diagnostics: Validated the synchronization of a pulsed laser (critical for beam polarimetry) to the accelerator clock, proving the feasibility of integrating such systems into existing facilities with minimal disruption.

4. Results

• Precision: The system achieved picosecond-level synchronization.
  • RMS Jitter: Measured at 1.4 ps RMS for a 10 m link.
  • Long-Term Stability: Over 16 hours, the system maintained a lock with a peak-to-peak drift of up to 20 ps.
  • 100 km Performance: At 100 km, the RMS delay increased slightly to 5.5 ps RMS (compared to lower values at shorter distances), attributed to the addition of an optical amplifier and environmental factors.
• Phase Noise:
  • The phase noise of the locked laser matched the reference clock closely.
  • A slight degradation in phase noise above 300 Hz was observed at 100 km, likely due to technical noise from added amplifiers in the measurement chain.
• Environmental Sensitivity:
  • The system exhibited drifts correlated with room temperature cycles (air conditioning) over ~1 hour periods.
  • The 100 km link showed a longer-term stability factor 1.5x worse than shorter links, suggesting that thermal enclosure and temperature compensation are necessary for optimal long-term performance.
• Allan Deviation: Measured at approximately $3.1 \times 10^{-12}$ at 1 second for the synchronized laser.

5. Significance and Future Outlook

• Scalability: This work paves the way for deploying low-cost, scalable timing networks in large-scale facilities (accelerators, large detectors) where femtosecond precision is not strictly required, but picosecond precision is essential.
• Applications: The system is directly applicable to:
  • Compton Polarimeters: For measuring beam polarization.
  • Photocathode Lasers: For generating electron beams.
  • Beam Diagnostics: Such as bunch-per-bunch position monitors.
  • Large Detectors: Synchronizing distributed detector components in cosmic ray or medical physics experiments.
• Next Steps: The authors plan to:
  • Encapsulate the boards in thermally controlled $\mu$TCA crates to mitigate environmental drift.
  • Implement software-based temperature corrections using probes on the Idrogen boards.
  • Develop an FMC module integrating the SI5362 chip for a fully compact, sub-picosecond arbitrary frequency generator.

In conclusion, the paper successfully demonstrates that the White Rabbit protocol, when enhanced with custom FPGA hardware and commercial clock generators, can provide a robust, cost-effective solution for picosecond timing synchronization over 100 km, significantly lowering the barrier for advanced diagnostics in particle physics.