Optical frequency transfer via a 660 km underground fiber link using a remote Brillouin amplifier

Area of Science:

• Physics
• Optical Engineering
• Metrology

Background:

• Long-distance optical frequency transfer is crucial for modern scientific applications.
• Remote bidirectional Erbium-doped fiber amplifiers are standard for mitigating signal loss.
• Existing methods face limitations in stability and reach for precise frequency dissemination.

Purpose of the Study:

• To demonstrate the first ultrastable optical frequency transfer using a remote fiber Brillouin amplifier.
• To assess the performance of this novel amplification method over long fiber links.
• To establish new benchmarks for optical frequency transfer instability.

Main Methods:

• Implemented a remote fiber Brillouin amplifier in a server room along a 660 km fiber loop.
• Achieved amplification at distances of 250 km and 160 km between amplifier sites.
• Conducted multi-day uninterrupted measurements to evaluate frequency transfer stability.

Main Results:

• Achieved an optical frequency transfer instability of approximately 1 x 10⁻¹⁹ for averaging times exceeding 3000 s.
• Modified Allan deviation reached 3 x 10⁻¹⁹ at 100 s averaging time.
• Demonstrated instability in the 10⁻²⁰ range for averaging times longer than 1000 s, limited by the interferometer noise floor.

Conclusions:

• The remote fiber Brillouin amplifier enables ultrastable, long-distance optical frequency transfer.
• This technology offers a viable alternative to traditional amplifiers, achieving high accuracy (1 x 10⁻¹⁹).
• The findings pave the way for enhanced precision in global timekeeping and scientific experiments.