Non-reciprocity in optical fiber links: experimental evidence

Area of Science:

• Physics
• Optical Engineering
• Metrology

Background:

• Non-reciprocal effects in fiber links limit precision by creating asymmetry between forward and backward signal paths.
• These effects stem from technical setup issues and fundamental phenomena like the Sagnac effect, which is relevant for enclosed fiber areas.

Purpose of the Study:

• To investigate phase noise and frequency stability in coherent optical frequency transfer within bi-directional fiber links.
• To identify and quantify technical and fundamental limitations impacting Two-Way optical frequency transfer.

Main Methods:

• Developed a model to predict noise contributions in bi-directional fiber links.
• Conducted experiments to verify model predictions on a 50 km spooled fiber link.
• Implemented a post-processing technique to address polarization-induced noise.

Main Results:

• Identified polarization asymmetry due to environmental factors (temperature, humidity) as the dominant low-frequency noise source.
• First experimental evidence of a flicker noise floor caused by non-reciprocal noise from polarization mode dispersion.
• Achieved significant reduction in polarization noise, improving long-term stability and removing frequency bias.

Conclusions:

• The study successfully characterized and mitigated polarization-induced noise in fiber links, a key hurdle for high-precision optical frequency transfer.
• The post-processing method enhances stability to 2.6 (±39) × 10^-22, enabling searches for fundamental physics phenomena.
• This work paves the way for detecting effects like the Sagnac effect and transient frequency variations from dark matter.