Towards Drift Free One-Way Optical Frequency Transfer Over 500-m Fiber Baseline*

Optical frequency transfer over existing telecom fiber networks is an important tool for precision optical metrology. To combat the added phase noise due to fiber-piston action and refractive index changes in optical fibers, a phase cancellation method is required. We detail the evolution of a one-way optical frequency stabilization technique to increase the stability by limiting the out-of-loop path in a traditional phase-tracking heterodyne interferometer¹. Each apparatus’s stability was first tested over a 4.09 km SMF-28 fiber spool. The second and third apparatus iterations were individually deployed over a telecom SMF-28 fiber link of 2923 m in round-trip length. The first apparatus was composed of an all-PM fiber setup with over 3.5 m of out-of-loop path and suffered high instability and cycle slippage. The second iteration was composed of an acoustically-insulated partial free-space and PM fiber apparatus that reduced out-of-loop path-length to 0.8 m and resulted in an in-loop instability of 3 × 10⁻¹⁹ at 1 ksec over the telecom link. The final evolution involved a custom micro-optic component to reduce the out-of-loop to 0.75 m and achieved in-loop instability of 2.5 × 10⁻¹⁹ at 1 ksec over the telecom link.