Towards Drift Free One-Way Optical Frequency Transfer Over 1.4 km 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 3x10^-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 2.5x10^-19 at 1 ksec over the telecom link.