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Polarization-Insensitive Coherent Receiving in Optical Fiber Links for Frequency Metrology

In recent years, the dissemination of optical frequency signals along phase-stabilized fibers has allowed the comparison of optical clocks at unprecedented levels of accuracy. This has a strong relevance in frequency metrology and boosted the measurement capabilities also in other research fields such as radioastronomy [1], geodesy [2] and fundamental physics [3]. Fiber-based frequency dissemination relies on the transmission of a continuous-wave, narrow-linewidth laser whose frequency is accurately measured with an atomic clock at the National Metrology Institute (NMI) where it is generated. Specific techniques are used to compensate for the acoustic and seismic noise and temperature variations, which degrade the phase stability of the transmitted signal. However, in existing implementations, it is not possible to detect and compensate for the birefringence variations of the optical fiber. These can induce variations in the state of polarization (SOP) of the transmitted signal, affecting the optical beatnotes at the receiving end. This not only degrades the achievable performances in terms of phase stability of the metrological signal, but can also lead to beatnote fading, with consequent failure of the entire system. This problem currently represents one of the strongest limitations to the reliability of fiber-based frequency dissemination.