Interoperability in optical frequency transfer fiber optic links

The paper addresses the interoperability problem in heterogeneous optical frequency transfer fiber links, which requires the coexistence of equipment with different designs. As we point out, a key component is the repeater laser station, which, for its operation, requires ensuring design-specific relationships between the frequencies of the optical signals at its ports. Ensuring their interoperability requires dedicated optical frequency shifters that adapt to existing differences. The second aspect of interoperability relates to the ability to recover without human intervention after a link failure, e.g., a loss of power or a fiber break. Both aspects were investigated experimentally using two different repeater laser station designs in a 500 km laboratory fiber link, demonstrating the interoperability of the tested devices in various configurations. Transfer stability and frequency inaccuracy were comparable to those obtained in homogeneous chains, reaching 10^-20 at averaging times of around 10⁴ seconds.