Status of the Fiber Link Stabilization Units at FLASH

State-of-the-art X-ray photon science with modern freeelectronlasers (FEL) like FLASH (free-electron laser inHamburg) and the upcoming European X-ray Free-ElectronLaser Facility (XFEL) requires timing with femtosecondaccuracy. For this purpose a sophisticated pulsed opticalsynchronization system distributes precise timing via lengthstabilizedfiber links throughout the entire FEL. Stationsto be synchronized comprise bunch arrival time monitors(BAM’s), RF stations and optical cross-correlators (OXC)for external lasers. The different requirements of all thosestations have to be met by one optical link stabilizationunit (LSU) design, compensating drifts and jitter in thedistribution system down to a fs-level. Five years of LSUoperation at FLASH have led to numerous enhancementsresulting in an elaborate system. This paper presents theseenhancements, their impact on synchronization performanceand the latest state of the LSUs