FLASH, the high-gain free-electron laser (FEL) in Hamburg, enables the generation of light pulseswith wavelengths in the soft X-ray region and durations down to a few femtoseconds. To fully exploitthis capability in time-resolved pump-probe experiments, and for the projected externally seededoperation, the critical components of the accelerator and several external laser systems have to besynchronized with a temporal accuracy at least in the same order of magnitude. This can not be realizedpurely with established RF-based systems and therefore, an optical, laser-based synchronizationsystem is required.In this thesis, the optical synchronization system of FLASH has been, based on previous successfulproof-of-principle experiments, massively extended. One major topic is the comprehensive characterizationof the timing reference of the system and a comparison of different types of such masterlaser oscillators, as well as studies on their short- and long-term stability. Similar investigations havebeen carried out for the upgraded and newly installed length-stabilized fiber links, which connect theremote locations at the accelerator to the optical timing reference. The successful demonstration ofan all-optical synchronization of a Ti:sapphire oscillator with sub-10 femtosecond timing jitter andthe connection of the photo injector laser system to the synchronization system mark further importantkey experiments of this thesis. The robustness of the actual implementations played a key role,as the synchronization system forms the basis for the future, operator-friendly arrival time feedback.