High-performance bunch arrival time monitors with fs precision at DESY

Pump-probe experiments at Free-Electron Laser facilities depend heavily on the relative timing precision of the pump and the probe, which determines the resolution of the observed ultrafast phenomena. The DESY's Bunch Arrival Time Monitors (BAM) are state-of-the-art sensors based on an electro-optical detection principle, that delivers information on the bunch timing with unprecedented femtosecond-level precision. Timing synchronization at machine level is achieved through a complex system of arrival time sensors, stabilized optical distribution, and feedback controls. Major advances in the performance, construction, and operation of the BAMs are discussed in detail. Integration of the sensor into the synchronization system, and important global optimization and interplay are mentioned as well. These improvements enabled a synchronization of the electron beam with a world-leading precision of less than 3fs at European XFEL.