Free-electron Lasers


Highly intense light sources, such as free-electron lasers (FEL), are key to cutting-edge research on the molecular level. With ultrashort pulses, they offer the necessary temporal and subatomic spatial resolution. For the light generation in a free-electron laser source, multiple electrical and optical components need to be synchronized to a master clock, the facility’s RF standard. The challenge is that these components are distributed over kilometer-long distances, and the application requires a synchronization precision in the few-femtosecond regime.

For such tasks, optical timing distribution systems (TDS) such as Cycle’s PULSE are utilized to distribute the timing signal in the optical domain which offers a stable transfer over long distances. First, the facility’s RF signal is imprinted to a low-noise mode-locked laser – the optical master oscillator – using a Balanced Optical Microwave Phase Detector (BOMPD). Subsequently, the laser signal is coupled to timing-stabilized polarization-maintaining (PM) optical fiber links that distribute the signal to the individual clients. The optical fiber network comprises a feedback system with optical delay lines and Balanced Optical Cross-correlators (BOC) to compensate fluctuations upon signal transmission. Finally, at the remote stations of the TDS, voltage-controlled oscillators are connected to another BOMPD to generate low noise microwave signals.

For example, this approach is used at the Dalian Coherent Light Source (DCLS) in China, which achieved a timing jitter of 20 fs RMS and below to synchronize their RF timing signals. Furthermore, implementing Cycle’s BOMPD for translating RF and optical timing signals at this modern FEL enables high-precision experiments and new insights into molecular dynamics.

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