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Precision laser-to-RF synchronization for ultrafast electron sources
Enabling femtosecond laser-RF timing with BOMPD and ESYNC
Ultrafast electron sources enable the investigation of atomic, molecular and material dynamics on femtosecond timescales, providing direct insight into fundamental processes in matter. Their ability to capture ultrafast structural dynamics makes them powerful tools for advancing our understanding of physical, chemical and biological processes [1].
Achieving this temporal resolution requires precise synchronization between ultrafast lasers and RF sources. Timing fluctuations between these subsystems directly contribute to the temporal uncertainty of the experiment. Conventional approaches rely on photodetection of the optical pulse train, followed by microwave phase detection. Their low phase sensitivity, typically only a few μV/fs, results in limited noise suppression and leads to residual timing jitter (typically > 100 fs). In addition, their high thermal sensitivity [2] leads to significant long-term timing drift (typically > 1 ps).
Cycle’s Balanced Optical Microwave Phase Detector (BOMPD) addresses these limitations by performing the timing detection directly in the optical domain. Based on a well-biased fiber interferometer, BOMPD converts the timing difference between an optical pulse train and a microwave signal into an intensity imbalance, which is then converted into an electrical error signal by balanced detection. This approach provides approximately 100× higher phase sensitivity and 100× lower thermal sensitivity than conventional microwave phase detection [3].
Figure 1 illustrates a typical ultrafast electron source. A laser oscillator provides the optical pulses for the system, which are amplified and used for electron generation at the photocathode and to provide the optical pump used for time-resolved measurements. The generated electron bunch is accelerated by the RF gun and subsequently compressed by the RF buncher before reaching the experiment. Precise timing between the laser and RF modules is therefore essential for the temporal resolution.
BOMPD provides the high-sensitivity timing detection between the laser oscillator and the facility RF reference. The resulting timing error signal is processed by ESYNC, Cycle’s electronic synchronization unit, which provides the feedback required to tightly lock the laser oscillator to the RF reference.
ESYNC combines low-noise control electronics with advanced functions to simplify the operation of the synchronization system. These include pre-locking, delay scanning, automated lock acquisition and advanced system monitoring. Together, BOMPD and ESYNC provide a turnkey laser-to-RF synchronization solution that can be integrated into ultrafast electron sources and operated reliably during demanding experiments.
A BOMPD and ESYNC synchronization system was installed at a particle accelerator facility to synchronize a commercial Ti:sapphire laser oscillator operating at 79.33 MHz to a facility 5.712 GHz RF reference. Following installation, the synchronization performance was characterized using a second BOMPD as an out-of-loop detector.
As shown in Figure 2, the measured 0.4 mV/fs timing sensitivity demonstrates the exceptional BOMPD sensitivity to small timing variations. This enables tight feedback control of the laser oscillator, resulting in only 14 fs RMS residual out-of-loop timing jitter. Over an 8 hour measurement period, the residual timing drift remained at approximately 3 fs RMS. This ultra-low drift demonstrates the excellent long-term stability of the BOMPD-lock and its low sensitivity to environmental variations.
The demonstrated combination of high timing sensitivity, femtosecond-level residual jitter and drift makes BOMPD + ESYNC a turnkey solution for stable, high-performance laser-to-RF synchronization in ultrafast electron sources. BOMPD is available for 800 nm, 1030 nm and 1550 nm laser systems.
