Jo Wonhyuk, Lee Sooheyong, Eom Intae, Landahl Eric C
Department of Physics, Soongsil University, Seoul 156-743, South Korea.
Korea Research Institute of Standards and Science, Daejeon 305-600, South Korea.
Rev Sci Instrum. 2014 Dec;85(12):125112. doi: 10.1063/1.4903967.
The ability to synchronize a femtosecond laser to x-ray pulses is crucial for performing ultrafast time-resolved x-ray scattering experiments at synchrotrons. Conventionally, the task has been achieved by locking a harmonic frequency of the laser oscillator to the storage ring master radio-frequency (RF). However, when the frequency mismatch between the two sources cannot be compensated by small adjustments to the laser cavity length, synchronization to a harmonic frequency requires modifying the optical components of the laser system. We demonstrate a novel synchronization scheme, which is a flexible alternative for synchronizing these two sources operating at arbitrarily different frequencies. First, we find the greatest common divisor (GCD) of the two frequencies that is still within the limited tuning range of the laser cavity length. The GCD is generated by dividing down from the storage ring RF, and is separately multiplied up to provide a feedback signal for synchronizing the laser cavity. Unique to our scheme, the GCD also serves as a harmonic RF source for the laser amplifier such that only laser oscillator pulses at fixed integer multiples of the storage ring RF are selected for amplification and delivery to experiments. Our method is implemented at the Photon Test Facility beamline of Pohang Light Source where timing-jitter less than 4 ps (r.m.s.) is measured using a new shot-to-shot method.
将飞秒激光与X射线脉冲同步的能力对于在同步加速器上进行超快时间分辨X射线散射实验至关重要。传统上，该任务是通过将激光振荡器的谐波频率锁定到储存环主射频（RF）来实现的。然而，当两个源之间的频率失配无法通过对激光腔长度进行小调整来补偿时，与谐波频率同步就需要修改激光系统的光学组件。我们展示了一种新颖的同步方案，这是一种灵活的替代方法，可用于同步以任意不同频率运行的这两个源。首先，我们找出两个频率的最大公约数（GCD），该最大公约数仍在激光腔长度的有限调谐范围内。最大公约数是通过对储存环射频进行分频生成的，并分别进行倍频以提供用于同步激光腔的反馈信号。我们方案的独特之处在于，最大公约数还用作激光放大器的谐波射频源，这样只有储存环射频固定整数倍的激光振荡器脉冲才会被选来放大并输送到实验中。我们在浦项光源的光子测试设施光束线实施了我们的方法，在那里使用一种新的逐次测量方法测得定时抖动小于4皮秒（均方根）。
