## 3 Synchronization of Laser
The timing monitor, which is based on a spatial decoding method using a transient optical transmittance change of gallium arsenide (GaAs) irradiated by intense X-ray pulses, can sort data sets of pump-probe measurements in the order of the arrival time to correct the error due to the jitter in post-processing [23; 24; 25; 36; 35]. Figure 14 shows the optical setup of the timing monitor at EH1. About 10% of the XFEL beam is split by a transmission grating and vertically focused on the GaAs target in the timing monitor by an elliptical mirror [24]. The horizontal incident angle is set at 45 degrees toward the target normal for the spatial decoding. As described in Section 1, 10% of the optical laser beam before the seven-pass amplifier is transported to EH1 and compressed by a pair of transmission gratings. We installed a linear stage delay line and a 1-m long multi-roundtrip delay line at EH1 to match the optical path length with that to the sample position at EH2 or EH4c. The optical path to the sample position is different according to an experimental hutch in use (EH2 or EH4c), an experimental setup, and a use state of the optical frequency conversion units (SH, TH, and OPA). The optical laser beam propagates along the surface normal of the target (the 45-degree crossing angle toward the XFEL beam) and probes the area irradiated by the XFEL beam. The target images are captured by a long working distance microscope and a charge-coupled-device (CCD) camera (OPAL-2000, Adimec, Eindhoven, The Netherlands). Figure 15a indicates the camera image of the timing monitor and the projection trace of the area irradiated by the XFEL beam. The precipitous edge in the projection trace was formed by the pulse front of the XFEL beam. The spatial decoding  
Figure 12: Jitter spectral density (red) measured by a phase noise analyzer (FSWP8, R&S) with a calibrated sensitivity of 0.22 mV/fs and integrated jitter (blue). method can convert the edge position to the relative arrival time of the XFEL pulse to the optical laser pulse. The coefficient for the conversion is calibrated as 2.478 fs/pixel by scanning the linear stage delay line. We measured a long-term shift of the relative timing using the timing monitor. Figure 15b,c show a histogram of 3000 shots and trends of averages and standard deviations in 3000 shots in 24 h, respectively. The timing fluctuations have been reduced down to 50 fs (rms) by the synchronization system using BOMPD. However, there still remains a long-term drift of \(\sim\)0.5 ps in a day. This drift can be suppressed by controlling an RF phase of the mode-locked oscillator using the 'out-of-loop' BOMPD output, which has a linear correlation with the timing monitor. Figure 14: Optical setup for the timing monitor at EH1. The optical laser beam, which is split from the main beam before the seven-pass amplifier, is transported to EH1 and compressed by the pair of transmission gratings. The optical stage delay line and the 1-m long multi-roundtrip delay line are set up to adjust the optical path length, which should be matched with that to the sample position at EH2 or EH4c. About 10% of the XFEL beam split by the transmit grating is vertically focused on the GaAs target in the timing monitor by the elliptical mirror. The GaAs target is mounted with the incidence angle of 45 degrees toward the XFEL beam for spatial decoding. The optical laser beam probes the area irradiated by the XFEL beam with a direction of the surface normal of the target. The target images are captured by the long working distance microscope and the charge-coupled device (CCD) camera. Figure 13: Block diagram of the timing delay unit (TDU). The TDU consists of counters and in-phase and quadrature (IQ) phase shifters. The input signals of 5712 MHz, 238 MHz, and 60 Hz are provided from the SACLA accelerator. The 960-Hz and 60-Hz signals required for the laser operation are produced by counting 238-MHz RF and re-clocking with 5.7-GHz RF.