## 2 Laser System  
The optical laser system has been installed in a thermo-stabilized laser hutch 1 (LH1) next to experimental hutches (EH1 and EH2) of SACLA and provides femtosecond pulses for time-resolved experiments at EH2 and EH4c of Beamline 3 (BL3) [26]. This laser system, which is based on a chirped pulse amplification [27] of a Ti:sapphire laser, comprises a mode-locked oscillator, a pulse stretcher, a regenerative amplifier, a multi-pass amplifier, and pulse compressors (shown in Figures 1 and 2). Optical pulses generated by the oscillator at 79.3 MHz are stretched up to 0.2 ns and amplified to 5 mJ by the regenerative amplifier at 960 Hz (Legend Elite, Coherent Inc., Santa Clara, CA, USA. An \"acousto-optic programable dispersive filter (AOPDF)\" ([PERSON], FASTLITE, Antibes, France) [28] before the stretcher can compensate and optimize dispersion in the optical path. The output pulses from the regenerative amplifier are divided by a Pockels Cell at the operation frequency of SACLA (60 Hz). Ten percent of these pulses split by a partial reflection mirror is directed to the experimental hutch 1 (EH1) for a timing monitor (described in Section 3). The seven-pass amplifier multiplies the rest of the pulses (90% in pulse energy) to 20 mJ at 60 Hz. The amplified main beam with about 4-mm diameter (FWHM, the full width at half maximum) is propagated without pulse compression to EH2 and EH4c. The power ratio between the two beams toward EH2 and EH4c is variable with a half-wave plate and a polarization beam splitter. The three transport lines toward EH1, EH2, and EH4c with distances of 3, 4, 16 m, respectively, are built on the floor of the experimental hall. The mirror holders of the transport lines, designed for long term stability ([PERSON], Thorlabs Inc., Newton, NJ, USA), are mounted on massive lead blocks to avoid vibration from the floor. These transport lines, including the optics, are covered with ducts to suppress fluctuation from air-flow. Figure 1: Laser system at LH1. Optical pulses generated by a mode-locked oscillator at 79.3 MHz are stretched up to 0.2 ns and amplified to 5 mJ by a regenerative amplifier at 960 Hz. Output pulses from the regenerative amplifier are picked up at 60 Hz by a Pockels Cell (PC) and multiplied to 20 mJ by a seven-pass amplifier. The amplified pulses without pulse-compression are transported to EH2 and EH4c and compressed in each experimental hutch (Figure 2). 10% of the output of the regenerative amplifier is used for the timing monitor at EH1. The synchronization system using a balanced optical-microwave phase detector (BOMPD) is installed to the oscillator. \(\lambda\)/2+PBS: a half-wave plate and a polarization beam splitter; FC: a fiber coupler; AOPDF: an acousto-optic programable dispersive filter; BS: a 90% partial reflection mirror for beam splitting. A compressor, a delay line, and a third-harmonic generation (THG) unit are installed on an optical bench in each of EH2 and EH4c, as shown in Figure 2. Before the grating-pair compressor, the optical beam is expanded to 6 mm in diameter (FWHM) not to damage the gratings. A typical spectrum of the compressed pulses is shown in Figure 3. Finally, ultrashort pulses of 12 mJ with a 40-fs pulse duration are provided for user experiments at EH2 and EH4c. The fluctuation of the output energy is about 4% in 24 h, as shown in Figure 4. A roof mirror on a 30-cm-long motorized stage (HPT-2, Kohzu Precision Co., Kawasaki, Japan) enables users to tune and scan the delay between the optical and the XFEL pulses with a 6.67-fs step in the range of 2 ns. This long stage has been customized to suppress pitching and yawing motion below five arcminutes (~1.5 mrad). Second-harmonic (SH) and third-harmonic (TH)--whose wavelengths are 400 nm and 267 nm, respectively--are generated by the THG unit on users' demand. A type-I \(\beta\)-BaB2O4 (BBO) crystal with 0.1-mm thickness is used for SH generation. A TH beam is generated as a sum-frequency mixing of the SH and the remaining fundamental beams by a 0.5-mm thick type-I BBO crystal.