## 3 Photon Beamlines
### Experimental Laser  
One of the unique advantages of an FEL is the ultrashort pulse duration in the X-ray range. Therefore a large number of experiments are time resolved. The SwissFEL experimental laser facility [112] provides lasers for both pump-probe experiments, as well as for certain specialized diagnostic techniques which provide feedback on FEL operation (see Section 3.3.2). The critical parameters are availability, performance, and stability. The laser systems are based on a commercial 100 Hz Ti:Sapphire amplifier system from Coherent. An enhanced diagnostics system ensures the long-term stability of the laser. The laser output is combined with nonlinear conversion stages and allows access to a broad spectral range, including UV, visible, near-IR, mid-IR, and THz, as well as the generation of ultrashort (<10 fs) pulses. In addition to the femtosecond laser systems a tunable (UV to IR), ns pulse-duration laser system which will be fibre-coupled to the experimental stations is also planned to allow experiments to take advantage of the short XFEL probe pulses, for example using \"diffract-before-destroy\" techniques [22; 113; 114], but with more efficient optical excitation resulting from the longer duration excitation pulses. The accessible timescales from ns to ms with this laser system are especially relevant for dynamics in proteins [115; 116; 117; 118; 119]. The laser infrastructure is distributed over two floors. The two identical laser amplifiers are located in a dedicated pump laser room (LHx), on the floor above the experimental stations (Figure 10), while the nonlinear conversion stages are installed on optical tables inside the experimental hutches, close to the experiment. This layout allows the simultaneous and independent operation of both end stations, and, in case of a failure of one system, the beam from the remaining laser can be redirected to both experimental stations and assure uninterrupted operation. Figure 10: Laser I and Laser II are located in the laser lab LHx, on the floor above the two experimental stations Alvra (ESA) and Bernina (ESB). The uncompressed amplifier output is sent through an evacuated transfer line from LHx to ESA and ESB (Figure 11). The pulse can be compressed to <30 fs and has a total pulse energy of 20 mJ. Arriving on the optical table inside the end station, the laser beam is split into two branches, one for diagnostics and one for the pump-probe experiment. Each branch is equipped with its individual optical compressor. Normally the available laser energy is split equally between the two branches. The diagnostics branch is used to operate the pulse arrival time and length monitor (PALM) [107; 108], as well as the spectral encoding (PSEN) [102; 102] for X-ray arrival time measurements (see Section 3.3.2). The major part of the diagnostics branch laser energy is used for the THz source to operate the PALM. It is based on the tilted pulse front scheme in LiNbO\({}_{3}\)[121] with a typical field strength of 100 kV/cm, centered at 0.5 THz [107]. The rest of the laser energy is used for chirped white light generation for the operation of the PSEN [102; 122]. For user operation the following modes are available: Mode 1 is the standard compressed output of the laser (10 mJ, <30 fs, 800 nm), which, when used in combination with an optical parametric amplifier (OPA), can generate wavelengths from 1100 to 2600 nm with up to 2 mJ pulse energy (Mode2a). Mode 2b combines the OPA output with subsequent conversion modules and extends the spectral range to the visible and UV (NirUVIS), to the IR (NDFG up to 15 \(\upmu\)m) and to the THz range (1-10 THz, depending on the crystal used [123; 124; 125]). Mode 3 is the short-pulse option, delivering <10 fs pulses with >200 uJ. It is based on a hollow core fiber compressor [126]. For the first user experiments, Mode1, Mode2a and the UV/VIS module will be available at ESA and Mode1, Mode2a, DFG, THz, and <10 fs will be available for ESB.