Optical laser

Pump-probe experiments can be performed with femtosecond temporal resolution using both FEL and optical laser radiation. The current optical source is the laser used to generate the FERMI seed, part of which is transported trough a 150 m long optical beamline to the end-stations, where the very high pointing stability is achieved by Rayleigh imaging and beam-position active stabilization, guaranteeing an added timing jitter as low as 3 fs RMS (Cinquegrana et al. Physical Review Special Topics - Accelerators and Beams, 17 040702 (2014)), 10 fs for DiProI in particular (Danailov et al. Optics Express, 22 12869 (2014)). A compact pulse compressor, integrated in the beam transport, controls the pulse length in the range 90-250 fs.
Optical laser transport
Three optical breadboards are available for laser beam manipulation, IR beam transport into the chamber and diagnostics. The first one, of size 900x600 mm2, provides different options fo matching the optical pulse parameters to the experiment. An optical attenuator (Atten.), made of half wave plate and two thin film polarizers, is used to adjust the exact pulse energy impinging on the sample in the 1-700 µJ range (measured on an energy meter (En.). A translation stage (Sc.D.) provides a variable pulse delay in the range of ±570 ps. A compact single-shot auto-correlator (SSAC), built from a Fresnel prism, a thin barium borate BBO second harmonic generation crystal and a CCD camera, is installed for input pulse duration diagnostics. The size of the beam at the sample can be adjusted by a lens telescope (T). The light polarization plane and state can be modified according to the user requirements (WP). A green pilot laser (PL), installed on the breadboard, can assist the preparation of the experiment when the seed laser is used by the other beamlines. After all parameters (energy, beam size, divergence and polarization) are set, the IR laser pulse passes on a small insertion breadboard with focusing lens and beam pointing stabilization system, compensating the beam pointing drifts. A compact second and third harmonic generation setup is available as well. The third breadboard is placed at the output port of the chamber allowing for characterization of the transmitted or reflected beams after interacting with the sample.
For time resolved experiments, two optical laser paths have been conceived inside the vacuum vessel, impinging on the sample plane at an angle of 45° (red line) or 15° (orange dashed line), that can be switched without breaking the vacuum by inserting a movable Second Path (SP) mirror. Both the reflected beam for 45° geometry and the transmitted beam through a Si3N4 thin film or membrane for the 15° one can be used to determine the time jitter between the FEL and IR pulses.
Last Updated on Friday, 18 October 2019 15:16