PHOTON SCIENCE

Ultrafast optical laser systems are available to users at both FLASH experimental halls. Those lasers provide ultrashort pulses in a wide wavelength range to be used for pump-probe experiments at the experimental endstations. They are independent light sources with respect to FLASH and hence require a proper synchronization with the FEL pulses. All those laser systems are synchronized to the FEL master clock (Master Laser Oscillator) by means of optical cross correlators and RF modulators. This technology alone guarantees an overall jitter between the laser and FEL pulses in the experimental hall in a range from 10 up to 40 fs. Recent developments, to monitor the actual difference in the arrival time of the FEL (Beam-Arrival-Monitor, BAM) and the laser (Laser-Arrival-Monitor, LAM) at the experimental end-station, allow the users to further compensate for the jitter during the data analysis phase offline from the experiment. The use of the optical cross-correlators for synchronization allows to control the relative temporal delay between the FEL and the laser pulses in a range of several nanoseconds with a resolution of few femtoseconds.

FLASH1 pump-probe laser systems 

In the FLASH1 experimental hall there is one laser system currently available and one under construction. The present laser system [1] serves as pump-probe laser for the PG end-stations (both PG1 and PG2). It consists of a fiber frontend laser which is used to feed a free space amplifier baser on Yb:YAG crystals with a central wavelength of 1030 nm. A fundamental feature is the burst-mode operation, mimicking the bunch patter of FLASH. It can produce 800 µs long bursts with a maximum intra-burst repetition rate of 1 MHz (hence allowing up to 800 pulses per burst) at 10 Hz burst repetition rate. The intra-burst repetition rate and the actual number of pulses per burst can also be selected as needed for the experiments. This laser system is the first facility laser which combines the high throughput of Yb:YAG amplifiers with the novel spectral broadening technology based on a Herriot type Multi-Pass-Cell: the subsequent pulse compression allows to have pulses as short as 60 fs in the fundamental (1030 nm) The laser is highly automated and remotely controlled. Feedback and controls allow to precisely shape the burst profile in terms of pulse intensity (variance less than 5% intra-burst) and temporal profile (pulse duration of the fundamental can be varied between 60 to 300 fs). Harmonic conversion modules allow the use of pulses with central wavelengths of 515 nm, 343 nm and 257 nm. Please refer to the table below for more details.
The new laser system which is planned to serve the new FL11 beamline (replacing BL3) is currently under construction and will be available starting from mid-2026. The new laser system will serve BL1/CAMP end-station in later stage.

FLASH2 pump-probe laser system

The FLASH2 pump-probe laser system is based on optical parametrical chirped pulse amplification (OPCPA) and novel spectral broadening technology based on a Herriot type Multi-Pass-Cell [2]. The OPCPA technology grants higher flexibility in pulse properties than conventional Ti:Saphire or Yb:YAG femtosecond lasers whereas the MPC technology grants up to 1 mJ pulse energy in the interaction region. The OPCPA serves the FL24, and FL26 beamlines in FLASH2. Depending on user request the laser can be prepared either in a short pulse operation mode using the full amplification bandwidth between 700 and 900 nm to produce sub-20 fs pulses or in a long pulse operation mode with reduced spectral bandwidth and pulse duration in the order of 40 fs up to 1 ps chirped (FWHM). In that case the central wavelength can be tuned between 700 and 900 nm. The wavelength stability is better than 5% of the bandwidth. Harmonic wavelength conversion modules (central wavelength 400 nm, 267 nm) are available at the FL24 endstation. Also, the second and third harmonic radiation can be tuned in central wavelength, please contact us for details. The laser provides a pulse pattern matched to the FEL pulse pattern with up to 80 pulses at 100 kHz repetition rate per 10 Hz pulse train, thus up to 800 pulses per second. The pulses can be divided in two different bursts and delivered to two end-station contemporarily allowing parallel pump-probe experiments. Lower pulse repetition rates within the 800 µs pulse train length are also possible. The laser delivered to the FL26 beamline can also be used to generate high harmonics in noble gases (HHG) in a wavelength range from 35 to 120 nm used for pump probe experiments [3]. The MPC output of the laser serves the FL23 beamline. Depending on user request the laser can be prepared either in a short pulse operation mode from 70 fs up to 3 ps chirped. The wavelength stability is better than 5% of the bandwidth. Harmonic conversion modules provide output at central wavelength 515 nm, 343 nm, and 257 nm, details and conversion efficiencies are reported in the table below.

Nominal specifications of all laser systems are listed in the table below.

Nominal parameters of the optical pump-probe lasers

CONTACT

Ingmar Hartl E-Mail: Ingmar Hartl Phone: +49 (0)40 8998 (9)2863 Location: 25B / 119

References:

[1] M. Seidel et al., Ultrafast MHz-Rate Burst-Mode Pump–Probe Laser for the FLASH FEL Facility Based on Nonlinear Compression of ps-Level Pulses from an Yb-Amplifier Chain, Laser Photonics Rev. 16, 2100268 (2022);
https://doi.org/10.1002/lpor.202100268

[2] A.-L. Viotti et al., Multi-pass cells for post-compression of ultrashort laser pulses, Optica 9, 197-216 (2022); https://doi.org/10.1364/OPTICA.449225

[3] E. Appi et al., Synchronized beamline at FLASH2 based on high-order harmonic generation for two-color dynamics studies, Rev. Sci. Instrum. 92, 123004 (2021); https://doi.org/10.1063/5.0063225