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Beamline Description

Laser emission source for LDM experimental station  

The seed pulse starts from a femtosecond Ti:Sapphire oscillator (Vitara, Coherent) which is repetition rate locked to the reference timing signal distributed over the facility. The main part of the oscillator output beam is amplified in a chirped-pulse regenerative amplifier-single pass amplifier tandem, which delivers pulses with an energy of up to 7 mJ and duration about 100 fs at wavelength 784nm (14 nm FWHM).
after leaving the Ti:Sapphire amplifier the IR pulse is split and about 70% of its energy is used to pump an OPA (optical parametric amplifier OPerA-Solo, Coherent) and for THG (third harmonic generation).  UV emission from OPA or THG is used as seed for FEL operation. In the other arm, the remaining IR pulse energy (typically about 1.5 mJ) propagates to the experimental stations along a total distance of about 150 m. The beam propagation to the beamlines preserves a very good beam quality, and introduces a slight positive chirp and pulse lengthening to about 120 fs due to material dispersion. The duration of the transported pulse can be varied in the range 80-250 fs range by a compact transmission grating pulse compressor installed at the end of the beam transport system.

 

LDM beamline optical setup description 

The optical setup used to prepare and deliver the external laser pulse for pump-probe experiments at the LDM end-station includes two optical breadboards which are used for laser beam manipulation, insertion into the chamber and diagnostics. The breadboards are covered with protective boxes and filled with nitrogen to prevent dust contamination if optical components. CCD1 controls the position and spatial parameters of the incoming beam after the vacuum window W1 and is incorporated in the beam transport system. The motorized mirror mount M1 and CCD2 allow to control the laser beam direction of propagation on the optical breadboard. An optical attenuator, made of half waveplate (l/2 @ 784 nm)WP1 and two thin film polarizers (PL1, PL2), allow adjustment of the exact pulse energy delivered to the region of interaction.  A translation stage TS1 (PI M403.8PD) provides a variable pulse delay relative to the FEL pulse in the range of +-570ps with minimum step delay of 1.6 fs. The setup includes a high-bandwidth copper coax cable based antenna. A directly at crossing position of laser and FEL beam which allows to estimate time delay between laser and FEL pulses with fast oscilloscope (typical resolution of pulse position less than 50 ps). The fast photodiode (PD) signal is used as a trigger source for the oscilloscope in this case.   A compact single-shot auto-correlator AC, build from Fresnel prism, thin BBO SHG crystal and CCD3 camera, is installed for input pulse duration diagnostics. It is possible to change laser wavelength to 392 nm or to 261 nm by generating second harmonic (SH) or third harmonic (TH)  in BBO crystals,. In the TH case there are additional calcite time delay plate TDP and double wavelength waveplate (l/2@780 nm,l@390 nm) WP2. Interference filter F1 (HT@390 nm, HR@780nm) is used to select for operation only second harmonic emission when required. Set of four mirror with HR@260 nm, HT@390 nm HT@780nm is used as filter F2 to select only the TH UV emission. Polarization state control of the laser light arriving in the chamber is performed by using rotation stage RS2 and flipper FL2, where there are installed waveplates. Half wave waveplates WP l/2 are installed on rotation stage RS2 andquarter wave waveplates WP l/4 are installed on flipper FL2. This design provides possibility to provide horizontal/vertical linear polarization or left/right circular polarization of the emission for any operating wavelength (changing set of l/2 and l/4 waveplates). Energy meter heads EM1 and EM2 allow to control pulse energy delivered in region of interaction. Optical system including three lenses L3, L4, L5 provide focusing laser beam to a spot diameter down to 80 μm (at level 1/e2). The lens L3 is mounted on motorized translation stage TS2 (Standa 8MT173-25), that allows to keep focus position at switching between the above mentioned of operation wavelengths or to increase beam diameter up to 500 μm (at level 1/e2). CCD cameras CCD4 and CCD5 are used as virtual focal points and allow controlling beam size and position in the interaction region. CCD4 is used for operation at fundamental (784 nm) and second harmonic (392 nm) wavelength. CCD5 and UV-visible converter UVC are used for operation at third harmonic (261 nm) wavelength. Motorized mount M2 with incorporated piezo Tip-Tilt mirror allow to precisely scan the beam position or used as pointing stabilization system with beam position stability (rms) less than 5μm.  IR pilot laser IRPS is installed on the breadboard to assist the preparation of the experiment when the SLU is used by other beam lines at Fermi. 
 

Main parameters (see also here)

Parameter Value

Laser emission wavelength (nm)

784 nm
392 nm
261 nm

Maximum pulse energy

750 μJ @ 784 nm
100-200 μJ @ 392 nm
20-50 μJ @ 261 nm

Pulse energy stability (rms)

<0.5 % @ 784 nm
<1.0 % @ 392 nm
<1.0 % @ 261 nm

Pulse duration, fs ( FWHM)

80-250 @ 784 nm
80-120 @ 392 nm
120-170 @ 261 nm

Pulse repetition rate

1-10 Hz, 50 Hz

Timing jitter relative to FEL (rms)

<10 fs

Scan range pulse delay relative to FEL 

-570…+570 ps

Minimal step delay

1.6 fs

Beam diameter at region of interaction (at level 1/e2)

80-500μm

Beam position stability (rms)

<3μm


Last Updated on Wednesday, 17 June 2015 16:06