60 years of laser 60 years of laser 60 years of laser

Laser 1 Allegra: 100 milijoule, 1 kHz

Laser L1 Allegra vyvíjel laserový tým centra ELI Beamlines. Laserový systém je navržen pro generování sub20fs (dvacet miliardtin z miliontiny sekundy!) pulzů s energií vyšší než 100 mJ při vysoké opakovací frekvenci (1 kHz). Koncepce laseru je zcela založena na zesílení frekvenčně rozmítnutých pikosekundových pulzů v řetězci OPCPA (Optical Parametric Chirped Pulse Amplification), který se skládá z celkem sedmi zesilovačů. Jednotlivé stupně OPCPA zesilovačů jsou čerpány přesně synchronizovanými pikosekundovými pulzy, které generují špičkové tenko-diskové Yb:YAG laserové systémy.
[Podrobné informace jsou k dispozici v anglickém jazyce.]

The L1 Allegra system architecture

The L1 laser system consists of three main blocks: the front end, the booster, and the main amplifier as shown schematically in the block diagram below.

L1 front end

The pulses of 6 fs duration from the Ti:sapphire oscillator are stretched to picosecond duration (10e-12 seconds) and amplified in the first three stages of OPCPA from 1 nJ to 2 mJ (two million times). A particular technological challenge in OPCPA amplification of picosecond duration pulses lies in the precise synchronization (sub-15 fs) of the amplified pulse with the pump pulse. This is achieved first by generating the seed pulses at 1,030 nm for the pump laser from the same oscillator as the broadband pulses amplified in the OPCPA and then by active measurement and compensation of the time difference between the pulses. A cross-correlator, nicknamed “JITKA,” has been developed for this purpose; for more details and to find out about performance see [1].

Another complex part of the front end is the pump laser-generating 16 mJ/1.6ps pulses at 515 nm. This pump laser consists of a fiber Bragg grating stretcher, a 1 kHz repetition rate Yb:YAG thin-disk-based regenerative amplifier, a multi-layer dielectric (MLD) grating compressor, and a lithium triborate (LBO) second harmonic generator.

The front end includes an accurate electronic timing system (ETS) and is designed to phase-lock the Ti:sapphire oscillator to the facility’s master clock with femtosecond precision. This feature allows for accurate synchronization with other lasers in the facility and a fully electronically controlled delay relative to the facility clock.

The booster

The 2 mJ pulses from the front end are further amplified in stage 4 of the OPCPA to pulse energies at the 10 mJ level. This amplification stage is pumped by another pump laser that has been developed in house with an output energy at 1,030 nm exceeding 100 mJ. These pulses are then compressed to 1.8 ps, converted to 515 nm, and synchronized with the broadband pulse using another “JITKA” cross-correlator.

The main amplifier

The high energy broadband pulses from the booster are then stretched further to 3 ps duration and transported in a vacuum to the main in-vacuum OPCPA amplifier, which consists of stages 5, 6, and 7 designed to reach output energy in excess of 100 mJ. The amplified broadband pulses are then compressed to below 20 fs using an array of chirped mirrors. Three commercial, high power thin-disk-based Yb:YAG amplifiers are used as part of the pump laser system for the main amplifier, with each beam compressed to about 3 ps in MLD grating compressors housed in a single vacuum vessel.

L1 beam line parameters

Designed parameters Actual parameters
Output pulse energy 100 mJ 20 mJ
Pulse duration <20 fs 15 fs
Repetition rate 1 kHz 1 kHz
Central wavelength 830 < λ < 860 nm 750 – λ – 920 nm
Output beam polarization Linear; horizontal; 100:1
Pump laser technology DPSSL
Output laser pulse energy RMS stability Better than 5% rms
Output laser beam RMS pointing stability < 10 µrad
Laser control system Labview and EPICS
Output pulse external synchronization relative to the facility clock <20 ps to RF clock with any delay relative to facility clock
Beam format circular, single aperture, Gaussian-like circular, single aperture, Gaussian-like
Beam diameter 27.5 mm at 1/e2 27.5 mm at 1/e2
Electrical power consumption <100 kW

References:

[1] Optics Express, Vol. 22, Issue 24, pp. 30281–30286 (2014)

Dr. Pavel Bakule, pavel.bakule@eli-beams.eu

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