The ELI ALPS HR-1 laser system is designed to produce sub-2 cycle, 1 mJ, CEP-stabilized laser pulses at 100 kHz repetition rate at 1030 nm. The system is an ytterbium fibre chirped-pulse amplifier system. The main amplifier uses 8 parallel (diode-pumped) large mode area photonic crystal fibre amplifiers, which are coherently combined to > 200 W and compressed to 280 fs. These pulses are then post-compressed in two subsequent multi-pass cells (MPC), first to < 35 fs and then to 6.2 fs. (Fig 1.) [1,2]
The HR-2 laser system is a five-times more powerful laser built on a similar architecture as HR-1. This system has a fibre CPA of 16 combined amplifier channels of upgraded design, capable of 10 mJ (260 fs) pulses at 100 kHz (1 kilowatt). The efficiency of the grating compressor has been significantly improved, too. HR2 is specified to provide 5mJ, 6 fs pulses. The development of the post-compression layout is still an ongoing work, but > 3mJ sub-10fs pulses have already been published .
Fig1. Schematic layout of the HR-1 laser.
Table 1 shows the output specifications of both laser systems. Currently the long pulse mode of HR1 is available for experiments at ELI-ALPS until 2021 Q1, when it will receive factory upgrades to reach the final specifications. HR2 is also in the last stages of development and it is expected to become available from 2021 Q2, while the full HR1 beam with the specified parameters is estimated to be available after 2021 Q3 for users.
Table 1. Parameters of HR 1 and HR 2 lasers as specified.
The average power of both systems can be lowered to circa 1 W, being able to provide a beam for pre-alignment of an optical setup. (In this pre-alignment case, the post-compression stages receive only minimum power, so the spectral bandwidth is limited.) Continuous power control of HR-1 (from 10% to 100% of power) without changing the other parameters is already available for the long pulse mode. Power control for the short pulse mode is under implementation.
Pulse duration can be slightly tuned by using lower power levels in the post-compression stages, resulting in longer pulses and a narrower wavelength range. Note that the refractive indices of all materials depend on the wavelength, and some mirrors may shift the spectral phase, too. This means that the shortest (few cycles) pulse duration will be only available in the plane of the experiment (i.e. focal plane). To achieve this, careful compensation of all optical elements is needed, including several pulse duration measurements and iterative optimization.
Due to the high average power, the optical and optomechanical elements of user experiments are prone to overheating (including heat generated by unintended reflections, leakage of dielectric mirrors, and scattered light). First of all, this has to be limited by blocking stray beams, and then, at the final target, the use of active pointing stabilization to the final target is recommended. Some optical elements can behave as a thermal lens. Some mirrors can locally heat up, then thermal expansion changes the surface curvature and thereby defocus the laser beam. This means that fine alignment may be necessary on the thermally stabilized experimental arrangement.
Fig. 2. Typical pulse duration and spectrum of the long pulse mode.
Fig.3. Typical D-scan trace of the HR-1 laser in the short pulse mode (6.05 fs measured pulse duration).
Fig. 4. Typical power stability and output beam profile of the HR-1 laser system.
1. Hädrich et al, Opt.Lett 41, 4332
2. T.Eidam et al, CLEO Europe 2017, (CJ_12_6).
3. T.Nagy et al, Optica Memorandum 6, p. 1423 (2019)