Research Technology

Research Technology

Lasers

Primary laser sources

The research infrastructure at ELI-ALPS is based on four main laser sources: three operating in the regime of 100 W average power in the near-infrared (NIR) and one at 10 W in the mid-IR (MIR). These systems are designed to deliver pulses with unique parameter combinations of pulse duration, repetition rate and pulse energy. Characteristic for this next generation laser architecture is the use of sub-ps fiber oscillators, pulse amplification in fibers, white light generated seeding pulses, which exhibit passive carrier-envelope phase (CEP) stability. Each laser will run synchronized to the central facility clock and is guaranteed to run continuously for at least 8 hours per day. There will be four lasers optimized for different regions in parameter space.

HR – High Repetition Rate system

This system is designed to produce sub 2 cycle (~6 fs) laser pulses at 100 kHz repetition rate with TW peak power at 1030 nm. This system will be used to drive two gas high harmonic secondary sources. The laser uses a carrier envelope phase stabilized fiber oscillator centered at 1030 nm at 80 MHz repetition rate, which is subsequently reduced to 100 kHz by acousto-optic modulators. These pulses are then pre-amplified, to an average power of 20 W, using large mode area (LMA) fiber amplifiers (65 µm, 1 m) and finally stretched to 2 ns by a double pass grating stretcher.

Figure 1: Schematic layout of the HR laser

The output of the frontend is split and parallel amplified in 8 LMA fibers each channel pumped by high power laser diodes resulting in pulse amplification of up to 60 W in each fiber. The resulting pulses are then coherently combined and compressed close to the transform limit (~200 fs). The pulses are then compressed below 2 cycles by two stages of hollow core fiber compressors, filled with noble gases, followed by chirped mirrors.

Table 1

Parameters

HR 1

HR 2 (targeted)

Center wavelength λc

1030 nm

1030 nm

Repetition rate

100 kHz

100 kHz

Average power

>100 W

>500 W

Pulse energy

>1 mJ

>5 mJ

Pulse duration (@ λc)

<6.2 fs (<1.85 cycles)

<6.0 fs (<1.8 cycles)

Output energy stability

<0.8% (rms)

<0.8% (rms)

Beam quality (Strehl ratio)

>0.9

>0.9

CEP stability

<100 mrad (rms)

<100 mrad (rms)

Beam pointing instability

<2.5% (diffr. limited div.)

<2.5%

 

SYLOS – Single cycle laser

The SYLOS single cycle laser system drives the emission of XUV and soft x-ray attosecond pulses through the process of high-harmonic generation in solid and gaseous media. The first version of this laser system currently generates 4.5 TW peak power, sub-10 fs duration pulses at a repetition rate of 1 KHz. SYLOS has already set new standards for the industry in terms of reliability, tunability and stability of pulse energy, pointing direction and carrier-envelope phase. These features are highly important as SYLOS will face the highest demand from users at ELI-ALPS when the final specifications, namely 20 TW, sub-2 cycle pulses are reached. At the heart of the SYLOS laser is the state-of-the-art non-collinear optical parametric chirped pulse amplification (NOPCPA) chain which amplifies a white light seed using a picosecond pump laser chain to generate sub 10 fs pulse centered at 880 nm.

Schematic layout of the SYLOS laser

The ultrashort pulses are initially generated in an Yb:KGW oscillator, which is synchronized to the facility clock and provides seeds for both the signal and the pump laser chain. In the next step, these weak seed pulses enter the front-end, where they reach µJ-level pulse energy and acquire a very broad spectrum (spanning from 600 nm to 1000 nm, centered at 880 nm) through a white-light generation (WLG) process. The front-end utilizes passive difference frequency generation (DFG), which provides excellent CEP-stabilization [*]. The combination of grism stretcher (gratings + prisms) and Dazzler (an acousto-optical modulator) ensures the matching of pump and signal pulse durations. The overall stability is primarily ensured by an advanced diode-pumped Nd:YAG pump system [**] that drives a sequence of NOPCPA stages [***]. This technology allows the central wavelength of the pulses to be easily tuned and the spectrum can be tailored by simply changing the pump pulse delay and the phase-matching angles of the NOPCPA crystals. The negatively chirped pulses are compressed using large aperture bulk glass blocks and then positively chirped mirrors under vacuum, ultimately yielding sub-10 fs, 45 mJ pulses at kHz repetition rate.

Table 2

Parameters

SYLOS 1

SYLOS 2 (targeted)

Peak Power

>4.5 TW

20 TW

Pulse duration

<4 cycles (<10 fs)

<2 cycles (<5 fs)

Center wavelength

880 nm (tunable)

900-1000 nm (tunable)

Repetition rate

1 kHz

1 kHz

CEP stability

<250 mrad

<200 mrad

Energy stability

<1.5%

<1.5%

Temporal contrast

> 1010

> 1010

Strehl ratio

>0.85

>0.85

PPL pulse duration

90 ps

<90 ps

HF – High Field laser

The HF laser is a dual arm system delivering laser pulses of a Petawatt (PW) peak power. The major, HF-PW arm will deliver high contrast, sub 20 fs, 2 PW pulses at 10 Hz, whilst the HF-100 arm will produce a slightly lower energy output of sub 4 cycle, 50 TW pulses but at 100 Hz. The common frontend seeding ensures that there is a high level of synchronization between the two laser arms.

Schematic layout of the HF laser including the HF-PW and HF-100 laser arms

The HF- frontend is designed to deliver two pulsed beams that will seed power amplifiers. It utilizes a combination of modern laser technologies that include Ti:Sapphire, fiber lasers , nonlinear optics and OPCPA. The device will generate millijoule energy and support 10 fs pulses with a high temporal contrast. The starting point is a sub-picosecond fiber-based pump laser generating 2 mJ 1030 nm pulses, a small fraction of which is used to generate whitelight. The remainder is used in a difference frequency generation stage, ensuring CEP stability. The output is subsequently frequency doubled, OPA amplified, recompressed then frequency doubled to reach the central wavelength of 800 nm. The beam is split to seed the two laser arms and the HF-100 seed is further amplified to ~ 2 mJ. Cross polarized wave generation (XPWG) is then used to ensure the highest possible temporal contrast of the seed.

The HF-PW power amplifier is based on Ti:Sapphire technology, which can support sub 20 fs pulses. Additional bandwidth correction is performed using additional spectral filters within the amplification stages to mitigate the gain narrowing effect. The final amplifier is pumped by two P60 lasers (60 J, 532 nm, 10 Hz). The output from the amplifier is recompressed, resulting in 34 J, 17 fs pulses with a repetition rate of 10 Hz.

The HF-100 arm, currently under design, will use a combination of OPCPA; in-house developed polarization encoded CPA and Ti:Sapphire thin disk technology.

Table 3

Parameters

HF-PW arm

HF-100 arm (targeted)

Peak Power

2 PW

50 TW

Pulse duration

<17 fs

<10 fs

Center wavelength

800 nm

800-850 nm

Repetition rate

10 Hz

100 Hz

CEP stability

NA

<250 mrad

Energy stability

<1.5%

<1.5%

ASE contrast

> 1011

> 1011

Strehl ratio

>0.85

>0.85

 

MIR – Mid-infrared laser

The MIR laser is a groundbreaking laser which is markedly different from the other ELI-ALPS primary lasers as it operates in a completely different window of the electromagnetic spectrum. The MIR will generate sub 4 cycle pulses centered at 3.1 µm at a 100 kHz repetition rate and with an average power of 15 W. The laser can be synchronized with other primary lasers and is ideal for coincidence experiments.

Schematic layout of the MIR laser

At the heart of the MIR laser system is an OPCPA chain which is pumped by two Ytterbium doped lasers operating at 100 kHz. These two pump lasers are seeded by a commercial femtosecond fiber oscillator and are synchronized to the ELI-ALPS master clock.

The first pump laser generates 200 µJ, 300 fs pulses from a 20 W fiber CPA (FCPA) system. A small fraction of the output is converted into an ultra broadband continuum, which is subsequently amplified in an OPA system. These pulses are then shaped and combined with the remaining output in a difference frequency generation device. The resultant 3 µm idler beam is combined in a OPA using the output of the second pump laser, a Yb-YAG CPA laser delivering 200 µJ, 300 fs pulse. Subsequent compression provides few cycle pulses close to the transform limit.

A third, non-CEP stable, beam (1.4 µm -1.75 µm) is generated as a byproduct of three wave mixing in the amplification process. This output could be compressed below 150 fs; would be energetically comparable to the MIR laser and could be a complementary radiation source for pump-probe experiments.

Operating wavelength region of the MIR laser (red shaded) compared to the absorption spectrum of air

Table 4 Working parameters of the MIR laser.

Parameters

MIR Laser

Center wavelength

3.1 μm

Average Power

15 W

Pulse duration

<4 cycles

Pulse energy

<150 μJ

Repetition rate

100 kHz

CEP stability

<100 mrad

Energy stability

<1.0%

Tunability

2.4 μm-3.9 μm

Strehl ratio

>0.5

 

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