Research Infrastructure

Primary laser sources

The research infrastructure at ELI-ALPS is based on four main laser sources: three operating in the 100 W average power regime in the near-infrared (NIR) and one 10 W mid-IR (MIR) source. These systems are designed to deliver pulses with unique combinations of pulse duration, repetition rate and pulse energy. The hallmarks of this next generation laser architecture is the use of sub-ps fiber oscillators; pulse amplification in fibers and 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.

Each lasers had been optimised for different regions in parameter space.

HR – High Repetition Rate system

This system is designed to produce sub 2 cycle (~6 fs) laser pulses with a 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 stabilised fiber oscillator centred 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.

The output of the frontend is split and parallelly amplified in 8 LMA fibers. Each channel is pumped by high power laser diodes, resulting in pulse amplification 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.

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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%

The working parameters of the High Repetition Rate lasers Phase 1 and 2.

SYLOS – Single cycle laser

The SYLOS single cycle laser system will create XUV and soft X-ray attosecond pulses by means of high-harmonic generation in gas and solid 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 industrial standards for reliability, tunability and stability of pulse energy, pointing direction and carrierenvelope phase. These features are highly critical as the SYLOS system will have the highest user demand when the final specifications of 20 TW, sub-2 cycle pulses are obtained. 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, centred at 880 nm.

The ultrashort pulses are initially generated in an Yb:KGW oscillator, synchronized to the facility clock, and provides seeds for both the signal and the pump laser chain. These weak seed pulses enter the front-end and obtain µJ-level pulse energy and a very broad spectrum (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 of SYLOS is ensured by an advanced diode-pumped Nd:YAG pump system [**] that drives a sequence of NOPCPA stages [***]. This technology allows the easy tuning of the pulse’s central wavelength and the tailoring of resulting spectrum by 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.

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Parameters

SYLOS 1

SYLOS 2 (targeted)

     

Peak Power

>4.5 TW

20 TW

Pulse duration

<4 cycles (<10 fs)

<2 cycles (<5 fs)

Central 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

     

SYLOS laser in Phases 1 and 2.

HF – High Field laser

The HF laser is a dual arm system delivering laser pulses of a Petawatt (PW) peak power. The main HFPW 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 at 100 Hz.

The HF frontend is designed to deliver two pulsed beams that will seed power amplifiers. It utilizes a combination of modern laser technologies including Ti:Sapphire, fibers , nonlinear optics and OPCPA. The device will generate millijoule energy and support 10 fs pulses with a high temporal contrast.

A sub-picosecond fiber-based pump laser initially generates 2 mJ 1030 nm pulses and a small fraction 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 gain narrowing effects. 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. 

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

     

 

The working parameters of the HF-PW and HF-100 lasers.

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

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. MIR will operate at 100 kHz repetition rate and deliver 15 W of average power with sub 4 cycle radiation centered at 3.1 µm. In addition, the output will be tunable between 2.4 µm and 3.9 µm whilst providing unmatched CEP (Carrier-Envelope Phase) stability as well. This laser will be synchronisable with other primary lasers and is ideal for coincidence experiments.

An OPCPA chain lies at the heart of the MIR laser system, pumped by a powerful commercial Ytterbium doped thin disk laser operating at 100 kHz repetition rate. This

pump laser generates extremely high quality 2 mJ, 1 ps pulses (200 W) in a thin disk regenerative amplifier. A small fraction of the output is converted into an ultra-broadband continuum which is further tailored and combined with the pump in a difference frequency generation (DFG) stage. The resultant 3.1 µm idler beam is amplified further in subsequent OPA stages to reach the target pulse energy. Finally compression in bulk material provides few cycle mid-IR pulses with excellent quality and stability [N].

A third, non-CEP stable beam (1.4 µm -1.75 µm) is generated as a complementary product of three wave mixing during parametric amplification. This output can be compressed below 150 fs with an energy comparable to the MIR idler wave (although not CEP stable) providing a complementary radiation source for pump-probe experiments.

 

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Schematic block diagram of the MIR laser.

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

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

   

Working parameters of the MIR laser.

 

Attosecond pulses

The primary focus of ELI-ALPS is the generation of high quality XUV attosecond pulses that set new working standards in terms of pulse energy, repetition rate and photon energy. This goal is only possible with the highest quality primary sources and expertly designed, innovative high-harmonic beamlines. ELI-ALPS will focus on two methods for generating high fluxes of attosecond pulse trains (APT) and isolated attosecond pulses (IAP) using Gas or Solid target High Harmonic Generation, GHHG and SHHG respectively. The following Table shows the predicted performance of the ELI-ALPS attosecond secondary sources and compares them with current competitive 

 

Rep. rate (Hz)

Pulse duration (fs)

Pulse energy (μJ)

Peak power (GW)

Tuning range (eV)

           

Synchrotrons

≥ 106

102

≈ 10-9

≤ 10-9

10-3 - 105

SASE FEL #

1-5000

30 - 300

1 - 500

0.03 - 16

28 - 295

Seeded FEL *

10

≈ 100

20 - 100

0.2 – 1

12 - 60

GHHG

103 - 105

0.07 - 0.5

0.01

≤ 10-3

10 - 120

 

10 - 100

0.07 - 0.5

0.1 - 10

10-3 - 1

10 - 120

GHHG HR 

105

≤ 0.5

≤ 10-3

≈ 0.002

17 - 90

GHHG SYLOS 

103

≤ 0.5

≈ 1

≥ 2

10 - 70

SHHG SYLOS 

103

≤ 1

≤ 3

≤ 3

8 - 60

           

 

Comparison of the main working parameters of current XUV pulse sources and the ELI-ALPS attosecond beam lines.

 Estimated from peak brilliance relative to FELs. ##Values for FLASH, *Values for FERMI at ELETTRA.  Guaranteed values, improved operating values expected.

Gas High Harmonic Generation – GHHG

Gas High Harmonic Generation is a well-established method for producing attosecond XUV pulses by focussing a IR laser pulse into a gas-cell or gas-jet, filled with noble gases. However, there are numerous experimental and technical challenges which need to be overcome in order to fully harness the outstanding pulse energy and repetition rates of ELI-ALPS primary lasers. One of the major problems is the ionization of the gas target which limits the permissible peak intensity of the driving pulses.

At ELI-ALPS, four beamlines will be dedicated to GHHG beamlines: Two driven by the SYLOS laser; the others by the HR laser. All four GHHG beamlines will be available for user experiments and to attosecond source researchers so that users’ needs can be accommodated whilst maintaining and developing cutting-edge source technology. Both SYLOS beamlines will use loose focusing to maximize the efficiency of attosecond pulse generation. This results in beamline lengths of tens of meters, based on optimisation studies. The LONG beamline will use an extremely loose laser focusing arrangement in conjunction with a long gas cell running at a very low gas pressure. The COMPACT beamline also uses loose focusing but uses a higher pressure, short medium to ensure phase matching by the short interaction length. Both systems utilise multiple generation regions to increase the output power by quasi phase matching. Unconverted IR laser light in these systems will be removed by using long propagation lengths for the LONG beamline; radial structuring of the generating laser beam in the COMPACT beamline and XUV reflecting, IR transmitting silica plates.

The Phase 1 SYLOS laser configuration results in pulses which are too long for IAP generation and require a gating technique to confine the XUV generation process to a single half cycle of the driving field. However, this may become redundant in the second implementation phase of SYLOS when the pulse duration is less than 2 optical cycles. Table XX shows the main characteristics of the SYLOS generated attosecond laser pulses.

    

Schematic design layout of the GHHG SYLOS COMPACT beam line.

The HR beamlines have modest pulse energies but the high repetition rate results in very high average power levels on the optical components. Custom optics with cooling elements have to be used which avoid CEP fluctuations as well as group delay dispersion (GDD).

 

         

Phase 1

Phase 2

 

Trains of attosecond pulses

Isolated attosecond pulses

Trains of attosecond pulses

Isolated attosecond pulses

         

Spectral range (eV)

17-30 eV (generating gas: xenon or krypton, aluminium filter)

Output energy at the end station interaction point (pJ)

15-50

5-15

85-250

25-90

Spectral range (eV)

25-55 eV (generating gas: argon, aluminium filter)

Output energy at the end station interaction point (pJ)

5-25

3-8

35-125

10-35

Spectral range (eV)

70-90 eV (generating gas: neon, zirconium filter)

Output energy at the end station interaction point (pJ)

3-10

1-3

15-45

4-15

 Predicted output pulse parameters for the GHHG SYLOS LONG beam line

Surface Plasma High Harmonic Generation – SHHG

High harmonic generation during the relativistic interaction of an intense ultrashort, high temporal contrast laser pulse with surface plasma can produce attosecond pulses in the reflection mode. The rising edge of an intense (I > 1017 Wcm -2) ultrashort laser pulse can form a thin and highly reflective plasma layer (Plasma mirror, PM) when interacting with an optically polished solid target. If the incident pulse is polarized in the plane of incidence, the main part of the pulse periodically drives and nonlinearly interacts with the plasma mirror. The PM induces periodic temporal spikes in the reflected light field due to periodic relativistic electronic dynamics.