Research Infrastructure

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.

 

 

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.

 

Figure 2: 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.

 

 

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.

 

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.

 

 

Table 4 Working parameters of the MIR laser.

Parameters

MIR Laser

Center wavelength

3.1 Î¼m

Average Power

15 W

Pulse duration

<4 cycles

Pulse energy

<150 Î¼J

Repetition rate

100 kHz

CEP stability

<100 mrad

Energy stability

<1.0%

Tunability

2.4 Î¼m-3.9 Î¼m

Strehl ratio

>0.5

 

 

Attosecond pulses

The primary focus of ELI-ALPS is the generation of the best quality XUV attosecond pulses to set new working standards in terms of pulse energy, repetition rate and photon energy. This goal is only achievable 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. Table 5 shows the predicted performance of the ELI-ALPS attosecond secondary sources and compares them with current competitive methods.

Table 5: Comparison of the main working parameters of current XUV pulse sources and the ELI-ALPS attosecond beam lines
  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

 

* 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 to produce attosecond XUV pulses in which an IR laser pulse is focused 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 in various studies and will also serve attosecond source research aims to accommodate to user needs and to maintain the the cutting-edge property of the technology. Both SYLOS beamlines will use loose focusing to maximize the efficiency of attosecond pulse generation resulting in beamline lengths of several tens of meters, based on optimization 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 whilst the COMPACT beamline also uses loose focusing but instead a higher pressure, short medium to ensure phase matching by the short interaction length. Both systems utilize 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 (LONG) and radial structuring of the generating laser beam (COMPACT) as well as XUV reflecting and 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 measure may become redundant in the second implementation phase of SYLOS (Phase 2) when the pulse duration falls below 2 optical cycles. Table 6 shows the main characteristics of the attosecond pulses generated from SYLOS.

    

 

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).

 

Table 6: Predicted output pulse parameters for the GHHG SYLOS LONG beamline

 

 

Phase 1

Phase 2

Attosecond

 pulse

trains

Isolated 

attosecond

 pulses

Attosecond

 pulse

trains

Isolated 

attosecond 

pulses

Spectral range (eV)

17-30 eV (generating gas: xenon or krypton, aluminum 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, aluminum 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

 

Surface Plasma High Harmonic Generation – SHHG

The high harmonic generation during the relativistic interaction of an intense ultrashort, high temporal contrast laser pulse with surface plasma provides a pathway to generate 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.

There are currently two well-understood competing mechanisms for SHHG where their relative predominance is dependent on the driving field intensity and the sharpness of the driven plasma mirror. Coherent Wake Emission (CWE) involves surface electron bunches being pulled into the vacuum by the combined laser/plasma field then pulled back into the overdense plasma. This generates excited charge density waves across the plasma density gradient in the wake of the returning electron bunches. These plasma oscillations emit the subcycle light pulse once every optical cycle and thus both the odd and even harmonics of the incident laser beam are generated. At higher laser intensities (Iλ2 â‰³ 1018 Wcm-2μm2), the driving laser field causes the electronic motion to become relativistic. The relativistic oscillation of the reflecting plasma mirror (the Relativistic Oscillating Mirror or ROM) surface leads to periodic Doppler shifts in the local reflected light field leading to the generation of phase locked ROM harmonics. Both the mechanisms have their relative advantages, leading to attopulses with complementary nature to those generated from gaseous media at substantially lower light intensities.

The complementary nature of SHHG and GHHG are shown in Table 7.

Table 7: Summary of the complementary nature of high-order harmonics generated from gaseous medium and via relativistic excitation of a plasma mirror, in the case of linearly polarized single color laser field

Properties

Gas HHG

Surface HHG

HHG spectrum

Only odd harmonics

Both odd and even harmonics

Attosecond bursts per cycle

Twice in every cycle and Ï€ out of phase

Once in every cycle and in phase

HHG Cut-off & intensity

Limited by saturation intensity. Harmonic intensity and cut-off drops after saturation intensity 

No such limit. Harmonic intensity scales up with laser intensity & harmonic cut-off either depends on the material (CWE) or increases with laser intensity (ROM).

Phase matching

GHHG occurs in transmission mode. Proper phase matching during propagation is needed for intense harmonics. Harmonic properties depends on phase matching condition 

SHHG takes place in the reflection mode. Phase matching condition is intrinsically satisfied 

Spatio-temporal coupling (STC)

HHG emission is fundamentally space-time coupled irrespective of the phase matching conditions 

There is no fundamental STC in SHHG emission 

HHG as a probe of the source

GHHG are being used to extract information about atomic, molecular and condensed matter structure and dynamics

SHHG have been used to extract information on attosecond electron dynamics in Î¼-scale plasma

 

At ELI-ALPS, there will be two beamlines developed to tap in the potential of SHHG: One driven by the SYLOS laser and the other by the HF laser. Both the beamlines would be development beamlines pushing the frontiers of state of the art technology in high intensity laser matter interaction and XUV science at relativistic intensities.

The SHHG SYLOS development beamline will be able to explore both CWE and ROM SHHG regimes by changing only the focusing and plasma conditions. A “low contrast mode” uses soft focusing to ensure that the incident laser intensity is below the ROM threshold (1018 Wcm-2) whilst a “high contrast mode” uses an additional plasma mirror to enhance the pulse contrast and this “clean” pulse is then tightly focused onto the target to reach intensities ~ 10­19 – 1020 Wcm-2. Figure 3 shows the proposed SYLOS SHHG beamline.

 

The capability to finely tune the plasma mirror density profile and the few cycle laser phase, allows for sub cycle control of the generated attopulses. The SHHG SYLOS would also be the first SHHG beamline working at kHz repetition rate, while working at relativistic intensities.

 

Table 8

Attosecond pulse specs

SYLOS Phase 1

SYLOS Phase 2

APT

SAP

APT

SAP

Spectral range*

8–40 eV

6–20 eV

8–60 eV

6–40 eV

Pulse energy at the SHHG source*

1–10 µJ

0.3–3 µJ

3–30 µJ

1–10 µJ

Pulse energy at the end station interaction region*

0.3–3 µJ

0.1–1 µJ

1–10 µJ

0.3–3 µJ

Pulse duration at the end station interaction region*

10 fs

1 fs

5 fs

1 fs

Spot size at the end station interaction region

10 µm

10 µm

10 µm

10 µm

Beam divergence emitted from SHHG source

1 rad

Polarization state

linear, horizontal

 

* Estimations based on single-shot long-pulse high-energy laser experiments

The SHHG HF development beamline will be able to operate at on target laser intensity of
10­21 – 1022 Wcm-2  exploring the extremely relativistic domain of laser plasma interaction where the efficiency of ROM SHHG process and its spectral window scales up, potentially giving access to more intense and shorter attopulses.

The combination of subtle control of the plasma mirror density profile, state of the art feedback stabilized high repetition rate targetry and a temporally and spatially high contrast on focus PW laser allows for opportunities of unique user experiments after the development phase. The SHHG SYLOS would also be the first SHHG beamline working at 10 Hz repetition rate, while working with a PW class ultrashort laser.

Finally the facility is equipped with its own electrical-, mechanical- and optical workshop in order to manufacture adequate custom components supplying the wide variety of experiments needs.