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Performance comparison of Yb:YAG ceramics
and crystal gain material in a large-area,
high-energy, high average–power diode-pumped
laser
M. D IVOKY, 1,* J. P ILAR , 1 M. H ANUS , 1 P. N AVRATIL , 1 M.
S AWICKA -C HYLA , 1 M. D E V IDO, 2 P. J. P HILLIPS , 2 K.
E RTEL , 2 T. B UTCHER , 2 M. F IBRICH , 3 J. T. G REEN , 3 M.
K OSELJA , 3 J. P RECLIKOVA , 4 J. K UBAT, 4 J. H OUZVICKA , 4 B. R US , 3
J. C OLLIER , 2 A. L UCIANETTI , 1 AND T. M OCEK 1
1 HiLASE Centre, Institute of Physics of the Czech Academy of Sciences, Za Radnici 828, 25241 DolníBřežany, Czech Republic
2 Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK
3 Extreme Light Infrastructure-Beamlines, Institute of Physics of the Czech Academy of Sciences, Za Radnici
835, 25241 Dolní Břežany, Czech Republic
4 CRYTUR, spol. s r.o., Na Lukách 2283, CZ-511 01 Turnov, Czech Republic* divoky@fzu.cz

Abstract: We compare for the first time the influence of different Yb:YAG gain media on theperformance of a large-area, high average–power laser system with an output energy of up to 6 J.
Monocrystalline slabs grown by a new technique without central growth defect are comparedwith ceramics. Small signal gain, maximum output energy and thermal lensing are comparedfor ceramic slabs with co-sintered amplified spontaneous emission (ASE) absorber cladding,monocrystalline slab with and without optically bonded ASE absorber cladding, and surface
structured monocrystalline slabs. We show that these large monocrystals with optically bonded
absorber cladding have similar performance to cladded ceramics, so far the only material forhigh-energy Yb:YAG lasers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1.

Introduction

High-energy, high average–power (HE-HAP) diode pumped laser systems offer unprecedented
energy stability and faster processing speeds in comparison to the more established flash lamp
pumped systems. The parameters of such lasers open up new application areas such as advanced
material processing and laser shock peening [1,2], large area laser induced damage threshold
testing, and pumping of high repetition rate ultra-high intensity femtosecond petawatt laser
systems [3].
Several geometries of such HE-HAP systems have been previously considered, but as of today,
only the multi-slab laser architecture has delivered pulse energies above 50 J at a 10 Hz repetition
rate. The principle of a multi-slab laser system is to divide the gain medium into multiple slabs
which are stacked longitudinally with gaps between within the laser head. A coolant flows
between the slabs, allowing efficient heat extraction from the gain media and higher accessible
average powers. The Mercury project demonstrated 61 J at 10 Hz using Yb:S-FAP crystal gain
media at room temperature [4]. The HAPLS project demonstrated 97 J at 3.3 Hz using Nd:APG-1
glass as gain medium [5]. It should be noted that HAPLS will achieve a much higher energy at
a 10 Hz repetition rate in the future, but it has not been demonstrated yet. The Bivoj/DiPOLE
system demonstrated 105 J at 10 Hz using cryogenically cooled Yb:YAG ceramics as the gain

#379713
Journal © 2020

https://doi.org/10.1364/OE.379713
Received 15 Oct 2019; revised 10 Jan 2020; accepted 15 Jan 2020; published 27 Jan 2020

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medium [6]. There are many more diode pumped laser systems around 10 J and 10 Hz level[7,8]. All above mentioned energies were obtained for nanosecond pulses.
The large aperture of the gain media used in such lasers limits the material choice to glassor ceramics. Also, high energy diode pumped systems suffer from amplified spontaneousemission (ASE) that decreases the gain of the system. To counter this, Sm or Cr doped ASE
absorbing cladding is applied to the gain medium to prevent reflections of spontaneously emitted
photons from the edges of the gain medium. Recently, a new method for growing large scale
Yb:YAG crystals with clear aperture larger than 140 mm was introduced by a crystal growing
company Crytur [9] and, together with optical bonding of Cr:YAG absorber onto the crystal, such
crystals slabs can be used for high energy high repetition rate laser systems. Monocrystals allow
depolarization control due to uniform crystal orientation [10]. Due to polycrystalline nature ofceramics, such control is not possible and therefore the high power laser community still puts
a lot of effort into monocrystals development. In this article, we use the first cryogenic mainpre-amplifier of the Bivoj/DiPOLE capable of delivering pulses with energy up to 10 Joules todirectly compare multiple Yb:YAG sets of gain media from ceramics or monocrystals to ourknowledge for the first time at these high energy levels and repetition rates. Output energy for
Yb:YAG crystal without ASE absorber and ceramics with ASE absorber was compared at 14 J
and 2 Hz at LUCIA [11] and small signal gain comparison for Nd:YAG crystal and Nd:LuAG
ceramics was performed at 10 Hz [12]. In addition to absorption-based ASE countermeasures,
we also tested a prototype one based on surface structuring of the gain media and compared them
all with plain slabs with no ASE suppression.
2.

System layout

The DiPOLE/Bivoj system [13] incorporates a low-energy, fiber-based front end oscillator (∼ nJ),
followed by a regenerative Yb:CaF2 amplifier that increases the output energy to the mJ level(PA1) and a rod Yb:YAG multi-pass booster amplifier to raise the output energy to multi tens mJlevel (PA2). Two diode pumped, helium gas cooled large aperture power amplifiers then increase
the output energy to 7 J (Main pre-Amplifier 1) and finally to 100 J (Main power Amplifier 2).
The schematic of the system is in Fig. 1.
The front end starts with a single frequency temperature stabilized tunable CW fiber oscillator.
The CW beam is then temporarily shaped in an A-O (acousto-optic) modulator to produce 200 ns
long pulses with a repetition rate of 10 kHz. Pulses are then amplified in a fiber amplifierand subsequently shaped by an E-O (electro-optic) modulator to produce 2-14 ns pulses with
arbitrary shape and a resolution of 200 ps. The output energy is around 10 nJ for a 10 ns longpulse. Then the pulses are sent to regenerative rod amplifier (PA1, Amplitude Systemes) basedon Yb:CaF2 that reduces repetition rate to 10 Hz and boosts the energy to ∼4 mJ. The 2 mm
Gaussian beam coming from the regenerative amplifier is then spatially shaped to a 8 mm × 8 mm
square super-Gaussian profile in a beam shaper consisting of π-shaper and serrated aperture witha spatial filter. Then the pulse is further amplified to ∼50 mJ in a 6-pass booster amplifier (PA2,
Lastronics GmBH) based on Yb:YAG. The booster amplifier preserves the square super-Gaussian
beam profile, which is subsequently expanded to 21 mm × 21 mm and injected into the 10 J mainpre-amplifier.
The 10 J main pre-amplifier (MA1) is based on a multi-slab design. It consists of four circular
Yb:YAG slabs with two doping levels of Yb (1.1 at.% for outer slabs, 2.0 at.% for inner slabs).
The dimensions of each circular slab are 45 mm in diameter and 5 mm in thickness. The Yb:YAG
is cladded with a 5 mm Cr:YAG absorber (absorption coefficient of 6 cm−1 ) which minimizes
loses due to ASE and parasitic oscillations. The pumped area is a square of 23 mm × 23 mm.
The pump beam is coming from two homogenized, diode pump modules operating at 939 nm
and each producing 29 kW of peak power in up to 1200 µs long laser pulses at a repetition rate of
10 Hz. The amplifier is cooled by a forced Helium gas flow with a pressure around 8 bar at a

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Fig. 1. (a) Schematic of Bivoj/DiPOLE100 amplifier chain showing maximum output performance after each amplifier stage, including free-space beam size and shape:
YDFO = Yb-silica fiber oscillator; YDFA = Yb-silica fiber amplifier (inc. temporal pulse
shaping); PA = room-temperature pre-amplifier (1 = Yb:CaF2 regenerative, 2 = Yb:YAG
multi-pass); MA = main cryogenic amplifier (ceramic Yb:YAG multi-slab). (b) 3D model of
DiPOLE100 system: D = Diode pumps; cGC = cryogenic gas coolers (Ref. [6], Fig. 1).

temperature of 150 K. The head housing the gain media is separated from the vacuum isolation
by sapphire pressure windows and the isolation vacuum from the ambient environment by fusedsilica vacuum windows.
The beam is injected into the amplifier through a dichroic mirror and then is image-relayed bya spatial filter (f = 0.9 m) to a back reflector and then relay-imaged back to the amplifier head.
Each pass is propagated by a set of separate mirrors and lenses. A square bimorph deformablemirror with 49 actuators and dielectric coating (Adaptica 7 × 7, 27 mm × 27 mm, stroke ± 30 µm)
is placed in the amplifier after the 3rd pass. This mirror, together with lens arrays placed on linear
stages, is used to compensate the thermal wavefront distortion acquired in the amplifier head.
The birefringence of the sapphire pressure windows changes the polarization of the beam andsets of quarter- and half-waveplates are used to compensate this on pass 1 and 5. The beam exits
the amplifier after 7 passes.
3.

Gain media comparison

To compare all gain media, we first inspected them in the laboratory to evaluate the level ofscattering before insertion into the amplifier head. Then we measured the small signal gain
(SSG) and amplification to maximum achievable energy. We also measured thermal aberrations
caused by the slabs in the form of wavefront distortion. For ceramics and cladded crystals, we
also calculated the wavefront distortion numerically.
3.1.

Gain media

The parameters of the gain media slabs are summarized in Table 1. All slabs had an outer
diameter of 55 mm. The Yb:YAG aperture was 45 mm for the ceramics, 37 mm for the cladded
monocrystals, full aperture of 55 mm for the un-cladded plain monocrystals, and 37 mm for the
structured monocrystals. The smaller clear aperture of the cladded monocrystals was caused by aResearch Article

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hexagonal shape of Yb:YAG part, which was necessary for optical bonding of Cr:YAG absorbercladding.
Table 1. Overview of gain media parameters
Parameter

Ceramics

Monocrystals
Cladded

Plain

Structured

Yb doping (at. %)1.1% and 2%

1.1% and 2%

1.1% and 2%

1.1% and 2%

Orientation

random

[100]

[100]

[100]

Cr absorption[cm−1 ]6, 6.35, 6.35, 6.45

6.1

NA

NA

Total thickness [mm]

19.88

21.09

21.89

21.83

Clear aperture [mm]

35

37

55

37

To visually assess the quality of the gain media, we observed them in collimated bright light.
Photographs from this observation taken with same exposition settings of a camera are shown in
Fig. 3. All slabs show scattering of the light in the clear aperture, but the visual levels are small
and comparable.
The micro-structure was a series of equidistantly spaced concentric rings (see Fig. 2), wherematerial was removed by laser ablation. A comparable structure is used by Crytur to enhanceout-coupling of a light generated in Cr:YAG scintillators. In our home-made code using Monte
Carlo approach, we calculated that the structure increases out-coupling efficiency for very broad
spectral range. In the scintillators, the light radiated from the crystal increases by 300-400% when
one of the faces is micro-structured. Additionally, mean free path of the rays in the scintillator
decreases 10 times, when one surface is micro-structured. These properties of the scintillators
were the primary motivation for trying similar micro-structuring as ASE prevention in laser slabs.

Fig. 2. Photographs of the micro-structure with 20 times magnification from optical
microscope.

3.2.

Small signal gain measurement

In this experiment, the gain media were pumped by a total energy of 50 J in a 1 ms long pulse
centered at 939.5 nm at a 1 Hz and 10 Hz repetition rate. The 10 ns signal pulse attenuated to
5 mJ coming from PA2 was amplified in a single pass through the amplifier head. The input andoutput pulse energy was simultaneously monitored by energy meters (input: Gentec QE25LP inleakage through a high reflectivity mirror, output: QE50LP directly in the beam). The gain wasmeasured at two temperatures, 120 K and 150 K. In each measurement, the signal wavelength
was adjusted to the peak of the gain curve for each temperature.

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Fig. 3. Photographs of the gain media taken with 0.5 s exposure for a) ceramics, b) cladded
monocrystals, c) plain monocrystals, and d) structured monocrystals.

During the measurement, the delay between the pump and signal pulses was changed, so
the signal pulse observed a different effective pump energy, but the heat load of the amplifier
remained constant. The ratio of input and output energies give the small signal gain values, which
are summarized in Fig. 4.
The SSG is heavily influenced by ASE, therefore slabs with ASE absorbers show better results
than plain slabs. Structured slabs performed poorly in this comparison, effectively on the samelevel as the plain monocrystals. We noticed that ASE coming out of the amplifier was roughly
three times higher for structured slabs in comparison to other gain media sets, which would be inline with initial predictions. However the reason for their poor performance is still not clear. The
temperature of the slabs also influences the SSG, with lower temperatures producing a higher
emission cross-section and thus higher gain if ASE is suppressed, as in the case of the ceramics
and cladded monocrystals. Conversely, if ASE is not suppressed, lower temperature decrease thegain even below the value obtained at higher temperatures, as in the case of the structured andplain monocrystals. The small discrepancy between ceramics and cladded monocrystals at 10 Hz
pumping probably originates from the higher inner temperature of cladded monocrystals as will
be explained later. The highest values of SSG are summarized in Table 2.
Table 2. Maximum SSG values for different temperatures and repetition rates.
Temperature

120 K

150 K

Rep. rate

1 Hz

10 Hz

1 Hz

10 Hz

Ceramics

6.05

4.75

4.3

3.6

Cladded monocrystals

5.8

4.5

4.2

3.5

Structured monocrystals

2.1

2.45

2.4

2.5

Plain monocrystals

1.8

1.85

2.05

2.1

3.3.

Amplification and wavefront aberrations

After the SSG measurement, the beam was propagated through the whole amplifier and the output
energy was measured. The amplifier was realigned completely with each new slab set. Theenergy output from PA2 was increased to 30 mJ and the pulse duration remained 10 ns. In thisResearch Article

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Fig. 4. Small signal gain for various temperatures and repetition rates a) 1 Hz 120 K, b) 1
Hz 150 K, c) 10 Hz 120 K, and d) 10 Hz 150 K for ceramics (square), cladded monocrystals(circle), plain monocrystals (triangle), and structured monocrystals (cross).

experiment, the gain media were pumped by a total energy of 30 J in a 600 µs long pulse (samepeak power as in SSG measurement) centered at 939.5 nm at 1 Hz and 10 Hz repetition rates. We
chose shorter pump pulse duration than what is used in regular operation to decrease the thermalloading of the amplifier and still be able to reach 6 J of output energy. The temperature of theamplifier was set to 150 K. Lower temperatures weren’t used, since self-lasing was observed atlower temperatures. Results are summarized in Fig. 5.
Before amplification, we set the lens arrays to positions where thermal lensing is notcompensated, and the deformable mirror to a flat surface. We captured wavefronts at the outputof the amplifier with no pumping, and then captured the wavefronts again while pumping. Then
we changed the delay between signal and pump pulse to obtain amplification and captured
wavefronts. By subtracting the corresponding wavefronts, we characterized the thermal lens atfull thermal load with no energy extraction and at maximum energy extraction. Then the lensarrays and deformable mirror surface were optimized to obtain the lowest aberrations on thebeam, captured wavefronts, and near and far field profiles.
Wavefronts reflecting thermal lenses (Fig. 6) and beam profiles for optimized wavefront (Fig. 7)
and are presented only for 10 Hz operation, because at 1 Hz all profiles are less affected by heatand show even less difference. The optimized wavefronts for all slab sets have similar peak tovalley (P-V) values below 0.8 µm and RMS below 0.08 µm, therefore they are not presented
here. All images were taken from a leak through a high reflective mirror. The leaked beam wasdown-collimated (1:10) and then imaged onto a detector (near fields, wavefronts) or focused by
f = 100 mm lens (far fields).

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Fig. 5. Amplification of 10 ns, 30 mJ signal beam in amplifier with ceramics (square),cladded monocrystals (circle), plain monocrystals (triangle), and structured monocrystals(cross) at temperature of 150 K for pumping at a) 1 Hz repetition rate and b) 10 Hz repetition
rate.

Fig. 6. Wavefronts at the output of the system for (top row) full thermal load with no energy
extraction and for (bottom row) thermal load lowered by maximum energy extraction fromthe amplifier. Wavefronts correspond from left to right to ceramics, cladded monocrystals,
structured monocrystals, and plain monocrystals.

The maximum output energy correlates with the SSG measurements. For 1 Hz operation,
the differences are smaller, the ceramics reached 6.5 J, the cladded monocrystals reached also
6.5 J, while the structured monocrystals reached only 1.05 J and the plain monocrystals only
0.65 J. For 10 Hz operation, the differences are larger and follow the previous trends, the ceramics
reached 4.3 J, the cladded monocrystals reached 3.9 J, while the structured monocrystals reachedonly 1.2 J and the plain monocrystals only 0.7 J. The cladded monocrystals show lower output
energy probably due to the higher temperature of the active area, which was calculated tobe 1.5-2.5 degrees above the ceramics due to closer proximity of the absorber to the beam.
The structured monocrystals had a higher output energy at 10 Hz operation due to increased

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Fig. 7. Near field (top) and far field (bottom) beam profiles for maximum output energy with
optimized wavefront at 10 Hz operation for from left to right ceramics, cladded monocrystals,
structured monocrystals, and plain monocrystals.

temperature that lowered ASE due to higher re-absorption and lower emission cross-section.
Similar behavior is observed for plain slabs.
Note that the output energies of the structured monocrystals are 71% and 61% higher than the
output energies of the plain monocrystals at 10 Hz and 1 Hz, respectively. Both values of thestructured monocrystals are well below the values of the cladded monocrystals and the ceramics.
This point deserves some further attention, which would, however, go beyond the scope of thispaper.
The thermal lens reflects the amount of heat generated in the slab. Slabs without an absorberlayer have the opposite sign for the thermal lens in comparison to slabs with an absorber. Since
most of the heat is generated in the absorber itself, it changes the thermal lens sign easily. The
cladded monocrystalline slabs show largest thermal lens of all sets, which is a result of the close
proximity of the absorber and the pumped area of the Yb:YAG.
3.4.

Thermal calculations

The experimentally obtained thermal lenses and assumptions about temperature of the slabs
were compared with a numerical model. We used a MATLAB code [14,15] to calculate the heatgenerated in the slabs. We note that the MATLAB code is not able to calculate heat in the slabs
without an absorber, since it relies on Monte Carlo ray tracing and absence of the cladding willtrap rays in the slab and cause very long computation times. Simple assumptions would yieldfaulty results, so we do not present data for slabs without an absorber here. The calculated heat
distribution was then imported to COMSOL to model temperature profiles and the thermal lenses
in the slabs in the amplifier head. In accordance with experiment, the coolant temperature was
set to 150 K, He pressure to 7.5 bar with flow of 30 m3 /h. The ceramic slabs were thinner (5 mm)
than the cladded monocrystals (5.27 mm) which mean the separation between ceramic slabs waslarger than between the cladded monocrystalline slabs. This increased gap between ceramics
slabs resulted in a decreased flow velocity of He between the slabs and a lower calculated heattransfer coefficient for the cladded ceramic slabs (1500 W/m2 /K) in comparison to the cladded
monocrystalline slabs (1800 W/m2 /K). For calculations we assumed that He flow was turbulent
and there was no contact of the slab with the holder. For 300 W pumping, in case of full thermal

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lens, the calculated heating in the pumped volume of the slabs was 28.5 W and in the absorbercladding 246 W. For full energy extraction it was 28.5 W and 186 W, respectively. The absorption
coefficient of the absorber was chosen to be 6 cm−1 in both cases. The surface temperature
profiles of the slabs are shown in Fig. 8.

Fig. 8. Temperatures on the surface of the slab at 10 Hz pumping with maximum energy
extraction for a) ceramics and b) cladded monocrystals. Black square in the picture represents
pumped area of the slab.

Thermal calculations also predict that the average temperature in the pumped area rises to154 K for ceramics and to 155.5 K for cladded monocrystals. The maximum temperature rises to155.5 K for ceramics and to 157.5 K for cladded monocrystals. Both calculated for maximum
energy extraction from the amplifier. The temperatures are even higher in the case of no energy
extraction (average temperatures 158.5 K and 160 K, maximum temperatures 160.5 K and
163.5 K for ceramics and cladded crystal, respectively).
The calculated wavefront of the beam after 7 passes through the amplifier head pumped at
10 Hz with no energy extraction is shown in Fig. 9. Peak to valley values of the wavefront with noenergy extraction and for maximum energy extraction are summarized in Table 3. All show good
match to experimentally obtained values and in shape and confirm our model of the gain medium.

Fig. 9. Calculated wavefront after 7 passes through amplifier head pumped at 10 Hz withno energy extraction for a) ceramics and b) cladded monocrystals.

Table 3. Calculated P-V values of wavefronts measured with 10 Hz pumping
Slab

Wavefront P-V value full thermal lensWavefront P-V value max energy extraction

Measured [µm]Calculated [µm]Measured [µm]Calculated [µm]Ceramics

2.4

2.48

1.4

1.35

Cladded crystal

4.2

4.32

2.8

2.73

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Summary

We compared several types of slab gain media based on Yb:YAG in a 10 J, 10 Hz laser system(cladded ceramics, and cladded, structured and plain monocrystals). ASE suppression had alarge influence on the performance of the amplifier, and Cr:YAG absorbers were much better
than micro-structured treatment of the surface of the slabs. Minor performance effects wereobserved due to the different geometry of the monocrystalline and ceramics claddings. For
similar geometries, the performance is expected to be the same.
We therefore conclude that the performance of cladded single crystal in a large area highaverage power amplifier is the same as of ceramics. This should grant the laser community more
flexibility in laser design and in sourcing gain media for high energy amplifiers.
Funding
European Regional Development Fund (CZ.02.1.01/0.0/0.0/15_006/0000674,
CZ.02.1.01/0.0/0.0/16_019/0000789); H2020 Spreading Excellence and Widening Participation(739573); Ministerstvo Školství, Mládeže a Tělovýchovy (LO1602).
Disclosures
Jana Preclikova, Jan Kubat: Crytur s.r.o. (E) and Jindrich Houzvicka: Crytur s.r.o. (I, E). The
authors declare that there are no conflicts of interest related to this article.
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