DiPOLE100/Bivoj Laser: High-Energy UV Source

• DiPOLE100/Bivoj is a high-energy, high-average-power diode-pumped solid-state laser using cryogenically cooled multi-slab Yb:YAG for nanosecond pulse generation.
• The system incorporates polarization control and adaptive optics to reduce thermally induced losses, achieving up to 96% polarization alignment for efficient nonlinear conversion.
• In third-harmonic generation, the laser delivers nearly 50 J at 343 nm at 10 Hz, demonstrating its potential for fusion drivers, LIDT testing, and industrial high-energy UV applications.

Searching arXiv for the cited DiPOLE100/Bivoj papers to ground the article. DiPOLE100/Bivoj is a high-energy, high-average-power diode-pumped solid-state laser based on cryogenically gas-cooled multi-slab Yb:YAG amplification, developed as a member of the DiPOLE family and operated as a nanosecond-class driver at approximately 1030 nm for large-aperture nonlinear conversion, laser-induced damage-threshold testing, and related high-energy-density applications. In the configuration documented for third-harmonic generation, the system delivers approximately 100 J-class pulses at 10 Hz with a large square beam and programmable temporal shaping, and it has been used to generate 343 nm output at the half-kilowatt level through a two-stage LBO conversion chain (Pilar et al., 2024).

1. Family placement and system identity

Bivoj is presented as a reference implementation of the DiPOLE100 architecture, a high-energy, high-average-power diode-pumped solid-state laser employing cryogenically cooled Yb:YAG in a multi-slab geometry. In the harmonic-conversion study, it is described as a cryogenically gas-cooled multi-slab Yb:YAG laser operated as a nanosecond-class driver for large-aperture nonlinear conversion to its third harmonic at 343 nm. In the adaptive-optics study, the same platform is treated as the high-energy kW-class DiPOLE100/Bivoj system, with a gain medium of cryogenically cooled Yb:YAG, central wavelength $\lambda \approx 1030\ \text{nm}$, energy range $1\text{–}150\ \text{J}$ at 1030 nm, pulse duration $2\text{–}10\ \text{ns}$, and repetition rate $1\text{–}10\ \text{Hz}$; for Bivoj specifically, the paper reports pulse energy up to $146\ \text{J}$, repetition rate $10\ \text{Hz}$, pulse duration $2\text{–}10\ \text{ns}$, and average power $1460\ \text{W}$ (Paliesek et al., 4 Aug 2025).

The system is directly linked to earlier DiPOLE100 work at RAL and is framed as a HE-HAP diode-pumped laser intended to drive large-aperture UV nonlinear conversion and to provide beams for fusion-class UV applications, large-aperture LIDT testing, and more generally high-energy-density applications. The same programmatic context connects Bivoj to installations at European XFEL and EPAC, and to prospective roles in future fusion drivers, while also situating the platform in industrial and space-oriented domains such as laser shock peening, space debris removal, and spacecraft propulsion (Pilar et al., 2024).

2. Fundamental architecture and operating envelope

The Bivoj amplifier chain is based on cryogenically gas-cooled multi-slab Yb:YAG amplifiers, with the main amplifier head cooled by flowing helium gas at cryogenic temperature; one operating point given for the main amplifier head is helium flow $150\ \text{g/s}$ at $120\ \text{K}$. The architecture is designed for high pulse energy at approximately nanosecond durations, high average power, and 10 Hz operation with a large beam aperture suitable for large nonlinear crystals. The harmonic-conversion paper characterizes the system as a typical chirp-free multi-slab Yb:YAG amplifier seeded by a narrowband Yb:YAG oscillator and using diode pumping in multiple heads, culminating in a large-aperture final amplifier (Pilar et al., 2024).

The front-end and power-amplifier partition are described in more detail in the adaptive-optics study. The front-end consists of a fiber front-end plus two preamplifiers and generates pulses up to $1\text{–}150\ \text{J}$0, 10 Hz, $1\text{–}150\ \text{J}$1, with adjustable pulse shape and beam profile. The first main amplifier, MA1, is a thin-disk-style multi-pass head with four Yb:YAG gain disks in a seven-pass relay-imaging angular multiplexing geometry. The second main amplifier, MA2, is a six-slab variably doped square Yb:YAG multi-slab head, cooled by cryogenic helium gas at $1\text{–}150\ \text{J}$2 flowing in $1\text{–}150\ \text{J}$3 gaps between slabs with turbulent flow at Reynolds number greater than 19000, and pumped from both sides by two pump units. The beam size in MA2 is $1\text{–}150\ \text{J}$4, with four passes through the head and Keplerian telescopes using 3 m focal-length lenses and a vacuum spatial filter with a 3 mm pinhole in the common focal plane (Paliesek et al., 4 Aug 2025).

At the fundamental, the harmonic-conversion study reports a central wavelength $1\text{–}150\ \text{J}$5, flat-top temporal profiles with duration of 10 ns, arbitrary pulse shaping in a 14 ns shaping window via a temporal pulse shaper with closed-loop optimization, and output bandwidth of approximately 200 MHz, limited by the temporal shaper; the oscillator itself operates with 70 kHz bandwidth. The beam at the laser exit has a $1\text{–}150\ \text{J}$6 square cross section and is described as a square flat-top beam profile, more precisely a square super-Gaussian with satisfactory energy uniformity after conversion. A Keplerian telescope de-magnifies this by a factor of 1.56 to $1\text{–}150\ \text{J}$7 at the LBO crystals (Pilar et al., 2024).

Parameter	Value	Context
Fundamental wavelength	$1\text{–}150\ \text{J}$8	DiPOLE100/Bivoj output
Repetition rate	$1\text{–}150\ \text{J}$9	Fundamental and UV operation
Pulse duration	$2\text{–}10\ \text{ns}$0 flat-top; $2\text{–}10\ \text{ns}$1 overall range	Conversion run; broader architecture
Beam size	$2\text{–}10\ \text{ns}$2 at exit; $2\text{–}10\ \text{ns}$3 at LBO	Harmonic-conversion setup
Bivoj pulse energy	up to $2\text{–}10\ \text{ns}$4	AO paper, system-specific
Fundamental energy into SHG crystal	$2\text{–}10\ \text{ns}$5 maximum	343 nm experiment

These values show that the term “DiPOLE100/Bivoj laser” refers not merely to a nominal 100 J-class source, but to a scalable HEHAP platform spanning front-end pulse synthesis, cryogenic multi-pass amplification, and downstream large-aperture frequency conversion. A plausible implication is that published parameter ranges reflect different operating modes and subsystems rather than a single fixed-point specification.

3. Polarization control and mitigation of thermally induced losses

A defining issue in DiPOLE100/Bivoj operation is thermal-stress-induced birefringence and the associated thermally-induced polarization changes driven power losses. In the analytical polarization paper, TSIB is described as producing a non-uniform polarization distribution across the beam, which becomes critical when that spatially varying polarization encounters polarization-selective elements such as thin-film polarizers or crystals with polarization-dependent gain or loss. In principle TIPCL can lead to approximately 50% loss, and in practice for high-energy high-average-power systems often $2\text{–}10\ \text{ns}$6 loss. For Bivoj/DiPOLE100, the problem is identified as especially significant in the last stage of the power amplifiers, where the heat load is largest and the beam subsequently encounters polarization-selective elements relevant to frequency conversion (Jochcová et al., 2024).

Earlier work on the same system, as summarized in the analytical paper, used Mueller-matrix polarimetry-based compensation of TSIB/TIPCL in the Bivoj amplifier chain and reduced power loss from greater than 33% to about $2\text{–}10\ \text{ns}$7, enabling very high-efficiency SHG to 515 nm. The 2024 analytical treatment replaces the earlier four-parameter numerical optimization with a direct closed-form calculation of the optimal input polarization and optimal output analyzer polarization, expressed through Jones and Mueller formalisms and directly connected to the experimentally measured Mueller matrix of the amplifier chain. The amplifier is modeled as a position-dependent unitary Jones matrix $2\text{–}10\ \text{ns}$8, equivalent to placing the entire amplifier between two elliptical polarizers (Jochcová et al., 2024).

For the Bivoj amplifier chain, the analytical method yields the optimal input polarization

$2\text{–}10\ \text{ns}$9

and the optimal output polarization

$1\text{–}10\ \text{Hz}$0

The paper states that these values are in excellent agreement with those obtained in the earlier numerical optimization. It further reports initial TIPCL greater than 33%, subsequent reduction to $1\text{–}10\ \text{Hz}$1 for CW beam and $1\text{–}10\ \text{Hz}$2 for pulsed beam, and then further reduction to approximately $1\text{–}10\ \text{Hz}$3 after more precise measurement and refinement (Jochcová et al., 2024).

In the harmonic-conversion experiment, polarization management is implemented with two pairs of zero-order waveplates. One pair compensates thermally induced polarization changes from the Bivoj final amplifier based on the polarimetric method of Slezák et al.; the second pair adjusts the polarization at the LBO input to align it with the principal plane required for type-I SHG and type-II SFG in LBO. After optimization, approximately 96% of the beam energy is in the polarization state suitable for frequency conversion and 4% is in the orthogonal component (Pilar et al., 2024). This suggests that polarization optimization is not ancillary but structurally coupled to high-efficiency UV conversion in the Bivoj program.

4. Third-harmonic conversion to 343 nm

The 343 nm source produced from DiPOLE100/Bivoj uses a classical SHG $1\text{–}10\ \text{Hz}$4 SFG scheme in large-aperture LBO. The first stage is second-harmonic generation from $1\text{–}10\ \text{Hz}$5 to $1\text{–}10\ \text{Hz}$6 in a type-I phase-matched LBO crystal placed in a temperature-stabilized oven. The second stage is third-harmonic generation by sum-frequency generation in a type-II phase-matched LBO crystal according to

$1\text{–}10\ \text{Hz}$7

with residual fundamental at 1030 nm and second harmonic at 515 nm as inputs. The THG crystal is an LBO crystal from Coherent Inc., with aperture $1\text{–}10\ \text{Hz}$8, thickness 12 mm, and cut for type-II phase matching at 1030 nm + 515 nm $1\text{–}10\ \text{Hz}$9 343 nm with angles $146\ \text{J}$0. Its front face has dual-band AR coating at 1030 nm and 515 nm with $146\ \text{J}$1 and $146\ \text{J}$2, while the back face has single-layer AR optimized for 343 nm with $146\ \text{J}$3 and off-design reflectances $146\ \text{J}$4 and $146\ \text{J}$5 (Pilar et al., 2024).

Both SHG and THG LBO crystals were operated at $146\ \text{J}$6, empirically found to give the highest overall THG efficiency. The THG mount provides temperature stabilization but uses only natural convection of ambient air for cooling, whereas the SHG oven includes thermoelectric temperature stabilization with some cooling capability. Diagnostics after the crystals use a water-cooled beam dump made from colored glass filters in a water tank and a sampling wedge reflecting approximately 1% at all three wavelengths to wavelength-selective lines equipped with Gentec-EO QE25LP energy meters and AVT Manta 145B near-field cameras. Because the calibration coefficient for 343 nm varied by more than 20%, attributed to heating-induced changes in the wedge reflectivity due to UV exposure, the third-harmonic energy was computed by energy conservation rather than read directly (Pilar et al., 2024).

The paper defines the relevant transmissions as

$146\ \text{J}$7

and calculates the third-harmonic energy exiting the THG crystal as

$146\ \text{J}$8

with scattering and absorption in the bulk neglected as an order of magnitude lower than coating losses (Pilar et al., 2024).

Performance is reported in two regimes. In a short-term, pre-thermalized regime, the maximum third-harmonic energy is approximately 55 J at 343 nm for input 1$146\ \text{J}$9 energy up to 86.5 J, corresponding to $10\ \text{Hz}$0, or 66% if only p-polarized input energy is counted. In final thermally stabilized operation after approximately 40–90 min of thermalization and feedback tuning, the output settles at 46–50 J at 343 nm, typically “almost 50 J,” with conversion efficiency $10\ \text{Hz}$1, or 55.5% when only p-polarization is counted. At 10 Hz this corresponds to $10\ \text{Hz}$2 average power at 343 nm, while the title and conclusion emphasize “Half-kilowatt high energy third harmonic conversion to 50 J @ 10 Hz at 343 nm” (Pilar et al., 2024).

The 343 nm pulses inherit the fundamental 10 ns flat-top temporal profile, so that 50 J in 10 ns corresponds to peak power on the order of 5 GW. Energy stability improves from 6% RMS and 22% peak-to-peak in the initial phase to $10\ \text{Hz}$3 RMS and $10\ \text{Hz}$4 peak-to-peak after thermal stabilization and SHG phase-matching adjustment. Near-field UV profiles at 50–55 J remain square super-Gaussian with generally good uniformity, though with hot spots indicative of thermal gradients and nonuniform conversion in the LBO crystals (Pilar et al., 2024).

5. Thermal behavior, feedback mechanisms, and adaptive optics

Thermal management is central to DiPOLE100/Bivoj at both the gain-medium and nonlinear-conversion levels. In the cryogenic multi-slab Yb:YAG architecture, helium-gas cooling at approximately 120 K provides high thermal conductivity and low thermo-optic coefficient in Yb:YAG, reduces thermal lensing and stress-induced birefringence, and permits operation at approximately 100 J and 10 Hz with a large high-quality beam. In the nonlinear stage, both LBO crystals are operated at $10\ \text{Hz}$5, but only the SHG oven has active thermoelectric stabilization with some cooling capability, whereas the THG mount relies on natural convection (Pilar et al., 2024).

The harmonic-conversion paper identifies a specific destabilizing mechanism: back-reflected 343 nm radiation from the THG crystal is reflected by AR coatings and crystal mounts, creating parasitic feedback into the SHG stage. The inferred consequence is local heating and temperature gradients in the SHG crystal, which destabilize the SHG oven’s temperature control and the conversion efficiency and require continuous adjustment of the SHG phase-matching angle during a $10\ \text{Hz}$6 min thermalization period. The paper also notes that small residual reflectances at 343 nm, approximately 1.5%, are sufficient to create noticeable feedback effects at these power levels. Future work is stated to include re-engineering the AR coatings and feedback geometry and using an uncoated wedge for UV sampling (Pilar et al., 2024).

At the amplifier level, the adaptive-optics study analyzes MA2 wavefront distortions under realistic thermal load. At the “100 J” equivalent operating point of 75% pump, 130 g/s helium flow, and 150 K, the static wavefront at MA2 output after defocus compensation has $10\ \text{Hz}$7, peak-to-valley approximately $10\ \text{Hz}$8, and Strehl ratio approximately 0.15. Aberrations are separated into static contributions from pump heating and optics imperfections and dynamic contributions from cooling-gas turbulence, room air conditioning, and nitrogen blow across cold head windows. Pulse-to-pulse RMS wavefront variation is reported as $10\ \text{Hz}$9, with no clear periodicities (Paliesek et al., 4 Aug 2025).

The original MA2 AO concept used a large ILAO $2\text{–}10\ \text{ns}$0 deformable mirror with 52 piston actuators, but this design proved inadequate. Bench characterization showed that the mirror was spatially capable of reproducing the inverse of MA2 thermal aberration with 58 nm RMS fitting error, but its response time of 1–5 s per adjustment cycle was orders of magnitude slower than the turbulence dynamics. Moreover, because the DM was not conjugated to the dominant aberration source planes, strong thermal gradients distorted imaging from the DM plane to the WFS plane and invalidated the control matrix under pumping (Paliesek et al., 4 Aug 2025).

A redesigned test AO architecture at the MA2 output employed a piezoelectric bimorph DM with an $2\text{–}10\ \text{ns}$1 square actuator grid, a clear aperture $2\text{–}10\ \text{ns}$2, and a Shack-Hartmann WFS running at 345 Hz on a CW alignment beam at 1030 nm under “100 J equivalent” thermal load. With the loop operating at 345 Hz, settling to Strehl approximately 0.9 required 62 ms, corresponding to 21 iterations. In this regime, RMS wavefront standard deviation improved by a factor of 10 and Strehl ratio improved by a factor of 11; when the loop was stopped while retaining the last static DM shape, improvement dropped to approximately $2\text{–}10\ \text{ns}$3 in Strehl and approximately $2\text{–}10\ \text{ns}$4 in RMS, indicating a substantial dynamic component (Paliesek et al., 4 Aug 2025).

The AO study also formulates explicit design rules. Simulations and measurements indicate that mean Strehl greater than 0.8 requires loop frequency approximately $2\text{–}10\ \text{ns}$5 in simulation and approximately 50 Hz in experiment, while Strehl greater than 0.9 requires approximately 300 Hz-class operation, realized experimentally at 345 Hz. Fitting-error analysis favors square actuator arrays over the original irregular 52-actuator pattern, with at least a $2\text{–}10\ \text{ns}$6 square array needed for acceptable fitting error over the $2\text{–}10\ \text{ns}$7 pupil. The geometric-optics validity criterion is applied using a worst-case scale $2\text{–}10\ \text{ns}$8 at $2\text{–}10\ \text{ns}$9, giving

$1460\ \text{W}$0

so a DM placed at the MA2 output must be within approximately 1.4 m in conjugate distance to the head plane to avoid strong scintillation that breaks the DM–WFS relation (Paliesek et al., 4 Aug 2025).

6. Applications, comparisons, and system significance

The principal application space identified for DiPOLE100/Bivoj in its 343 nm configuration is high-energy UV operation. The harmonic-conversion paper explicitly links recent achievements in indirectly driven thermonuclear fusion to the need for high-repetition-rate, high-energy UV lasers and for large-aperture UV optics whose LIDT must be tested with large beams. In that context, Bivoj/DiPOLE at 343 nm is positioned as a natural platform for such tests. Additional applications cited include UV annealing of silicon in semiconductor processing and, in broader context, UV micromachining, ablation, air ionization, and use as a pump for OPCPA or other short-pulse systems (Pilar et al., 2024).

Within the DiPOLE/Bivoj program, beam quality is a recurrent requirement because high near-field uniformity and high far-field quality are necessary to avoid hot spots on optics, to achieve precise focus on targets, and to efficiently pump secondary systems. The AO paper adds industrial and space-oriented applications—laser shock peening, LIDT testing, space debris removal, and spacecraft propulsion—and argues that real-time wavefront correction is integral to sustaining these use cases under kW-class average power and multi-slab thermal loads (Paliesek et al., 4 Aug 2025).

The published comparisons in the 343 nm paper place Bivoj against earlier and alternative sources at the same wavelength. DiPOLE100 THG at RAL had demonstrated 65 W average power and 65 J per pulse at 343 nm from a similar multi-slab Yb:YAG system. Femtosecond fiber and thin-disk sources cited include 100 W average and 28.5 $1460\ \text{W}$1J at 343 nm, 120 W average and 118 mJ at 343 nm, and 234 W average at 343 nm from a 7.7 ps thin-disk system at 300 kHz. Against this background, the present Bivoj/DiPOLE system is reported to achieve $1460\ \text{W}$2 at 343 nm and is explicitly described as “more than 2× increase to the published state-of-the-art” and “4× increase in terms of average power to the state-of-the-art” for high-energy UV systems at this wavelength (Pilar et al., 2024).

The same sources also outline the principal limitations. For UV conversion, the stabilized 343 nm operating point is lower than the transient maximum because of thermal load and feedback in the SHG LBO, thermal gradients in the LBO crystals, and dynamic coupling between SHG and THG. The authors state explicitly that further increase of 343 nm energy in the reported experiment was limited by the available 1030 nm energy rather than by immediate damage thresholds. For polarization optimization, the analytical model assumes a non-depolarizing Jones system at each point and is tied to a measured Mueller matrix at a particular operating condition; changes in average power, repetition rate, or beam size may therefore require remeasurement or reoptimization. For adaptive optics, the demonstrated real-time correction used a CW alignment beam rather than full-energy pulses, and the study notes that at full energy deformable-mirror damage thresholds and thermal loading of optics would become critical (Jochcová et al., 2024).

Taken together, these results define DiPOLE100/Bivoj as a modular HEHAP laser platform whose significance lies in the conjunction of three capabilities: cryogenic multi-slab amplification at the $1460\ \text{W}$3, 10 Hz level; polarization-engineered suppression of thermally induced depolarization losses; and large-aperture nonlinear conversion to stable, high-energy 343 nm output in the half-kilowatt regime. A plausible implication is that the Bivoj implementation functions not only as a single laser source but as a systems testbed for the coupled problems of polarization control, UV conversion, and real-time wavefront correction in next-generation DiPOLE-class architectures.