High energy, diode pumped solid state lasers (HE-DPSSLs) have the potential to deliver high peak power combined with high average power, high repetition rate and high efficiency. Such lasers open up a new and expanding applications space currently inaccessible to more established flashlamp pumped lasers, which are intrinsically inefficient and typically restricted either to high pulse energy at low repetition rate, or low energy at high repetition rate. Applications range from advanced materials processing and laser shock treatment of high value mechanical components [1], to pumping of ultra-high intensity fs petawatt-class lasers to generate efficient and compact radiation ($x$-ray, $\gamma$-ray) and particle (electron, proton, ion, muon) sources. These have potential applications for novel medical therapies [2] and in high-resolution radiography and advanced imaging for industrial and security sectors [3].

A variety of geometries have been considered to exploit the inherent power scalability of DPSSLs, all of which employ master oscillator power amplifier (MOPA) architectures with multiple amplification stages. To date the best reported performance has been achieved from multi-slab amplifier geometries, where residual heat is removed by flowing helium gas across the slab surfaces at high speed. The Mercury project demonstrated amplification to 61 J at 10 Hz (610 W) at 1047 nm over several minutes, with an efficiency of $\sim 8\%$, using slabs of Yb:S-FAP cooled at room temperature [4]. More recently, the HAPLS project has adopted a similar architecture to demonstrate 70 J at 3.3 Hz (231 W) at 1053 nm. This is being used as a pump source for a Ti:sapphire based PW laser for the European Extreme Light Infrastructure (ELI) Beamlines Facility, this time using Nd:APG-1 glass as the gain material [5].

At the Central Laser Facility (CLF), the DiPOLE team has been developing multi-slab cryogenic gas cooled amplifier technology based on ceramic Yb:YAG. The combination of low temperature operation, which reduces reabsorption loss and increases absorption and emission cross-sections in Yb:YAG, and the low quantum defect (pump and emission wavelengths are 940 nm and 1030 nm, respectively), enables efficient energy extraction and potential scalability to high average power. A prototype single-stage cryo-amplifier cooled to 140 K demonstrated amplification of 10 ns pulses at 1029.5 nm to 10.8 J at 10 Hz (108 W), with an efficiency of over 22% [6]. More recently, we demonstrated 107 J at 1 Hz (107 W) from a two-stage cryo-amplifier system DiPOLE100 [7], developed for the HiLASE [8] facility at Dolní Břežany in the Czech Republic. Following delivery and assembly at HiLASE, the laser has now been commissioned to its full specification of 100 J at 10 Hz by a joint HiLASE/CLF team.

Figure 1(a) shows a schematic of the architecture of DiPOLE100, where design details for each amplification stage have been reported previously [9]. A 3D model showing the main system components is given in Fig. 1(b). An output energy of 105 J at 10 Hz (1.05 kW average power) was obtained when the final cryo-amplifier (MA2) was seeded with 6 J pulses of 10 ns duration from the cryo pre-amplifier (MA1). This was achieved with a pump energy of 465 J, corresponding to a final-stage optical-to-optical conversion efficiency of 22.5%, as shown in Fig. 2(a). Both cryo-amplifiers were operated at a helium gas temperature of 150 K. This is the world’s first demonstration of a kW-level HE-DPSSL.

 

Fig. 1. (a) Schematic of DiPOLE100 amplifier chain showing typical output performance after each amplifier stage, including free-space beam size and shape: $\mathrm{YDFO}=\mathrm{Yb}$-silica fibre oscillator; $\mathrm{YDFA}=\mathrm{Yb}$-silica fibre amplifier (inc. temporal pulse shaping); $\mathrm{PA}=\mathrm{room}$-temperature pre-amplifier ($1=\mathrm{Yb}\text{:}{\mathrm{CaF}}_2$ regenerative, $2=\mathrm{Yb}\text{:}\mathrm{YAG}$ multi-pass); $\mathrm{MA}=\mathrm{main}$ cryogenic amplifier (ceramic Yb:YAG multi-slab). (b) 3D model of DiPOLE100 system: $\mathrm{D}=\mathrm{Diode}$ pumps; $\mathrm{cGC}=\mathrm{cryogenic}$ gas coolers.

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Fig. 2. (a) Energy conversion; (b) temporal profile measured with 4 GHz bandwidth limit; (c) far-field intensity image and (d) spatial uniformity profile of output beam with cross-section profiles from DiPOLE100 operating at 1029.5 nm and 10 Hz.

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Typical output temporal and far-field profiles measured at maximum energy are shown in Figs. 2(b) and 2(c), respectively. The FWHM angular spread of the central maximum in the far-field image was $28\mathrm{\mu rad}(x\text{-}\mathrm{axis})\times 20\mathrm{\mu rad}(y\text{-}\mathrm{axis})$, corresponding to 2.3 and 1.7 times the diffraction limit for a square top-hat beam, respectively. Further improvement in output beam quality is expected when adaptive wave front correction is fully implemented.

The intensity uniformity of the output (measured away from MA2 amplifier image plane) was confirmed by imaging a portion of the output beam scattered from a ceramic plate. The corresponding spatial profile measured at maximum energy is shown in Fig. 2(d). A total of $4.3\times {10}^4$ shots at energies in excess of 100 J have been completed over a series of runs, with long term energy stability of 1% RMS recorded over a continuous 1 h period as seen in Fig. 3.

 

Fig. 3. Long term energy stability over 1 h at 10 Hz ($3.6\times {10}^4\mathrm{shots}$). Inset shot-to-shot energy stability at 10 Hz (10 shots).

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The combination of efficient and stable operation under high average power conditions confirms the power scalability of multi-slab cryogenic gas-cooled amplifier technology, and demonstrates its potential as an advanced laser driver for future scientific, industrial and medical applications. The DiPOLE100 laser is now operational at HiLASE and is being used by a growing community of scientific and industrial users.