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Abstract
We report on the successful amplification of 10 ns pulses to 10 J energy at 10 Hz in a DiPOLE laser amplifier using crystalline Yb:YAG/Cr:YAG composite slabs manufactured using adhesive-free bonding (AFB) technology. We demonstrate that bonded slabs are suitable for operation in high energy cryogenic laser amplifiers. We also report on frequency doubling of the beam amplified in the bonded slabs. When the pulse energy of the output infrared beam is set to 5 J, a pulse energy of 3.9 J is achieved in the green (corresponding to 78% conversion efficiency). Results demonstrate that AFB technology is suitable for producing large-sized gain material slabs and can overcome current limitations in the manufacture of large-aperture gain material pieces. We believe this work will facilitate energy scaling of high energy lasers where aperture scaling of optical elements is not achievable via conventional manufacturing techniques.
Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
Lasers capable of amplifying nanosecond pulses to high energies (multi-J) at high pulse rates (multi-Hz) are required for a wide range of applications. These include the processing of industrial materials [1], development of inertial confinement fusion power plants [2,3] and high energy density physics research [4]. These systems are also required for pumping high pulse rate petawatt-class femtosecond amplifiers, which in turn enable the generation of laser-driven high brightness and compact radiation (X-ray and $\gamma$-ray) and particle (electron, proton, ion, muon) sources [5–7] for remote high-resolution imaging and medical applications [8–10].
Over the last decade, high energy diode pumped solid state laser (DPSSL) technology has the performance required for the implementation of these applications. Ytterbium-doped yttrium aluminum garnet (Yb:YAG) is one of the most suitable gain materials for high energy, high pulse rate DPSSLs. Recent research showed that both ceramic and crystalline Yb:YAG can meet the optical quality requirements for operation in high energy lasers [11,12].
Production of large-aperture, high optical quality gain material is a prerequisite for high-energy lasers, as such systems require amplification stages with apertures in excess of 10 cm to produce the required energies. The DiPOLE technology, based on diode-pumped cryogenic gas cooled multi-slab Yb:YAG, demonstrated amplification of nanosecond pulses to 150 J at 10 Hz pulse rate [13]. This was achieved using 8.5 mm thick composite ceramic slabs, with a 100 x 100 mm$^2$ Yb:YAG central region surrounded by a 10 mm wide index-matched Cr:YAG cladding for the absorption of amplified spontaneous emission (ASE). However, slab apertures of this size are currently close to the size limit achievable via conventional crystalline and ceramic gain material manufacturing techniques. In the case of YAG in ceramic form, the main limitation to aperture scaling is the size of available sintering ovens, with the development of larger ovens requiring significant upfront investments. Instead, the main constraints towards aperture scaling of single crystal YAG are long growth time and challenges in maintaining good material uniformity across the crystal boule [14,15]. As a result, there is increasing demand for innovative and cheaper solutions to scale the size of gain material elements.
Recently, we demonstrated that adhesive-free bonding (AFB) is a promising technique to manufacture Yb:YAG components from smaller starting pieces. We showed that small-scale crystalline AFB Yb:YAG samples exhibit mechanical strength comparable to monolithic Yb:YAG, good optical properties and good resilience to high fluence laser irradiation [16]. The work presented in this paper builds on these earlier results and reports on the manufacturing of composite crystalline Yb:YAG/Cr:YAG slabs for a cryogenic DiPOLE laser amplifier. We show that bonded slabs can withstand thermal cycling between room temperature and 120 K in a high pressure helium gas atmosphere. We show that the bonded material can also withstand non-uniform heating and temperature gradients within the slab volume arising from a combination of laser pumping and cryogenic cooling. The slabs successfully supported the amplification of 10 ns pulses to 10 J pulse energy at 10 Hz. As several applications of high energy lasers require frequency conversion, we also report on frequency doubling of a 5 J infrared beam amplified by the bonded slabs to 3.9 J (corresponding to 78% conversion efficiency) at 1 Hz pulse rate.
2. Materials and methods
2.1 Bonded slabs preparation
A set of 4 slabs was manufactured by Onyx Optics (US) using the AFB technique. This technique, described in previous publications [17–19], allows joining crystalline, ceramic and glass materials without the use of adhesives. Bonding between starting components is achieved thanks to the formation of London-van der Waals forces between two surfaces, polished to a high flatness and low roughness, brought into close contact. The bonding is facilitated by the application of moderate heating.
As shown in Fig. 1(a), slabs are composed of a central 40 x 40 mm$^2$, 5 mm thick, crystalline Yb:YAG region. The Yb$^{3+}$-doping concentration is 2.0 $\pm$ 0.02 at-% for two of the slabs and 1.1 $\pm$ 0.02 at-% for the other two. An AFB interface runs through the middle of the central region, joining together two equally-sized 40 x 20 mm$^2$ Yb:YAG pieces. This bonding interface is exposed to pump and seed beams during laser operation. This way, the impact of the AFB interfaces on the quality of the output beam and on the overall performance of the laser can be assessed. The central region is surrounded by a 5 mm wide index-matched crystalline Cr:YAG cladding with an absorption coefficient of 6 $\pm$ 1.5 cm$^{-1}$ at 1030 nm. The cladding, also bonded using the AFB technique, acts as an index-matched absorber to suppress ASE. Figure 1(a) shows the position of the five AFB interfaces (red dotted lines) and of the area exposed to laser irradiation during laser operation (yellow square).
Fig. 1. Drawing showing composite slab geometry, position of AFB interfaces (red dotted lines) and area exposed to laser irradiation (yellow area). The green arrows indicate the crystal orientation (a). Photo of one of the bonded slabs under bright white light illumination. The bonding interface is visible at the centre of the slab (b).
All YAG pieces were grown via Czochralski method, with growth along the [111] direction. The slabs are cut perpendicular to the [211] direction. The individual pieces are harvested parallel to the [111] direction with an orientation accuracy of 1$^{\circ }$. The [211] and [110] directions are perpendicular to the [111] direction within a tolerance of 1$^{\circ }$. In the <111> plane (perpendicular to the [111] direction), the [211] and [110] directions are perpendicular to each other with a tolerance of 5$^{\circ }$. Although the demonstration described in this paper uses the [211] crystalline cut, we believe this bonding technique is equally applicable to other orientations as well as ceramic Yb:YAG. The joined Yb:YAG pieces were harvested from adjacent sections of the same crystal boule and their compatibility in terms of uniform doping concentration was verified by measuring refractive index changes by interferometric techniques. The orientation accuracy when bonding the pieces together is within 0.1$^{\circ }$.
Figure 1(b) shows a photo of one of the composite slabs used for the experiments. The central AFB interface can be seen as a faint feature when slabs are exposed to bright white light illumination at certain incidence angles. As discussed in [16], the AFB interface is visible because of Fresnel reflections caused by a small localised variation in refractive index. Bonded slabs were individually characterised using a Zygo VeriFire interferometer in transmission mode. Transmitted wavefront distortion is within $\lambda$/10 for all slabs across the entire Yb:YAG region, with no sign of strong aberrations. Transmitted wavefront measurements showed that the localised refractive index change $\Delta n$ at the central bonding interface is $\Delta n \leq 4.5 \cdot 10^{-6}$ for all slabs. It is worth noting that the observed material inhomogeneity is comparable to the homogeneity achieved in the highest quality fused silica bulk material, which is around 3 ppm. Minimising $\Delta n$ is required to avoid the onset of parasitic lasing caused by the formation of closed ring paths within the slab material.
After the AFB procedure, both 50 x 50 mm$^2$ surfaces were polished to a root-mean-square roughness of 0.5 nm. The polished surfaces were anti-reflection (AR) coated with a dielectric layer multi-stack deposited via magnetron sputtering and designed to achieve $< 0.2$% reflectivity between 920 nm and 1050 nm for angles of incidence between $0^{\circ }$ and $10^{\circ }$.
2.2 Experimental setup
2.2.1 DiPOLE 10 J, 10 Hz laser amplifier
Laser amplification experiments were performed on the DiPOLE prototype 10 J, 10 Hz laser at the STFC Central Laser Facility.
Figure 2(a) shows a schematic of the system summarising the stages of the laser chain. As design details were reported previously in [20], the description of the setup in this section is limited to those design features of relevance for the understanding of this work.
Fig. 2. Schematic of DiPOLE 10 J amplifier chain showing typical output performance after each stage: YDFO / YDFA = Yb-silica fibre oscillator / amplifier; PA = room-temperature pre-amplifier (1 = Yb:CaF$_2$ regenerative, 2 = Yb:YAG multi-pass); MA = main amplifier (Yb:YAG multi-slab). (b) Layout of MA: VSF = vacuum spatial filter, LD = diode laser pump. Adapted with permission from [20] The Optical Society.
A rendering showing the layout of the main DiPOLE multipass amplifier is shown in Fig. 2(b). Bonded slabs are held within an amplifier head containing a pressure vessel which forms part of a closed helium circuit. Slabs are are mounted in aerodynamically-shaped titanium-alloy vanes using a berillium-copper spring strip, allowing thermal movement (Fig. 3). To achieve equalised pump energy absorption and uniform gain and thermal loading, the 2 at-% Yb$^{3+}$-doped slabs are positioned at the centre of the slab set [21]. Slabs are oriented inside the amplifier head to achieve an alternate orientation of the central bonding interfaces, as shown in Fig. 3(b).
Fig. 3. Rendering of vane assembly (a) and schematic showing the alternate orientation of central bonding interfaces (red lines) in the laser setup (b).
Slabs are cooled to cryogenic temperatures by a stream of helium gas at a pressure of 10 bar flowing at a mass flow rate of 35 g/s. Two 7 mm thick sapphire windows allow both the pump and the seed beams to propagate through the pressure vessel. The pressure vessel, as well as the entire helium circuit, is contained inside a vacuum envelope providing thermal insulation. Optical access is provided by two 12 mm thick fused silica windows. The slabs are face-pumped from both sides by two 940 nm wavelength diode pump sources (Ingeneric, Jenoptik and Amtron), each delivering 20 kW peak power in 1 ms pulses (corresponding to a pump pulse energy of 20 J) at 10 Hz pulse rate. Pump beams are low divergence and coupled into the amplifier head by $45^{\circ }$ dichroic mirrors, reflecting at 940 nm and transmitting at 1030 nm (Fig. 2(c)). At the centre of the amplifier head, the pump beam profile is a 20 x 20 mm$^2$ top hat. A 18 x 18 mm$^2$ super-Gaussian seed beam with a wavelength of 1029.5 nm propagates through the amplifier head 6 times thanks to an angular multiplexed geometry. At each pass, Keplerian 1:1 telescopes using 900 mm focal length lenses perform relay-imaging of the beam onto the centre of the amplifier head. To maintain good beam quality, weak spatial filtering is implemented on every pass by placing a tantalum plate with a 2 mm diameter aperture at the focal plane of the telescopes.
2.2.2 Second harmonic generation (SHG) setup
As shown in Fig. 4, the output from the DiPOLE laser, after propagating through a quarter- and a half-waveplate pair (QWP and HWP) for polarisation optimisation, is down-collimated to a size of 14 x 14 mm$^2$ using a Keplerian 1:1 relay-imaging telescope (lenses L1 and L2). To prevent air breakdown at the focus of L1, the beam propagates through a vacuum tube with fused silica windows AR coated at 1030 nm. The beam from the DiPOLE amplifier is re-imaged on a frequency-doubling lithium triborate (LBO, Cristal Laser) crystal with a diameter of 30 mm and a thickness of 13 mm. The crystal cut ($\theta$ = 90$^{^{\circ }}$, $\phi$ = 13.6 $^{\circ }$) is chosen to achieve type-I phase matching. To suppress reflections, both crystal surfaces are AR coated for both 1030 nm and 515 nm. The crystal is held in a temperature-controlled oven, described in [22], set at $30^{\circ }$C to increase the stability of conversion which would otherwise be degraded by thermal effects arising from small absorption of radiation in the LBO material. The reported absorption coefficients at 1064 nm and 532 nm wavelength are 15 ppm/cm and 20 ppm/cm, with similar values expected for 1030 nm and 515 nm [23].
A partial reflector (PR), designed to reflect 0.5% at both 1030 nm and 515 nm, directs a small fraction of the green and residual unconverted infrared beams to a diagnostic channel. The leakage through the partial reflector is terminated in a beam dump. A dichroic mirror (DM) with reflectivity of > 99.9% at 515 nm and a transmission of > 99% at 1030 nm separates the green and unconverted infrared beams. Each beam is reduced to a size of 2.5 x 2.5 mm$^2$ by Keplerian telescopes (L3 and L4, L5 and L6) and re-imaged on CCD cameras (CAM1, CAM2, Allied Vision Manta G-145) to provide near-field images of green and unconverted infrared beams. CAM1 was fitted with a long pass filter (FGL1000M, Thorlabs), while a line filter (FL514.5-3, Thorlabs) was placed in front of CAM2.
Fig. 4. Experimental layout of the frequency conversion stage. QWP, HWP: quarter- and half- waveplate, HR1, HR2: high-reflection mirrors @ 1030 nm, L1-L6: lenses, PR: partial reflector, DM: dichroic mirror, UC1, UC2: uncoated fused silica substrates, TR1, TR2: 90$^{\circ }$ total reflectors, CAM1, CAM2: CCD cameras.
The energy of the infrared and green beams is measured with two energy meters (EM1, EM2, Gentec, QE50LP-S-MB) placed behind uncoated substrates UC1 and UC2. The energy measurement from EM1 was calibrated by temporarily placing the energy meter directly in the main beam before the LBO crystal. The green beam energy measurement was calibrated by temporarily inserting EM2 after two dichoric mirrors (HR @ 515 nm, HT @ 1030 nm) placed in series between the LBO crystal and the PR to isolate the green radiation from the unconverted infrared.
3. Results and discussion
3.1 Resilience to thermal cycling and temperature gradients
Bonded slabs were thermally cycled several times from room temperature down to cryogenic levels, with 120 K the lowest temperature tested, using a flow of helium gas at a pressure of 10 bar and a cooling rate of 10 K/min. No degradiation to the bonding interfaces, such as delamination, was observed as result of thermal cycling. This confirmed the suitability of AFB YAG for applications over this temperature range.
In addition to thermal cycling, slabs experience mechanical stresses during laser operation as a result of thermal gradients arising from the interplay between heating due to the pumping process and cooling by the helium gas. Figure 5(a) shows the expected heat density distribution in one of the slabs when the slab set is pumped by the two diode lasers, each emitting 20 J pulses of 1 ms duration at 10 Hz pulse rate, in the absence of pump energy extraction. The calculation was carried out as described in [24]. A total of 100 W is absorbed in each slab, 91.3 W of which is deposited in the Cr:YAG as a result of the absorption of amplified spontaneous emission. Figure 5(b) shows the temperature distribution when slabs are cooled down by a flow of helium gas at a temperature of 145 K and a mass flow rate of 35 g/s. The temperature distribution was calculated assuming a uniform heat transfer coefficient of 1534 W/m$^2$/K derived from the fluid properties of helium [25] and from the cooling geometry [24,26]. Pumping and cooling parameters used for these calculations have been chosen based on the typical operating conditions for high energy amplification. A maximum temperature of 164.1 K is observed in the Cr:YAG cladding and an overall temperature differential of 12.7 K is observed across the slab.
Fig. 5. Heat deposition map (a) and temperature distribution under typical operating conditions (b).
The bonded slabs survived pumping in the DiPOLE amplifier, with no evidence of delamination of the cladding, where the largest temperature gradients are predicted.
3.2 Small signal gain measurements
Small-signal gain measurements were performed at 1 Hz and 10 Hz pulse rate at cooling gas temperatures between 120 K and 145 K. Seed pulses had an energy of 30 mJ, a wavelength of 1029.5 nm and a temporal duration of 10 ns. Each pump source delivered 1 ms pulses with an energy of 20 J (corresponding to 20 kW peak power). The energy of the amplified beam was measured after one pass through the amplifier head. As shown on Fig. 6, the small signal gain does not show sign of saturation over the temperature range under investigation. Also, measurement results are in line with previously measured small signal gain values measured on DiPOLE with conventional, monolithic slabs [27]. This is an indirect confirmation that AFB interfaces do not introduce excessive reflections and that therefore ASE is under control. Small signal gain at 1 Hz is higher than at 10 Hz as the reduced heat deposition from the pumps at lower pulse rate leads to lower slab temperature and therefore higher gain cross-section.
Fig. 6. Small signal gain at 1 Hz (blue data points) and 10 Hz (red data points) at cooling gas temperatures between 120 K and 145 K.
3.3 High energy amplification at 1029.5 nm
For high energy amplification experiments, the helium cooling gas temperature was set to 145 K and the amplifier was seeded with 30 mJ pulses of 10 ns duration. An output energy just under 10 J at 10 Hz was obtained after 6 passes through the amplifier head when pumping the slabs with 40 J of total pump energy and 1 ms pump pulse duration (Fig. 7(a)). This corresponds to 25% optical-to-optical efficiency. The measured performance is in line with results previously achieved using monolithic slabs [20]. Figure 7(b) shows the output energy measurements at 10 J, 10 Hz operation over 3600 shots (6 minutes). The data shows an average output energy of 9.87 J with rms stability of 0.3% and a peak-to-valley variation of 0.2 J, mostly dominated by slow drift caused by ambient temperature variations affecting the performance of optical components. No sign of laser-induced damage onset was observed, confirming that bonding interfaces are resilient to laser irradiation at the operating fluence of 3.1 J/cm$^2$. Figure 7(c) shows the temporal profile of the output pulses. No attempt was made to optimise the seed temporal profile to generate a flat-top output profile during these experiments.
Fig. 7. Output energy (blue data points) and optical-to-optical efficiency (red data points) at 10 Hz operation (a); energy stability over 3600 shots (b); normalised output temporal pulse profile (c).
Figures 8(a) shows the output near-field measured at 10 J using a diagnostic camera. The beam clipping on the left edge of the beam is caused by clipping of the beam in the diagnostic channel. Figure 8(b) shows a burn pattern of the output beam captured by placing Kodak type 1895 Linagraph photographic paper and a neutral density filter (OD = 1) directly in the output beam. The beam profile shows faint traces of the central bonding interfaces (pointed by arrows in Fig. 8(b)). These are particularly visible on the burn patter, most likely due to the non-linear response of the photographic paper to incident beam intensity. The imprints of the bonding interfaces appear blurred as a result of filtering of high spatial frequencies by the pinholes in the spatial filters and by the recording of the profile slightly outside of the image plane.
Fig. 8. Near-field (a), burn pattern (b) and far-field (c) of the output beam at 10 J, 10 Hz operation. The insert in (c) is the far-field spot recorded at 7 J 10 Hz operation.
The presence of the modulation on the beam fluence distribution caused by the interfaces did not result in laser-induced damage to any of the optical components in the laser chain. The beam profile does not show noticeable beam fluence imbalances in the right/left or top/bottom halves of the beam thus confirming close matching in the Yb-doping concentration of the bonded pieces.
Figure 8(b) shows the output far-field (FF) profile recorded without any wavefront correction in place. The full-width at half maximum (FWHM) angular spread of the central maximum in the FF image is 114 $\mu$rad along the x-axis and 178 $\mu$rad for the main peak along the y-axis. This corresponds, respectively, to 2.2 and 3.5 times the diffraction limit for a square 18 x 18 mm$^2$ top-hat beam. Implementation of wavefront compensation techniques is expected to lead to an improvement of the FF profile. Information on the phase-step introduced by a bonding interface is reported in [16], where it is shown that a bonding interface in a 7 mm thick sample introduces a modulation of about 0.01 $\lambda$ in the transmitted wavefront.
Far-field patterns are strongly affected by thermal and cooling gas flow effects in the DiPOLE amplifier head, causing shot-to-shot variations in the far-field patterns. Far-field images recorded at lower output energy levels show a single spot (see insert in Fig. 8(c)), as opposed to the spot distortion observed in the image recorded at 10 J. This allows to rule out bonding interfaces being the cause of the far field distortion observed in the far-field at 10 J operation.
3.4 SHG results
The output beam at a reduced pulse rate of 1 Hz was subsequently frequency doubled using the setup described in section 2.2.2. When carrying out the experiments, the DIPOLE laser was operated at a reduced output energy of 5 J to limit the fluence of the 14 x 14 mm$^2$ beam to 2.6 J/cm$^2$ to prevent risk of laser-induced damage to optics. As shown in Fig. 9(a), the energy of the frequency doubled beam was 3.9 J, corresponding to a conversion efficiency of 78%. This conversion efficiency level is comparable to the one achieved under similar conditions using monolithic slabs in the DiPOLE 10 J, 10 Hz amplifier [22]. The NF profile of the green beam, recorded at 3.9 J, shows evidence of bonding interfaces. The higher spatial frequencies and the circular diffraction patterns are caused by interference from, and contamination of, the camera filters.
Fig. 9. Energy of input infrared beam (red line), frequency converted beam (blue line) and conversion efficiency (black line) recorded during the SHG experiments (a). NF profile of the green beam at 3.9 J (b).
4. Conclusion
In this paper we report on the successful amplification of nanosecond pulses to 10 J energy at 10 Hz pulse rate using crystalline Yb:YAG/Cr:YAG composite slabs manufactured via the AFB technology.
The experiments confirmed that bonded slabs are capable of withstanding temperature cycling down to 120 K as well as thermally-induced stresses arising during laser operation. During high energy operation the bonding interfaces did not suffer from laser-induced damage onset up to the operational fluence of 3.1 J/cm$^2$.
The infrared beam was frequency doubled, with operation up to 3.9 J in the green demonstrated when 5 J infrared pulses were delivered to a LBO frequency conversion crystal. The beam profiles of both the 1030 nm amplified beam and of the 515 nm frequency doubled beam present traces of the bonding interfaces. This is the only appreciable difference in the performance of the amplifier using bonded slabs compared to regular, monolithic slabs. While the resulting modulation of the beam fluence profile is likely to be low enough for most applications, if deemed necessary the output beam profile could be smoothed by means of beam homogenisation techniques [28]. Further investigation on the impact of refractive index inhomogeneity at the bonding interface on B-integral effects may be required if the AFB technique were to be used in the manufacturing of optics for high intensity systems based on chirped pulse amplification.
These experiments confirm the viability of AFB technology for the manufacture of large-sized YAG slabs from smaller starting components, thus overcoming current limitations imposed by conventional manufacturing techniques. We believe that this approach will pave the way towards energy scaling of high energy DPSSLs. In addition, the flexibility in the design of optical elements inherently afforded by the AFB technique could enable the production of geometries and composition distributions not easily obtainable via conventional manufacturing techniques, with potential benefits in terms of laser operation and optimisation.
Funding
Royal Commission for the Exhibition of 1851 (Industrial Fellowship); H2020 Spreading Excellence and Widening Participation (739573); European Regional Development Fund (CZ.02.1.01/0.0/0.0/15 006/0000674).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available in Ref. [29]
References
1. A. Azhari, S. Sulaiman, and A. K. Prasada Rao, “A review on the application of peening processes for surface treatment,” IOP Conf. Ser.: Mater. Sci. Eng. 114, 012002 (2016). [CrossRef]
2. M. Dunne, “A high-power laser fusion facility for Europe,” Nat. Phys. 2(1), 2–5 (2006). [CrossRef]
3. R. Kodama, P. A. Norreys, K. Mima, et al., “Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition,” Nature 412(6849), 798–802 (2001). [CrossRef]
4. M. McMahon, U. Zastrau, K. Appel, et al., “Conceptual Design Report: Dynamic Laser Compression Experiments at the HED Instrument of European XFEL”, XFEL.EU TR-2017-001 (2017).
5. S. Kneip, S. R. Nagel, C. Bellei, et al., “Observation of synchrotron radiation from electrons accelerated in a petawatt-laser-generated plasma cavity,” Phys. Rev. Lett. 100(10), 105006 (2008). [CrossRef]
6. S. P. D. Mangles, C. D. Murphy, Z. Najmudin, A. G. R. Thomas, J. L. Collier, A. E. Dangor, E. J. Divall, P. S. Foster, J. G. Gallacher, C. J. Hooker, D. A. Jaroszynski, A. J. Langley, W. B. Mori, P. A. Norreys, F. S. Tsung, R. Viskup, B. R. Walton, and K. Krushelnick, “Monoenergetic beams of relativistic electrons from intense laser-plasma interactions,” Nature 431(7008), 535–538 (2004). [CrossRef]
7. H. Schwoerer, S. Pfotenhauer, O. Jckel, K. U. Amthor, B. Liesfeld, W. Ziegler, R. Sauerbrey, K. W. D. Ledingham, and T. Esirkepov, “Laser-plasma acceleration of quasi-monoenergetic protons from microstructured targets,” Nature 439(7075), 445–448 (2006). [CrossRef]
8. C. M. Brenner, S. R. Mirfayzi, D. R. Rusby, et al., “Laser-driven X-ray and neutron source development for industrial applications of plasma accelerators,” Plasma Phys. Controlled Fusion 58(1), 014039 (2016). [CrossRef]
9. U. Masood, M. Bussmann, E. Cowan, W. Enghardt, L. Karsch, F. Kroll, U. Schramm, and J. Pawelke, “A compact solution for ion beam therapy with laser accelerated protons,” Appl. Phys. B 117(1), 41–52 (2014). [CrossRef]
10. J. M. Cole, J. C. Wood, N. C. Lopes, K. Poder, R. L. Abel, S. Alatabi, J. S. J. Bryant, A. Jin, S. Kneip, K. Mecseki, D. R. Symes, S. P. D. Mangles, and Z. Najmudin, “Laser-wakefield accelerators as hard x-ray sources for 3D medical imaging of human bone,” Sci. Rep. 5(1), 13244 (2015). [CrossRef]
11. D. Albach and J.-C. Chanteloup, “Large size crystalline vs. co-sintered ceramic Yb3+:YAG disk performance in diode pumped amplifiers,” Opt. Express 23(1), 570 (2015). [CrossRef]
12. M. Divoky, J. Pilař, M. Hanuš, P. Navrátil, M. Sawicka-Chyla, M. De Vido, P. J. Phillips, K. Ertel, T. Butcher, M. Fibrich, J. T. Green, M. Koselja, J. Preclikova, J. Kubat, J. Houzvicka, B. Rus, J. Collier, A. Lucianetti, and T. Mocek, “Performance comparison of Yb:YAG ceramics and crystal gain material in a large-area, high-energy, high average-power diode-pumped laser,” Opt. Express 28(3), 3636 (2020). [CrossRef]
13. M. Divoky, J. Pilař, M. Hanuš, P. Navrátil, O. Denk, P. Severová, P. Mason, T. Butcher, S. Banerjee, M. De Vido, C. Edwards, J. Collier, M. Smrž, and T. Mocek, “‘150 J DPSSL operating at 1.5 kW level’,” Opt. Lett. 46(22), 5771 (2021). [CrossRef]
14. J. Kajan, M. Volkov, G. Damazyan, I. Mukhin, T. Gregor, and O. Palashov, “Fabrication and characterization of high-dimension single-crystal Yb:YAG ingots grown by horizontal directed crystallization method,” Cryst. Res. Technol. 55(12), 2000105 (2020). [CrossRef]
15. J. Preclíková, K. BartoŠ, J. Kubát, M. KoŠelja, B. Rus, M. Divoký, T. Mocek, and J. HouŠvička, “ inLarge Scale Single Crystal Growth,” Laser Congress 2018 (ASSL), OSA Technical Digest (Optical Society of America, paper AM4A.1 (2018).
16. M. De Vido, D. Meissner, S. Meissner, K. Ertel, P. J. Phillips, P. D. Mason, S. Banerjee, T. J. Butcher, J. M. Smith, C. Edwards, C. Hernandez-Gomez, and J. L. Collier, “Characterisation of adhesive-free-bonded crystalline Yb:YAG for high energy laser applications,” Opt. Mater. Express 7(2), 425 (2017). [CrossRef]
17. H. E. Meissner and O. R. Meissner, “Solid state lasers with composite crystal or glass components” U.S. Patent No. 5,852,622. 22 Dec. 1998.
18. H. Lee, H. E. Meissner, and O. R. Meissner, “Adhesive-free bond (AFB) CVD diamond/sapphire and CVD diamond/YAG crystal composites,” Defense and Security Symposium. International Society for Optics and Photonics (2006).
19. H. C. Lee and H. E. Meissner, “Characterisation of AFB sapphire single crystal composites for infrared window application,” Proc. SPIE 6545, 65450K (2007). [CrossRef]
20. S. Banerjee, K. Ertel, P. D. Mason, P. J. Phillips, M. De Vido, J. M. Smith, T. J. Butcher, C. Hernandez-Gomez, R. J. S. Greenhalgh, and J. L. Collier, “DiPOLE: a 10 J, 10 Hz cryogenic gas cooled multi-slab nanosecond Yb:YAG laser,” Opt. Express 23(15), 19542 (2015). [CrossRef]
21. K. Ertel, S. Banerjee, P. D. Mason, P. J. Phillips, M. Siebold, C. Hernandez-Gomez, and J. C. Collier, “Optimising the efficiency of pulsed diode pumped Yb:YAG laser amplifiers for ns pulse generation,” Opt. Express 19(27), 26610 (2011). [CrossRef]
22. J. P. Phillips, S. Banerjee, K. Ertel, P. Mason, J. Smith, T. Butcher, M. De Vido, C. Edwards, C. Hernandez-Gomez, and J. Collier, “Stable high-energy, high repetition rate, frequency doubling in a large aperture temperature-controlled LBO at 515 nm,” Opt. Lett. 45(10), 2946 (2020). [CrossRef]
23. “Cristal Laser”, private communication and website https://www.cristal-laser.com/products/non-linear-optics/lbo-crystal.html.
24. M. De Vido, P. D. Mason, M. Fitton, R. W. Eardley, G. Quinn, D. Clarke, K. Ertel, T. J. Butcher, P. J. Phillips, S. Banerjee, J. Smith, J. Spear, C. Edwards, and J. L. Collier, “Modelling and measurement of thermal stress-induced depolarisation in high energy, high repetition rate diode-pumped Yb:YAG lasers,” Opt. Express 29(4), 5607 (2021). [CrossRef]
25. “Thermophysical Properties of Fluid Systems”, NIST Chemistry WebBook, SRD 69, https://webbook.nist.gov/chemistry/fluid/.
26. P. D. Mason, M. Fitton, A. Lintern, S. Banerjee, K. Ertel, T. Davenne, J. Hill, S. P. Blake, P. J. Phillips, T. J. Butcher, J. M. Smith, M. De Vido, R. J. S. Greenhalgh, C. Hernandez-Gomez, and J. L. Collier, “Scalable design for a high energy cryogenic gas cooled diode pumped laser amplifier,” Appl. Opt. 54(13), 4227 (2015). [CrossRef]
27. S. Banerjee, K. Ertel, P. D. Mason, P. J. Phillips, M. Siebold, M. Loeser, C. Hernandez-Gomez, and J. L. Collier, “High-efficiency 10 J diode pumped cryogenic gas cooled Yb:YAG multislab amplifier,” Opt. Lett. 37(12), 2175 (2012). [CrossRef]
28. F. M. Dickey and S. C. Holswade, Laser Beam Shaping: Theory and Techniques (Marcel Dekker, 2000), Chap.6.
29. M. De Vido and J. Phillips, "eData: STFC Research Data Repository," Science and Technology Facilities Council (2023), https://edata.stfc.ac.uk/handle/edata/935 .
