Scalable, massive fabrication of complex three-dimensional (3D) structured highly transparent Nd: YAG (Y3Al5O12) ceramics was achieved by a lithography-based (photopolymerization) digital light processing or digital light projection (DLP) 3D printing method. 3D-printed ceramic green bodies with a printing layer height of 25 µm and an xy resolution of 40 µm were fabricated by using a photocurable slurry with 39 vol% solid loading. The developed slurry exhibited a stable high viscosity of 80–340 Pa s over a large shear rate range. A specially designed pre-sintering treatment based on YAM (Y4Al2O9) phase formation was applied to prevent cracking and bubbling formation in the DLP 3D printing approach for fabricating highly transparent Nd: YAG ceramics. This thermal treatment also reduced the average grain size, leading to a higher optical quality. Following a series of thermal treatments, the sintered transparent ceramics reached a relative density of 99.9% with a maximum in-line transmittance of 80% in the visible wavelength range. Moreover, the fabricated Nd-doped YAG exhibited 1064-nm emission under 808-nm laser excitation.

Three-dimensional printing of transparent ceramics by lithography-based digital projection

The construction of a novel user facility employing laser-driven plasma acceleration with superior beam quality will require an industrial grade, high repetition rate petawatt laser driver which is beyond existing technology However, with the ongoing fast development of chirped pulse amplification and high average power laser technology, options can be identified depending on the envisioned laser–plasma acceleration scheme and on the time scale for construction Here we discuss laser requirements for the EuPRAXIA infrastructure design and identify a suitable laser concepts that is likely to fulfill such requirements with a moderate development of existing technologies

A viable laser driver for a user plasma accelerator

Silicate bonding is a flexible bonding method that enables room-temperature bonding of many types of materials with only moderate flatness constraints. It is a promising approach for bonding components in high power laser systems, since it results in a thin and low-absorption interface layer between the bonded materials. Here we demonstrate for the first time silicate bonding of a sapphire window to a SEmiconductor Saturable Absorber Mirror (SESAM) and use the composite structure to mode-lock a high-power thin-disk laser. We characterize the fabricated devices both theoretically and experimentally and show how the thermally induced lens of the composite structure can be tuned both in magnitude and sign via the thickness of the sapphire window. We demonstrate mode-locking of a high-power thin-disk laser oscillator with these devices. The altered thermal lens allows us to increase the output power to 233 W, a 70-W-improvement compared to the results achieved with a state-of-the-art SESAM in the same cavity.

Silicate bonding of sapphire to SESAMs: adjustable thermal lensing for high-power lasers

High erbium content yttrium aluminum garnet (Er:YAG) and yttrium scandium aluminum garnet (Er:YSAG) ceramics have been fabricated from Er:YAG and Er:YSAG powders, respectively. The powders have been synthesized via a reverse precipitation technique, processed by uniaxial pressing followed by cold isostatic pressing, and sintered in a vacuum. TEOS (tetraethoxysilane) was used as a sintering additive. After vacuum sintering, all of the samples were processed by annealing in air to increase the transmittance and polished on both sides. The influence of the Sc3+ content and the synthesis conditions on the microstructure and optical transparency of the Er:YAG and Er:YSAG ceramics have been investigated in detail. It has been found that changing Al3+ for the bigger Sc3+ ion leads to the transmittance increasing to up to 60% at a wavelength of about 1500 nm.

The influence of the Sc3+ dopant on the transmittance of (Y, Er)3Al5O12 ceramics.

In the recent years, bonded components have seen a rise in popularity in solid state laser gain media as a way to mitigate thermal issues which are detrimental to their performances and limit power scaling. The bonding technique is currently selected based on a trade-off between cost and performance. Hydroxide catalysed bonding is a rather young technique coming from the field of gravitational research which has not really made an impact in photonics applications. Yet it is believed that thanks to its low cost and overall good performance, there is a non-negligible share of applications for which it could prove as the most advantageous process.
In this work phosphate glass and YAG are selected as materials of interest due to their common use as host matrices for solid-state lasers gain media. Both these materials have a different chemistry from the materials of interest in previous studies on hydroxide catalysed bonding and the bonding mechanisms are to a large extent unknown. Therefore the work has to be carried out from first principles. The highlighted properties to study are the reflectivity and the Light Induced Damage Threshold (LIDT) of the bond which are both of interest when the interface has to be crossed by a laser beam. The mechanical strength is added as a way to check the robustness of the component, to obtain an insight into the curing kinetics and to address concerns associated with manufacturability. Results on how the parameters of the solution used affect both the optical and mechanical properties of the bonded component, and which solution is the most suitable for photonics applications, are presented. 
The beginning of this work largely draws on a presentation of the science case and a review of the relevant literature, from the description of the heat generation phenomenon in laser operation, to designs and techniques that allows mitigation of this heating. 
Initial trials were carried out on phosphate glass. The parameters for the aqueous solution used in hydroxide catalysed bonding are not standardized and therefore various solutions were used to estimate the range of properties that can be obtained with the technique. Successful bonding of phosphate glass with either pure hydroxide (HCB) or silicate-enriched hydroxide (SB) was achieved. The strength of the bond was tested using 4-point flexure technique and strengths of about 13 MPa and 3 MPa were obtained for SB and HCB, respectively, from the first week of curing and with no significant variation with more time. The reflectivity of the bond at 532 nm was measured using an in-house built setup fully described in this work and the values obtained were of the order of magnitude of 0.01 % (HCB) and 0.1 % (SB). Since the lower strength can be a limitation to handle and manufacture components, it was decided to focus on SB samples for the following trials. The parameters of the solutions were changed and as a result the reflectivity was reduced to values between 0.01 % and 0.1 % depending on parameters. The reflectivity trials also allowed an estimation of the bond thickness and refractive index in a non-destructive way using an optical model based on thin-layer interference described in this work. The kinetics of the change of these properties with curing time was studied. These gave valuable insight into the bonding mechanisms, and hypotheses were made on the chemistry occurring. The LIDT trials were outsourced and values obtained were about 1.75 GW/cm2 in the near infrared long pulses regime with no significant influence from the parameters of the solution used.
The attention was then shifted towards YAG again with a major focus on the reflectivity of the bond and improvement of the existing setup. Mechanical strengths of about 24 MPa (SB) after 2 months of curing were obtained. The reflectance obtained for the interface was about 0.2 % which makes it borderline for photonics applications without further work. However this value is relatively low compared to the ones obtained in similar studies on sapphire, and further investigations on the chemistry were carried out using x-ray analysis to understand why. From the results, bonding mechanisms that differ from the ones presented until then are suggested – mechanisms that form a hybrid layer of bonding chemical and substrate material - and the outlooks of the use of the techniques for new materials are discussed. 
Finally various studies are presented on parameters that could affect the transfer of the process from a laboratory to a production environment such as that at Gooch and Housego. Reduction of curing time by applying elevated temperature would allow decreasing the lead time of the process. Control over the settling time of the bond would allow better yield on parts by making the operation easier. Last but not least, a UV/Ozone cleaning process, as an alternative to the current abrasion process used, would open the technique to a new range of samples designs while increasing the yield for larger batches.

Hydroxide catalysed bonding applied to new materials for photonics applications

We report on the application of the adhesive-free bonding (AFB) technique to form Yb-doped crystalline yttrium aluminum garnet (Yb:YAG) gain media slabs. We performed experiments to characterise mechanical strength, optical quality and laser-induced damage threshold (LIDT) of bonded substrates. We demonstrate that mechanical properties of bonded samples are similar to those of monolithic substrates. We show that the presence of a bonding interface does not introduce unwanted wavefront deformations and does not increase the probability of laser-induced damage onset. Results indicate that the AFB technique constitutes a viable alternative for producing large aperture gain media slabs required for high-energy laser systems.

Characterisation of adhesive-free bonded crystalline Yb:YAG for high energy laser applications

The bonds established with Onyx’s adhesive-free bonding (AFB) technique between optical materials, such as trivalent rare-earth ion doped YAG, un-doped YAG, or sapphire, rely on Van der Waals forces. The AFB technique is being applied to fabricate true crystalline fiber waveguides for high power, high beam quality laser emission, and walk-off compensated nonlinear crystal stacks for high efficiency, high beam quality optical nonlinear conversion.

Adhesive-free bond (AFB) true crystalline fiber waveguides and walk-off compensated nonlinear crystal stacks: optical components for high performance lasing and frequency conversion

We describe the application of the adhesive-free bonding (AFB) technique to form crystalline Yb:YAG gain media slabs. We demonstrate that the AFB technique is a viable alternative for producing large aperture slabs for high-energy lasers.

Demonstration of Adhesive-Free Bonded Crystalline Yb:YAG for High Energy Laser Applications

A nonlinear optical crystal (NLO) with a phase matching angle that is corrected with a source laser beam for harmonic conversion. The source laser only has to be within a wavelength range depending on the dispersion of the crystal while the crystal is tilted to the calculated expected conversion angle of the source laser as reference. The angle correction is accomplished with a parallel kinematic motion device to which a nonlinear crystal is mounted on a platform, to determine the wavelength- and temperature-specific angle with active laser alignment and subsequent precision resurfacing. The invented phase matching angle correction is applicable to any uniaxial and biaxial NLO crystals in a wide range of wavelengths, e.g., from far ultraviolet to visible to far infrared. It is of most value for NLO crystals of large walk-off and is applicable to any prior art frequency converting architectures.

Nonlinear optical crystal with corrected phase matching angle

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Characterisation of adhesive-free bonded crystalline Yb:YAG for high energy laser applications

Adhesive-free bond (AFB) true crystalline fiber waveguides and walk-off compensated nonlinear crystal stacks: optical components for high performance lasing and frequency conversion

Demonstration of Adhesive-Free Bonded Crystalline Yb:YAG for High Energy Laser Applications

Nonlinear optical crystal with corrected phase matching angle