LIDT Evaluation of Bonded Crystals for J-Class Energy System

Joule-class amplifier systems for compact laser-driven particle accelerators [1] require an efficient heat management approach [2]. Due to the fact that these systems commonly use Yb:YAG crystal as the main gain medium, the use of cryogenic cooling is crucial, which significantly increases the overall system size. Previously, we proposed to implement a distributed face cooling (DFC) approach that was successfully demonstrated for CW and pulsed laser regimes [3], where periodic bonding of a single- or polycrystalline Nd:YAG ceramic gain medium to a sapphire crystal could preserve room temperature operation. The heat generated inside the gain medium is effectively removed via the attached sapphire crystal surface. The crystal bonding was performed using an inter-layer (e.g., amorphous layer) assisted surface activated bonding (il-SAB) method, which is suitable for optical materials, such as YAG or Al
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crystals. Two 21-crystal DFC chips comprised of single Nd:YAG and sapphire crystals in the main amplifier showed 2 J operation at 25 Hz repetition rate [4]. This bonding applied at room temperature, has sufficient bonding strength and is suitable for joining dissimilar materials, as no additional stresses are introduced to the bonded structure. Given that the system consists of multiple bonded interfaces, concerns regarding high-energy operation could arise and comparison between the bonded interface and bulk crystal is required. In our previous work we characterized LIDTs for single-crystal and polycrystalline Nd:YAG ceramics [5]. A number of measurements were performed at the surface, bulk, and bonded interface for the materials bonded using the il-SAB method. The damage threshold value of bonded crystals had a slight degradation for both types of Nd:YAG materials.