BBO-sapphire sandwich structure for frequency conversion of high power lasers

We report on successful joining of a beta barium borate crystal by plasma-activated direct bonding. Based on this technology, a sandwich structure consisting of a beta barium borate crystal, joined with two sapphire heat spreaders has been fabricated. Due to the high thermal conductivity of sapphire, the sandwich structure possesses superior thermal properties compared to the single crystal. Simulations based on the finite element method indicate a significant reduction of thermal gradients and the resulting mechanical stresses. A proof of principle experiment demonstrates the high power capability of the fabricated structure. A pulsed fiber laser emitting up to 253 W average power has been frequency doubled with both a single BBO crystal and the fabricated sandwich structure. The bonded stack showed better heat dissipation and less thermo-optical beam distortion than the single crystal. The work demonstrates the huge potential of optical sandwich structures with enhanced functionality. In particular, frequency conversion at average powers in the kW range with excellent beam quality will be feasible in future.