Enhanced thermal conductivity of distributed face-cooled composite laser medium included thermal resistance at the bonding interface

A new concept of the effective thermal conductivity (<i>κ</i>_eff) of the laser gain medium with the distributed face-cooled composite synthesized by the inter-layer surface activated bonding (il-SAB) has been proposed. The thermal resistances at the bonded interface (<i>R</i>) between 1at.% Nd:YAG and a c-cut sapphire single crystal in several composites were experimentally confirmed and it was found that il-SAB brings negligibly small <i>R</i>. On the contrary, <i>R</i> in the bi-layer composite of 1at.% Nd:YAG and sapphire sandwiching indium foil fabricated by 6.0-kN uniaxial pressing reached 1.4 × 10^-5 m²K/W at 25°C. In the case of the simple contact of 1at.% Nd:YAG and a sapphire single crystal, <i>R</i> at 25°C was 4.3 × 10^-4 m²K/W. Consequently, effective thermal conductivities in bi-layered composites with 1at.% Nd:YAG and c-cut sapphire with the same thickness fabricated by il-SAB, sandwiching indium foil, and simple contacting without bonding were evaluated to be 15.3 W/mK, 13.9 W/mK, and 3.65 W/mK, respectively. Negligible <i>R</i> of composite gain media by il-SAB indicates that <i>κ</i>_eff of 1at.% Nd:YAG will be improved from 10 W/mK to more than 30 W/mK if sapphire parts are thicker than 12.2 times of YAG parts in composite gain media with the distributed face-cooling structure.