Fabrication and characterization of room-temperature-bonded composite lasers

Summary form only given. Thermal effects such as thermal lens and thermally induced birefringence are critical in solid-state lasers, degrading the beam quality and preventing higher-power operations. The composite lasers, which consist of laser-ion-doped and undoped materials, have been successful in suppressing those thermal effects because the heat generated in the doped region can be effectively removed into the undoped segment. Composite structures have been fabricated mainly with the diffusion bonding [1], and a hybrid composite of a doped single crystal and undoped ceramics has been also recently reported [2]. However, it is difficult for the diffusion-bonding technique to bond materials with different thermal-expansion coefficients, and the undoped ceramics are limited to isotropic materials. In this paper we report fabrication of composite lasers using another technique: room-temperature bonding (RTB). The RTB, also called as the surface-activated bonding (SAB), is a versatile method to bond a variety of materials including dielectrics and compound semiconductors without any degradation of crystal qualities [3]. Since the RTB is a process at room temperature which enables us to bond the materials with different thermal expansion coefficients, we believe that this technique is promising to develop new composite lasers with superior thermal properties. We have succeeded in fabrication and laser oscillation of composite Nd:YAG and Yb:YAG. Undoped YAG which is a typical heat-sink material was used in this work to characterize the fundamental properties. Bonding was performed in a vacuum of ~2 x 10-5 Pa after the surfaces of the crystals to be bonded were irradiated by the Ar-atom beams and chemically activated (Fig. 1(a)). One-side and both-side end-capped composite lasers were fabricated. For the Nd:YAG composite, the sizes of the doped (1 at.%) and undoped YAG were 3 x 3 x 3 mm3 and 3 x 3 x 2 mm3, respectively (Fig. 1(b)). As for the Yb:YAG composite, the si- es of the doped (25 at.%) and undoped YAG were 2 x 2 x 0.6 mm3 and 10 x 10 x 2 mm3, respectively (Fig. 1(c)). The transmittance spectra of the composite lasers measured in the near-infrared region showed negligible degradation compared with the noncomposite crystals, which indicates that the bonded interfaces have good optical quality free from the scattering by air gaps or voids [4]. Figure 2 shows the laser characteristics for the Nd:YAG. The composite Nd:YAG lasers achieved higher power and slope efficiency than the noncomposite one. We also obtained the same results for the composite Yb:YAG. In near future we will develop composite lasers using heat-sink materials with higher thermal conductivities.