Fabrication of a high power Faraday isolator by direct bonding

With increasing output power of lasers, absorption in optical components grows larger and demands on heat withdrawal become challenging. We report on the fabrication of a Faraday isolator for high power fiber laser applications (P = 1 kW) at a wavelength of 1080 nm and operation at ambient conditions. We investigate direct bonding of Terbium Gallium Garnet to sapphire disks, to benefit from the good heat spreading properties (having a 6-fold higher thermal conductivity than TGG) at high transparency of the latter. Successful bonding was achieved by extensive cleaning of the plane and smooth surfaces prior to low pressure plasma activation. The surfaces to be bonded were then contacted in a vacuum environment at elevated temperature under axial load. Our measurements show that the bonded interface has no measurable influence on transmission properties and bonded samples are stable for laser output powers of at least 260 W. As compared to a single Terbium Gallium Garnet substrate, wavefront aberrations were significantly decreased by bonding sapphire disks to Terbium Gallium Garnet