Optical rib waveguides in Yb-doped KY(WO₄)₂ epilayers

Channel waveguide structures are the building blocks of future integrated optics devices. In order to allow for high functionality in optical routing and processing, such devices have to be compact and highly efficient in both passive and active operation modes. On the one hand, increased integration density and light bending capability require the use of waveguides with high refractive-index-contrast. On the other hand, operation as a waveguide laser or amplifier is more efficient in low refractive-index-contrast waveguides with low diffraction losses. Therefore, there is a need of fabricating active epilayers with flexible refractive-index contrast. Here we report, for the first time, on the fabrication of flexible refractive-index-contrast channel waveguides based on Yb-doped $KY(WO_4)_2$ epilayers (hereafter KYW). We used liquid-phase epitaxy to grow 1.7 at.% Yb-doped KYW layers on undoped (010) KYW substrates. The corresponding refractive-index contrast is as low as Δn = 6x10-4, which is suitable for demonstrating highly efficient planar waveguide laser action [1]. These planar layers were structured using reactive ion etching (RIE) and standard photolithography processes for producing high-quality ribs with low surface roughness. Channels of 15-μm width and 5-μm heigth were etched in a 10-μm thick layer, creating single-mode waveguides with mode-beam diameters of 17x12 μm2.