KY(WO4)2 and other materials of the double tungstate crystal family have been used for decades in active optical applications because of their relatively high refractive index (n≈2-2.04 @ 1550 nm), high transparency window (0.3- 5 μm), excellent gain characteristics when doped with rare-earth ions and reasonably high thermal conductivity (~3.3 Wm-1K-1). Low-contrast (Δn<0.02) on-chip amplifiers and lasers in this material with good performance have been shown in recent years. Higher refractive index contrast can further improve this performance, and allow easier integration with other integrated optics platforms due to their smaller footprint. Because double tungstate materials cannot be directly grown on many prospected substrates, other methods to fabricate optical waveguides with a thickness of 1-2 μm need to be investigated. In this work, swift heavy ion irradiation has been used to produce a planar waveguide by introducing a buried layer of lower refractive index in the KY(WO4)2 at a depth of ~2.5 μm. After the irradiation, an annealing step was introduced to reduce the scattering losses. The refractive index profile, effective refractive indices and absorption spectra of the planar waveguides have been investigated for several annealing temperatures, and end-facet free-space coupling of 1550 nm has been used to measure the losses. For a fluence of 3·1014 ion/cm2 of 9 MeV C ions, propagation losses <1.5 dB/cm have been demonstrated at 1550 nm after an annealing step at 350°C.