MeV4He+implantation into single crystals of KNbO3has been used to fabricate planar optical waveguides by generating a buried layer of lowered refractive index. We characterize the optical loss of these waveguides as a function of the ion doseD, and find a minimum loss of (1.0±0.2) cm−1after implantation of 2 MeV ions at a dose of 5×1014cm−2. This is by far the lowest dose ever for which efficient waveguiding has been reported. For good guides, the observed losses are mainly due to tunneling through the refractive index barrier with nontunneling contributions of less than 30%. The reduction &Dgr;nbof the refractive indexnbin the region of the nuclear damage peak is proportional to the dose and we find &Dgr;nb/D=(7.8±1.6)×10−17cm2.