The optical tuning of the power splitting ratio in an integrated optical Y-splitter fabricated on the substrate of a photorefractive lithium niobate crystal (LiNbO3) has been studied. The areas of the Y-splitter that are most sensitive to local external illumination are determined. Tuning of the splitting ratio by 2% was experimentally demonstrated, which can be effectively used to increase the extinction ratio in Mach–Zehnder modulators.