A precursor composition of TiO2 doped with erbium (Er) and yttrium (Y) for forming a film used in a planar optical waveguide amplifier. The precursor composition includes 100 mol % TiO2 precursor compound, about 0.1-10 mol % erbium ion (Er3+) precursor compound, and about 1-50 mol % yttrium ion (Y3+) precursor compound, thereby forming a doped TiO2 film co-doped with erbium and yttrium an amorphous structure to achieve the enhancing effect on photoluminescence properties.