A varifocal lens using an electrooptic KTa1−xNbxO3 crystal with a microsecond order response time

We report a varifocal lens made of potassium tantalate niobate (KTa_1-xNb_xO₃, KTN) that exhibits a huge second-order EO effect, namely the Kerr effect. The lens consists of a KTN block on which four strip film electrodes are mounted. A voltage applied with these electrodes forms an electric field distribution, modulates the refractive index of the KTN block, and bends light rays. The lens power can be controlled from zero to 1.4 m^-1. By inserting the KTN lens in front of a fixed glass lens with a focal length of 250 mm, we were able to move the focus by 87 mm. It was confirmed experimentally that the focus of the lens moved as theoretically predicted with a response time of faster than 2 μs. Owing to the anisotropy of the Kerr effect, the lens is polarization dependent. We also present a technique for eliminating this dependence.