Enhancement of optical transparency in Bi<sub>2</sub>O<sub>3</sub>-modified (K<sub>0.5</sub>Na<sub>0.5</sub>)<sub>0.9</sub>Sr<sub>0.1</sub>Nb<sub>0.9</sub>Ti<sub>0.1</sub>O<sub>3</sub> ceramics for electro-optic applications