A voltate sensor for optical fibers using a transversal BSO (Bi12SiO20) modulator is presented. The sensitivity of the sensor depends on the BSO crystal shape and the polarization states of the polarizer and the analyzer. The crystal shape is designed to reduce the insertion loss in the optical fibers. We optimized the polarizer and the analyzer to maximize the sensitivity by establishing a method to calculate the sensitivity. We found that one of the best solutions is to set the polarizer for circular polarization and the analyzer for linear polarization rotated by half of the optical rotatory power to the normal of the electrodes. To minimize the insertion loss, we used a polarizer which consists of a TiO2 single crystal (rutile) plate with a quarter wave plate and an analyzer which consists of a rutile plate with a half wave plate. By using these components, we obtained 0.28%/V sensitivity at 0.85 μm wavelength.