PURPOSE: To elevate the measuring accuracy with a higher utilization efficiency of light, by providing polarized beam splitters and total reflection mirrors to have two polarized components perpendicular to each other undergoing a modulation.

CONSTITUTION: A photovoltage sensor is made up of optical fibers 1 and 10, microlenses 2 and 9, polarized beam splitters (PBS)3 and 8, total reflection mirrors 4 and 7, a 1/4 wavelength plate 5 and an electrooptic element 6. A linearly polarized light 11 transmitted through the PBS3 is modulated in the phase with an electrooptic element 6 via the 1/4 wavelength plate and modulated in the light intensity with the PBS8, entering the optical fiber 10. On the other hand, the linearly polarized light 12 is reflected with the total reflection mirror 4, after the optical path is turned by 90°, it is modulated in the phase with the electrooptic element 6 via the 1/4 wavelength plate 5 and aftaer reflected with the total reflection mirror 7, it is modulated in the light intensity with the PBS8 to be combined with the linearly polarized light 11. This can double the power of light entering the optical fiber 10 with a decrease in the light loss thereby elevating the measuring accuracy.