High voltage fiber optic sensor for the measurement of an alternating electric field

The present invention is relative to a method to measure an alternating electric field having a frequency range between 10 Hz and 500 MHz, wherein said method includes: - Realizing a first diffraction grating (1) in a first location, in a core of a silica-based optical fiber (2) free from any coating; - Measuring the peak reflection wavelength of said first diffraction grating (1); - positioning said optical fiber (2) along a direction having a non-zero component of an electrical field generated by an alternating voltage to be measured; - coupling a substantially monochromatic light to said optical fiber (2) surrounded by said electric field; - Measuring a parameter dependent on the shift of said peak reflection wavelength due to mechanical deformation or refractive index change of the material in which said optical fiber (2) is realized due to the electric field; - Calculating a value of the electric field causing such a measured deformation or refractive index change.