Electric field distribution fluctuation period measuring method and electric field distribution fluctuation period measuring device

An electric field distribution variation cycle measuring apparatus that measures a variation cycle of an overlap of an electric field distribution between first and second higher-order modes propagating through an optical fiber, the electric field distribution variation cycle measuring apparatus comprising:

• a mode multiplexing and demultiplexing device that converts pump light into the first higher-order mode and converts probe light having a predetermined optical frequency difference from the pump light into the second higher-order mode and causes the pump light to be incident on one end of the optical fiber and the probe light to be incident on the other end of the optical fiber;
• and
• a gain analysis device that receives, at one end of the optical fiber, the probe light in the state of the second higher-order mode after passing through the optical fiber and including a stimulated Brillouin scattering generated when the pump light and the probe light collide in the optical fiber in a distributed manner in a longitudinal direction of the optical fiber, acquires a Brillouin gain of a desired wave mode of the stimulated Brillouin scattering, and sets a variation cycle of the Brillouin gain in the longitudinal direction of the optical fiber as the variation cycle of the overlap of the electric field distribution between the first and second higher-order modes.