Patent ID: 8836950

Claim:
A Sagnac interferometer fiber-optic sensor, comprising: a light source; a fiber coupler optically connected with the light source; a sensor head unit including a light sensing fiber formed in a loop and a mirror provided an end of the light sensing fiber; a phase modulator to carry out phase modulation to the light propagating in the light sensing fiber; a phase modulator driving circuit to control the driving of the phase modulator; an optical filter optically connected between the phase modulator and the fiber coupler; a light photodetector to detect the light propagating in the light sensing fiber and convert the detected light into a detected light signal; a synchronous detection circuit to carry out synchronous detection of the detected light signal with a phase modulation angular frequency; and a signal processing circuit to calculate and output the magnitude of current to be measured using the signal detected in the synchronous detection circuit; and wherein the phase modulator driving circuit controls a phase modulation depth so that the amplitude of the second-order harmonics and the fourth-order harmonics obtained by carrying out the synchronous detection of the detected light signal with the phase modulation angular frequency becomes the same, the signal processing circuit includes a normalization circuit to calculate a reference value by dividing the amplitude of the third-order harmonics with the amplitude of one of the second-, fourth- and sixth-order harmonics or the sum of the amplitudes of the second- and fourth-order harmonics, in which the absolute values of the amplitude of the second- and fourth-order harmonics are controlled to become same by the phase modulator driving circuit, and the normalized reference values is outputted as an output signal proportional to the current to be measured, and the optical filter is optically connected with the phase modulator for converting light from a light source to a linearly polarized light, the optical filter including; a polarizer, a first Lyot depolarizer configured by a first polarization maintaining fiber provided on the light source side of the polarizer and the propagate the light by defining an optic axis of the polarizer, a second polarization maintaining fiber provided on the phase modulator side of the polarizer to propagate the light by defining an optic axis of the polarizer, the phase modulator including a third polarization maintaining fiber provided on the polarizer side to propagate light by defining an optic axis, and a second Lyot depolarizer configured by the second and third polarization maintaining fibers, wherein the full length ratio of the first Lyot depolarizer and the second Lyot depolarizer is set to be 1:2n or 2n:1 using the positive integer n.