Detection of the phase disturbance in fibre optic Sagnac interferometer

ABSTRACT Unified physical interpretation of spatial and frequency characteristic of fibre optic ioop mterferometer is presented.The existence of insensitiveness regions of this interferometer and its periodic frequency characteristic is shown. It is alsodemonstrate that such an interferometer may be used as an distributed sensor for localization of a disturbance, because itsresponse is linearly dependent on the position ofthis disturbance in relation to the loop. 1. INTRODUCTION The construction of fiber optic interferometers is similar to the construction of classic optical interferometers with open light flux. They are designed for measurements of phase shift caused by an interaction of external fields on the opticalfibre.' The optical configuration of interferometers, namely propagation of light in closed optical waveguide, is alsoadopted from classical interferometry. The main components of fibre optic interferometers are: semiconductor laser, a coupler, a detector and a demodulator. The MachZender interferometer, shown in Fig. 1, represents typical configurationoffibre optic interferometer. The optical part ofthis device is single-mode optical fibre.Fig. 1 The configuration offiber optic Mach-Zender interferometerBoth arms of the mterferometer are uniformly supplied by light passing from a source through a coupler S,. Those armsare called signal or measuring arm and reference arm, respectively. The phase shift in measuring arm, in relation to re-ference arm, is caused by a function of signal 4(t) measured by transducer (which couples field acting on optical fibre withphase disturbance of light wave). This phase shift is converted into modulation oflight intensity detected by photodetec-tors as a result of interference oftwo optical waves in a coupler S2. The modulation factor is a function of phase shift. Thesignal function 4(t) is reproduced in the demodulator by means of electronic conversion of modulation function. A changeof forces acting on a transducer may be determined by measurement of a change of characteristic functions. The magni-tude of these changes does not depend on a position of the transducer and an optical circuit length.The above mentioned basic metrological fimction is fuffihled by all interferometers irrespective of configuration,