A Dual Fabry-Perot Optical Fiber Sensor

Interferometric fiber—optic sensors, while capable of high resolution .have in general been accompanied by a limited unambiguous operating range. They also have had limited applicability for measurement temperature, strain and displacment. Owing to the problem of initiating the system, i. e. the interferometer switch off, all the information about the measured phase will be lost and new initial conditions must be determined when system is turn on again(1). A. S. Gerges had presented a system which is a Michelson interferometer with two equal length arms. A digital feedback servo is used to track the zero optical path difference(2). However, the optical fiber in Gerges’s system are inherently distributed sensors. This can be a disadvantage when point measurment Eire required. G. Beheim had presented another system in which a Fabry-Perot cavity is used as a sensing cavity(3), a Michelson interferometer is used to measure the spacing of the sensing cavity. Let us call it FPM system. The central peak of the interferometric fringe in FPM system is nearly as higher as other peaks. This makes the system very difficult to lock in the central peak. Also, the central peak's half-width of the FPM system is comparatively larger than that of dual F-P system ( DFP ) . This limits the resolution of FPM system. G. Beheim has also presented a dual F-P cavities system. But its electronics CEUI not make sure that the system is locked in the central peak of the transmitted optical signal(4).