Research On Sagnac Interometer Based Fiber Optic Current Sensor

The research achievements of fiber-optic current sensors (FOCS) during the last fewyears are particularly surveyed in this paper. The Sagnac fiber-optic current sensor isdeeply researched and the complete system theory model has been established. The factorswhich affect the system measuring precision are comprehensively analyzed and simulated.Some acting rules which influence the system precision are acquired. The functionelements of the measuring system are designed. A new method to produce circularpolarization light is proposed. The passive phase bias to the interferometric system with a3×3 coupler is theoretically probed and the related system model is established. The initialexperiment research is carried out with the sensor's electronic circuit designed. Firstly, considering all the factors such as the effect of two delayers axescoincidence error, the effect of the delay error of the phase delayers and the effect ofintrinsic circular birefringence etc., which would affect the measuring precision, thecomplete sensor system theory model is established. The acting rules of all the factors tosystem response with birefringence existing in sensing fiber are analyzed. The effects ofeach factor in the model on measuring precision are simulated and the curves and thetypical error data which represent the effecting rules are theoretically acquired. Accordingto the simulation results, it is found that the system response error is increasing with thetwo delayers axes coincidence error, the delay error of the phase delayers and the intrinsiccircular birefringence etc. Compared with the other error factors, the effect of the intrinsiccircular birefringence is relatively small. The effects of all the factors on system error arerelated to the linear birefringence of sensing fiber, and the larger the linear birefringence is,the larger the effects of the factors are. The birefringence should not be larger than πwhich is a specific value of the sensing fiber birefringence. Birefringence is the main errorsource of the measuring system, and the other error sources will be easy to predict andprocess if the system with no birefringence.Secondly, two key function elements of the system, λ/4 phase delayer and phasemodulator(bias element), are theoretically probed. A method to get λ/4 phase delay withpolarization maintain fiber is proposed. The polarization state of a linear polarization lightpropagating in a polarization maintaining fiber is changed by twisting a length of the fiberat the output end. It is theoretically shown that light propagating in polarizationmaintaining fiber will be changed to circular polarizations with half of the beat of the fiberoutgoing end twisted π/2 angle. The passive phase bias with 3×3 coupler is theoreticallyanalyzed and the theory model related to the sensor configuration is established. Theinfluence of the 3×3 coupler on sensor sensitivity is simulated and analyzed. The resultsshow that the bias phase is dependent on coupling ratio of the coupler. With a certaincoupling ratio, the perfect bias could be acquired and high sensitivity measuring of smallsignal can be carried out. A group of ratio values which can make perfect measuringsensitivity is calculated and acquired.Finally, the system electronic circuit including laser driver, preamplifier and bandpass filter is designed. The initial experiment research of the model is carried out. Themeasuring sensitivity of the experimental system is analyzed and the relation betweenresponse and the current measured is acquired. The noise of the system is theoreticallyanalyzed and the noise value is estimated. The experiment results initially prove the theorymodel. But the stability and linearity of the measuring data are not good enough, and thesystem is far from practice application. The system output shows that electromagneticdisturbance is the main noise resource and much larger than the noise level estimated.The research in this paper is expected to provide theoretical basis to development ofSagnac fiber-optic current sensors and to help for practice product design.