Study On Optical Fiber Electric Field Sensing Based On Liquid Dielectric

With the development of power technology, electric field monitoring is becomingmore and more important. Most of traditional optical electric field sensors are based onthe electro-optical crystal and the system is composed of discrete components, leadingto bulky volume. It also suffers from the external interference and temperature causedcross effect. Integrated optical sensors aggregate all devices to compact the sensingsystem while their fabrication processes are complicate. All fiber electric field sensorshave attracted considerable advantages, including compactness, immunity toelectro-magnetic interference and ease of remote measurement. By now, the proposedall fiber electric field sensors are mostly based on piezoelectric materials and liquidcrystal. Piezoelectric materials based sensors are polarization dependent wherepolarization maintaining fiber and polarization related problems should be taken intoaccount, complicating the signal demodulation. And sensors based on liquid crystal aresensitive to temperature and the measurement range is relatively small. In this paper,three kinds of all fiber electric sensors including large offset Mach-Zehnderinterferometer, Fabry-Perot interferometer and etched fiber Bragg grating are proposedon the basis of Kerr effect and flowing of liquid dielectric by electric field.Mach-Zehnder interferometer with wide measure range can measure the electric fieldat any place of liquid dielectric. The packaged Fabry-Perot interferometer disconnectthe cavity and dielectric, avoiding the effect of temperature. Also, both the packagedFabry-Perot and FBG sensors can be used as voltage sensors, which are used tomeasure pulsed voltage and power voltage. The main content of this thesis are asfollowings:①The principle of electro-hydro dynamics is studied and the Kerr effect isanalyzed from the micro view point. The phenomena that electrophoretic force canpush the fiber is found, verifying the possibility of all fiber electric field measurementby using Kerr effect or electro-hydro dynamics.②An all fiber Mach-Zehnder interferometer with large offset is firstly proposedbased on Kerr effect, which is fabricated by fusion splicing two pieces of single modefiber at each end of another piece of single mode fiber with lateral offset of62.5μm.The electric field produced by the pulse voltage ranged from10.67kV to23.3kV ismeasured by immersing this sensor into liquid dielectric. A linearly increasing relationship between electric field and voltage is obtained from the initial to peakvoltage. While at the end of the voltage, electric field distorts, i.e., electric fielddecreases with voltage sharply and then increases. The electric field reaches up to150kV/cm when the voltage is23.3kV with the sensitivity of0.01μW/(kV/cm). Thereason for electric field distortion lies in that the insulating particles are appealed to thesurface of electrode to form resistance layer, decreasing the electric field while theelectric field increases after it decreases to0when the particles are re-injected into theliquid.③In order to decrease the error of interference beam length which caused byliquid flow, two kinds of liquid flow sensitive sensors are fabricated. One is fabricatedby using two pieces of single mode fiber and capillary tube while the other utilizes theetched fiber Bragg grating, which are then packaged to be employed as voltage sensors.The Fabry-Perot interferometer is used to measure the pulsed voltage with peakamplitude from10kV to76kV and the experimental results show that optical intensityreaches peak with1ms lag behind voltage and recovers to the initial value afterremoving voltage. The sensitivity is~0.36μW/(V.s). Then Fabry-Perot interferometerand fiber Bragg grating are employed to measure power frequency voltage withfrequency of50Hz and peak of7kV. Experimental results show that optical signalchanges periodically with frequency of50Hz while it also shows0.5ms lag withrespect to voltage.