Research On Liquid Temperature Sensor Based On Long Period Fiber Grating

The fiber grating is one kind of optical passive components with broad prospects fordevelopment. In addition to traditional advantages of fiber grating, long period fiber grating(LPFG) has advantages such as small reflection performance, low insertion loss, widestop-band bandwidth, and it is very sensitive to changes in environment. So LPFG has wideapplication in communication field and sensing technology field. Due to reason such ascross-sensitivity, LPFG has not been popularization and application in temperature sensingresearch, so it has high research value and practical significance.In this paper one-time writing LPFG on single-mode fiber using the high frequencyCO2laser was successfully fulfilled; on basis of large numbers of experiments, liquidtemperature sensing properties of LPFG was studied; considering temperature sensitivityenhancement and cross-sensitivity, LPFG liquid temperature sensor was designed andimplemented.The main contents of this paper include:Firstly, from the basic principle of LPFG transmission, the formation mechanism ofLPFG and the micrograph of LPFG, this paper concludes that the internal causes and theexternal causes makes the LPFG transmission spectrum producing loss peak at the resonantwavelength. The internal causes,such as residual stress releasing, glass densification,dopant diffusion and so on, result in the periodic refractive index modulation and themelting deformation. And the external causes, for example, physical deformation, makeschanges in the structure of optical waveguide. The fabrication system of the LPFG is set upby using high frequency CO2laser on the ordinary single mode optical fiber. The analysisof the specific fabrication process is analyzed, and two main parameters which are theenergy radiated on the fiber by the high frequency CO2laser and the axial stress of the fiberare studied.Secondly, the temperature sensing characteristics measurement system of LPFG in airand liquid is designed and built. The results show that the LPFG temperature characteristicsin liquid environment are consistent with which in the air, with the temperature increasing,the resonant wavelength drift towards the long wavelength direction, and with thetemperature decreasing, the resonant wavelength drift towards the short wavelength direction, The curves between wavelength and temperature are linear and the fluctuation ofloss of the peak amplitude is not obvious.The parameter P is defined to measure thetemperature features of the LPFG, and the effect of the axial stress on temperaturecharacteristics of the LPFG is studied though many experiments.Finally, the LPFG liquid temperature sensing demodulation system is designed andrealized. Considering the temperature sensitivity enhancement and the removal ofcross-sensitivity, the structure of the LPFG liquid temperature sensor is designed andimplemented. Through a large number of temperature demodulation experiments, thetemperature error of the LPFG liquid temperature sensor is less than3oC.