Research On The NaCl Solution Concentration Sensor Based On Long Period Fiber Grating

With the development and progress of the fiber grating technology, the fiber gratinghas been more and more famous in application fields like optical fiber communication,optical fiber laser and optical fiber sensors.This paper is focused on the long period fiber grating. Deep research has been doneon the coupling theory and sensing performance of long period fiber grating. Themain work done in the paper includes the aspects below:(1) Research has been done on the theory of fiber grating mode coupling theory. Thetheory is derived based on the three layer fiber mode. The fiber grating coupling modeand field expressions are discussed. We deduced the coupling efficiencies between thefiber core guide modes itself and the clad coupling modes. The coupling modes oflong period fiber grating are studied based on the coupling equations. Thetransmission spectra of long period fiber grating are simulated with the transfer matrixmethod.(2) The sensing characteristics of LPFG are studied based on the derivation of thecoupling mode theory and simulation results of the transmission spectra. The factorslike length, period and the refractive index modulation depth are discussed. Therefractive index change infect on the transmission spectra is analyzed.(3) A solution concentration sensing system is designed and improved. A fixing groove ismade to prevent the fiber grating from the impact of the stress. The temperaturemaintain21℃in the progress of the experiment. The transmission spectra between1350nm to1650nm are measured. The study shows the transmitted optical powervaries linearly along with the concentration change. We have got the optical powerexpression through the linear fit of the experimental data. The experiment system isimproved so that we can monitor the concentration change visa optical power changeat one certain wavelength. Experiment shows, the optical power expression could beobtained. The measure range reaches40g/L with a resolution of5g/L. The opticalpower sensitivity is0.06W/g/L.