Research On The Temperature Sensing Characteristics Of Photonic Crystal Fiber Based Long-period Gratings

By means of the coupled-mode theory and the transfer matrix method,the sensoringcharacteristics of photonic crystal fiber (PCF) based long-period gratings (LPGs) are analyzedin the thesis. Firstly, the the spectral characteristics of phase shift long-period grating areanalyzed,the transmission matrix of two parallel placed long period gratings is deduced, theaffect of introduction different non-uniform parameters include phase shift, pitch detuning,chirping and apodization is theoretical studied.Secondly, the mode properties of PCF, therefractive index of the PCF core mode and part of the cladding modes, and the temperaturesensitive properties before and after filled with temperature-sensitive refractive-index liquidare studied; Furthermore, the sensoring characteristics of a single LPG and a LPG pairinscribed in a PCF filled with temperature-sensitive refractive-index liquid are studied,respectively.The details are as follows:(1) The transfer matrix method for coupled LPGs derived from the coupled-mode theory isemployed to study the effects of phase shifts which are distributed along the LPG on thetransmission spectra. The analysis shows that with a single phase shift introduced along aLPG two transmission dips are formed in the lauching fiber's transmission spectrum bycreating a reflection peak. The position of the reflection peak is determined by the valueof the phase shift. While the phase shift is π, the grating transmission spectrum issymmetrical. Meanwhile, the grating transmission spectrum is broadened with theposition of phase shift approaching the grating center; and the number of transmissiondips increases with the number of the phase shifts.(2) The impact of various non-uniform grating parameters on the transmission spectra ofcoupld long-period gratings is analyzed. The non-uniform parameters include phase shift,pitch detuning, chirping and apodization. It is demonstrated that the bandwidth of thetransmission dips/peaks is directly influenced by the phase shift, pitch detuning and chirp.The bandwith broadenes with the increasing of the non-uniform parameters,simultaneously significant side lobes appear in the passing band. The index apodizationcan be used to suppress the sidelobes effectively.(3) Based on a single LPG formed in an index guiding PCF that is filled with a liquid oftemperature-sensitive refractive index, a novel temperature sensor is proposed. Theelectric modulation characteristic of photonic crystal fiber is researched experimentally.Using a full-vector finite element method, the modal profiles and the dispersion curves ofthe core mode and the cladding mode of a large mode area photonic crystal fiber(LMA-PCF) are calculated with its hexagonally arrayed air-holes filled and unfilled withliquid ethanol, respectively. By means of the coupled-mode theory, the temperaturedependence of the transmission spectrum of the PCF-LPG is studied. The result shows that in the temperature range from-20℃to80℃the PCF-LPG resonance wavelengthvaries almost linearly with the temperature when the PCF is filled with liquid ethanol andthe temperature sensitivity is1.766nm/℃.(4) In analogy with the Mach-Zehnder (M-Z) interferometer, a novel PCF-LPGs temperaturesensing structure is proposed. The sensor is constructed by cascading two identical LPGson a single index guiding PCF filled with liquid ethanol. Study shows that thetransmission spectrum of the two cascaded LPGs is sensitive to the temperature variationand the bandwidth of the transmission dips can be significantly reduced in comparisonwith that of the single LPG structure in (3), which will impove the measurement accuracyof the sensor.