Study On The Fabrication Of Mechanically-induced Long-period Fiber Gratings And Its Application In Refractive Index Sensing

With the quickly development of optical fiber and photon devices fabricationtechnology, fiber grating has become one of the most representative and promisingfiber-optic passive components and has been widely used in many fields. Long-periodfiber gratings (LPFGs) as a new kind of optical passive devices，which can promote thecoupling between the core mode and the cladding modes, resulting in a notch in thetransmission spectrum at a specific wavelength, have been studied by many researchersall around the world recently. LPFGs have been widely used in optical communicationowing to its many advantages such as low back reflection, low insert-loss, easy tofabrication, small size, high sensitivity, anti-electromagnetic interference andcompatibility with fiber communication networks. On the other hand, the resonantwavelength and coupling strength of LPFGs are strongly influenced by externalconditions, such as temperature, strain, bending, and as well as refractive index,Therefore, LPFG-based sensors have attracted wildly attention and research of homeand abroad scholars. In this dissertation, the fabrication of the mechanically-inducedLPFG (MI-LPFG) and its application in refractive index sensing are researched. Themain contents of this dissertation are given as follows:1. Fiber grating development and classification are summarized. Several methodsof LPFG fabrication are showed and their advantages and disadvantages are compared.LPFG-based refractive index sensing scheme are also summarized. Current researchstatus and application prospect of the MI-LPFG are introduced.2. On the basis of the analysis of mode field in the multi-layer circular uniformoptical waveguide, generalized functions of arbitrary refractive index distribution in thefiber cross section are given by using the method of the discretization of refractive indexring. Combined with the coupled mode theory, the coupling equations of LPFG with theuniform and non-uniform refractive index modulation in the cross-sectional of fiber arederivated. Finally, the spectra characteristics of the two kinds of gratings are analysisand discussion through the numerical methods. 3. A simple method for fabricating MI-LPFG owing to periodic elastic opticaleffects and microbending effect by pressing two periodically grooved plates fromdouble sides is proposed, and the transmission spectra of the LPFG is investigatedexperimentally. The results show that the characteristics of the LPFG can be modifiedby adusting the pressure and a peak transmission loss of24.6dB has been achieved. Amaximum PDL of7.42dB at1551.9nm for LP13cladding mode and the resonancewavelength seperation of1.9nm are measured, which increase with the order of thecladding mode. In addition, the characteristics of the MI-LPFG formed over a twistedfiber are also investigated. And the experimental results show that the resonances shiftto shorter wavelength as the applied twist increases. Finally, the coupling efficiency ofthe LPFG can be improved by etching the cladding of the fiber.4. Phase shifted LPFG (PS-LPFG) has attracted a great of concern because of itsspectra design with more flexibility. Based on the micro-bend effect and photo-elasticeffect, the mechanically-induced π PS-LPFGs are fabricated in the dissertation. Theinfluence of the grating parameters on the transmission spectra of the π PS-LPFG aredetailed discussed. The results show that: the resonance peak amplitude of π PS-LPFGdeepens with the applied pressure increases, the bandwidth of the pass-band increasewith the number of the π phase-shifts increases, and the central wavelength of thepass-band remains unchanged. For LP13mode coupling, the pass-band width of theLPFG with number of π phase shifts of nine can achieve32.34nm.5. A novelty fiber-optic refractive index sensor based on a MI-LPFG in acomposite waveguide is first proposed and experimentally demonstrated in thisdissertation. And a series of organic reagents with different refractive index and NaClsolution with a concentration of0to25%are used in the experiment to test thesensitivity of the sensor. The results show that: a higher sensitivity of the sensor can beobtained by using either a higher-order cladding mode coupling for a given gratingperiod, or a longer grating period under the condition of the same order cladding mode.The LPFG of580μm for the LP14cladding mode has a sensitivity of72nm/RIU, whenrefractive index changes from1.33to1.434, which is10times than that of the LP11cladding mode.