| Microstructured fiber(MSF), also called as photonic crystal fiber(PCF) or holy fiber(HF), becomes a hot research topic in the field of fiber communication, fiber sensor and fiber laser because of its flexible structural design and unique optical waveguide character. Combining MSF with fiber grating and interferometer technique, the variety of fiber sensor is enriched and the performance is improved. Besides, long period fiber gratings(LPFG), which is based on the fundamental core mode being resonantly coupled to the cladding modes that are absorbed by producing resonant dips in the transmission spectrum, is one of the most important fiber sensors, and the inscription of LPFG in MSF, what can improve the sensing quality and the resolution of sensor. LPFG fabricated upon CO2 laser pulses in all-solid bandgap fiber(AS-PBGF) was presented in this paper, and its sensing characteristics were studied; The relationship of period and resonant dips of LPFG was simulated by using mode coupling theory. LPFG in single mode fiber(SMF) and grapefruit microstructure fiber(GMF) by femtosecond laser exposure was fabricated, strain and temperature sensing test of the LPFG in GMF was studied as well. We also used this GMF to fabricate a Mach-Zehnder interferometer(MZI) by lateral offset splicing both ends of GMF with SMF, and its sensing test was studied. The main contents are arranged as follows:(1). Review of the optical fiber sensor and the basic step and the general steps to solve the mode property of MSF by FEM in detail.(2). Sensing characteristics of LPFG fabricated by CO2 laser pulses with AS-PBGF based sensor was investigated. Mode field distributions and bandgap characters of this fiber were analyzed by theory and experiment. The resonant dip of this LPFG is at the wavelength of 1335.76 nm,insert loss is about 5.5 d B, fringe contrast reaches 15 d B, and the 3 d B wavelength width from the bottom is about 2.6 nm. In experiment, we tested sensing characteristics of this LPFG to strain, surrounding refractive index(SRI), temperature and twist.(3). Characteristics of LPFG fabricated by femtosecond laser pulses was investigated, the relationship of grating period and resonant dip of LPFG is analyzed by mode coupling theory, and experimental verification by inscribed LPFG in SMF using 800 nm femtosecond laser pulses, the results are coordinated with the theory simulation. We fabricated LPFG in GMF with the period of 250 μm, the resonant dip was obtained to be 1449.4 nm, insert loss is about 1.397 d B, fringe contrast reaches 11.36 d B and the 3 d B wavelength width from the bottom is about 3.87 nm. When the period was designed to be 230 μm, the resonant dip shifted at the wavelength of 1325.8 nm, which is coordinated with law of LPFG in SMF. LPFG inscribed in GMF with the period of 250 nm was applied to do the sensing researches, strain sensitivity of-1.88 pm/με and temperature sensitivity of 0.267 pm/℃ were obtained. The results show this LPFG can decrease the crosstalk of temperature in strain sensing.(4). Sensing characteristics of MZI fabricated by lateral offset splicing the both ends of GMF with SMF for one dimensional micro bending was investigated. Mode field distributions and dispersion of GMF was analyzed by FEM, and the transmission spectra characteristics of this MZI with different length of GMF was studied in experiment, the results demonstrated that the interferential pattern of this MZI was generated by the modes of LP01 and LP11. Sensing characteristics of this MZI to micro bending, temperature and surrounding refractive index were tested, bending sensitivities of 0.441 nm /m-1 and-0.754 nm /m-1 at 0° and 180° bending orientation of 1513.67 nm are obtained with 3.4 cm length of MZI. Temperature sensitivity is-8.9 pm/℃, and the crosstalk of temperature at 0° and 180° bending orientation are 2.02?10-2 m-1 /℃ and 1.18?10-2 m-1 /℃, respectively. Besides, the response of this MZI to SRI is low. Therefore, this MZI can decrease the crosstalk of temperature and SRI in the micro bending measurement. |
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