High-temperature Properties And Optimization Of The Bandpass Long-period Fiber Gratings

The high temperature characteristics of three kinds of long-period fiber gratings（LPFGs） are studied in this thesis, including lateral offset induced band pass LPFG,transmission LPFG and reflective band pass LPFG. And the disadvantages of thecommon high temperature sensors are also discussed and overcomed in this paper. Themode coupling theory of LPFG is firstly analyzed in this thesis, followed by thefabrication of three kinds of LPFGs with band pass characteristics and the analysis ofthe high temperature characteristics. A solution for further improving the band passLPFGs by incorporating fiber ring laser is finally proposed in this thesis, which isemployed to narrow the line width of the sensor and enables the sensor demodulated bythe methods that applied to the FBG. The main contents of this thesis are as followings:1. A band pass LPFG is fabricated by fusion-splicing two LPFGs with sameresonant wavelength by setting a lateral offset. The transmission ratio is determined bythe overlap area of the two fiber claddings. Compared with the symmetric LPFG,asymmetric LPFGs written by CO₂laser have higher transmission ratio in this way sincethe cladding mode distribution in the cross section of the fiber is asymmetric. Then ahigh temperature experiment of such a band pass LPFG is conducted and theexperimental results show that the temperature sensitivity is⁰.091nm/℃within100℃-900℃, which is comparable with a common LPFG based temperature sensor.2. The second band pass LPFG is fabricated by fusion splicing two LPFGs withsame resonant wavelength with a piece of the hollow core fiber （HCF） amid them. Theguidance mechanism of the HCF is that the core has no guidance while the claddingcould guide the light propagating along the fiber. The diameter of the hollow core is13μm and the diameter of the cladding is the same with the single-mode fiber, i.e.,125μm. The light in the core decreases even disappears with the increment of the lengthof the HCF since the HCF core cannot guide the light. The cladding mode coupled bythe LPFG propagates along the fiber with the guidance of the cladding and then iscoupled back to the core by the other LPFG, resulting in pass band. Such a band pass isalso put into a oven to take high temperature experiment and the results show that thesensitivity is comparable to that of a single LPFG.3. The reflective band pass LPFG is fabricated by fusion splicing a piece of HCFinto the LPFG end by a special process and then the HCF is cut using a cleaver. The key point is to eliminate the reflection of the core mode at the fusion point. Limited by theinstrument, the highest temperature measured by such a reflective band pass LPFG is600℃and the sensitivity is0.095nm/℃.4. To solve the problems of band pass LPFGs including wide linewidth,complicated demodulation and inconvenient for demodulation, two methods areproposed, the first one is increasing the period number of LPFG and the second isincorporating the band pass LPFGs into fiber ring laser where a spectrum like FBG isobtained. Thus the methods for demodulating the FBG sensors can be used for thedemodulation of band pass LPFGs. Moreover such a method enables the LPFG to beconvenient for long distance measurement.