Research On Refractive-Index Sensing Technology Based On Optical Fiber Gratings

Fiber grating sensors offer numerous advantages over traditional transducers due to their small size, high sensitivity, immunity to electromagnetic interference, environmental ruggedness, wavelength multiplexing, possibility of distributed sensing, and have been successfully used in many aspects, such as civil, aerospace systems for structural health engineering and smart structures, mainly focusing on the sensing of strain and temperature. Fiber gratings, especially long-period gratings (LPGs) are sensitive to the change of external refractive index (ERI), which can be used as refractometers. Moever, refractive index (RI) sensing is very important for biological and chemical applications in environmental monitoring, medical diagnosing and liquid hydrocarbon leak detecting, etc., in which case a number of substances can be detected through measurements of the refractive index. Therefore fiber-grating-based biochemical detecting technologies have been attracted considerable attentions recently.Refractive-index (RI) sensing is the essence of fiber-grating-based biochemical sensors, and achieving high sensitivity and solving potential temperature cross-sensitivity are the two major problems. In this work, we employed fiber Bragg grating (FBG), long-period grating (LPG), and LPG based M-Z interferometer to carry out the RI sensing, mainly focusing on improving the sensitivity to ERI and simultaneous measurement of RI and temperature. We completed the following research work and brought forward some innovative schemes.Firstly, the spectral properties of FBG and LPG were analyzed with the coupled mode theory and transfer matrix method, then the photosensitivity of fiber was introduced and the fabrication techniques of fiber gratings were summarized. During the project, we have developed a high-pressure hydrogen-loading set-up and established a fiber grating fabricating system, which provide a good experimental platform for the research work.Then, FBG refractive index sensing was investigated theoretically and experimentally. The relationship between the Bragg wavelength and fiber cladding diameter was analyzed, as well as the change of the effective index of the guided mode of fibers with different cladding diameters as ERI changes. In the experiment, we designed and fabricated an experimental device for both etching and RI measuring, etched two FBGs with different etching degrees and then tested their sensitivities to ERI. The RI measurement results have shown that thinner FBG possesses higher sensitivity.The co-directional coupling between the guided mode and cladding mode in LPG makes it inherently sensitive to ERI. The effect of reducing fiber cladding of LPG on the shift of resonance wavelength and on the sensitivity to ERI was theoretically analyzed. We etched two groups of LPG with different degrees, and then finished the RI measurement with the etched LPGs. The results have shown that the sensitivity of LPG to ERI can be greatly improved by etching the fiber cladding. Furthermore, it was observed for the first time thatthe sensitivity of the LPG to ERI is increasing with etching degree, while the order of the cladding mode, which involved in the coupling with guided mode at the certain wavelength, is decreased.Two identical LPGs with ~3 dB transmission consist of an LPG-based Mach-Zehnder interferometer (LPG-MZI), which is sensitive to ERI due to its special mode coupling mechanism. Comparing with a single LPG, LPG-MZI possesses narrower resonance peaks and can improve the spectral quality of the loss-dip for high accuracy wavelength measurement. To improve the sensitivity of LPG-MZI to ERI, we proposed two novel schemes, one is etched LPG-MZI and the other one is fiber-taper seeded LPG-MZI.By etching the fiber cladding of LPG-MZI, more evanescent field of cladding mode are distributed and interacts with the surrounding medium, which improves the sensitivity. We theoretically and experimentally characterized both the spectrum of the etched LPG-MZIs and their responses to the change of ERI. The results have shown that the response of the interference dip at the wavelength around 1550 nm of the etched LPG-MZI with less cladding diameter is still more sensitive to the change of ERI, though the order of the involved cladding mode is decreased.In a fiber-taper seeded LPG-MZI, the fiber-taper section made between the two LPGs can significantly improve the sensitivity of the sensor because the evanescent field of the selected cladding mode in the surrounding medium is increased by fiber tapering. The experimental results have shown that the sensitivity of LPG-MZI to ERI can be efficiently improved by a fiber taper, and can be flexibly tailored by the tapering degree. Enhanced sensitivity of the fiber-taper seeded LPG-MZI with a taper length of 16 mm and a taper waist of 38.75 [im, which was five times higher than that of a normal LPG-MZI, was achieved in the experiment.Finally, the simultaneous measurement of temperature and ERI was presented by using a sandwiched structure of LPGs. An LPG-MZI with co-directional coupling between the guided mode and a low-order cladding mode acts as a temperature sensor with low RI sensitivity, whereas another in-between LPG which couples the guided mode with a higher order cladding mode performs as an RI sensing element. The experimental results show such a balanced all-LPG structure is not only suitable for use as a compact temperature-compensated RI sensor, but also convenient for fabrication.