| With the increasingly important role of optical fiber sensors in today's society,it has become an inevitable trend to continuously improve the performance of optical fiber sensors.Because of its special structure,multi-core optical fiber?MCF?can often achieve better sensing performance and meet special sensing needs.Based on the National Natural Science Foundation of China?NNSF?,this doctoral dissertation has made in-depth research on the fabrication of new MCF microstructures and the application of MCF sensors.The main research work and innovations are summarized as follows:1.A novel T-shaped taper was fabricated to form the Mach-Zehnder interferometer?MZI?with an all-solid twin-core fiber?TCF?.The fabrication process of the sensor is simple and suitable for micro sensing applications.We made the novel T-shaped taper between TCF and single mode fiber?SMF?by using commercial fusion splicer.Because of the multipath interference of the beam in the TCF,the mode interference can be used to measure the sensing parameters.The refractive index?RI?,strain and temperature response characteristics of different peaks of the sensor are investigated.For the RI measurement,within the range of 1.3388 to 1.3908,the maximum wavelength drift of2.4 nm will occur for 1%change of RI.For strain measurement,the maximum strain sensitivity of the sensor is 4.61 pm/??.Moreover,the temperature stability of the sensor is good,and the temperature sensitivity is about 0.002 nm/°C in the range of 30°C to80°C,which will facilitate the realization of more accurate sensor measurement applications.2.A miniature optical fiber strain sensor based on TCF was developed and experimentally demonstrated.On the basis of multicore fiber sensor with T-shaped taper,we used the cleaver to cut the TCF from the middle,and rotate one of the TCFs by 90°,then reassemble them.The optimized interference spectrum was obtained by the new optical fiber sensor.It presents a method to improve the sensor performance for the special structure of TCF.The experimental results show that the improved sensor structure has better stains sensing sensitivity.Under different strains,the wavelength sensitivity and intensity sensitivity were 6.11 pm/??and 9.9×10-3 d B/??,respectively.The temperature stability of the sensor is good;with different temperatures,the corresponding wavelength sensitivity is about 0.69 pm/°C,and the intensity sensitivity is about 0.0053 d B/°C.In addition,the proposed technique for making a new T-shaped taper is highly repeatable,which can provide an effective way to fabricate different types of MCF microstructures.3.A new RI sensor based on four-core fiber?FCF?was fabricated,which has a taper structure.In our experiment,we analyzed the influence of different fiber parameters on the sensor performance,and found that the taper structure significantly enhanced the sensitivity of the sensor's RI sensing.According to the test,the maximum refractive index sensing sensitivity of the sensor is 171.2 nm/RIU,and the corresponding RI resolution is 2.92×10-4 RIU.In addition,by reducing the size of the taper structure,the sensitivity of the penetrating sensor can be improved,so as to meet more application scenarios,such as chemical sensing or biological sensing with high sensitivity.4.An ultra-compact chemically etched TCF based RI sensor was proposed.The etched fiber was fabricated by immersed in an aqueous solution of hydrofluoric acid?HF?to etch the cladding.The numerical simulations demonstrated that only the cladding modes strongly interacted with the surrounding media,and a high-order cladding mode was more sensitive to external medium.In experiment,the RI response characteristics of different peaks of the sensor were investigated.A maximum wavelength shift of 3.51 nm for 1%change of RI was measured in the range of 1.3388to 1.3981,and the temperature response characteristics of the sensor were also investigated,which demonstrated that the sensor has a low cross-sensitivity to a temperature of 1.06×10-6RIU/°C.Experimental results show that the etched TCF plays an important role in improving the sensitivity of the RI sensors,which can provide a useful reference for designing and optimizing the sensing characteristics.Due to easy fabrication and low temperature cross-sensitivity,the sensor can be a suitable candidate in biochemical sensing.5.A new type of optical fiber sensor was designed.The sensing head was fabricated by splicing a short section of TCF between two sections of multimode fiber?MMF?.When the beam propagate from the SMF to the MMF,a variety of high-order modes were excited,and these high-order modes continue to propagate to the TCF,which enhance the mode interference in the sensor structure and improve the sensor sensing performance.Through experimental tests,a maximum RI sensitivity of 125.5 nm/RIU is achieved,and a strain sensitivity of 3.11 pm/??.The temperature induced wavelength shift perturbation is less than 0.76 nm as temperature varies from 30°C to 90°C,achieving a low temperature sensitivity of 13.36 pm/°C.All sensors fabricated in this paper show good linearity in terms of the spectral wavelength shift.6.A miniature optical fiber temperature sensor consisting of a seven-core fiber?SCF?and a few mode fiber?FMF?was proposed and experimentally demonstrated.During the FMF region,few modes can be excited and will propagate within the SCF.A strong interference with the highest extinction ratio of 27 d B was achieved.The temperature response characteristics of the sensor have been investigated,obtaining a temperature sensitivity of 91.8 pm/°C in the temperature range of 25°C to 110°C.We also looked at the fast Fourier transform?FFT?spatial frequency spectrum of the sensor,with the change of temperature,the sensitivity of 1.57×10-2 a.u./°C was obtained.Since the fabrication process of the sensor is much simpler and faster,the sensor can be a good candidate for sensing applications.7.A novel Fabry-Pérot interferometer?FPI?based on a section of TCF was proposed and experimentally demonstrated for micro-displacement sensing.By mounting the sensor on translation stages and moving the feedback fiber in different directions,we tested different reflection spectras to obtain the variation of the interferometer and realized the sensing measurement.In experiments,different sensitivity of micro-displacement along X-axis,Y-axis and Z-axis has achieved,and the displacement sensitivity along the Y-axis can reach 0.954 nm/?m,in conjuction with a higher resolution 0.052?m.Due to the different sensitivity of the sensor in different displacement directions,the sensor has the potential to realize multi-dimensional micro-displacement sensing.8.A new temperature sensor based on SCF was fabricated.Two segments of SCF were encapsulated in a glass capillary using silicone rubber to form an F-P cavity structure,which used as a sensing head.In the experiment,we analyzed the reflection spectrum of different cavity lengths in detail,and used this characteristic to realize sensing detection.By analysing the experimental data,we obtained the temperature sensing characteristics of the sensor.9.An image processing method has been developed to obtain multicore fiber geometric parameters.According to the characteristics of multicore fiber,we use MATLAB to process the sectional view of the MCF,and the algorithm mainly concludes the following steps:filter out image noise,edge detection,use of an appropriate threshold for boundary extraction and an improved curve-fitting algorithm for reconstruction the cross section,then we get the relative geometric parameters of the MCF in pixels.We also compare different edge detection operators and analyze each detection results,which can provide a meaningful reference for edge detection.Automatic analysis of optical fiber parameters with digital image processing technology will be an important frontier trend,which is of great value to design and analyse of MCF fibers. |
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