Research On Mode Interferometric Optical Fiber Sensing Technology Based On Twist Microstructure

After decades of research and development in the field of optical fiber sensing,more mature theories and production processes have been formed.On the one hand,mature grating optical fiber sensors represented by fiber gratings have been widely used in engineering practice,such as geology monitoring,power system monitoring,clinical medicine monitoring,building structure monitoring,chemical and chemical testing,etc.On the other hand,scientists are also committed to continuously improving the structure of fiber-optic sensors,reducing the size of sensors,enhancing the sensitivity of sensors,and developing new fiber-optic sensors in the direction of simultaneous measurement of multiple parameters.For example,various sensors including mode interference sensors,microstructured fiber sensors,photonic crystal fiber sensors and so on have been proposed.These new types of fiber sensors have greatly enriched the connotation of fiber-optic sensing technology,expanded the application of fiber-optic sensors,and enabled fiber-optic sensors to have a wider application prospect.How to improve the measurement sensitivity and multi-parameter measurement has become one of the research hotspots of optical fiber sensors.In this thesis,based on the combination of a mode interference sensor and a micro-structure,not only a series of mode interferometric devices for twisted and grating micro-structure are developed to improve the strain measurement sensitivity of conventional mode interferometric sensors and the resolution of strain and temperature dual-parameter measurements,but also low-frequency vibration measurement is realized using a grating-type SMS structure.The study is summarized as follows:Firstly,we give in-depth theoretical analysis of the mode interference sensor,in particular the theoretical analysis and simulation of the single mode-multimode-singlemode?SMS?based multimode interference sensor,and the self-imaging effect and light field distribution including self-imaging point position influence factors are discussed.According to the the simulation compared between conventional SMS and twisted SMS structure,the feasibility of improving the sensor performance is theoretically confirmed.Secondly,by combining the SMS structure with the twisting process,a SMS-type mode interferometer strain sensor based on a twisted structure was fabricated,which improved the low sensitivity of the conventional mode interferometric structure measurement.The production process was introduced in detail,and the influence of the process of twisting speed,preheating time and heating time on the performance of the process was discussed.Different samples were made and compared by experiments.Different strain sensitivities were obtained when twisting 5 turns,8 turns,and 16 turns,and the linear relationship between the number of twist turns and the sensitivity response was obtained.The maximum sensitivity reached-42.5 pm/??when twisted for 16 laps,which is40 times that of conventional conventional SMS.Thirdly,dual-parameter measurement of strain and temperature using a twisted SMS interferometric sensor is proposed.Different from the traditional way of measuring dual-parameters cascaded by multiple devices,the structure does not require the use of a reference sensor,and only one sensor device is required to distinguish between strain and temperature parameters.Due to the introduction of the twisted structure,higher-order modes of different orders are excited,and the effective refractive index difference between the modes is larger than that of the conventional SMS structure,resulting in a larger difference in the wavelength shift corresponding to the Dip value of the different loss peaks of the interference spectrum during measurement.Spectral signals were demodulated by using wavelength and spatial frequency respectively to achieve high-resolution dual-parameter measurement of strain and temperature.The resolution of strain and temperature obtained by frequency demodulation approached±2.14??and±0.89°C,respectively.Fourthly,an MZ double-parameter measurement sensor using a single-mode fiber based on the double twist structure was prepared.This structure uses only one common single-mode fiber.By introducing two twisted points to form an MZ interferometer,the higher-order cladding modes of different orders are controlled to participate in the interference by controlling the number of twisted turns,causing different sensitivity drifts of different Dip resonant wavelengths.Different from the traditional MZ mode interferometer,the shear force caused by the twisting process combines the change of the refractive index with elasto-optical effect,resulting in different Dips drifts in the opposite direction,so that the dual-parameter high resolution measurement of temperature and strain can be realized.The resolution of the double-parameter measurement is further improved,and the measurement resolution of strain and temperature obtained in the experiment is±0.564??and±0.357°C,respectively.Finally,a mode interference sensor based on CO₂ laser writing SMS grating structure is proposed to realize low-frequency vibration measurement.By using a CO₂ laser,a long-period grating microstructure with a period of 500?m is written on the surface of a multimode fiber of the SMS structure,and its strain sensitivity is measured to be-0.13dB/??.The cantilever beam structure is used for transducing energy to achieve vibration measurement with a measurement range of 0.05 Hz to 38 Hz.A flat frequency response curve is obtained within the first-order resonance frequency of the cantilever beam,and the low-frequency vibration measurement below 0.1 Hz is achieved.