| Sensor is an important component part of the Internet-of-things.The development of the Internet-of-things has put forward new and higher requirements for sensing technology,such as long range,distributed and multi parameter measurement.Fiber optic sensor is easy to be networked and has lots of advantages,such as anti-electromagnetic interference and corrosion resistance,and multi-point and multi-parameter measurement can be realized with it.So fiber optic sensor has bright development prospect in the era of the Internet-of-things.However,the measurement parameters of distributed fiber optic sensor based on Raman scattering,which has been extensively researched and applied,is only temperature and that of distributed fiber optic sensor based on Brillouin scattering are just temperature and strain,and quasi-distributed fiber optic sensor based on traditional fiber Bragg grating is hard to be networked.These sensors are difficult to meet the needs of the Internet-of-things.According to the demands of the new fiber optic sensor technology and the problems existing in the distributed fiber optic sensor,in this paper,we used the on-line preparing technology of grating in drawing tower and fabricated identical weak grating array on the birefringence fiber.The quasi-distributed sensing properties of both temperature and pressure of the grating array were studied and fiber optic multi-parameter quasi-distributed sensing based on the combination of special optical fiber and identical weak grating array was attempted.By employing the technology of on-line dynamic continuous fabrication of grating,the grating array has advantages of no splicing joint and high tensile strength.By using the optical wavelength domain reflection demodulation method,the recognition of grating spectral in the whole grating array is realized.By using birefringence fiber and measuring the central wavelength of reflection peak and the distance between reflection peaks,the decoupling of sensing signals with two parameters is realized.The main work and results of this dissertation are as follows:?1?The formation mechanism and sensing properties of birefringence in birefringence fiber were studied theoretically.The intrinsic birefringence of the panda type birefringence fiber is all attributed to the stress birefringence induced by the internal stress in stress region,and there is no geometric birefringence.The intrinsic birefringence of the double side holes type birefringence fiber is the consequence of the joint action of geometrical birefringence produced by structure,stress birefringence produced by structure and geometrical birefringence produced by deformation.The double side holes type birefringence fiber has a sensitive response to the static pressure of the homogeneous environment,and the sensitivity of the fiber to the external static pressure is proportional to the stress birefringence produced by the structure of the fiber.?2?The characteristics of birefringence fiber grating were theoretically deducted and analyzed.The relationship between the phase mask period,the center wavelength of the dual polarization reflection peak and the birefringence of the fiber is obtained as:?xx-?yy=2??nx-ny?=2B?=B?pm.On the basis of the above theoretical formula,the writing of gratings on birefringence fiber can be used as a method to measure the birefringence of the fiber.The gratings were written on two kinds of fibers respectively,and the distance between two peaks was measured to calculate the value of birefringence.The birefringences of the two kinds of fibers are5.37×10-4 and 7.4629×10-5,respectively.?3?The panda type fiber Bragg grating array which consists of 10 gratings and the distance between gratings is 1 m,was prepared using traditional method.The double side holes type fiber Bragg grating array was prepared which consists of 6gratings and the distance between gratings is 1 m.The fluctuating range of the center wavelength is 0.034 nm and the fluctuating range of the reflectivity is 2.4 dB.Two kinds of double side holes type fiber Bragg grating arrays was fabricated using on-line preparation system.For the first kind of the double side holes type fiber Bragg grating array with a circular core,the birefringence is 7.4629×10-5,the diameter of the fiber is 125?m,the diameter of the side holes is 26.2?m,the mode field diameter is6?m and the distance between the inner edge of side holes and the fiber center is 6.7?m.When loosely placed,the central wavelength of grating array is about 1553 nm,the reflectance is about-32 dB and the sidelobe suppression ratio is about 20 dB.The fluctuating range of the central wavelength is 0.09621 nm and the fluctuating range of the reflectivity is±1.4 dB.The second kind of double side holes type fiber Bragg grating array with an elliptical core has higher birefringence.The birefringence is 1.67×10-4,the diameter of the fiber is 125?m and the diameter of the side holes is 36.0?m.The shape of the core is elliptic,the long axis of mode field is about 15 nm,the short axis is about 5.8 nm and the distance between the inner edge of side holes and the fiber center is 9.8?m.The separation of double polarization reflection peaks can be observed in reflection spectral of elliptical core double side holes type fiber Bragg grating array and the central wavelengths of two peaks are 1551.7 nm and 1551.9 nm,respectively.The fluctuating range of the central wavelength of reflection peak of fast axis and slow axis are both 0.17 nm.The fluctuating range of the distance between reflection central wavelength of fast axis and slow axis is 0.04 nm.?4?Based on the demodulation principle of wavelength-division/time-division optical wavelength time domain reflection,we theoretically analyzed various crosstalk and delay of optical signals in the identical birefringence fiber Bragg grating array.The conclusions are got as follows:1)The shadow effect weakens the intensity of the reflected signal of the back-end grating array,and it also influences the consistency of reflection signal intensity in grating array;2)The multiple reflection reduces the signal to noise ratio of the system;3)In same demodulation condition,the grating array of lower reflectivity can be accessed to more grating sensing units.4)If the number of gratings is fixed,the influence of the crosstalk to signal can be reduced to a minimum when the grating reflectivity in the identical grating array is the same.5)Although the polarization crosstalk reduces the signal to noise ratio of the system,it does not have a significant impact on the collection of real sensing signal.6)By analyzing the time delay,it can be found that the time delay difference of weak gratings is less than 1 ps,which has no obvious effect on the demodulation system.7)It is the polarization mode delay that mainly influences the time-division multiplexing demodulation system.Through derivation of formula and numerical calculation,we obtained the corrected formula to compensate the time delay difference.?5?We used four kinds of birefringence grating arrays,and reflection spectrum of each grating in the grating array was acquired by optical wavelength time domain reflection.Based on the distance between the central wavelength of the grating reflection spectrum and the distance between the two polarization reflection peaks,we conducted multi-parameter distributed sensing experiments.1)Simultaneous distributed temperature and strain sensing of panda type fiber Bragg grating array.The temperature sensitivity of fast axis is 13.310 pm/°C and of slow axis is 12.385pm/°C.The strain sensitivity of fast axis is 0.98501 pm/??and of slow axis is0.99380 pm/??.The error in the experiment is mainly from the manual adjustment of the translation table and the temperature fluctuation of the temperature controller.2)Distributed measurement of static pressure of the double side holes type fiber Bragg grating array with smaller air holes.The sensitivity of the distance between the two peaks to static pressure is 3.34 pm/MPa.3)On-line fabrication of a double side holes type fiber Bragg grating array with a circular core in drawing tower.The average sensitivity of the temperature is 10.79 pm/°C and the sensitivity of static pressure is 4.75 pm/MPa.4)On-line fabrication of a double side holes type fiber Bragg grating array with an elliptical core in drawing tower.It has higher birefringence and its double polarization reflection peaks appear an obvious separation.Distributed temperature sensing experiment shows the average temperature sensitivity of reflection peak of fast axis is 9.8025 pm/°C and of slow axis is 9.8125 pm/°C.The difference of sensitivity is 0.01 pm/°C,which means the sensitivity of the distance between two peaks to the temperature is 0.01 pm/°C.Distributed static pressure sensing experiment shows the response sensitivity of static pressure is 6.43 pm/MPa.The distance between the double polarization reflection peaks of double side holes type fiber Bragg grating array is insensitive to temperature,and the sensitivity of ambient static pressure is 643 times higher than the temperature sensitivity. |
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