Study On Novel Fiber-optic Magnetic Sensors

In oil and gas exploration and production logging industry,magnetic positioning logging is used to obtain accurate the CCL curve as the basic data of calibration depth in logging project.It is great significance to quickly and accurately locate the casing structure,the logging equipment and the damage in the oil casing.Optical fiber sensors have advantages of small size,high sensitivity,corrosion resistance,easy to reuse,multi-parameter monitor,capable of working under the logging and no pollution to the measured objects.The theoretical basis of this paper is the mode interference theory of tapered fiber and FBG.Several novel kinds of fiber Directional dependent magnetic field sensors are proposed and fabricated,which are theoretically analyzed and experimentally studied.The research contents are mainly as follows:1.The category and research status of fiber magnetic field sensor were reviewed in the background section.In the theoretical part,the mode-coupling characteristics of micron tapered fiber and cladding FBG were analyzed,and the sensing theory of the Mach-Zehnder interference and cladding FBG magnetic field sensor were discussed.Meanwhile,several fabrication methods of micron conical fiber are introduced.2.A WGM magnetic field sensor based on a tapered single mode fiber and optical micro resonators is proposed and experimentally demonstrated.The harmonic peaks of the proposed sensor are measured and theoretically analyzed at different magnetic field strength.The results show that the magnetic field sensitivity reaches up to-0.04 dB/Oe in the range of 40-140 Oe.3.Two kinds of MZI magnetic field sensors based on a tapered fiber and magnetic fluid are proposed and experimentally demonstrated.?A compact fiber-optic magnetic field sensor based on a tapered thin-core fiber and magnetic fluid is proposed.The modal interference effect is caused by the core mode and excited eigen modes in the TTCF cladding.The transmission spectra of the proposed sensor are measured and theoretically analyzed at different magnetic field strength.The results show that the magnetic field sensitivity reaches up to-0.10 dB/Oe in the range of 40-160 Oe.The cross sensitivity of magnetic field and temperature of the sensor was analyzed.?A compact fiber-optic magnetic field sensor based on a tapered two-mode fiber?TTMF?and femtosecond laser refractive index modulation?RIM?is proposed.The additional refractive-index-modulation on TMF taper lateral is introduced by femtosecond laser for enhancing the interference intensity.The modal interference effect is mainly caused by LP₀₁ and LP₁₁ modes,and moreover the RIM provides the sensor a significant direction-dependence due to its asymmetric position relatively to fiber axis.The spectral response to magnetic field intensity and direction of the proposed sensors have been investigated.And,the results show that the wavelength-referenced magnetic field sensitivity reaches up to 71.98 pm/Oe in the range of 40-120 Oe and the intensity-referenced sensitivity reaches up to 0.11 dB/Oe.Meanwhile,the sensor presents great directional resolution with intensity-reference?0.11dB/deg?.The temperature test results of the sensor show that the error caused by the temperature change to the magnetic field measurement is small and can be ignored.The sensor is a non-cross sensitive magnetic field sensor.4 The FBG was simultaneously written on the core and cladding of multi-clad Fiber?MCLF?based on the femtosecond laser and phase mask technology.Two kinds of magnetic field sensors were made based cladding FBG.?1?The high-order cladding mode of the cladding FBG on MCLF showed great sensitivity and direction on dependence for the magnetic field measurement.The results show that the magnetic field sensitivity reaches up to-0.14 dB/Oe in the range of 60-110 Oe.And the temperature sensitivity reaches up to 12 pm/oC.The sensor can simultaneously measure both magnetic fields and temperature.?2?The low-order cladding mode of the cladding FBG on MCLF showed great sensitivity and direction on dependence for the bending measurement.The magnetic field sensor is made based on the PW-080 linear magnetic material and the cladding FBG.The results show that the sensor have high sensitivity and direction on dependence to magnetic field.The intensity-based interrogation of proposed magnetic field sensor improved the stability compared with the wave length demodulation.Meanwhile,these sensors have advantages of compact structure,high sensitivity,and orientation relativity,which provides the basis for the development of magnetic field sensor in oil logging industry.