Research On Highly Sensitive Optical Fiber Magnetic Field Sensors Based On Ferrofluids And Nanodiamonds

Optical fiber magnetic field sensors based on magnetically sensitive materials are the main research direction of magnetic field sensing technology because of their miniaturization,high sensitivity and high integration.Improving the performance of optical fiber magnetic field sensors is of great significance to the development of magnetic field sensing technology and even the current intelligent industries.In this paper,ferrofluids and diamond nitrogen-vacancy（NV）centers are used in optical fiber magnetic field sensing as a starting point to obtain a new highly sensitive optical fiber magnetic field sensors by studying the sensing mechanisms,sensor structures and sensitization methods for the demand of high sensitivity magnetic sensing.The main research work of this paper can be summarized as follows:（1）A highly sensitive vector magnetic field sensor based on ferrofluids and half-side gold-coated（HSGC）hetero-core fiber structure is proposed.The sensor is composed of a HSGC hetero-core fiber structure encapsulated with ferrofluids.The mode field distribution characteristics of the sensor are analyzed in terms of theoretical simulation.It is demonstrated that the modes have non-circular symmetry of the optical field distribution as well as the enhancement of the evanescent field.The magnetic field response characteristics of the sensor are experimentally tested,and the results show that the sensor is sensitive to both magnetic field direction and intensity,demonstrating the vector magnetic field sensing capability.The sensor can achieve magnetic field direction sensitivity of-988 pm/°（180-248°）and magnetic field intensity sensitivity of 1008 pm/Oe（20-120 Oe）,the sensitivity of the magnetic field intensity is the highest value of the current type of sensors.In addition,the stability and dynamic performance of the sensor are tested and analyzed,and the limitations of this type of magnetic field sensor are pointed out.（2）The magnetic detection mechanism of the diamond NV centers is described,and its magnetic detection methods as well as sensitivity are specifically analyzed.Focus on the continuous wave optical detection magnetic resonance（CW-ODMR）method,and lead to the improved LI-ODMR method based on microwave modulation used in this paper,the effect of the system noise,the maximum slope of the spectral line and the equivalent noise bandwidth on the magnetic detection sensitivity are analyzed,which provide the theoretical support for the research of the optical fiber magnetic field sensor based on the nanodiamonds NV centers in this work.（3）A highly sensitive optical fiber magnetic field sensor integrated with nanodiamonds NV centers is proposed.The sensor is composed of a tapered optical fiber and nanodiamonds NV centers modified on the surface of the fiber,in which the tapered fiber is prepared by chemical etching and the nanodiamonds are modified on the surface of the tapered fiber by chemical modification and covalent bonding coupling method.An experimental system is built based on the principle of LI-ODMR to investigate the two main factors（nanodiamonds dispersion concentration and modified time）affecting the performance of the modified sensors.When the nanodiamonds dispersion concentration is increased from 0.25 mg/m L to 1mg/m L,the magnetic field sensitivity of the sensor is increased from 1068 n T/Hz^1/2@1 Hz to 105 n T/Hz^1/2@1 Hz;when the modification time is increased from 1 h to 7 h,the magnetic field sensitivity of the sensor is increased from 3121 n T/Hz^1/2@1 Hz to191 n T/Hz^1/2@1 Hz.Further combined with the flux concentrator,the magnetic field sensitivity of the sensor is significantly improved to 0.57 n T/Hz^1/2@1 Hz,which is the highest value available for an optical fiber magnetic field sensor based on the nanodiamond NV centers.