Characterisation and performance of a Terfenol-D coated femtosecond laser inscribed optical fibre Bragg sensor with a laser ablated microslot for the detection of static magnetic fields

Research Progress on Magneto-Refractive Magnetic Field Fiber Sensors

The magnetic field is a vital physical quantity in nature that is closely related to human production life. Magnetic field sensors (namely magnetometers) have significant application value in scientific research, engineering applications, industrial productions, and so forth. Accompanied by the continuous development of magnetic materials and fiber-sensing technology, fiber sensors based on the Magneto-Refractive Effect (MRE) not only take advantage in compact structure, superior performance, and strong environmental adaptability but also further meet the requirement of the quasi-distributed/distributed magnetic field sensing; they manifest potential and great application value in space detection, marine environmental monitoring, etc. Consequently, the present and prevalent Magneto-Refractive Magnetic Field Fiber Sensors (MR-MFSs) are briefly summarized by this paper, proceeding from the perspective of physicochemical properties; design methods, basic performance and properties are introduced systematically as well. Furthermore, this paper also summarizes key fabrication techniques and future development trends of MR-MFSs, expecting to provide ideas and technical references for staff engaging in relevant research.

Magnetic field sensor based on a dual-frequency optoelectronic oscillator using cascaded magnetostrictive alloy-fiber Bragg grating-Fabry Perot and fiber Bragg grating-Fabry Perot filters

A magnetic field sensor using a dual-frequency optoelectronic oscillator (OEO) incorporating cascaded magnetostrictive alloy-fiber Bragg grating-Fabry Perot (MA-FBG-FP) and FBG-FP filters is proposed and demonstrated. In the OEO resonant cavity, two microwave signals are generated, whose oscillation frequencies are determined by the FBG-FP filter and MA-FBG-FP filter filters with two ultra-narrow notches and two laser sources. Due to the characteristics of MA and FBG, the two generated microwave signals show different magnetic field and temperature sensitivities. By monitoring the variations of two oscillating frequencies and the beat signal using a digital signal processor, the simultaneous measurement for the magnetic field and temperature can be realized. The proposed sensor has the advantages of high-speed and high-resolution measurement, which make it very attractive for practical magnetic field sensing applications. The sensitivities of the proposed OEO sensor for magnetic field and temperature are experimentally measured to be as high as -38.4MHz/Oe and -1.23 or -2.45 GHz/°C corresponding to the MA-FBG-FP filter and FBG-FP filter, respectively.

Magnetic field sensor of enhanced sensitivity and temperature self-calibration based on silica fiber Fabry-Perot resonator with silicone cavity

In this paper, a magnetic field sensor with enhanced sensitivity based on a fiber Fabry-Perot (F-P) cavity formed by a pair of identical fiber Bragg gratings (FBGs) is demonstrated. The F-P cavity which was filled with silicone rubber was bonded to a magnetic alloy at two positions such that when longitudinal strain is applied, the cavity is lengthened while the FBGs was virtually strain-free, effectively magnified the magnetic-field induced strain of the magnetic alloy. The FBGs could also be used for temperature-compensation because the FBG spectrum change is negligible compared to the F-P spectrum.

Enhancing the humidity response time of polymer optical fiber Bragg grating by using laser micromachining

The humidity sensors constructed from polymer optical fiber Bragg gratings (POFBG) respond to the water content change in the fiber induced by varying environmental condition. The water content change is a diffusion process. Therefore the response time of the POFBG sensor strongly depends on the geometry and size of the fiber. In this work we investigate the use of laser micromachining of D-shaped and slotted structures to improve the response time of polymer fiber grating based humidity sensors. A significant improvement in the response time has been achieved in laser micromachined D-shaped POFBG humidity sensors. The slotted geometry allows water rapid access to the core region but this does not of itself improve response time due to the slow expansion of the bulk of the cladding. We show that by straining the slotted sensor, the expansion component can be removed resulting in the response time being determined only by the more rapid, water induced change in core refractive index. In this way the response time is reduced by a factor of 2.5.

3 × 3 coupler based interferometric magnetic field sensor using a TbDyFe rod

An all-fiber magnetic field sensor by using a TbDyFe rod as the sensing element for the measurement of the weak alternating magnetic field is presented and experimentally demonstrated. The sensor is constructed with a 3×3 coupler based Mach-Zehnder interferometer, and the time-varying phase shift induced by the applied magnetic field is recovered by using a passive demodulation method. The experimental results show that the sensor exhibits excellent linearity and reversibility within 0-75 μT (residual mean square), and a high sensitivity of 69.83 mrad/μT (residual mean square) with a resolution of 2.14  nT/√Hz (residual mean square) at 200 Hz is experimentally achieved. The frequency response and the temperature characteristic of the sensor are also investigated.

Femtosecond laser ablation of microstructures in fiber and application in magnetic field sensing

A novel (to our knowledge) 3D microstructure manufactured by a femtosecond laser in fiber Bragg grating (FBG) fiber cladding is proposed. The special spiral parameters including single thread and double thread with certain pitches of 60 and 80 μm are controlled by the feed and rotation speed of a rotary fixture. Moreover, supermagnetostrictive TbDyFe film with a thickness of nearly 6 μm is deposited on microgrooves of a FBG by magnetron sputtering technology to form the magnetic field sensing probe. Experimental results demonstrate that a FBG with a double-thread microstructure has a sensitivity of 1.1  pm/mT responding to a magnetic field, and in a theoretical situation, it is approximately six times higher than the original optical fiber grating (approximately 0.2  pm/mT). This new microstructure and method show great prospect in the magnetic field sensing domain.

A noncontact force sensor based on a fiber Bragg grating and its application for corrosion measurement

A simple noncontact force sensor based on an optical fiber Bragg grating attached to a small magnet has been proposed and built. The sensor measures the force between the magnet and any ferromagnetic material placed within a few millimeters of the sensor. Maintaining the sensor at a constant standoff distance, material loss due to corrosion increases the distance between the magnet and the corroded surface, which decreases the magnetic force. This will decrease the strain in the optical fiber shifting the reflected Bragg wavelength. The measured shift for the optical fiber used was 1.36 nm per Newton. Models were developed to optimize the magnet geometry for a specific sensor standoff distance and for particular corrosion pit depths. The sensor was able to detect corrosion pits on a fuel storage tank bottom with depths in the sub-millimeter range.

Magnetic field sensor based on fiber Bragg grating with a spiral microgroove ablated by femtosecond laser

A novel magnetic field sensor based on Terfenol-D coated fiber Bragg grating with spiral microstructure was proposed and demonstrated. Through a specially-designed holder, the spiral microstructure was ablated into the fiber Bragg grating (FBG) cladding by femtosecond laser. Due to the spiral microstructure, the sensitivity of FBG coated with magnetostrictive film was enhanced greatly. When the spiral pitch is 50 μm and microgroove depth is 13.5 μm, the sensitivity of the magnetic field sensor is roughly 5 times higher than that of non-microstructured standard FBG. The response to magnetic field is reversible, and could be applicable for magnetic field detection.

Femtosecond-laser-induced highly birefringent Bragg gratings in standard optical fiber

We report highly birefringent fiber Bragg gratings in standard single-mode optical fiber realized with UV femtosecond pulses and line-by-line inscription. By controlling the three-dimensional positioning of the focused laser beam with respect to the fiber core, we achieve very high birefringence at the grating location in a single exposure. A maximum birefringence value of 7.93×10(-4) has been reached for 10th-order gratings when using 2 μJ pulses, which is to our knowledge the highest birefringence value reported so far. This birefringence results from UV-induced high-densification lines shifted from the center of the core, increasing the asymmetry of the induced-stress lines. With a Bragg wavelength spacing reaching more than 800 pm between polarization modes, such gratings are particularly well suited for selective filtering or, as demonstrated here, for temperature-insensitive transverse-strain measurements.

Optofluidic magnetometer developed in a microstructured optical fiber

A directional, in-fiber optofluidic magnetometer based on a microstructured optical fiber (MOF) Bragg-grating infiltrated with a ferrofluidic defect is presented. Upon application of a magnetic field, the ferrofluidic defect moves along the length of the MOF Bragg grating, modifying its reflection spectrum. The magnetometer is capable of measuring magnetic fields from 317 to 2500 G. The operational principle of such in-fiber magnetic field probe allows the elaboration of directional measurements of the magnetic field flux.

Magnetic field measurements based on Terfenol coated photonic crystal fibers

A magnetic field sensor based on the integration of a high birefringence photonic crystal fiber and a composite material made of Terfenol particles and an epoxy resin is proposed. An in-fiber modal interferometer is assembled by evenly exciting both eigenemodes of the HiBi fiber. Changes in the cavity length as well as the effective refractive index are induced by exposing the sensor head to magnetic fields. The magnetic field sensor has a sensitivity of 0.006 (nm/mT) over a range from 0 to 300 mT with a resolution about ±1 mT. A fiber Bragg grating magnetic field sensor is also fabricated and employed to characterize the response of Terfenol composite to the magnetic field.