Enhanced sensitivity of temperature and magnetic field sensor based on FPIs with Vernier effect.

A kind of temperature and magnetic field sensor using Fabry-Perot interferometers (FPIs) and Vernier effect to enhance sensitivity is proposed. The sensor structure involves filling the FP air cavities with polydimethylsiloxane (PDMS) and magnetic fluid (MF) to create the PDMS and MF cavities for temperature and magnetic field detection, respectively. The two cavities are reflective structures, which are interconnected in series through a fiber-optic circulator. Experimental data demonstrates that the Vernier effect effectively enhances the sensor sensitivity. The average temperature sensitivity of the sensor is 26765 pm/°C within the range of 35∼39.5°C. The magnetic field intensity sensitivity is obtained to be -2245 pm/mT within the range of 3∼11 mT. The sensitivities of the temperature and magnetic field using the Vernier effect are about five times larger than those of the corresponding single FP cavity counterparts.

In-line temperature-compensated vector magnetic field sensor with side-polished fiber.

A novel, to the best of our knowledge, vector magnetic field sensor with temperature compensation is proposed and investigated. The proposed sensor is realized by side polishing a multi-mode optical fiber and adopting the surface plasmon resonance (SPR) effect. The side-polished surface is coated with a magnetic fluid (MF) and polydimethylsiloxane (PDMS) successively along the fiber axis. The as-fabricated sensor can be used not only for magnetic field strength and direction sensing, but also for temperature detection. The achieved magnetic field intensity sensitivities are 1720 pm/mT (90° direction) and -710 pm/mT (0° direction), and the temperature sensitivity is -2070 pm/°C. On top of its temperature compensation ability, the easy fabrication and very high sensitivity of the proposed sensor are attractive features for vector magnetic field sensing applications.

Dual-channel-in-one temperature-compensated all-fiber-optic vector magnetic field sensor based on surface plasmon resonance.

All-fiber-optic magnetic field sensor integrated with magnetic fluid has been investigated for decades, accompanied by the commitment to vectorization, miniaturization, integration and solving the temperature cross-sensitivity caused by thermo-optic effect of magnetic fluid. A kind of dual-channel-in-one temperature-compensated all-fiber-optic vector magnetic field sensor was proposed and investigated theoretically in this work. Three optical surfaces, including two sensing surfaces (plated with gold film of 40 nm thickness and then coated with magnetic fluid and polydimethylsiloxane, respectively, referred as CH1 and CH2) and one reflective surface, were integrated on a single-mode fiber tip to facilitate the dual-channel-in-one design. The Kretschmann configurations were formed by the waveguide fiber, gold film and functional materials at the sensing surfaces (CH1 and CH2). Surface plasmon resonance was excited in different wavelength bands corresponding to CH1 and CH2. Attenuation wavelengths corresponding to CH1 and CH2 depend on the magneto-induced and temperature-induced refractive index change of functional materials, respectively, which makes the temperature-compensated magnetic field sensing possible. The non-centrosymmetric evanescent field generated by micro-fiber-tip-prism enables the vector magnetic field sensing. Especially, the length of the sensing area is only 115.5 µm, which achieves ultra-integration and miniaturization. The current work provides a novel scheme for designing all-fiber-optic vector magnetic field sensing based on magnetic fluid and demonstrates the realization of lab-on-a-fiber and then promotes the industrial application of all-fiber-optic vector magnetic field sensing devices.

Dual-channel temperature-compensated vector magnetic field sensor based on lab-on-a-fiber-tip.

Fiber-optic magnetic field sensors based on magnetic fluid (MF) is encountering with thermal effects and demand for vectorization for several years. A common solution is to use axially processed fiber cascaded with fiber Bragg grating (FBG). However, the length of such sensors is usually in centimeter-level, which restricts the sensing applications in narrow space and gradient field cases. In this work, we present an ultracompact reflection-type dual-channel sensor for vector magnetic field (Channel 1, referred as CH1) and temperature (Channel 2, referred as CH2) monitoring, which is composed of a pair of gold-plated wedge-shaped multimode fiber (MMF) tip and gold-plated multimode-no-core fiber (MNF) tip. The surface plasmon resonance (SPR) effect was adopted. The two sensor probes are coated with magnetic-field-sensitive MF and temperature-sensitive polydimethylsiloxane (PDMS), respectively. The issue of vector magnetic field and temperature cross-sensitivity is tactfully resolved. Importantly, the proposed sensing probes are ultracompact and the spatial resolution is extremely small (615 µm for CH1 based on wedge-shaped fiber tip and 2 mm for CH2 based on MNF), which is very helpful for narrow space and gradient magnetic field detection. The obtained magnetic field intensity sensitivities are 1.10 nm/mT (90° direction) and -0.26 nm/mT (0° direction), and temperature sensitivity is -3.12 nm/°C.

Fiber-Optic Vector-Magnetic-Field Sensor Based on Gold-Clad Bent Multimode Fiber and Magnetic Fluid Materials.

A kind of bent multimode fiber (MMF) vector magnetic sensor based on surface plasmon resonance (SPR) was proposed. By plating gold film on the curved part of the bent multimode fiber, the surface plasmon mode (SPM) was excited via a whispering gallery mode (WGM). Fabricating the structure only required bending the fiber and plating it with gold, which perfectly ensured the integrity of the fiber and made it more robust compared with other structures. The sensor used magnetic fluid (MF) as the magnetically sensitive material. Through monitoring the shift of the surface plasmon resonance dip, the as-fabricated sensor not only had a high magnetic field intensity sensitivity of 9749 pm/mT but could also measure the direction of a magnetic field with a high sensitivity of 546.5 pm/°. The additional advantages of the proposed sensor lay in its easy fabrication and good integrity, which make it attractive in the field of vector-magnetic-field sensing.

Magnetic Field Sensing Based on Whispering Gallery Mode with Nanostructured Magnetic Fluid-Infiltrated Photonic Crystal Fiber.

A kind of novel and compact magnetic field sensor has been proposed and investigated experimentally. The proposed sensor consists of a tapered single mode fiber coupled with a nanostructured magnetic fluid-infiltrated photonic crystal fiber, which is easy to be fabricated. The response of magnetic fluid to magnetic field is used to measure the intensity of magnetic field via whispering gallery mode. The magnetic field-dependent shift in resonance wavelength is observed. The maximum magnetic field intensity sensitivity is 53 pm/mT. The sensor sensitivity is inversely proportional to the thickness of the photonic crystal fiber cladding.

Vector magnetic field sensor based on U-bent single-mode fiber and magnetic fluid.

A novel, compact, and easy fabrication vector magnetic field sensor has been proposed and investigated. The proposed sensor consists of a U-bent single-mode fiber fixed in a magnetic-fluid-filled vessel. Neither mechanical modification nor additional fiber grating is needed during the sensor fabrication. The results show that the response of magnetic fluid to magnetic field can be used to measure the direction and intensity of magnetic field via whispering gallery modes supported by the U-bent fiber structure with suitable bending radius. The sensitivity of direction is 0.251 nm/°, and the maximum magnetic field intensity sensitivity is 0.517 nm/mT. Besides, the results of this work prove the feasibility for realizing vector magnetic sensors based on other bending structures (such as bending multimode interference, bending SPR structure) in the future.

All-fiber-optic vector magnetic field sensor based on side-polished fiber and magnetic fluid.

A kind of compact all-fiber-optic vector magnetic sensor is proposed and demonstrated. The sensor consists of a side-polished-fiber (SPF)-integrated with singlemode-no core-singlemode (SNS) fiber structure. A section of side-polished fiber breaks the axially symmetry of the composite structure. The as-fabricated sensor supports vector sensing and has a magnetic field strength sensitivity of up to -2370 pm/mT over 2-6 mT range. The physical mechanism is that the modal interference is strongly influenced by the refractive index (RI) near the side-polished surface. The advantages of the proposed sensor lie in low cost, simple structure and easy manufacture, which make it attractive in the field of magnetic field vector sensing.

Gold-Film-Thickness Dependent SPR Refractive Index and Temperature Sensing with Hetero-Core Optical Fiber Structure.

A simple hetero-core optical fiber (MMF-NCF-MMF) surface plasmon resonance (SPR) sensing structure was proposed. The SPR spectral sensitivity, full width of half peak (FWHM), valley depth (VD), and figure of merit (FOM) were defined to evaluate the sensing performance comprehensively. The effect of gold film thickness on the refractive index and temperature sensing performance was studied experimentally. The optimum gold film thickness was found. The maximum sensitivities for refractive index and temperature measurement were obtained to be 2933.25 nm/RIU and -0.91973 nm/°C, respectively. The experimental results are helpful to design the SPR structure with improved sensing performance. The proposed SPR sensing structure has the advantages of simple structure, easy implementation, and good robustness, which implies a broad application prospect.

Fiber-Optic Magnetic Field Sensing Based on Microfiber Knot Resonator with Magnetic Fluid Cladding.

A kind of all-fiber magnetic field sensing structure is proposed and demonstrated here. The sensing element includes a microfiber knot resonator (MKR) cladded with magnetic fluid (MF). The low-index MgF2 slab is adopted as the substrate. The sensitivity increases with the decrease of the MKR ring diameter. The achieved maximum magnetic field sensitivity is 277 pm/mT. The results of this work have the potential to promote the development of magnetically controllable optical devices and the design of ultra-compact cost-effective magnetic field sensors.

Investigation of optical force on magnetic nanoparticles with magnetic-fluid-filled Fabry-Perot interferometer.

The optical force acting on the magnetic nanoparticles (MNPs) is investigated with the magnetic-fluid-filled fiber-optic Fabry-Perot interferometer. The shift of interference spectra is related with the local refractive index variation in the light path, which is assigned to the optical-force-induced outward movement of MNPs. The influence of magnetic fluid's viscosity, ambient temperature, strength and orientation of the externally applied magnetic field on the optical-force-induced MNPs' movement is studied in details. The results of this work provide a further understanding of interaction between light and MNPs and clarify the dynamic micro-processes of MNPs within magnetic fluid under external stimuli. It may have the potentials in the fields of light-controllable magnetic-fluid-based devices and vector magnetic field detection.

Ultrasensitive Magnetic Field Sensing Based on Refractive-Index-Matched Coupling.

An ultrasensitive magnetic field sensor is proposed and investigated experimentally. The no-core fiber is fusion-spliced between two pieces of single-mode fibers and then immersed in magnetic fluid with an appropriate value of refractive index. Under the refractive-index-matched coupling condition, the guided mode becomes leaky and a coupling wavelength dip in the transmission spectrum of the structure is observed. The coupling wavelength dip is extremely sensitive to the ambient environment. The excellent sensitivity to the refractive index is measured to be 116.681 µm/RIU (refractive index unit) in the refractive index range of 1.45691-1.45926. For the as-fabricated sensors, the highest magnetic field sensing sensitivities of 6.33 and 1.83 nm/mT are achieved at low and high fields, respectively. The sensitivity is considerably enhanced compared with those of previously designed, similar structures.

A Photonic Crystal Magnetic Field Sensor Using a Shoulder-Coupled Resonant Cavity Infiltrated with Magnetic Fluid.

A kind of photonic crystal magnetic field sensor is proposed and investigated numerically. The shoulder-coupled resonant cavity is introduced in the photonic crystal, which is infiltrated with magnetic fluid. Through monitoring the shift of resonant wavelength, the magnetic field sensing is realized. According to the designed infiltration schemes, both the magnetic field sensitivity and full width at half maximum increase with the number of infiltrated air holes. The figure of merit of the structure is defined to evaluate the sensing performance comprehensively. The best structure corresponding to the optimal infiltration scheme with eight air holes infiltrated with magnetic fluid is obtained.

Reflective all-fiber magnetic field sensor based on microfiber and magnetic fluid.

A kind of reflective all-fiber magnetic field sensor based on a non-adiabatically tapered microfiber with magnetic fluid is proposed and experimentally demonstrated. The modal interference effect is caused by the abrupt tapers, which result in an approximately sinusoidal spectral response. The reflection spectra of the proposed sensor under different magnetic field strengths have been measured and theoretically analyzed. The maximum sensitivity of 174.4 pm/Oe is achieved at wavelength of around 1511 nm. Besides, an intensity tunability of -0.02 dB/Oe is also achieved. Comparing with the traditional sensors operating at transmission mode, the presented sensor in this work owns the advantages of smaller size and higher sensitivity and resolution due to the enhanced extinction ratio. The proposed structure is also promising for designing other tunable all-in-fiber photonic devices.

Magnetic field sensing based on magnetic-fluid-clad multimode-singlemode-multimode fiber structures.

Magnetic field sensing based on magnetic-fluid-clad multimode-singlemode- multimode fiber structures is proposed and experimentalized. The structures are fabricated out using fiber fusion splicing techniques. The sensing principle is based on the interference between the core mode and cladding modes. Two interference dips are observed in our spectral range. Experimental results indicate that the magnetic field sensing sensitivities of 215 pm/mT and 0.5742 dB/mT are obtained for interference dip around 1595 nm. For interference dip around 1565 nm, the sensitivities are 60.5 pm/mT and 0.4821 dB/mT. The response of temperature is also investigated. The temperature sensitivity for the dip around 1595 nm is obtained to be 9.93 pm/°C.

Magnetic field sensing based on magnetic-fluid-clad fiber-optic structure with taper-like and lateral-offset fusion splicing.

A kind of magnetic field sensor composed of magnetic fluid surrounding a segment of singlemode fiber is proposed. The taper-like and lateral-offset fusion splicing techniques are employed. The sensing principle is based on cladding mode interference. The interference valley wavelength or transmission loss of the sensing structure is sensitive to the external magnetic field, which is utilized for magnetic field sensing. The linear response regions are obtained in the range of 38-225 Oe and 250-475 Oe. For the valley-wavelength-shift-type sensing, the sensitivities are 14.1 pm/Oe and 26 pm/Oe at low and high field ranges, respectively. For the transmission-loss-variation-type sensing, the sensitivity of -0.024 dB/Oe is achieved for the magnetic field strength ranging from 250 to 475 Oe.

Iron-oxide nanoparticles embedded silica microsphere resonator exhibiting broadband all-optical wavelength tunability.

In this Letter, a novel silica microsphere resonator (MSR) embedded with iron-oxide nanoparticles, which possesses broadband all-optical wavelength tunability, is demonstrated. It is generated by using in-line 1550 nm laser ablation of a microfiber with the assistance of magnetic fluid. To the best of our knowledge, this simple method of fabricating such MSRs is reported for the first time. Prominent photothermal effect is realized by the iron-oxide nanoparticles absorbing light pumped via the fiber stem, leading to a wavelength shift of over 13 nm (1.6 THz). Moreover, a linear tuning efficiency up to 0.2 nm/mW is realized. With excellent robustness and being fiberized, the spheres can be attractive elements in building up novel micro-illuminators, point heaters, optical sensors, and fiber communication modules.

All-optical tuning of a magnetic-fluid-filled optofluidic ring resonator.

An all-optical tunable optofluidic ring resonator (OFRR) is proposed and experimentally demonstrated. The all-optical control of a silica microresonator is highly attractive, but it is difficult to realize because of the relatively weak Kerr effect and the absence of a plasma dispersion effect of silica. Here, we infuse a silica microcapillary-based optofluidic ring resonator with a magnetic fluid, into which pump light is injected by a fiber taper. Iron oxide nanoparticles dispersed in the magnetic fluid produce a strong pump light absorption, and this leads to a resonance shift of the silica microresonator due to the photothermal effect. To the best of our knowledge, this is the first scheme for all-optical tuning of an OFRR. A tuning sensitivity of up to 0.15 nm mW(-1) and a tuning range of 3.3 nm are achieved. With such excellent performance, the magnetic-fluid-filled OFRR has great potential in filtering, sensing, and signal processing applications.

Magnetic field sensing based on singlemode-multimode-singlemode fiber structures using magnetic fluids as cladding.

Magnetic field sensing based on magnetic fluid (MF) and a singlemode-multimode-singlemode (SMS) fiber structure is proposed. The sensitivity of the proposed sensing system can be enhanced by corroding the cladding of the multimode fiber of the SMS fiber structure. The achieved maximum magnetic field sensitivity of our experimental structures is -16.86 pm/Oe as the fiber is corroded for 1680 s. The visibility of the interference dip for the MF-clad SMS fiber structure decreases with corrosion time. Considering the trade-off between sensitivity and visibility, the figure of merit of the sensing system is employed to evaluate the sensing performance comprehensively. In our experiments, the structure corroded for ~1620 s is found to have maximum sensing performance.

Magnetic field sensing based on capillary filled with magnetic fluids.

A kind of magnetic field sensing system based on capillary tube filled with magnetic fluids is developed in this work. The analytical expressions for the sensing system are derived in detail. The sensitivity and other sensing properties of the system are investigated numerically and experimentally. The focal line position of the emergent light after the capillary is related with the strength of the externally applied magnetic fields and recorded and judged by the CCD to sense the magnetic field indirectly. The sensing mechanism is analyzed and ascribed to the magnetically tunable refractive index of magnetic fluids. This kind of sensing unit has the advantages of miniaturization of device, easy operation, and lower dosage of sensing media.