Magnetic nanoparticle-composites as sensing media for metering and monitoring ac currents and ac magnetic fields using fiber-optic sensor

Abstract

A sensor for monitoring AC currents and / or AC magnetic fields includes an optical fiber structure and a cladding / sensing layer coupled to the optical fiber structure, the cladding / sensing layer comprising a composite embedded media member, wherein the composite embedded media member includes magnetic nanoparticles embedded in a host media. Also, systems and methods for monitoring AC currents and / or AC magnetic fields that incorporate such a sensor.

Description

MAGNETIC NANOPARTICLE-COMPOSITES AS SENSING MEDIA FOR METERING AND MONITORING AC CURRENTS AND AC MAGNETIC FIELDS USING FIBER-OPTIC SENSORCROSS-REFERENCE TO RELATED APPLICATIONS:

[0001] This application claims priority to U.S. Provisional Patent Application Serial No.63 / 636,218, filed on April 19, 2024, and titled “Magnetic Nanoparticle-Composites as Sensing Media for Metering and Monitoring AC Currents and AC Magnetic Fields Using Fiber-Optic Sensor,” the disclosure of which is incorporated herein by reference.STATEMENT OF GOVERNMENT INTEREST:

[0002] This invention was made with government support under grant # DE-EE0009632 awarded by the Department of Energy (DOE). The government has certain rights in the invention.FIELD OF THE INVENTION:

[0003] The disclosed concept pertains to sensors for sensing AC currents and / or AC magnetic fields, and, in particular, to a sensor for sensing AC cunent and / or AC magnetic fields that employs a magnetic nanoparticle composite as a cladding / sensing material of a fiber optic-based sensor.BACKGROUND OF THE INVENTION:

[0004] Optical fiber based electromagnetic field sensing is a rapidly expanding field in fiber sensor technology, offering diverse sensing architectures (e.g., gratings, interferometers, specialty fibers), functional materials (e.g., magneto-optical, magnetostrictive, and diamond NV centers-quantum materials), and interrogation methods (shift in intensity, resonance wavelength, polarization, and phase) for application specific needs of bandwidth, sensitivity, and range. The applications include biomedical instruments, space and navigation, geomagnetism, electric power grid monitoring, and quantum applications, among others. Immunity to electromagnetic interference (EMI) due to dielectric silica fiber materials, compactness, low size and weight, and compatibility for remote and distributed sensing modalities are some of the advantages touted over their electronic counterpart current transducers.

[0005] With the advancement in nanomaterial synthesis and their functionalization, magnetic fluids (MFs) are a widely investigated and used functional material in applications ranging from biosensing, hyperthermia treatments for cancer therapy,magnetic resonance imaging, and fiber-optic-based magnetic field sensing. MFs, also know n as ferrofluids, are colloidal suspensions of spontaneously magnetized singledomain nanoparticles of ferromagnetic materials, typically magnetite (FeaO-i) and maghemite (y-FesC ), uniformly dispersed in a nonmagnetic carrier liquid (e.g., water, kerosene, or heptane) with the help of thin layer coating of surfactant (e.g., oleic acid, ethylene glycol, and other polymers). When the diameter of ferromagnetic nanoparticles is below the “critical diameter"’ (5-15 nm), the magnetic nanoparticles (MNPs) in ferrofluids exhibit superparamagnetic behavior characterized with zero coercivity, remanence, and hysteresis loss. The surfactant and the Brownian motion keep the homogeneous dispersion of nanoparticles stable against particle sedimentation and agglomeration.

[0006] Owing to their customizable and tunable magneto-optical properties, such as tunable refractive index, birefringence, and transmittance. MFs are popular sensing materials used in fiber-optic magnetic field sensing, but only in connection with DC magnetic-field sensing. The long response time of MFs has often been cited as a barrier to the application of MFs to AC field sensing (see, e.g., Du, B.; Yang, D.; Bai, Y.; Yuan, Y.; Xu. J.; Jiang, Y.; Wang, M., “Ultralong time response of magnetic fluid based on fiber-optic evanescent field,” Appl. Opt. 2016, 55 (21), 5585-5590).SUMMARY OF THE INVENTION:

[0007] In one embodiment, a sensor for monitoring AC currents and / or AC magnetic fields is provided that includes an optical fiber structure and a cladding / sensing layer coupled to the optical fiber structure, the cladding / sensing layer comprising a composite embedded media member, wherein the composite embedded media member includes magnetic nanoparticles embedded in a host media.

[0008] In another embodiment, a method of monitoring an AC magnetic field is provided that includes providing an optical interrogation signal to a sensor as recited above when the sensor is within the AC magnetic field, detecting an optical response signal generated by the sensor in response to the interrogation light, generating a voltage or current signal based on the detected optical response signal that is indicative of an intensity or polarization response of the detected response light, and generating a signal that is proportionate to a field strength of the AC magnetic field based on the voltage signal or current signal.

[0009] In still another embodiment, a system for monitoring an AC magnetic field includes a sensor as just described above, a light source for providing an interrogation light to the sensor, a detector structured and configured to detect a response light generated by the sensor in response to receiving the interrogation light and the AC magnetic field, the detector being further structured and configured to generate a voltage or current signal based on the detected response light that is indicative of an intensity or polarization response of the detected response light, and a signal processing apparatus structured and configured to receive the voltage or current signal and generate a signal that is proportionate to a field strength of the AC magnetic field based on the voltage or current signal.BRIEF DESCRIPTION OF THE DRAWINGS:

[0010] A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying draw ings in which:

[0011] FIG. 1 is a schematic diagram of an optical fiber-based sensor for monitoring AC currents and / or AC magnetic fields according to a non-limiting exemplary embodiment of the disclosure concept;

[0012] FIG. 2 is a schematic diagram of a sensing system according to an exemplary embodiment of the disclosed concept; and

[0013] FIG. 3A is a schematic diagram of an interrogation set-up for evaluating the performance of an exemplary sensor as shown in FIG. 1, FIG. 3B shows the response of that exemplary sensor in the time and frequency domains, and FIG. 3C is a composite image which sums up the finding and analysis of the evaluation of the exemplary' sensor;

[0014] FIGS. 4A-4H are plots showing the response to several types of current profiles of an exemplary sensor as shown in FIG. 1 using the interrogation set-up of FIG. 3, including the response to burst current (FIG. 4A), intermittent spikes / pulses (FIG. 4B), symmetric and asymmetric current profiles during short circuit faults (FIGS. 4C and 4D), second-order step and impulse currents (FIGS. 4E and 4F), and exponential fall and rise currents (FIGS. 4G and 4H);

[0015] FIG. 5 illustrates the response of an exemplary sensor according to one particular embodiment, which has been also shown to successfully detect AC frequencies up to 15kHz;

[0016] FIG. 6 shows a calibration curve for the interrogation set-up of FIG. 3 when evaluating the DC bias method of the disclosed concept;

[0017] FIGS. 7A-7F are plots showing the effect of the DC bias method of the disclosed concept on the response signal of an exemplary nano-ferrofluid-cladded sensor as shown in FIG. 1 studied using the integration set-up of FIG. 3;

[0018] FIGS. 8 A, 8A'-8F, 8F' are various signals waveforms illustrating the purification of AC frequency as detected by an example sensor of FIG. 1 upon various DC bias fields;

[0019] FIGS. 9A-9D are plots showing the response to several types of current profiles of an exemplary sensor as show n in FIG. 1 using the DC bias method of the disclosed concept;

[0020] FIG. 10 is a schematic diagram of an alternative sensing system according to an exemplary embodiment of the disclosed concept that implements the DC bias field method of the disclosed concept:

[0021] FIG. 11 is a schematic diagram of an optical fiber-based sensor for monitoring AC currents and / or AC magnetic fields according to an alternative non-limiting exemplary embodiment of the disclosed concept;

[0022] FIG. 12 is a schematic diagram of an alternative sensing system according to an exemplary embodiment of the disclosed concept that uses the sensor of FIG. 11;

[0023] FIG. 13 is a schematic diagram of an optical fiber-based sensor for monitoring AC currents and / or AC magnetic fields according to another alternative non-limiting exemplary embodiment of the disclosed concept;

[0024] FIG. 14 is a schematic diagram of an alternative sensing system according to an exemplary embodiment of the disclosed concept that uses the sensor of FIG. 13;

[0025] FIG. 15 is a schematic diagram of a system for quasi-distributed sensing for 3- phase electrical current and current-induced magnetic field detection and monitoring according to another alternative non-limiting exemplary embodiment of the disclosed concept;

[0026] FIG. 16 is a schematic diagram of an alternative sensing system according to another exemplary embodiment of the disclosed concept that uses the sensor of FIG. 13;

[0027] FIG. 17 are plots showing sensitivity dependance on the amount of transmitted power through the sensors of the disclosed concept.DETAILED DESCRIPTION:

[0028] As used herein, the singular form of “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise.

[0029] As used herein, the statement that two or more parts or components are “coupled” shall mean that the parts are joined or operate together either directly or indirectly, i.e., through one or more intermediate parts or components, so long as a link occurs.

[0030] As used herein, “directly coupled” means that two elements are directly in contact with each other.

[0031] As used herein, the term “number” shall mean one or an integer greater than one(i.e., a plurality)

[0032] As used herein, the term “no-core fiber” or “NCF” shall mean an optical fiber in which there is no core / cladding structure such that the medium surrounding the fiber serves as the effective cladding.

[0033] As used herein, the term “single mode fiber” or “SMF” shall mean an optical fiber in which a dominant single propagating mode is guided within the fiber.

[0034] As used herein, the term “multimode fiber” or “MMF” shall mean an optical fiber in which numerous (i.e., a plurality of) modes are guided within the fiber.

[0035] As used herein, the term “quasi-distributed fiber optic sensing” shall mean sensing based on measurements of discrete sensor element(s) provided within or coupled to one or more fiber optic cables at a plurality7of distinct locations to allow for measuring parameters both temporally and in a spatially distributed manner.

[0036] As used herein, the term “quasi-distributed fiber optic sensor” shall mean a fiber optic cable sensing device that employs quasi -distributed fiber optic sensing.

[0037] As used herein, the term “distributed fiber optic sensing” shall mean sensing parameters along the length of a fiber optic cable wherein the entire fiber optic cable acts as an array of sensing elements.

[0038] As used herein, the term “distributed fiber optic sensor” shall mean a fiber optic cable sensing device that employs distributed fiber optic sensing.

[0039] As used herein, the term “nanoparticle” shall mean an object that behaves as a whole unit with respect to its transport and properties having a size in one direction (e.g.. diameter or width) less than 100 nm (in particular embodiments, nanoparticles may be further limited to objects having a size of less than 20nm).

[0040] Directional phrases used herein, such as, for example and without limitation, top, bottom, left, right, upper, lower, front, back, and derivatives thereof, relate to the orientation ofthe elements shown in the drawings and are not limiting upon the claims unless expressly recited therein.

[0041] The disclosed concept will now be described, for purposes of explanation, in connection with numerous specific details in order to provide a thorough understanding of the disclosed concept. It will be evident, however, that the disclosed concept can be practiced without these specific details without departing from the spirit and scope of this innovation.

[0042] The AC magnetic field response of ferrofluids or MNP containing solids (e.g., superparamagnetic ferrofluids or MNP containing solids) is an interplay between the Neel and Brownian relaxation processes, which depends on the t pe of ferromagnetic nanoparticles, the strength of the magnetic field, the size of the nanoparticles, the hydrodynamic volume / surfactant layer coating of the nanoparticles, and the viscosity of the carrier liquid or / polymer solid media. When magnetic nanoparticles are in a colloidal solution form as in ferrofluids / magnetic fluids, both Neel and Brownian relaxation contribute to the AC magnetic field response, whereas when magnetic nanoparticles are in solid (e.g., polymer composite) form, the Brownian relaxation seizes and Neel relaxation is the only process responsible for the AC field response. As described in detail herein, the disclosed concept provides an optical fiber-based sensor that is able to sense AC currents and / or AC magnetic fields using a composite sensing layer having embedded magnetic nanoparticles. The sensing layer as described herein enables the optical detection of the dynamic response of the magnetic nanoparticles, which in turn enables the monitoring of AC currents and / or AC magnetic fields. In particular, as described herein, the disclosed concept provides two specific types of composite sensing layers: (i) a composite sensing layer comprising magnetic nanoparticles dispersed in a liquid media (e.g., a ferrofluid / magnetic fluid), and (ii) a composite sensing layer comprising magnetic nanoparticles in a solid media (e.g., a polymer or silica matrix).

[0043] Accordingly, as described in detail herein, the disclosed concept provides a fiber optic-based AC current and / or AC magnetic field sensor in which the sensing mechanism and the sensing material are engineered for fast optical response to enable the sensor to respond to time varying currents and magnetic fields. In the exemplary embodiment, the sensing material is composed of superparamagnetic nanoparticles embedded in a host media. As noted above, the host media can be a liquid or a solid, with examples of liquids including, without limitation, w ater or other solvents such as kerosene or heptane, and with examples of solids including, without limitation, an organic polymer matrix orinorganic dielectric materials such as a silica matrix. One particular exemplary embodiment includes a fiber-optic interferometric structure that is tuned to work at telecommunication wavelength bands (e.g., telecommunication C-band wavelengths of 1530 nm-1565 nm). The optimized sensor, sensing materials, and the method of fabrication of the disclosed concept enables the detection and metering of AC current- induced magnetic fields up to at least 15 kHz frequency. The disclosed concept thus enables the monitoring of electrical current bearing assets of an electrical power grid system, such as transformers, inductors, and power lines, for detection of potential current-faults in the circuits to ensure the smooth delivery of electrical power to consumers. Alternatively, the magnetic field sensing technology of the disclosed concept may also be applied in connection with other technologies that require the detection and metering of AC current-induced magnetic fields, such as, without limitation, electric vehicles (EVs) and / or autonomous driving technology (where networks of sensors play crucial roles in detecting cunent, position, speed, etc ).

[0044] Moreover, by providing the engineered magnetic nanoparticles of the disclosed concept in either a fluid or a solid matrix, the magnetic response time can be reduced to sub-milliseconds so as to make the sensor of the disclosed concept suitable for sensing AC currents and / or current induced AC magnetic fields in the kHz range. Such AC magnetic field sensing responses have, in connection with the disclosed concept, been demonstrated to show a magneto-optical response with a similar high frequency capability, making the disclosed concept useful for fiber optic sensing and potentially other photonic sensing devices (e.g.. planar waveguides, free space optical sensors, etc.).

[0045] The sensors of the disclosed concept are able to use the same optical signal that is used in fiber-optic communications to also monitor AC currents and fields, thereby obviating the need for costly extra instrumentation for sensor interrogation. In one exemplary embodiment, described in greater detail herein, the sensor structure comprises a multimode interferometer (MMI) structure optimized for its 4th self-imaging spectral response where the sensor produces a filterlike spectral response and is highly sensitive to changes in magneto-optical properties of the surrounding medium. This optimization leads to an enhanced signal to noise ratio (SNR) of the sensor signal and enables detection of AC currents and / or AC magnetic fields in a wide range of AC amplitudes and frequencies.

[0046] A primary advantage of the disclosed concept is optical detection by harnessing the high frequency response of a magnetic nanoparticle-embedded media used as thecladding / sensing layer of, for example, a fiber-optic interferometric structure. The demonstrated capability includes the use of magnetic nanoparticles embedded within various media, namely fluid phases as in ferrofluids and solid phases as in polymers and silica matrices, to perform AC cunent and / or AC magnetic field sensing of various magnitudes and frequencies. As noted above, one non-limiting exemplary embodiment of the disclosed concept involves a ferrofluid / magnetic fluid-cladded MMI fiber optic sensor optimized for 4th self-imaging spectral response so that the sensor can be interrogated at telecommunication C-band wavelength (1530 nm-1565 nm), the same band used in long distance fiber-optic communications. Other examples of the disclosed concept include magnetic nanoparticle / nanocomposite incorporated evanescent wave sensors for point sensing and / or distributed sensing applications, and other magnetic nanoparticle / nanocomposite incorporated interferometric sensors used for point and quasi-distributed sensing applications, such as fiber Bragg gratings. The disclosed concept may also be implemented as sensing / cladding layers in connection with fiber structures such as Mach-Zehnder Interferometers (MZIs), Sagnac Interferometers (Sis), or Fabry-Perot Interferometers (FPI)s in transmission or reflection mode.

[0047] In use, the fiber optic-based sensor technology of the disclosed concept may be deployed in an environment in which the sensor is subjected to an AC magnetic field. In response to the AC magnetic field, the photo-intensity or polarization response of the light signal transmitted through the sensor will change in a linear fashion below saturation due to a combined effect of absorption, scattering cross-section and refractive index change. The intensity or polarization response changes can be detected and converted to photo-currents or voltages which are transmitted to a suitable data acquisition (DAQ) or signal processing apparatus (e.g. oscilloscope) that is configured to determine and quantity' the amplitude of the magnetic field that corresponds to the received photocurrents or voltages.

[0048] FIG. 1 is a schematic diagram of an optical fiber-based sensor 5 for monitoringAC currents and / or AC magnetic fields according to a non-limiting exemplary embodiment of the disclosed concept. Sensor 5 includes a multimode interferometer (MMI) structure 10 and a composite embedded media member 1 that, as seen in FIG. 1. is coupled to and surrounds a multi-mode fiber portion of MMI structure 10 and. as such, forms a cladding / sensing layer of sensor 5. MMI structure 10 includes a first single mode fiber 20 acting a first lead fiber (e.g., an input fiber), a second single mode fiber 25 acting as a second lead fiber (e.g., an output fiber), and a multimode fiber section 30 (withoutcladding) that is provided in between first single mode fiber 20 and second single mode fiber 25. To create MMI structure 10, an end of first single mode fiber 20 is fusion spliced to a first end of multimode fiber section 30, and an end of second single mode fiber 25 is fusion spliced to a second, opposite end of multimode fiber section 30. In one particular exemplary embodiment, multimode fiber section 30 may be a multimode fiber or a nocore fiber (NCF) acting as a special case of a multimode fiber with light propagation characteristics due to multimode interference.

[0049] Assuming circular symmetric fiber structures for single mode fibers 20 and 25 and multimode fiber section 30 and no lateral offset between the axes of single mode fibers 20 and 25 and multimode fiber section 30 after fusion splicing, the single mode input from single mode fiber 20 will excite the multiple linearly polarized higher order fiber modes in multimode fiber section 30. Over the length of multimode fiber section 30. the multiple modes propagating with different phases and velocities interfere within multimode fiber section 30 and the modulated transmission optical response at the end of multimode fiber section 30 will be coupled to single mode fiber 25. When the multiple modes are excited within multimode fiber section 30, the input field is replicated in periodic interv als along the length of multimode fiber section 30. The replicated input fields along the length of multimode fiber section 30 are also known as “self-images.” The significance of such “self-images” to an aspect of the disclosed concept is described elsewhere herein.

[0050] As seen in FIG. 1, composite embedded media member 15 surrounds at least a portion of multimode fiber section 30. Composite embedded media member 15 comprises magnetic nanoparticles embedded in a host media. In one embodiment, the magnetic nanoparticles are permanently magnetized single-domain nanoparticles made of a ferromagnetic material, such as, without limitation, magnetite (FesC ) or maghemite (y- FesCh). The magnetic nanoparticles may, in a specific embodiment, be superparamagnetic nanoparticles that have a diameter of 5-15 nm. In addition, as noted elsewhere herein, the host media can be a liquid (in which case composite embedded media member 15 is a ferrofluid) or a solid. For example, the host media can be water (e.g., deionized water) or another solvent such as, without limitation, kerosene or heptane. In one particular embodiment where the host media is a liquid, composite embedded media member 15 comprises a sealed capillary tube whose ends are closed (e.g., with UV-curable glue), wherein a ferrofluid is provided within the sealed capillary tube. In other examples, the host media can be a solid such as, without limitation, an organicpolymer matrix, or an inorganic dielectric material, such as silica matrix, having magnetic nanoparticles dispersed / embedded therein.

[0051] FIG. 2 is a schematic diagram of a sensing system 35 according to an exemplary' embodiment of the disclosed concept. Sensing system 35 is configured for monitoring AC currents within and / or current induced AC magnetic fields emanating from an AC electrical current bearing asset 55. For example, and without limitation, AC electrical current bearing asset 55 may be an electrical current bearing asset of an electrical power grid system, such as a transformer, an inductor, or a power line, in which case sensing system 35 may be used for monitoring an electrical power grid system for potential current-faults to ensure the smooth delivery of electrical power to consumers. AC electrical current bearing asset 55 may also be a conductor forming a part of another electrical system such as, without limitation, an EV or an autonomous driving system. Sensing system 35 includes a sensor 5 as described herein in various exemplary’ embodiments that is placed in proximity to AC electrical current bearing asset 55. Sensing system 35 also includes a light source 40 having an output that is coupled to an input end of first single mode fiber 20 of sensor 5 and a photodetector 45 having an input that is coupled to an output end of second single mode fiber 25 of sensor 5. In the nonlimiting exemplary embodiment, light source 40 is a narrow linewidth laser within the telecommunications window, such as a DFB laser. Also in the non-limiting exemplary embodiment, photodetector 45 is a switchable gain amplifying photodetector that is configured to convert received light signals to electrical signals.

[0052] As seen in FIG. 2, sensing system 35 further includes a signal processing apparatus 50 such as oscilloscope or a data acquisition (DAQ) system) that comprises a programmable analog and / or digital device (including an associated memory part or portion) that can store, retrieve, execute and process data (e.g., software routines and / or information used by such routines), including, without limitation, a field programmable gate array (FPGA), a complex programmable logic device (CPLD), a programmable system on a chip (PSOC), an application specific integrated circuit (ASIC), a microprocessor, a microcontroller, a programmable logic controller, or any other suitable processing device or apparatus. The memory portion can be any one or more of a variety’ of types of internal and / or external storage media such as. w ithout limitation. RAM, ROM, EPROM(s), EEPROM(s), FLASH, and the like that provide a storage register, i.e., a non-transitory machine readable medium, for data and program code storage such as in the fashion of an internal storage area of a computer, and can be volatile memory ornonvolatile memory. Signal processing apparatus 50 is configured to process the electrical voltage and / or current signals output by photodetector 45 in order to analyze / enable the monitoring of AC currents within and / or current induced AC magnetic fields emanating from AC electrical current bearing asset 55.

[0053] In operation, light source provides a light signal to sensor 5. In response to AC magnetic fields emanating from AC electrical current bearing asset 55, the photo-intensity or polarization response of the light signal transmitted through sensor 5 will change in a linear fashion below saturation due to a combined effect of absorption, scattering loss and refractive index change in sensor 5 caused by composite embedded media member 15. The intensity or polarization response changes are detected by photodetector 45 and converted to photo-currents or voltages by photodetector 45, which are transmitted to signal processing apparatus 50. Signal processing apparatus 50 is configured by way of one or more programs to determine and quantify the amplitude of the magnetic field that corresponds to the received photo-currents or voltages. In one aspect, signal processing apparatus 50 may be programmed to reveal harmonics components within the received sensor output signals via a fast Fourier transform (FFT) algorithm, which performs discrete Fourier transform (DFT) of the time domain signal into a frequency domain signal. The signal can be further processed in signal processing apparatus 50 to single out any particular frequency component via methods of digital filtering such as band pass, high pass, or low pass filtering. As a result, sensing system 35 is configured for monitoring AC currents within and / or AC magnetic fields emanating from AC electrical current bearing asset 55.

[0054] The present inventors evaluated the dynamic response of an exemplary sensor 5 as shown in FIG. 1 subjected to time vary ing AC H-field excitations at various frequencies and amplitudes using the interrogation set-up shown in FIG. 3A. As seen in FIG. 3A, a narrow linewidth optical signal with central wavelength 1554.5 ± 0.02 nm from a DFB laser (Thorlabs, S3FC1550) is transmitted through the sensor 5 and is converted into an electrical signal through an InGaAs switchable gain amplifying photodetector (PD) (Thorlabs, PDA10CS2). The electrical signal is then analyzed using a multichannel digital oscilloscope (Keysight, DSOX1204G, 200 MHz bandwidth, 2GSa / s sample rate). An AC magnetic field is applied to the sensor 5 parallel to sensor length using a Helmholtz (HH) coil (MH-6, Lakeshore, Max. current 2 A), which can produce a uniform magnetic field up to 55 G with a responsivity of 27.5 G / A. The current to the HH coil is supplied through an AC+DC power supply source (Instek, 2100R). The in situ magneticfield is measured via a Hall probe and a gaussmeter (Model 425, LakeShore). The electrical analog signal from the gaussmeter, also connected to the oscilloscope channel, is utilized as the reference sensor signal. Synchronization of two signals utilizing the “trigger’ function in the oscilloscope helps to analyze relative evolution of the dynamic signals and the sensor’s response to the AC field.

[0055] Using the interrogation set-up just described, an exemplary nano-ferrofluid- cladded sensor 5 was investigated for its response when exposed to pure 60 Hz sinusoidal AC H-field supplied via the current through the HH coil as indicated by the Hall probe signal and its FFT spectrum in FIG. 3B. The other higher order harmonics are invisible in linear scale and only visible in the logarithmic scale as the amplitude of the 60 Hz frequency component is the dominant one as evinced by the Fast Fourier transform (FFT) spectrum of the input signal. The MF sensors response, however, converts 60 Hz input frequency to a 120 Hz frequency as the dominant spectrum containing about 82% of the signal’s total amplitude. This is because the AC response of the sensor reverses the direction for each falling and rising edge of the sinusoidal signal irrespective of the direction of applied H-field as explained above. FIG. 3C shows the response of the exemplary sensor 5 to a 60 Hz H-field of various strengths, sensor produced AC signals at various magnitudes of applied 60 Hz input H-field, their FFT amplitudes, and linearity of FFT amplitudes with magnetic field. The sensor responds linearly to the increased magnetic field with output signal’s Vpp value and FFT amplitudes increasing at higher field applied up to 55 Gauss. The sensor is most sensitive at even harmonics frequencies and the highest sensitivity is measured to be 9.91 mV / Gauss at dominant harmonics of 120 Hz with near perfect linearity (R-square value = 0.995).

[0056] Table 1 provides the quantification of different harmonics components, their contribution to the total sensor’s signal, sensitivity and response linearity with H-field. The tabulated calculations correspond to the 52.19 pW transmitted power through the exemplary sensor 5 when a 2.48 mW of input power from a DFB laser at 1550 nm was fed to the exemplary sensor 5.Table 1. Sensitivity' and response-linearity at 60 Hz harmonics(mV (iauss)a1 0.021 ± 6'615" () .06 '"T 0 ?02 d 0 '.002 '""" 0.002R-Square 0.99 0 0.9405 0.8734 (49688 0.9836 6 9714

[0057] Using the same interrogation set-up of FIG. 3 A, an exemplary nano-ferrofluid- cladded sensor 5 was also investigated for its ability in detecting different current profiles that exist in different electrical circuits and components, including those during short circuit current faults events in power transmission lines and other assets of power grids systems. Since the base supply frequency is 60 Hz, the focus has been imparted on hardware simulation to generate different current profiles at the same frequency. The current is supplied again through the HH coil, and the response is recorded in terms of electrical voltage output through the oscilloscope. As shown in FIGS. 3B (time and frequency domains plots), for a base 60 Hz input field frequency, 120 Hz is the major frequency component in sensor signal of the exemplary sensor 5. However, the 60 Hz spectral component still possesses measurable amplitude, which can be used to extract the 60 Hz current profile embedded in the signal by filtering other frequency components For example, in FIG. 4A, for the burst current profile fed to the HH coil and measured by the Hall probe, the exemplary' sensor 5 produces the response signal represented by the bottom graph. After applying a narrow bandpass filter between 55 and 65 Hz to the output sensor signal, the filtered signal (top graph) can replicate the exact profile as the input current profile. The sensors of the disclosed concept thus can be used to monitor several current profiles that result from different anomalies in the circuits. In addition, several types of current profiles were generated mimicking an intermittent spike current profile (FIG. 4B), symmetric and asymmetric fault current profiles (FIGS. 4C and 4D) of short-circuit events in power transmission lines, two types of impulse current profiles (FIGS. 4E and 4F), and exponential fall and rise current profiles (FIGS. 4G and 4H). The exemplary sensor 5 replicates the exact same current / H-field profile as long as the polarity / direction of the current / H-field is in the same direction such as the current profiles of FIGS. 4D-4H.

[0058] One particular embodiment of the disclosed concept involves fabrication and optimization of a sensor 5 such that the full width half maxima (FWHM) of the interference maxima / peak of the transmitted spectrum of sensor 5 coincides with (i.e., includes) a predetermined desired wavelength (e.g., 1550 nm) of a fiber-optic communication band (e.g., 1260 nm to 1625 nm). As noted elsewhere herein, when the multiple modes are excited within multimode fiber section 30 of MMI structure 10 ofsensor 5, the input field is replicated in periodic intervals along the length of multimode fiber section 30. This phenomenon is also know n as self-images of the input optical field. Moreover, at every fourth self-image, multimode fiber section 30 will produce a near perfect minor image of the input field with a filter-like spectral response with a broad FWHM. In addition, every such fourth self-image is the transmitted spectrum that is most sensitive to external stimuli / perturbations to the sensing material comprising composite embedded media member 15. Also, the broader the FWHM of the interference peak, the wider the wavelength band it can work for in the optical telecommunication window. Thus, one specific embodiment of sensor 5 that may provide optimal performance is a sensor 5 configured to be interrogated by a light signal having a particular interrogation wavelength w herein the MMI structure 10 of the sensor 5 is optimized to transmit a fourth self-imaging spectral response of the MMI structure 10 having a FWHM that contains the particular interrogation wavelength.

[0059] In order to produce such an optimized sensor 5 having the fourth self-imaging spectral peak at the desired wavelength (e.g., 1550 nm), the multimode fiber section 30 of the MMI structure 10 will need to have a specific length. The specific length needed depends on both the diameter of the multimode fiber section 30 and the desired wavelength of the fourth self-imaging peak. Precise control of the specific length of the multimode fiber section 30 of the MMI structure 10 can be achieved by adjusting and measuring the fiber length using a microscope and then cleaving the fiber precisely w ith a fiber cleaver. In the exemplary embodiment, fabrication and optimization at the fourth self-imaging condition involves a two step-process. First, a reference sensor having a multimode fiber section 30 with the same core diameter as the sensor 5 to be produced is needed to measure the w avelength-shift of the fourth self-imaging peak w hen the multimode fiber section 30 is covered in air (i.e., no cladding) vs when the multimode fiber section 30 is covered by the composite embedded media member 15 as a cladding / sensing layer. Second, the actual optimized sensor 5 is fabricated by accurately controlling and fusion splicing the length of the multimode fiber section 30 so that the fourth self-imaging peak at the desired w avelength in an air cladding (i.e., before the addition of the composite embedded media member 15) is positioned a distance from the end of the multimode fiber section 30 that is equal to the wavelength shift determined in the first step. As a result, when the composite embedded media member 15 is added to multimode fiber section 30 as a cladding / sensing layer to produce the optimized sensor 5, the fourth self-imaging spectral peak at the desired wavelength will be output by sensor 5.

[0060] Ideally, in sensor 5 an interference peak overlapping with the interrogation wavelength (such as 1550 nm) is desired for highest sensitivity. However, as long as the operating w avelength falls within the FWHM of the interference peak (such as the fourth self-imaging peak), the sensor 5 will perform well.

[0061] Furthermore, in one particular embodiment of the disclosed concept, composite embedded media member 15 is a ferrofluid that is prepared by diluting polyethylene glycol-coated magnetic nanoparticles of 10 nm diameter of high saturation magnetization (990 Oe) in DI water. The method allows the tuning of saturation magnetization as a result of varying concentration of magnetic nanoparticles in the colloidal solution, and the relaxation time can be reduced to less than a millisecond, enabling composite embedded media member 15 to provide a response to a kHz frequency7range of AC current / magnetic fields. The moment vs magnetizing field for various levels of dilution of such ferrofluids (PBG-900, Msat. -990 Oe), including mixing with DI water, demonstrates a higher saturation moment for ferrofluids with higher concentrations of magnetic nanoparticles and vice versa. The steeper slope for higher concentration of magnetic nanoparticles indicates that ferrofluids with high saturation magnetization are preferred for a higher sensitivity / response to per unit changes in magnetic field. However, very high absorption at higher volumes of magnetic nanoparticles would yield very low transmitted photointensity and photocurrent. Therefore, the preparation of optimized ferrofluid solutions for optical sensing is a tradeoff between having a measurable transmitted intensity vs sensitivity. Nevertheless, high concentrations of magnetic nanoparticles can be incorporated by7using either higher laser power to interrogate sensor 5 or by amplifying the signal transmitted form sensor 5 using an amplifying photodetector. One particular ferrofluid that may be used in composite embedded media member 15 is a 33% diluted PBG-900 solution where an original 17.9% magnetic nanoparticle concentration by volume is lowered to -5.97 %. In addition, it has been found that the optical response time of sensor 5 can been decreased to -Ims by using an optimized ferrofluid of high saturation magnetization. As a result, sensor 5 is able to successfully detect, measure and monitor AC current and H-fields to at least the 10kHz frequency range. The implication of this finding is also useful in real-time monitoring of current and current faults in electrical assets of power grid systems.

[0062] In another particular embodiment, composite embedded media member 15 comprises magnetic nanoparticles in a polymer composite that is created using a water- soluble polyvinyl alcohol (PVA) synthetic polymer that is dissolved in a water-basedmagnetic fluid solution. The concentration of nanoparticles, which dictates the sensitivity of the AC response, can be tuned by mixing the PVA, the magnetic fluid and DI water. Such a magnetic nanoparticle-polymer composite solution can be optimally prepared such that the refractive index of the composite is closer to the RI of the fiber core and such that the FWHM of the interference maxima of the sensor 5 covers the desired wavelength band in a desired telecommunication window. As shown in FIG. 5, an exemplary7sensor according to this embodiment has been also shown to successfully detect AC frequencies up to 15kHz. This result is entirety new and has not been shown elsewhere in the art.

[0063] The response signal of sensor 5 contains various harmonics of the input AC magnetic field signal (H-field) being measured (e.g., the current induced AC magnetic fields emanating from an AC electrical current bearing asset 55), with its amplitude being divided among harmonics with the major component at double the input frequency. This is due to the indiscriminate response of sensor 5 to the alternating direction of the magnetic field for an AC field. The transmitted light intensity through sensor 5 increases with increasing magnitude of the magnetic field for both polarities of the AC field. This causes the sensor signal of sensor 5 to double in frequency as compared to the input magnetic field AC frequency. Thus, even though sensor 5 of the disclosed concept enables qualitative monitoring of an AC current profile and quantitative measurement of field strength, it comes short for detection and replication of the exact frequency of the input AC magnetic field.

[0064] The disclosed concept therefore provides a method for enabling sensor 5 to not only respond proportionately to the field strength of the input AC magnetic field, but also to replicate the exact profile of the input AC magnetic field in the time domain. According to the method, sensor 5 is subjected to a DC bias static magnetic field (such as by applying an external permanent magnet) so that sensor 5 will experience input AC magnetic fields in only one direction, meaning the net magnetic field experienced by the sensors is a DC with AC as ripple. In particular, this is achieved by applying a DC bias field that is greater in magnitude than the peak-amplitude of the input AC magnetic field so that the superimposed field is a AC ripple-w aveform with DC offset.

[0065] The effect of the above-described DC-bias method on the response signal of an exemplary nano-ferrofluid-cladded sensor 5 was studied using the integration set-up of FIG. 3 for a 60 Hz frequency H-field applied via HH coil by supplying the current through the AC+DC pow er supply unit (ASR 2100) and by changing the voltage of the electrical pow er. The calibration curve of FIG. 6 show s that for every Volt of the inputpower, the supply unit delivers 0.051 amperes of current through the HH coil that generates -1.39 Gauss of H-field (equivalently - 27.27 Gauss every' ampere of current).

[0066] During the study, a 60 Hz magnetic field with an RMS amplitude of 21.5 Gauss(power supply RMS voltage of 15 V) was applied parallel to both the length of the exemplary sensor 5 and the propagation direction of light. The electrical analog signal as measured by the Hall probe through the oscilloscope is a sinusoidal waveform of peakvoltage ± 3 V as show n in FIG. 7 A. The DC bias component was added to the 60 Hz AC signal through the same power supply. The response signal is a typical signal in the presence of a pure AC field and absence of any DC bias. FIGS. 7B-7F show the impact on the response of the exemplary sensor 5 upon increasing the DC bias voltage in steps of 3V.

[0067] The signal waveforms shown in FIGS. 7A-7F were recorded in DC coupling mode in the oscilloscope, which measures net amplitude of the signal combining both the DC and AC components. As seen in FIGS. 7A-7F, increasing the DC-bias results in an increase in the DC offset component for both the sensor 5 and Hall probe signals. For example, the zero DC offset value of the hall probe signal at no DC bias (case of DC bias: 0V) gradually increases at higher DC bias voltage and the waveform shifts towards the positive side of the graph. As a result, the negative component of the input H-field signal (portion of the waveform under the shaded area) gradually decreases, and the net H-field becomes unidirectional when the DC bias voltage is enough to nullify the negative peak amplitude. As the amplitude of the H-field on the negative half cycle of the waveform diminishes, so does the response of the sensor 5 to this polarity of the H-field. The response signal of the sensor 5 gradually takes the shapes of the input 60 Hz waveform as the DC bias is increased and eventually matches the input H-field profile when the net H- field is unidirectional. Vo and Vo’ are the DC offset voltage of the sensor 5 and Hall probe signal, respectively, for the DC bias case of 15V.

[0068] Moreover, the AC coupling mode setting of the oscilloscope subtracts the DC offset value of the signal in the DC coupling mode and displays the w aveform which then oscillates about the zero Y-axis. The signal waveforms of FIGS. 8A-8F are the pure AC component of the signals whereas plots of FIGS. 8A'-8F' are their STFTs (short time Fourier transform). The STFTs extract the frequency components within the signal w aveforms or change in frequency of signals over the time. At 0V DC-bias, the 120 Hz component / harmonics in the sensor signal is the major frequency component among other harmonics that constitute the net signal. Increasing the DC-bias voltage or H-fieldsuppresses the 120 Hz component and shifts the signal amplitude to the 60 Hz component. As the net H-field is unidirectional (case of Dc-bias: 15V), the 60 Hz component consists of -98% of the signal amplitude, with the rest distributed among other harmonics components, thereby replicating, and detecting the pure 60 Hz signal supplied to the HH coil. The DC bias approach of the disclosed concept thus enables detection of AC H-field up to kHz frequency range in its purest form preserving both waveform, frequency and amplitude.

[0069] In addition, the efficacy of the performance of an exemplary nano-ferrofluid- cladded sensor 5 in detecting the correct frequency and the waveform of the input H-field signal was evaluated against various transient current profiles generated via the power supply mimicking different current fault events. A set of cylindrical ring magnets applied a DC-bias field parallel to the HH coil’s field. The comparison of the exemplary nano- ferrofluid-cladded sensor 5 and Hall probe signals (FIGS. 9A-9D) shows that the exemplary nano-ferrofluid-cladded sensor 5 successfully traces input H-field profiles of the input signals, such as a symmetric current fault profile (FIG. 9A), an asymmetric current fault profile (FIG. 9B), a burst current profile (FIG. 9C), and a second order impulse current (FIG. 9D). in terms of frequency, amplitude and phase. Without the DC- bias, the extraction of the exact frequency component and waveform was only possible after signal processing, such as implementing bandpass filtering. Thus, the DC-bias approach of the disclosed concept extends the usefulness of sensor 5 in tracking the true AC H-field waveform both qualitatively and quantitatively, with competitive performance metrics to commercial sensors such as Hall probes.

[0070] FIG. 10 is a schematic diagram of an alternative sensing system 35' according to an exemplary' embodiment of the disclosed concept that implements the DC bias field method just described. Sensing system 35' is similar to sensing system 35, and like components are labeled with like reference numerals. However, as seen in FIG. 3, sensing system 35' further includes a permanent magnet 60 placed in proximity to sensor 5. Permanent magnet 60 applies a DC bias static magnetic field that has a magnitude that is greater than the peak-magnitude of the input AC magnetic field and that is opposite in direction to one polarity of the input AC magnetic field. As a result, sensing system 35' is able to detect and replicate the exact frequency of the input AC magnetic field emanating from AC electrical current bearing asset 55.

[0071] Thus far, the disclosed concept has been described in connection with a sensor 5 comprising and MMI structure 10. However, as noted elsewhere herein, this is meant tobe exemplar^’ only, and it will be understood that the disclosed concept including a magnetic nanoparticle composite as a cladding / sensing material of a fiber optic-based sensor can also be implemented in connection with other fiber optic sensor structures, A number of which are described below.

[0072] FIG. 11 is a schematic diagram of an optical fiber-based sensor 65 for monitoringAC currents and / or AC magnetic fields according to an alternative non-limiting exemplary’ embodiment of the disclosed concept. Sensor 65 includes a fiber optic evanescent wave structure 70 and a composite MNP-embedded media member 15 as described herein that, as seen in FIG. 11, is coupled to and surrounds a portion of fiber optic evanescent wave structure 70 and, as such, forms a cladding / sensing layer of sensor 65. Fiber optic evanescent wave structure 70 includes a single mode fiber or a multimode fiber having a core 75 and a cladding 80, wherein a few centimeters of a central portion of cladding 80 is removed, exposing core 75. Composite embedded media member 15 surrounds the exposed portion of core 75 as shown in FIG. 11. Referring to FIG. 12, sensor 65 may be employed in an alternative sensing system 85 that is similar to sensing system 35 or 35' (like components are labelled with like refence numerals) for monitoring AC currents within and / or current induced AC magnetic fields emanating from an AC electrical current bearing asset 55.

[0073] FIG. 13 is a schematic diagram of an optical fiber-based sensor 90 for monitoringAC currents and / or AC magnetic fields according to another alternative non-limiting exemplary embodiment of the disclosed concept. Sensor 90 includes a fiber optic fiber grating structure 95 (including a number of fiber gratings such as fiber Bragg gratings (FBGs) or long period gratings (LPGs)).and a composite MNP-embedded media member 15 as described herein that, as seen in FIG. 13, is coupled to and surrounds a portion of grating structure 95 and, as such, forms a cladding / sensing layer of sensor 90. Grating structure 95 includes a fiber having a core 100 including a plurality of Bragg reflectors 105 and a cladding 1 10, wherein a grating portion of cladding 110 is removed, exposing core 100. Composite embedded media member 15 surrounds the exposed portion of core 100 as shown in FIG. 13. Referring to FIG. 14, sensor 90 may be employed in an alternative sensing system 115 that for monitoring AC currents within and / or current induced AC magnetic fields emanating from an AC electrical current bearing asset 55 that includes a light source 40, a photo detector 45, a signal processing apparatus 50, and a circulator 120 as shown.

[0074] FIG. 15 is a schematic diagram of a system 125 for quasi-distributed sensing employed for 3-phase electrical current and current-induced magnetic field detection and monitoring that uses three sensors 5 (or, alternatively three sensors 70 or 90) as described herein (one for each phase) according to a further exemplary embodiment of the disclosed concept. The three sensors 5 may also further include a permanent magnet for generating a DC bias static magnetic field as described herein to implement the DC bias method of the disclosed concept. As seen in FIG. 15, system 125 includes light source 40, a 1x3 three optical coupler 130 connected to a light source 40. the three sensors 5, three photodetectors 45, and a signal processing apparatus 50. System 125 is thus configured to monitor the AC currents within and / or current induced AC magnetic fields emanating from each of the phase lines 135 of a 3-phase electrical system.

[0075] FIG. 16 is a schematic diagram of an optical time domain reflectometry (OTDR) - based quasi-distributed H-field sensing system 130 for monitoring AC currents and / or AC magnetic fields according to another alternative non-limiting exemplary embodiment of the disclosed concept. System 140 includes light source 40, an optical modulator 145, an optical amplifier 150, a circulator 120, a plurality7of sensors 90 (each configured to measure an AC magnetic field H at the location of the sensor 90) coupled to one port of the circulator 120 as shown, a photodetector 45 coupled to another port of the circulator 120 as shown, and signal processing system 50 (e.g., a DAQ-data acquisition) coupled to the output of photodetector 45. Signal processing system 50 is structured and configured to measure each of the AC magnetic fields H based on the intensity and timing of the light reflected from each sensor 90. The sensors 90 may also further include a permanent magnet for generating a DC bias static magnetic field as described herein to implement the DC bias method of the disclosed concept.

[0076] A further aspect of the disclosed concept is a method to increase the sensitivity of the sensors of the disclosed concept (in various alternative embodiments) by using laser of higher power. The higher the input laser power, the more light transmits through the sensors of the disclosed concept and interacts with the magnetic fluid, thereby increasing the sensitivity7. Thus, the sensitivity7can be further increased by reducing the optical coupling loss at the interfaces and at fusion splicing joints of the sensors of the disclosed concept or by using higher power laser. To investigate the sensitivity dependance on the amount of transmitted power through the sensors of the disclosed concept, the FFT amplitude output signal of an exemplary7sensor 5 at the 120 Hz harmonic when subjected to 60 Hz H-field (case of no DC bias) was recorded by varying the transmitted powerthrough the exemplary sensor 5 using different input laser powers provided to the exemplary sensor 5 with H-fields applied between 0 to 55 Gauss. The results are shown in FIG. 17. The inset table of FIG. 17 shows the transmitted power corresponding to different input laser powers with transmittance of only -2.045% (-16.89dB). The sensitivity values (slopes of the fitted curves of FIG. 17 plotted against the transmitted power through the exemplary sensor 5 show n in the plot on the right of FIG. 17) shows that the sensitivity7is linearly dependent on the amount of transmitted pow er through the sensor, as shown in the plot on the right of FIG. 17. The sensitivity was measured to be 0. 1909 ± 0.00451 mV / Gauss per pW (equivalently, 240.3 ± 5.68mV / Gauss per dBm) of transmitted power through the exemaplry sensor 5. The higher the input laser power, the more light transmits through the sensor 5 and interacts w ith the magnetic fluid, thereby increasing the sensitivity. Thus, the sensitivity can be further increased by reducing the optical coupling loss at the interfaces and at fusion splicing joints of the sensor 5 of the dislcosed cocnept or by using higher power laser light.

[0077] While specific embodiments of the invention have been described in detail, it w ill be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof.

Claims

What is claimed is:1 . A sensor for monitoring AC currents and / or AC magnetic fields, comprising: an optical fiber structure; and a cladding / sensing layer coupled to the optical fiber structure, the cladding / sensing layer comprising a composite embedded media member, wherein the composite embedded media member includes magnetic nanoparticles embedded in a host media.

2. The sensor according to claim 1 , wherein the optical fiber structure is an MMI structure that includes a first single mode fiber section, a second single mode fiber section, and a multimode fiber section provided between the first single mode fiber section and the second single mode fiber section, and wherein the composite embedded media section surrounds at least a portion of the multimode fiber section.

3. The sensor according to claim 2, wherein the multimode fiber section is a multimode fiber without cladding.

4. The sensor according to claim 2, wherein the multimode fiber section is a no-core fiber.

5. The sensor according to claim 1, wherein the composite embedded media member includes a ferrofluid wherein the host media is a liquid.

6. The sensor according to claim 5, wherein the liquid is water, kerosene, or heptane.

7. The sensor according to claim 6, wherein the water is deionized water.

8. The sensor according to claim 5, wherein the composite embedded media member includes a sealed capillary tube, wherein the ferrofluid is provided within the sealed capillary tube.

9. The sensor according to claim 1, wherein the host media is a solid.

10. The sensor according to claim 9, wherein the solid comprises an organic polymer.

11. The sensor according to claim 10, wherein the solid comprises an organic polymer matrix.

12. The sensor according to claim 9, wherein the solid comprises an inorganic dielectric material.

13. The sensor according to claim 12, wherein the solid comprises a silica matrix.

14. The sensor according to claim 1. wherein the optical fiber structure comprises a fiber-optic interferometric structure.

15. The sensor according to claim 14, wherein the fiber-optic interferometric structure is tuned to work at a specific wavelength band.

16. The sensor according to 15, wherein the fiber-optic interferometric structure is tuned to work at a C-band wavelength range of 1530 nm-1565 nm.

17. The sensor according to claim 14, wherein where the optical fiber structure is configured to transmit a spectrum including a filter-like spectral response having a peak.

18. The sensor according to claim 17, wherein the sensor is configured to be interrogated by an input light signal having an interrogation wavelength, and wherein the fiberoptic interferometric structure is optimized such that the spectrum comprises a fourth selfimaging spectral peak of the input light signal, and wherein the interrogation wavelength is within a full width half maximum (FWHM) of the spectrum comprising the fourth self-imaging spectral peak.

19. The sensor according to claim 1, wherein the magnetic nanoparticles are permanently magnetized single-domain nanoparticles of ferromagnetic materials.

20. The sensor according to claim 19, wherein the magnetic nanoparticles are superparamagnetic magnetic nanoparticles.

21. The sensor according to claim 20, wherein the superparamagnetic magnetic nanoparticles have a diameter of 5-15 nm.

22. The sensor according to claim 1, wherein the sensor is configured to be interrogated by a light signal having an interrogation wavelength, and wherein the interrogation wavelength is within a full width half maximum (FWHM) of an interference peak of the multimode fiber section.

23. The sensor according to claim 1. wherein the sensor structure comprises an evanescent wave structure.

24. The sensor according to claim 1, wherein the sensor structure comprises a number of fiber gratings including fiber Bragg gratings (FBGs) or long period gratings (LPGs).

25. The sensor according to claim 1, wherein the sensor is configured for monitoring AC currents and / or AC magnetic fields having a frequency of up to 15 KHz.

26. The sensor according to claim 1, wherein optical coupling loss within the optical fiber structure and / or between the optical fiber structure and the cladding / sensing layer is reduced in a manner that maximizes transmission of light through the optical fiber structure and the cladding / sensing layer.

27. The sensor according to claim 1, further comprising a DC bias source positioned adjacent to the optical fiber structure and the cladding / sensing layer, wherein the DC bias source is structured and configured to provide a DC bias static magnetic field.

28. The sensor according to claim 27, wherein the sensor is configured to sense an input AC magnetic field having a first magnitude, wherein the DC bias static magnetic field has a second magnitude that is greater than the first magnitude, and wherein the DC bias static magnetic field is opposite in direction to either polarity of the input AC magnetic field.

29. The sensor according to claim 27, wherein the DC bias source comprises a permanent magnet.

30. A system for monitoring an AC magnetic field, comprising: a sensor according to claim 1; a light source for providing an interrogation light to the sensor; a detector structured and configured to detect a response light generated by the sensor in response to receiving the interrogation light and the AC magnetic field, the detector being further structured and configured to generate a voltage or current signal based on the detected response light that is indicative of an intensity7or polarization response of the detected response light; and a signal processing apparatus structured and configured to receive the voltage or current signal and generate a signal that is proportionate to a field strength of the AC magnetic field based on the voltage or current signal.

31. The system according to claim 30, wherein the processing apparatus is structured and configured to determine a magnitude of the AC magnetic field or an AC current that produced the AC magnetic field based on the voltage or current signal.

32. The system according to claim 30, wherein the sensor further comprises a DC bias source positioned adjacent to the optical fiber structure and the cladding / sensing layer of the sensor, wherein the DC bias source is structured and configured to provide a DC bias static magnetic field, and wherein the signal that is proportionate to the field strength of the AC magnetic field is an exact profile of the input AC magnetic field in the time domain.

33. The system according to claim 32, wherein the sensor is configured to sense an input AC magnetic field having a first magnitude, wherein the DC bias static magnetic field has a second magnitude that is greater than the first magnitude, and wherein the DC bias static magnetic field is opposite in direction to one polarity of the input AC magnetic field.

34. The system according to claim 32, wherein the DC bias source comprises a permanent magnet.

35. A method of monitoring an AC magnetic field, comprising: providing an optical interrogation signal to a sensor according to claim 1 when the sensor is within the AC magnetic field;detecting an optical response signal generated by the sensor in response to the interrogation light; generating a voltage or current signal based on the detected optical response signal that is indicative of an intensity or polarization response of the detected response light; and generating a signal that is proportionate to a field strength of the AC magnetic field based on the voltage signal or current signal.

36. The method according to claim 35. further comprising determining a magnitude of the AC magnetic field or an AC current that produced the AC magnetic field based on the voltage signal or current signal.

37. The method according to claim 35. further comprising providing a DC bias static magnetic field to the sensor, wherein the signal that is proportionate to the field strength of the AC magnetic field is an exact profile of the input AC magnetic field in the time domain.

38. The method according to claim 37, wherein the sensor is configured to sense an input AC magnetic field having a first magnitude, wherein the DC bias static magnetic field has a second magnitude that is greater than the first magnitude, and wherein the DC bias static magnetic field is opposite in direction to one polarity of the input AC magnetic field.

39. The method according to claim 35. wherein a power of the optical interrogation signal is chosen to maximize transmission of optical interrogation signal through the optical fiber structure and the cladding / sensing layer.

40. The method according to claim 39. wherein optical coupling loss within the optical fiber structure and / or between the optical fiber structure and the cladding / sensing layer is reduced in a manner that maximizes transmission of light through the optical fiber structure and the cladding / sensing layer.

41. A system for monitoring a three-phase electrical system having a first phase conductor, a second phase conductor and a third phase conductor, comprising: a first sensor according to claim 1 positioned within a first AC magnetic field emanating from the first phase conductor responsive to a first AC current in the first phase conductor;a second sensor according to claim 1 positioned within a second AC magnetic field emanating from the second phase conductor responsive to a second AC current in the second phase conductor; a third sensor according to claim 1 positioned within a third AC magnetic field emanating from the third phase conductor responsive to a third AC cunent in the first phase conductor; a light source for providing an interrogation light to the first, second and third sensors; one or more detectors structured and configured to detect a first response light generated by the first sensor in response to receiving the interrogation light and the first AC magnetic field, a second response light generated by the second sensor in response to receiving the interrogation light and the second AC magnetic field, and a third response light generated by the third sensor in response to receiving the interrogation light and the third AC magnetic field, the detector being further structured and configured to generate a first voltage signal or first current signal based on the detected first response light that is indicative of an intensity or polarization response of the first detected response light, a second voltage signal or second current signal based on the detected second response light that is indicative of an intensity or polarization response of the second detected response light, and a third voltage signal or third current signal based on the detected third response light that is indicative of an intensity or polarization response of the third detected response light; and a processing apparatus structured and configured to receive the first voltage signal or first current signal and generate a first signal that is proportionate to a field strength of the first AC magnetic field based on the first voltage signal or first current signal, to receive the second voltage signal or second current signal and generate a second signal that is proportionate to a field strength of the second AC magnetic field based on the second voltage signal or second current signal, and to receive the third voltage signal or second current signal and generate a second signal that is proportionate to a field strength of the second AC magnetic field based on the second voltage signal or second current signal.

42. The system according to claim 41, wherein the processing apparatus is structured and configured to determine a magnitude of the first AC magnetic field or a first AC current that produced the first AC magnetic field based on the first voltage signal or first current signal, a magnitude of the second AC magnetic field or a second AC current that produced the second AC magnetic field based on the second voltage signal or second current signal, and a magnitude ofthe third AC magnetic field or a third AC current that produced the third AC magnetic field based on the third voltage signal or third current signal.

43. The system according to claim 41, wherein the first sensor further comprises a first DC bias source structured and configured to provide a first DC bias static magnetic field, and wherein the first signal that is proportionate to the field strength of the first AC magnetic field is an exact profile of the first AC magnetic field in the time domain, wherein the second sensor further comprises a second DC bias source structured and configured to provide a second DC bias static magnetic field, and wherein the second signal that is proportionate to the field strength of the second AC magnetic field is an exact profile of the second AC magnetic field in the time domain, and wherein the third sensor further comprises a third DC bias source structured and configured to provide a third DC bias static magnetic field, and wherein the third signal that is proportionate to the field strength of the third AC magnetic field is an exact profile of the third AC magnetic field in the time domain.

44. The system according to claim 43, wherein the first, second and third sensors are configured to sense an input AC magnetic field having a first magnitude, wherein the first, second and third DC bias static magnetic fields each have a second magnitude that is greater than the first magnitude, and wherein each of the first, second and third DC bias static magnetic fields is opposite in direction to one polarity' of the input AC magnetic field.

45. The system according to claim 43, wherein each of the first, second and third DC bias sources comprises a permanent magnet.

50. An optical time domain reflectometry (OTDR) -based quasi-distributed sensing system for monitoring a plurality’ of AC magnetic fields, comprising: a plurality' of sensors, wherein each of the sensors comprises an optical fiber structure including a number of fiber gratings and a cladding / sensing layer coupled to the optical fiber structure, the cladding / sensing layer comprising a composite embedded media member, wherein the composite embedded media member includes magnetic nanoparticles embedded in a host media; a light source for providing an interrogation light to each of the sensors; a detector structured and configured to detect a response light generated by each of the sensors in response to receiving the interrogation light and an AC magnetic field at alocation of the sensor, the detector being further structured and configured to generate a voltage or current signal based associated with each sensor based on the detected response light of each sensor that is indicative of an intensity or polarization response of the detected response light; and a signal processing apparatus structured and configured to receive the voltage or current signal associated with each sensor and generate a signal that is proportionate to a field strength of the AC magnetic field at the location of the sensor based on the voltage or current signal associated with the sensor.

51. The sensing system according to claim 50, the number of fiber gratings is a number of fiber Bragg gratings (FBGs).

52. The sensing system according to claim 50, the number of fiber gratings is a number of long period gratings (LPGs).

53. The sensing system according to claim 50, further comprising an optical modulator, and optical amplifier and an optical coupler couple to an output of the light source for providing the interrogation light to each of the sensors.

54. The sensing system according to claim 53, wherein the optical coupler is also structured and configured to provide response light generated by each of the sensors to the detector.

55. The sensing system according to claim 50, wherein the signal processing apparatus comprises a data acquisition system (DAQ).

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