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The AC magnetic field response of the superparamagnetic nano-ferrofluid is an interplay between the Neel and Brownian relaxation processes and is generally quantified via the susceptibility measurements at high frequencies. The high frequency limit is dictated by these relaxation times which need to be shorter than the time scale of the time varying magnetic field for the nano-ferrofluid to be considered in an equilibrium state at each time instant. Even though the high frequency response of ferrofluid has been extensively investigated for frequencies up to GHz range by non-optical methods, harnessing dynamic response by optical means for AC magnetic field sensing in fiber-optic-based sensors-field remains unexplored. Instead, the incorporation of nano-ferrofluid as sensing materials has been only limited to DC magnetic field sensing, often citing their long response time as a limiting factor to AC field sensing. This work reports the finding of high frequency (up to 15 kHz) AC magnetic field sensing capability of nanomagnetic fluid as the cladding material of a fiber-optic multimode interferometry (MMI) structure optimized for the fourth self-imaging spectral response. The key parameter enabling high frequency response is the short response time (<1 ms) achieved by optimizing both the sensing structure and nano-ferrofluid solution. Focus has been imparted on 60 Hz line-frequency profiles of various current/magnetic fields to test the efficacy of these sensors in metering and monitoring current and current-induced magnetic fields in the electrical power grid systems. The magnetic field sensitivity of 240 mV/Gauss per dBm of transmitted power was achieved for 60 Hz field applied via Helmholtz coil, whereas the 60 Hz AC current sensitivity of 2.83 mV/A was measured due to magnetic field induced by current in a straight conducting wire. [ABSTRACT FROM AUTHOR]

We demonstrate a multiparameter distributed sensing system based on optical frequency-domain reflectometry (OFDR) with a bandwidth-division multiplexing (BDM) technique. With the BDM-assisted OFDR, the sensor system can help interrogate multiple parallel sensing optical fibers by scanning the fibers within distinct spectral ranges without any cross talk. A dual-parameter distributed sensing system was set up by multiplexing two types of sensing fibers for the simultaneous independent measurement of strain and temperature. The rms deviations between the applied and measured values of the two parameters are $19~ {\mu } {\varepsilon } $ and 1.4 °C, respectively, for a sensing spatial resolution of 10 cm. The proposed multiparameter distributed sensor system offers inexpensive potential solution to continuously monitor various parameters and is effective for the health monitoring of complex distributed structures. [ABSTRACT FROM AUTHOR]

We propose a weighted singular value decomposition (WSVD) to reduce the random noise in the Rayleigh backscattering signal of the phase-sensitive optical time domain reflectometry (Φ-OTDR) to enhance the detection performance of the distributed vibration sensing. A 2D image is formed by assembling the raw Rayleigh backscattering traces into a matrix form, and slowly varying fluctuation and random noise can be removed using the WSVD. Consequently, the location information and the frequency of vibration induced by the external vibration event can be extracted. A vibration event with 9 m spatial resolution is detected along a 2.4 km single mode fiber. The signal-to-noise ratio (SNR) of location information for the 102 Hz physical vibration and the 525 Hz acoustic vibration was found to be 10.7 and 12.2 dB, respectively. The SNR of the vibration events demonstrate an increase of 6–7 dB compared to the conventional method, showing the excellent denoising capability of this new approach. [ABSTRACT FROM AUTHOR]

In this article, we present comparative study on the transmission and the sensing characteristics of an all-fiber Sagnac loop interferometer (SLI) made by different kinds of the highly-birefringent (HB) two-core photonic crystal fiber (TCPCF), respectively for strain, torsion, and temperature sensing. The fiber device is analyzed theoretically and experimentally. Transmission spectrum of the SLI using the HB TCPCF shows six spatial frequencies due to superposition of multiple two-beam interferences originating from the intra-core and the inter-core interactions between the fundamental polarization modes propagating in the two birefringent cores. Our results reveal that the transmission characteristics of the fiber device and the sensing features of its multiplexed interferences are strongly dependent on the optical and the structural properties of the two birefringent cores such as their intrinsic birefringences, the effective index contrast between them, and the relative orientation between the (slow) birefringence axes. Finally, the application of the SLI made by the specific HB TCPCF is discussed in the simultaneous measurement of three parameters. [ABSTRACT FROM AUTHOR]

A new fiber sensor configuration based on the cascaded in-line fiber interferometers has been proposed and experimentally demonstrated for simultaneous measurement of strain and temperature. In order to make the cascaded in-line interferometers, we used two different types of two-core photonic crystal fibers (TCPCFs) having two cores in the symmetry and asymmetry structures. The TCPCFs have different transmission characteristics owing to the distinct geometric structures of the integrated cores, this causes the different strain and temperature sensitivities and thus the two parameters can be discriminated. The measurement resolutions are achieved to be $11.1~\mu \varepsilon $ and 1.10 °C for strain and temperature, respectively, in the 10 pm wavelength resolution. [ABSTRACT FROM AUTHOR]

We propose a bend-loss free, high-resolution, fiber-optic distributed sensor by using a new sensing fiber namely Ge-doped-core photonic crystal fiber (PCF) in optical frequency domain reflectometer (OFDR). PCF is fabricated with high concentration of Ge-doping in the small core leading to large refractive index difference between core and air-silica cladding. We achieved negligible macrobending loss in our Ge-doped-core PCF for a bend radius of curvature down to 1 mm in experiment and the results are validated by numerical simulation. By using OFDR, with a bend radius of 1 mm, the strain and temperature sensitivities in Ge-doped-core PCF are measured to be 0.138 GHz/ $\mu \varepsilon $ and 1.46 GHz/°C, respectively, with 5 cm spatial resolution. [ABSTRACT FROM PUBLISHER]

A highly sensitive and simple all-fiber interferometric two-dimensional (2D) bending vector sensor is demonstrated. Fiber interferometer works in reflection mode and formed by manually splicing a small section of elliptic two-core photonic crystal fiber at the end of a single-mode fiber. Due to the birefringence nature of the two cores, the fiber device exhibits orientation-dependent bend sensitivity which is different for different polarization states. A high sensitivity of 6.49 nm/mm is achieved for bending along the x-direction, and the rms deviations for bending in the x- and y-directions are estimated to be 0.015 and 0.098 mm, respectively, by using the matrix method. The device is immune to bend-induced power fluctuation. [ABSTRACT FROM AUTHOR]

A highly sensitive temperature sensor based on an all-fiber Mach–Zehnder interferometer using a selectively polymer-filled two-core photonic crystal fiber is experimentally demonstrated. The sensor fiber was made by the manual gluing and subsequent infiltration of polymer; the cladding air holes surrounding one core were selectively filled with a polymer material with a high thermo-optic (TO) coefficient while leaving those of the other core unfilled, and this led to the large TO mismatch between two cores. It was found from measurements that a very high temperature sensitivity of 1.595 nm/°C could be achieved, which was more than two orders of magnitude higher than that of the sensor fiber before the selective filling process. [ABSTRACT FROM PUBLISHER]

We present a fiber-optic multibeam Mach-Zehnder interferometer (m-MZI) for simultaneous multi-parameter measurement. The m-MZI is comprised of a section of photonic crystal fiber integrated with two independent cores of distinct construction and birefringence properties characterized for torsion, strain and temperature sensing. Due to the presence of small core geometry and use of a short fiber length, the sensing device demonstrates inter-modal interference in the small core alongside the dominant inter-core interference between the cores for each of the orthogonal polarizations. The output spectrum of the device is characterized by the three-beam interference model and is polarization-dependent. The two types of interferometers present in the fiber m-MZI exhibit distinct sensitivities to torsion, strain and temperature for different polarizations, and matrix coefficients allowing simultaneous measurement of the three sensing parameters are proposed in experiment., Competing Interests: The authors declare no conflict of interest.

We have experimentally investigated the multi-parameter sensing characteristics in a novel all-fiber Sagnac loop hybrid interferometer based on a highly birefringent photonic crystal fiber with two asymmetric cores. The sensor device was based on a combination of two types of in-fiber interferences, the intra-core-mode Sagnac interference and the inter-core-mode Mach-Zehnder interference due to the distinct birefringent properties associated with the asymmetric cores. Fast Fourier transform analysis on the transmission spectra of the device exhibited six clear peaks in the spatial frequency domain. By examining the phase shift responses of two distinct Sagnac and one Mach-Zehnder interference peaks, the response matrix that enable simultaneous measurement of torsion, strain, and temperature could be obtained. The proposed all-fiber Sagnac loop hybrid interferometer has the advantages such as simplicity of the device structure, compact device size, and capability for simultaneous sensing of multiple parameters.