A high performance fiber-optic current sensor (FOCS) based on Faraday rotation in a toroidal sensing coil is proposed and demonstrated. The sensor performance has been improved by forming a toroidal sensing head to experience large magnetomotive force for a very small signal of electric current. In order to improve the sensor performance even further, the effective optical path length is increased by making a fiber coil, and the operating wavelength has been shifted to shorter wavelength (1064 nm) compared to the conventional telecom wavelengths (1550 nm) to benefit from a higher value of Verdet constant in conventional single mode fibers. Several toroid-core structures have been simulated using finite element analysis (COMSOL Multiphysics) to obtain enhanced sensitivity. The FOCS design includes an optimized 3D printed core structure along with toroidal windings and fiber loop inside it. Faraday rotator mirror (FRM) compensates for the birefringence along the sensing arm of the setup, while laser amplitude modulation is implemented using an electro-optic modulator (EOM) to enhance the signal to noise ratio at a particular modulated frequency. The developed FOCS set-up with four layers of copper wire windings in toroidal sensing head configuration is capable of detecting low currents of the order of 50 mA within a tested dynamic range of operation 0-10A. Detection of even lower order current (as low as several mA) could be achieved by tuning the design of sensing head.
Femtosecond laser irradiation allows to modify the optical properties of transparent materials with high accuracy. In many applications, optical scattering produced by the laser irradiation is one of the major limiting factors. However, there are situations when the scattering is responsible for the basic principle of operation of the optical element. This report reviews two research directions where laser-induced scattering can be successfully exploited. First, spectroscopic measurements can be performed by analyzing the speckle patterns created by the scattering medium. The measurements are made possible by the strong dependence of the speckle pattern on the wavelength of light. A scattering chip created thanks to a femtosecond laser makes allows addressing the stability problem faced by many scattering spectrometers. The volumetric scattering centers are induced in silica substrate via micro-explosions caused by the focused laser beam. Such a spectrometer can be successfully used for interrogating fiber Bragg gratings or interferometers. Second example is found in optical reflectometry. This technology allows turning an optical fiber into a distributed microphone or thermometer. A single optical fiber can monitor a stretch of several tens of kilometers with an accuracy of several meters. Such systems have wide range of applications in civil engineering, geosciences and other fields. Reflectometry measurements are performed by observing the back-scattered light produced by the glass medium of the optical fiber. Femtosecond laser writing allows effectively increase backscattered light whilst introducing minimal additional losses. In this way, the sensitivity of reflectometric systems can be increased or their range can be extended.
KEYWORDS: Reflectors, Acoustics, Sensing systems, Signal attenuation, Data acquisition, Spatial resolution, Distance measurement, Sensors, Structural health monitoring, Time metrology
The detection range of Distributed Acoustic Sensor (DAS) systems is limited by signal attenuation to approximately 75 km. The ability to increase the detection range is of great commercial interest to the offshore wind farm operators interested in structural health monitoring of their subsea cables. In most cases, the operators are interested in monitoring 200km~400km subsea cable where the fibres can be accessed at two ends of the cable. In this paper, we present a new, commercially viable, ultra low-loss sensing element, comprising of discrete broadband reflectors. Optical time domain reflectometry measurements were performed on a 100 m sample of the fibre. The sample contained reflectors placed at 3 m intervals. At reflector sites, the recorded trace revealed increases in the backscatter signal two hundred times that of the unmodified regions of the fibre. Theoretically, the spatial resolution of a system utilising this new element is only restricted by the ability to resolve two reflector points. Therefore, the ultra low loss fibre also offers the potential for high spatial resolution measurements over large distances, as long as sufficient data acquisition and processing techniques are employed. The significant enhancement means no amplification of the reflected signal is required, further reducing the cost of the system. To verify the long distance capability of the fibre, the sample was subjected to optical side scattering radiometry measurements. The largest side-scattered loss from a reflector point was 10^-4 dB per reflector. If a reflector was placed every meter, the total fibre attenuation is predicted to be 0.3 dB per km.
A Distributed Vibration Sensor Based on Phase-Sensitive OTDR is numerically modeled. The advantage of modeling the building blocks of the sensor individually and combining the blocks to analyse the behavior of the sensing system is discussed. It is shown that the numerical model can accurately imitate the response of the experimental setup to dynamic perturbations a signal processing procedure similar to that used to extract the phase information from sensing setup.
A multi-port microcoil resonator magnetic field sensor based on a microfiber coupler coil resonator (MMCR) is presented. The microfiber coupler coil is fabricated by coiling a four-port microfiber coupler with a uniform waist region around a low index support rod. The MMCR is embedded in a low refractive index polymer to increase the robustness and operation stability. The enhanced sensor response to the magnetic field is ascribed to the diverse MMCR response to the light polarization state. The MMCR magnetic field sensor is compact and low cost, and exhibits a magnetic field sensitivity of 37.09 dB/T with an estimated minimum detection limit (DL) of ~ 27 μT.
A distributed optical fibre sensor is demonstrated which is capable of quantifying acoustic and dynamic strain disturbances along a 1km sensing fibre. A phase-OTDR technique is used to detect the dynamic perturbations using the phase-difference between the backscattered light from two separate sections of the sensing fibre. The demonstrated sensor detects multiple dynamic perturbations simultaneously within a frequency range of 200Hz to 5000Hz with a frequency resolution of 10Hz and a spatial resolution of 1m.
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