Fiber optic technologies have strong potential to augment and improve existing areas of sensor performance across many applications. Magnetic sensing, in particular, has attracted significant interest in structural health monitoring and ferromagnetic object detection. However, current technologies such as fluxgate magnetometers and inspection gauges rely on measuring magnetic fields as single-point sensors. By using fiber optic distributed strain sensors in tandem with magnetically biased magnetostrictive material, static and dynamic magnetic fields can be detected across long lengths of sensing fiber. This paper investigates the relationship between Fiber Bragg Grating (FBG)-based strain sensors and the magnetostrictive alloy Metglas® 2605SC for the distributed detection of static fields for use in a compact cable design. Sentek Instrument’s picoDAS system is used to interrogate the FBG based sensors coupled with Metglas® that is biased with an alternating sinusoidal magnetic field. The sensing system is then exposed to varied external static magnetic field strengths, and the resultant strain responses are analyzed. A minimum magnetic field strength on the order of 300 nT was able to be resolved and a variety of sensing configurations and conditions were also tested. The sensing system is compact and can be easily cabled as both FBGs and Metglas® are commercialized and readily acquired. In combination with the robust and distributed nature of fiber sensors, this demonstrates strong promise for new means of magnetic characterization.

We present sapphire fiber densely multiplexable scattering array interferometer (DMSAI) for robust high-temperature sensing. Independent full-range measurement between $23^{\circ}\mathrm{C}$ and $1500^{\circ}\mathrm{C}$ is demonstrated on six sensors multiplexed on a single sapphire fiber. © 2024 The Author(s)

Acoustic frequency domain reflectometry (AFDR) is a novel sensing technique that employs frequency-modulated continuous wave (FMCW) methods in acoustic waveguide reflectometry. This approach is particularly effective for dispersion and phase compensation due to the method's frequency domain approach. By applying this compensation, distortion due to waveguide dispersion and frequency sweep nonlinearity is partially deconvolved from the spatial profile of the waveguide. Unlike conventional acoustic FMCW-based methods, AFDR uses a single acoustic transducer for both transmission and reception, which enables the use of lower cost electronic systems than in standard time-domain pulse-echo-based systems. Furthermore, AFDR allows for SNR improvement by extending the sweep time rather than increasing the applied acoustic amplitude, which can aid in small signal detection without signal distortion. AFDR was successfully demonstrated for distributed temperature sensing, achieving a temperature resolution of about 1 degrees C and a spatial resolution of 20 cm, with a theoretical spatial resolution of about 10 mm, illustrating its scalability in system parameters, sensor density, and methodology. Given its flexibility, simplicity, and low cost, AFDR has significant potential for a wide range of distributed sensing applications using acoustic waveguides.

Fiber-optic sensing has shown promising development for use in detecting magnetic fields for downhole and biomedical applications. Coupling existing fiber-based strain sensors with highly magnetostrictive materials allows for a new method of magnetic characterization capable of distributed and high-sensitivity field measurements. This study investigates the strain response of the highly magnetostrictive alloys Metglas® 2605SC and Vitrovac® 7600 T70 using Fiber Bragg Grating (FBG) acoustic sensors and an applied AC magnetic field. Sentek Instrument’s picoDAS interrogated the distributed FBG sensors set atop a ribbon of magnetostrictive material, and the corresponding strain response transferred to the fiber was analyzed. Using the Vitrovac® ribbon, a minimal detectable field amplitude of 60 nT was achieved. Using Metglas®, an even better sensitivity was demonstrated, where detected field amplitudes as low as 3 nT were measured via the strain response imparted to the FBG sensor. Distributed FBG sensors are readily available commercially, easily integrated into existing interrogation systems, and require no bonding to the magnetostrictive material for field detection. The simple sensor configuration with nanotesla-level sensitivity lends itself as a promising means of magnetic characterization and demonstrates the potential of fiber-optic acoustic sensors for distributed measurements.

Magnetic field sensing has the potential to become necessary as a critical tool for long-term subsurface geophysical monitoring. The success of distributed fiber optic sensing for geophysical characterization provides a template for the development of next generation downhole magnetic sensors. In this study, Sentek Instrument’s picoDAS is coupled with a multi-material single mode optical fiber with Metglas® 2605SC cladding wire inclusions for magnetic field detection. The response of acoustic sensing fibers with one and two Metglas® 2605SC cladding wires was evaluated upon exposure to lateral AC magnetic fields. An improved response was demonstrated for a sensing fiber with in-cladding wire following thermal magnetic annealing (~400 °C) under a constant static transverse magnetic field (~200 μT). A minimal detectable magnetic field of ~500 nT was confirmed for a sensing fiber with two 10 μm cladding wires. The successful demonstration of a magnetic field sensing fiber with Metglas® cladding wires fabricated via traditional draw processes sets the stage for distributed measurements and joint inversion as a compliment to distributed fiber optic acoustic sensors.

ABSTRACT: Fiber optic technologies such as distributed temperature sensing, distributed strain sensing and distributed acoustic sensing are becoming increasingly commercially available for rock mechanics and soil mechanics studies. However, for applications that require reliable quantitative mapping of subsurface saturation or ice content, these strain or seismic methods must often be augmented with electrical and magnetic methods. Some examples include monitoring the role of saturation in slope stability, as well as geomechanical monitoring in permafrost regions (e.g. for well integrity or infrastructure). Our team has developed novel fiber optic distributed magnetic sensing fibers, which include embedded rods of materials that expand in the presence of magnetic fields, a change known as magnetostriction. These fibers include a single mode core, so they can be connected end-to-end with standard fiber optics, and measured using the same distributed acoustic sensing optical interrogator unit. This talk will cover the design, manufacturing, and practical use of these fibers with multiple magnetostrictive materials embedded. We will show theoretical and numerical models of the fiber response that explain features observed in controlled laboratory experiments, as well as techniques (e.g. Bragg gratings) to improve sensitivity. We will show some initial field trial results and potential deployment styles for multi-physics fiber optic sensing as we move forwards.

Magnetic sensing optical fiber is a recently sought after technology able to withstand harsh environments, capable of distributive sensing, and readily adapted to different magnetic field strengths. A multi material optical fiber system utilizing magnetostrictive nanowires embedded in the fiber cladding was designed, tested, and characterized. The sensor takes advantage of fiber Bragg gratings and magnetostriction induced strain to sense changes in magnetic field amplitude. The distributed optical fiber sensor was shown to detect AC magnetic field strengths as low as 260 μT while still being able to be fusion spliced despite containing ferromagnetic material inside the fiber. Experiments using nickel and galfenol nanowires contained within the optical fiber sensor were conducted to demonstrate different available sensing ranges for the fiber. The results showed good feasibility for an adaptable fiber design and easy integration to available optical sensing interrogation systems.

An intrinsic sapphire fiber scattering array interferometer (SAI) is presented. SAI generates robust interferometric signal with a ~20 dB signal-to-noise ratio and is immune to blackbody radiation-induced noise. Multiplexable high-temperature sensing to ~1500°C is demonstrated.

The etching rates and activation energies associated with the etching of a-plane sapphire surfaces using sulfuric acid, phosphoric acid, and a 3:1 molar mixture of the two were determined for single crystal sapphire optical fibers. Significant differences in the reported etch rates of the c-plane and a-plane sapphire surfaces were observed and possible explanations for this behavior are discussed. The chemical and morphological changes occurring throughout heating of the etching acids are described, including the effects of these changes on the etching behavior of a-plane sapphire surfaces. The formation of insoluble etch products, that effectively mask the sapphire surface, was avoided by removing the fiber from the etching solution at temperature and cladding the fiber with a borosilicate glass tube at the etchant-air interface. The ability to tailor the size and shape of single crystal sapphire fibers via hot-wet acid etching will help expand the sensor functionality and application.

We present a multiplexable intrinsic sapphire fiber interferometric sensor using an array of scattering points with longitudinal periodicity to generate spectral interference with coherent scattering. The sensor, a scattering array interferometer (SAI), is fabricated by femtosecond (fs) laser point-by-point inscription technique. Parallel fabrication and thermal annealing are shown to be critical in producing detectable and reliable interference signals in sapphire fibers, where a signal-to-noise ratio (SNR) up to 20 dB can be achieved. The SAI signals can be interpreted from the point of view of both interferometers and fiber gratings. High-order harmonics of the SAI signal are studied to examine the coherence properties of sapphire fibers in comparison with silica multimode fibers (MMFs). Three cascaded SAIs with different pitches are fabricated on a single sapphire fiber to demonstrate multipoint temperature sensing. With a low-cost, fully multimode interrogator that uses a visible-band LED and a charge-coupled device (CCD)-based compact spectrometer, reliable multipoint temperature sensing up to 1500 °C is demonstrated.

We study the point-by-point inscription of sapphire parallel fiber Bragg gratings (sapphire pFBGs) in a fully multimode system. A parallel FBG is shown to be critical in enabling detectable and reliable high-order grating signals. The impacts of modal volume, spatial coherence, and grating location on reflectivity are examined. Three cascaded seventh-order pFBGs are fabricated in one sapphire fiber for wavelength multiplexed temperature sensing. Using a low-cost, fully multimode 850-nm interrogator, reliable measurement up to 1500°C is demonstrated.

This paper presents parallel FBGs in sapphire fiber to significantly boost the signal intensity of high order FBGs. Cascaded parallel FBGs are fabricated and achieve multi-point high temperature sensing.

We report a flexible method to integrate 3D optical resonators inside optical fibers. A prism-coupled cylindrical resonator and a free-space-coupled asymmetric resonator cavity are demonstrated in optical fibers with a quality factor up to 3.53×10 5 .

A novel multimaterial fiber optic magnetic sensing design is presented and evaluated in this letter. The multimaterial optical fiber has demonstrated $ < 0.2\ n\epsilon $ sensitivity to external magnetic fields over a 2-m gauge length. Intended for immediate use in unconventional oil and gas (UOG) and measuring while drilling (MWD), this instrument could have wide-ranging subsurface sensing applications. Composed of fused silica cladding, internally distributed ferromagnetic nickel, and a series of fiber Bragg gratings, the multimaterial structure has been consistently drawn to over 1 km of length on a fiber draw tower. Due to the consistency of manufacturability and high magnetic sensitivities over long lengths, the proposed design may be considered a competitive magnetic sensor in UOG and MWD applications.

The coatings produced by an atmospheric plasma spray process (APSP) must be of uniform quality. However, the complexity of the process and the random introduction of noise variables such as fluctuations in the powder injection rate and the arc voltage make it difficult to control the coating quality that has been shown to depend upon mean values of powder particles’ temperature and speed, collectively called mean particles’ states (MPSs), just before they impact the substrate. Here, we use a science-based methodology to develop a stable and adaptive controller for achieving consistent MPSs and thereby decrease the manufacturing cost. We first identify inputs into the APSP that significantly affect the MPSs and then formulate a relationship between these two quantities. When the MPSs deviate from their desired values, the adaptive controller is shown to successfully adjust the input parameters to correct them. The performance of the controller is tested via numerical experiments using the software, LAVA-P, that has been shown to well simulate the APSP.

A chirped pulse-based gas sensing technique was developed. With a short, 11.2 ns frequency chirped pulse generating differential absorption within the pulse duration, inter- pulse calibrated methane sensing up to 20.5% was demonstrated.

Previously, a fused-quartz acoustic waveguide named the suspended core waveguide with tight-field confinement in a small diameter was developed and its sensing potential in harsh environment was demonstrated with the fabricated periodic acoustic fiber Bragg grating. In this paper, the acoustic sensor was exposed to around 70 Gy/h gamma radiation at room temperature for 3550 h at Oak Ridge National Laboratory. For comparisons, a thermocouple and two optical fiber Bragg gratings were tested under the same conditions. The central frequency reading of the acoustic sensor was found to encounter a relatively fast decrease of 0.5 kHz in the first 800 hours. The reading then became stable, centering at 478.5 kHz with a fluctuation of ±0.2 kHz, and responded to small environmental temperature variations less than 1.2 °C. The major effect to the fluctuation was concluded to be radiation-induced material compaction and expansion. For optical fiber Bragg gratings, the same one-directional fast change of readings in the first 500 hours were observed as well. Although the optical gratings were able to track temperature changes, they also showed continuous drifts. The survivability and consistency of the acoustic sensor under long-term gamma radiation could lead to new sensing methods in nuclear applications.

Thermally sprayed coatings improve component lifespan and protect the underlying substrate in high temperature and corrosive environments. The coating adhesion is one of the most important properties with respect to coating performance. Before coating, surfaces are typically cleaned and roughened via manual grit blasting to promote mechanical interlocking of the coating with substrate. With recent interest in implementing ceramics and ceramic composites in the hot section of advanced gas turbine engines, there is a need to better understand how to prepare these nonmetal and composite materials before being coated. In this study, an automated grit blasting system was utilized to observe the effects of parameters such as blast pressure, angle, and nozzle traverse speed on multiple surface roughness parameters of reaction-bonded silicon carbide (rb SiC). The effect of these parameters on the substrate thickness loss during grit blasting was also analyzed. Blast pressure had the largest effect on surface roughness, as well as the highest linear correlation with roughness parameters. The automated robotic system allowed for the controlled study of traverse speeds higher than that typically used for grit blasting (≤ 350 mm/s). Increasing nozzle traverse speed was found to greatly reduce material loss with minimal effect on surface roughness.