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.

We demonstrate a silica optical fiber-based temperature sensor with an ultra-wide detection range from 1.7 to 1353 Kelvin (K) and temperature resolutions of 1.534 K at 3.3 K, 0.022 K at 296.2 K, and 0.966 K at 1353 K, respectively. © 2024 The Author(s)

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.

AbstractMicrostructures of additively manufactured metal parts are crucial since they determine the mechanical properties. The evolution of the microstructures during layer-wise printing is complex due to continuous re-melting and reheating effects. The current approach to studying this phenomenon relies on time-consuming numerical models such as finite element analysis due to the lack of effective sub-surface temperature measurement techniques. Attributed to the miniature footprint, chirped-fiber Bragg grating, a unique type of fiber optical sensor, has great potential to achieve this goal. However, using the traditional demodulation methods, its spatial resolution is limited to the millimeter level. In addition, embedding it during laser additive manufacturing is challenging since the sensor is fragile. This paper implements a machine learning-assisted approach to demodulate the optical signal to thermal distribution and significantly improve spatial resolution to 28.8 µm from the original millimeter level. A sensor embedding technique is also developed to minimize damage to the sensor and part while ensuring close contact. The case study demonstrates the excellent performance of the proposed sensor in measuring sharp thermal gradients and fast cooling rates during the laser powder bed fusion. The developed sensor has a promising potential to study the fundamental physics of metal additive manufacturing processes.

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.

Flexible and stretchable strain sensors are in high demand in sports performance monitoring, structural health monitoring, and biomedical applications. However, existing stretchable soft sensors, primarily based on soft polymer materials, often suffer from drawbacks, including high hysteresis, low durability, and delayed response. To overcome these limitations, a stretchable miniature fiber sensor comprised of a stretchable core tightly coiled with parallel conductive wires is introduced. This fiber sensor is flexible and stretchable while exhibiting low hysteresis, a remarkable theoretical resolution of 0.015%, a response time of <30 milliseconds, and excellent stability after extensive cycling tests of over 16 000 cycles. To understand and predict the capacitive sensor response of the proposed sensor, an analytical expression is derived and proved to have good agreements with both experimental results and numerical simulation. The potential of the strain sensor as a wearable device is demonstrated by embedding it into belts, gloves, and knee protectors. Additionally, the sensor can extend its applications beyond wearable devices, as demonstrated by its integration into bladder and life safety rope monitoring systems. The sensor is envisioned to have applications in the field of sports performance evaluations, health care monitoring, and structural safety assessments.

Introducing a dual Fabry-Perot sensor system for non-contact pipe burst monitoring. With a lab-standard dual-ferrule probe, we measure absolute distance from the pipe and differentiate bursting events from absolute drifting through relative measurements.

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.

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.

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.

Small-scale robots capable of remote active steering and navigation offer great potential for biomedical applications. However, the current design and manufacturing procedure impede their miniaturization and integration of various diagnostic and therapeutic functionalities. Herein, submillimeter fiber robots that can integrate navigation, sensing, and modulation functions are presented. These fiber robots are fabricated through a scalable thermal drawing process at a speed of 4 meters per minute, which enables the integration of ferromagnetic, electrical, optical, and microfluidic composite with an overall diameter of as small as 250 µm and a length of as long as 150 m. The fiber tip deflection angle can reach up to 54o under a uniform magnetic field of 45 mT. These fiber robots can navigate through complex and constrained environments, such as artificial vessels and brain phantoms. Moreover, Langendorff mouse hearts model, glioblastoma micro platforms, and in vivo mouse models are utilized to demonstrate the capabilities of sensing electrophysiology signals and performing a localized treatment. Additionally, it is demonstrated that the fiber robots can serve as endoscopes with embedded waveguides. These fiber robots provide a versatile platform for targeted multimodal detection and treatment at hard-to-reach locations in a minimally invasive and remotely controllable manner.

This paper reports on a wire horn structure to enhance acoustic-optical fiber coupling. Compared with the conventional schemes, it was proved to enhance the ultrasonic wave peak-to-peak amplitude coupling efficiency by up to 7.9 times.

This special issue contains a collection of papers on optical fiber sensors that were originally presented and published in a more succinct form in conjunction with the 27th International Conference on Optical Fiber Sensors (OFS) held in Alexandria, Virginia, United States, from 29th August to 2nd September, 2022.

There has been intense interest in fiber optic sensors as quench detectors for superconducting magnets and electrical power distribution because of their inherent advantages such as high measurement resolution, small size, and immunity to electromagnetic radiation. Unfortunately, their true potential has been limited by their lack of thermal sensitivity at cryogenic temperatures due to their dependence of properties of fused silica. In this study, we evaluate the response of fiber Bragg grating temperature sensors inscribed in fibers drawn with metal wires in the fused silica cladding and single layer fluoroacrylate coatings upon immersion in liquid nitrogen. Inclusion of the metal (Cu, Ni) wires in the cladding yielded little to no improvement in the temperature response, as compared to traditional bare and acrylate coated fibers. Conversely, the temperature response of fiber sensors with a fluoroacrylate coating exhibited an improved temperature response upon immersion in liquid nitrogen. Application of fluoroacrylate coatings on optical fibers during the draw process avoids the complexities associated with post-processing steps for sensors that rely on specific metals or polymers, which ultimately increase their footprint and/or decrease their reliability. The proposed sensing fiber design with a high thermal expansion coating that takes advantage of mature manufacturing techniques makes fully distributed or multiplexed measurements plausible and sets the stage for adoption of fiber optic sensors for health and condition monitoring of superconducting magnets.

Small-scale robots capable of remote active steering and navigation offer great potential for biomedical applications. However, the current design and manufacturing procedure impede their miniaturization and integration of various diagnostic and therapeutic functionalities. Here, we present a robotic fiber platform for integrating navigation, sensing, and therapeutic functions at a submillimeter scale. These fiber robots consist of ferromagnetic, electrical, optical, and microfluidic components, fabricated with a thermal drawing process. Under magnetic actuation, they can navigate through complex and constrained environments, such as artificial vessels and brain phantoms. Moreover, we utilize Langendorff mouse hearts model, glioblastoma microplatforms, and in vivo mouse models to demonstrate the capabilities of sensing electrophysiology signals and performing localized treatment. Additionally, we demonstrate that the fiber robots can serve as endoscopes with embedded waveguides. These fiber robots provide a versatile platform for targeted multimodal detection and treatment at hard-to-reach locations in a minimally invasive and remotely controllable manner.

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.

Point-by-point (PbP) cross-axis Bragg gratings in a step-index two-mode fiber are fabricated by tightly focused femtosecond laser pulses. The grating is formed by inscribing a line of periodic damage points crossing the fiber axis at a certain tilting angle. We experimentally show that the cross-axis structure can introduce multiple mainband resonances associated with the higher-order modes, with wavelength spacing proportional to the tilting angle. We also develop a numerical model for the PbP-inscribed gratings based on pulse-wave approximation and the standard coupled-mode theory, the resulting simulation exhibits a good agreement with the experimental findings. This work suggests the higher-order fiber modes, synergizing with the PbP technique, has the potential to add a new dimension to designing gratings with desired spectral spacing; such an inverse problem of PbP gratings can be assisted by the numerical modal we developed.