This PDF file contains the front matter associated with SPIE Proceedings Volume 13044, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Brillouin optical time domain analysis (BOTDA) sensor systems play a pivotal role in distributed sensing, which enable precise measurements of strain and temperature across extensive fiber lengths. Nonetheless, challenges emerge as distances grow due to signal attenuation and noise interference resulting in measurement errors. This research offers a comprehensive strategy to extend the sensing range of BOTDA systems beyond tens of kilometers while maintaining high spatial resolutions. Such enhanced sensing is realized through hybrid system that employs the integration of distributed Raman amplification, and inline amplification using erbium-doped fiber amplifiers (EDFA), as well as advanced noise reduction techniques. By optimizing power levels for the Brillouin pump and probe signals, as well as for Raman pump, hybrid BOTDA system ensures the robustness of Brillouin scattering signals, effectively countering attenuation-induced losses. Distributed amplification not only conquers attenuation but also suppresses nonlinear effects that could undermine measurement accuracy. Additionally, enhancing weak Brillouin signals by strategically placing EDFAs at optimal fiber locations keeps sensor sensitivity high. Leveraging inherent redundancy in measured data as a function of frequency and fiber distance, the non-local means (NLM) filter removes noise while preserving essential physical information. This approach proves particularly advantageous in BOTDA systems, where accurate measurement of Brillouin scattering signals is paramount for long-range sensing with high spatial resolutions. In summary, this research has shown a holistic exploration of extending BOTDA’s distance sensing capabilities up to 150km with spatial resolutions of 8 meters, and Brillouin frequency shift error of 1MHz.

We introduce a new noise analysis for distributed Brillouin fiber sensors comparing the prominent approaches to recovering the Brillouin resonance frequency. We demonstrate that all of these techniques, despite differences in mathematical recovery of the Brillouin frequency shift, have largely similar noise performance. We then use this noise framework to show the essential components of low noise Brillouin fiber sensing measurements and show how optimization of these components can improve sensitivity.

Inertial Navigation Units (INU) preferentially rely on Fiber Optic Gyro (FOG) technology, which has been proven from a performance perspective. Producibility has also improved with the information processing function well in hand and Photonic Integrated Circuits (PIC) advancing to TRL 9 or higher. The one remaining stubborn production bottleneck is winding the optical fiber Sagnac effect sensor coil. Present day coil winding is high skill, arduous, and slow because; 1) winding pattern is complex, 2) total precision is necessary, and 3) use of adhesive during winding makes coil winding even more demanding. Several attempts have been made to automate winding quadrupole gyro coils, but these attempts have succeeded only for very low performance coils. Because quadrupole winding is such slow, painstaking work, the number of properly trained and high-performance coil capable technicians in the United States can be counted on the fingers of two hands. This personnel environment puts a severe constraint on the attainable production volume and associated negative feedback severely distorts the gyro cost structure. 4S envisions the development of a gyro coil winding that uses multicore optical fiber. This advance, along with Photonic Integrated Circuit (PIC) technology, which is enabling, creates a new paradigm in FOG production.

Anello Photonics is a Silicon Valley based startup developing next generation navigation technologies. The heart of the Anello’s products is the silicon photonics optical gyro, the SiPhOG™ which is a 10X reduction in SWAP-C compared to current products.

We present a new approach for high-resolution spectral analysis based on Brillouin lasing. Unlike conventional Brillouin spectrometers, we exploit the wavelength dependence of the Brillouin frequency shift which changes by ~55kHz for every 1GHz of optical frequency. This enables continuous monitoring of the entire C-band (4THz or 25nm) with 28MHz (or 0.2pm) resolution in just 5ms. Our all-fiber system utilizes off-the-shelf components, providing a simple solution for high-resolution, broadband spectral analysis.

This research demonstrates femtosecond (FS) laser-written distributed fiber Bragg gratings (FBGs) sensors within sapphire crystalline fiber, tailored for steelmaking applications. The study precisely assesses sensor stability during a 72-hour exposure to severe conditions, including temperatures reaching 1600°C. The FBGs exhibit excellent signal strength and a maintained high signal-to-noise ratio (SNR) by averting external surface reactions with the sapphire fiber. Extensive annealing at 1600°C purifies the sheathing material. By utilizing an extended 1-meter sapphire fiber, this work surmounts the challenges of cascading FBGs in highly multimode waveguides, enabling FBG signal capture in demanding applications. This research enhances our comprehension of FBG performance in high-temperature environments and paves the way for robust optical fiber systems in steelmaking applications, including tundish probes and submerge entry nozzles (SEN) for molten metal casting. Additionally, the exceptional efficiency and precision of sapphire FBG sensors, in contrast to conventional thermocouples, offer the potential to boost productivity, lower energy consumption, and reduce the carbon footprint in the steel industry.

This study presents an advancement in high-temperature Raman spectroscopy, specifically for analyzing molten materials. It introduces an approach by integrating a fiber-optic Raman probe with a copper block protection system designed to endure extreme thermal conditions. The copper block features an open port designed to accommodate an external telescope with a 3cm focal length, enabling Raman spectra collection in challenging high-temperature environments. A built-in gas channel ensures a continuous flow of argon gas to prevent flux intrusion. The robust copper block acts as a reliable shield, safeguarding the fiber-optic Raman probe within molten materials. This enhancement maintains the probe's integrity and significantly improves its resilience, making it ideal for rigorous investigations of molten substances. This advancement is particularly relevant in metallurgy, where flux materials impact production quality and efficiency. The ability to acquire Raman signals under elevated thermal conditions offers opportunities for studying molecular dynamics, compositional changes, and chemical interactions within molten substances. This introduced direct immersion probing technique has implications, benefiting both scientific and industrial fields. It holds promise for advancing research and exploration in various contexts, from fundamental scientific inquiries to practical applications in metallurgical processes, where flux materials are critical for optimizing production quality and efficiency. This approach enhances the capabilities of high-temperature Raman spectroscopy, making it a valuable tool for investigating molten materials and their properties in diverse settings.

This study focuses on the critical aspect of interfacial heat transfer during the solidification process in metal casting, aiming to optimize these manufacturing processes. Fiber-optic sensors were employed to provide continuous real-time monitoring of mold gaps and temperature profiles during the solidification of A356 aluminum in a permanent mold-casting environment. A specially designed mold system, constructed from unheated, uncoated tool steel, facilitated the seamless integration of these advanced fiber-optic sensors. One key technique used was the Extrinsic Fabry-Perot interferometric (EFPI) sensor, which uniquely utilized molten metal as the second reflection interface for measuring mold gaps. This method yielded impressively accurate results, with a maximum error of just 2μm compared to physical measurements. Additionally, using the Rayleigh backscattering (RBS) technique, a stainless steel-encased fiber provided real-time temperature measurements with an impressive spatial resolution of 0.65mm. The study demonstrates that combining high-resolution temperature profiles with gap evolution measurements significantly enhances our understanding of heat transfer dynamics at the mold-metal interface, proving particularly beneficial for optimizing complex-shaped castings and continuous casting processes. Furthermore, the capability to monitor the shape of the casting in real-time as it exits a continuous casting mold introduces a novel tool for quality control and process safety improvement by early detection of conditions that might lead to slab cracking and breakouts, ultimately enhancing overall process efficiency and reliability.

This study presents the application of three distinct fiber optic sensor (FOS) technologies for temperature monitoring during Electric Arc Furnace (EAF) operations. This work looks into the application of fiber Bragg grating (FBG), Brillouin-based, and Rayleigh-based distributed FOS technologies. Through the deployment of these sensors in steel mills, we have successfully achieved distributed temperature monitoring within the bottom anode and side walls of the EAF. Our approach involves data collection from mock foundry trials and real-world EAF operations in a steel mill. The real-time temperature monitoring of the EAF’s bottom anode provides insights for early detection of temperature anomalies in the refractory layer, while the monitoring of the side wall is primarily for pinpointing hotspots within the furnace wall for effective and efficient water-spray cooling. The integration of these advanced FOS technologies brings forth a transformative solution for the steel-making industry. By providing real-time, distributed temperature profiles and enabling proactive anomaly detection, our work contributes to enhanced operational efficiency and, more critically, improved safety in EAF facilities. This research not only showcases the potential of FOS applications but also demonstrates their ability to facilitate timely interventions in the high-temperature, high-stress environment of EAFs, ultimately bolstering overall steel production and safety standards in steel mills.

Recent studies have shown that a compact self-mixing interferometer can be used for the characterization of shock waves. It measures dynamically (> 10MHz) the changes in the refractive index induced by the shock wave. Associated to an appropriate acousto-optic model, the pressure profile is computed with a 34mbar resolution. In the present work, we compare shock wave induced refractive index variations measurements by another method using a Michelson-type fiber-optic interferometer with phase analysis that has been developed for Photonic Doppler Velocimetry applications. The output signals of this system are processed in triature, which consists in analyzing the phase shift between the three interferometric signals. This bulkier system provides, in theory, a better resolution than the self-mixing interferometry sensing scheme. In the present paper, we compare these two optical methods to measure a shock wave pressure through experiments that were carried out with an open shock tube instrumented with commercial, bandwidth limited, pressure sensors. This configuration creates a spherical shock wave similar to those observed during on-field experiments with explosives. We describe the two measurement systems and the experimental setup design used for overpressure characterizations. Both sensing approaches have been carried out in the same experimental conditions and with shock wave pressure peak amplitudes of a few bars. We detail the two types of signal processing and we discuss the results obtained with the two optical methods, which are also compared to a piezoelectric reference sensor.

The performances of a single-mode polymer chirped fiber Bragg grating (CFBG) made in a microstructured PMMA fiber is investigated to measure shock and detonation velocities. The polymer material makes the fiber sensor more sensitive at lower pressure values than with silica. The optical characterization of the microstructured polymer CFBG is discussed. The single-mode fiber offers a stable reflected optical spectrum over time and the manufacturing process used provides a constant chirp rate along the grating. The maximum reflectivity of the microstructured polymer CFBG provides enough signal for the photoreceiver without significantly increasing the optical source power. The actual limitations include the global shape of the reflected spectrum, which comprises many dips, and the optical losses that reduce sensor sensitivity towards the end of the grating. Anyhow, a first detonation experiment intended to measure shock and detonation velocities in a wedge test was completed. The X-T diagram showing the shock and detonation wave positions as a function of time presents two slopes corresponding to a shock velocity of 4695m/s and a steady-state detonation velocity of 8392m/s. These values are very similar to the ones obtained with 48 piezoelectric pins, but the uncertainties remain high (>2%) for this first experiment. Nevertheless, the experience proves that the PMMA material is suitable for detonation physics experiments. Technical solutions were identified to improve sensor performance. First, optical losses could be reduced within the grating and a more constant reflectivity level could be obtained. Sensitivity would be similar along the full length of the grating. The second point of focus will be to prevent any dips in the reflected optical spectrum. With these improvements, we should achieve uncertainties of less than ±1% (at k=2) for shock and detonation velocity measurements.

Optical frequency domain reflectometry (OFDR) is a fiber optical sensor interrogation scheme widely used in distributed sensing for temperature and strain measurements. The measurements are mostly performed in single-mode optical fibers and rely on a high-performance tunable laser with a narrow line width and wide wavelength tuning range. This paper explores OFDR distributed sensing principles in a wider domain using both single-mode and multi-mode waveguides in both optical and microwave domains. Enabled by laser inscription of back-scattering structures in optical and RF waveguides, this paper shows that distributed sensing can be performed using low-cost wave sources in a wide array of guided structures to meet harsh environment sensing applications.

Online monitoring of a process is essential for industrial scale production, in particular the separation and processing of tritium at the Savannah River Site in Aiken, SC. Current technologies that support hydrogen isotope quantification rely on single “grab samples” that are analyzed offline or active sampling from production. Here, we describe progress towards an online gas monitoring technique using hollow-core waveguide Raman spectroscopy. The system is being initially developed for use in hydrogen processing systems but will have wider applicability. Commercially available fibers are employed for the enhancement of collecting Raman scattered light through optical alignment and high-power visible laser systems. A 532nm diode pumped solid state laser system focuses light through a hollow core waveguide that is arranged as a flow cell for gaseous samples. Reverse-scattering detection is coupled to a Horiba iHR320 spectrometer (0.318m; 2400 lines/mm grating) and an air-cooled CCD detector (–60 °C). Gaseous samples include argon carrier gas with ammonia (NH3), methane (CH4), and nitrogen (N2) in addition to hydrogen isotopes (H2 and D2). We have demonstrated the ability to simultaneously measure H2, HD, and D2 with sensitivities and dynamic range from 100s to 10,000s of parts per million.

This study presents a pioneering technique for fabricating highly cascaded first-order fiber Bragg gratings (FBGs) using a femtosecond laser-assisted point-by-point inscription method in highly multimode optical fibers, specifically Sapphire crystalline fiber, and pure silica coreless fiber. Notably, it marks the first successful demonstration of a distributed array comprising 10 FBGs within highly multimode fibers. This achievement is facilitated by a high-power laser technique that yields larger reflectors characterized by a Gaussian intensity profile. These first-order FBGs offer various advantages, including enhanced reflectivity, reduced fabrication time, and simplified spectral characteristics, enhancing their accessibility for interpretation when contrasted with higher-order FBGs. In addition to that it encompasses a comprehensive analysis of the robustness and efficacy of these FBGs, with particular emphasis on their ability to endure extreme temperatures. These FBGs demonstrate an advantageous capability for localized multi-point temperature monitoring, reaching temperatures up to 1500°C with sapphire crystalline fiber and 1100°C with pure silica coreless fiber. This resilience makes them suitable for deployment in harsh environmental conditions. This innovative approach substantially broadens the potential applications of highly multimode optical fibers, particularly in the arena of sensing and communication, where challenges related to thermal gradients and harsh environments prevail. These groundbreaking first-order FBGs signify a substantial advancement in the realm of distributed temperature sensing, offering supreme capabilities for temperature monitoring and signal stability. As such, our work holds the promise of a substantial impact on industries and applications that demand unwavering reliability under extreme conditions.

This work presents a distributed optical fiber specklegram sensor (FSS) specifically designed for the detection and localization of water leaks. The sensor analyzes specklegram images generated by a No-Core Fiber (NCF) under different water leak conditions, employing a low-cost CCD camera as an interrogation unit. To enhance the accuracy of leak detection, a convolutional neural network (CNN) model is employed to post-process the specklegram images for monitoring the different water leak conditions. The sensor demonstrates high sensitivity, accurately detecting water volumes as small as 0.1 mL. In the initial series of experiments, the sensor achieved a remarkable 100% accuracy in predicting the location of leak spots situated 1 cm apart. However, in subsequent rounds of the experiment, a slight reduction in accuracy was observed (87.5%) due to the issue of water droplet overflow across the Kapton tape used to mark the various test leak spots after multiple cycles of water addition and removal. Therefore, employing an impermeable material for the demarcation will mitigate the water droplet overflow problem. In summary, the proposed sensor offers an efficient approach for water leak detection through the application of machine learning-based specklegram analysis. The findings of this research underscore the potential of FSS as a low-cost, easily implementable, and real-time monitoring system for the detection and localization of water leaks.

Single crystal (SC) optical fiber has promising potential to be used for optical fiber sensing applications in harsh conditions due to its robustness to high temperature, high radioactivity, and resistance to chemical corrosion as compared to optical sensors using silica fiber. However, SC fiber grown via the laser-heated pedestal growth (LHPG) technique innately does not have a core-cladding structure found in standard optical fiber, resulting in optical losses. In this work we investigate optimization of the growth parameters of a two LHPG process used to grow SC fiber with a graded index via introduction of dopants to the feedstock material. Feedstock material is fabricated with the first LHPG device, then sol-gel dip-coated to deposit outer films of dopant material. The dip-coated feedstock is used to grow SC fiber in which segregation of the dopant constituents occurs, resulting in a graded index of refraction across the fiber, and an effective core-cladding structure. Hardware and software improvements to both LHPG systems are presented and the growth parameters for short pieces of ~320-330μm diameter YAG fiber has been established. Characterization techniques/procedures have also been established for future grown SC fiber. These improvements and preparations are anticipated to result in a significant increase in grown fiber quality with a similar growth rate to that previously established.

Distributed temperature sensing (DTS) is demonstrated using an enhanced scattering optical fiber (ESF). The temperature measurement is based on measuring the intensities of the backscattered light with two broadband lights. The spectra of the broadband light partially overlap with the backscattered spectrum of the ESF. Since the spectrum of the ESF shifts with temperature, the intensities of the backscattered light will change. The temperature coefficients of the system are -0.011 dB/°C and 0.022 dB/°C. Using the system, a low cost and high speed DTS system can be accomplished.

This paper examines the efficacy of quasi-distributed acoustic sensors (q-DAS) in identifying damage within pipeline structures, placing a substantial emphasis on generating synthetic q-DAS measurements in active ultrasonic testing setting and bridging the gap between synthetic and real q-DAS measurements. Our research utilizes simulation software to model the ultrasonic guided wave propagation and its interaction with pipeline defects. The pipeline structural health monitoring setup is based on the pulse-echo method utilizing a torsional symmetric mode T(0,1) at 32kHz, with an aim to identify corrosion and weld irregularities over extensive pipeline lengths. We have prioritized the calibration of simulation models against experimental data, fine-tuning the simulation processes to reflect actual conditions with higher fidelity. The study specifically highlights the simulation’s accuracy in capturing the distinct signatures of critical pipeline features and the subsequent detection capabilities within an operational context. By focusing on the experimental validation, we have advanced the understanding and application of structural health monitoring for essential infrastructure, ensuring the simulations' predictive strength aligns closely with real-world sensor data and observed phenomena.

Large natural gas and product transmission pipelines exist throughout the United States. These pipelines are susceptible to failure through corrosion and cracking due to internal and external factors. As these transmission pipelines age, the likelihood of a catastrophic failure increases. The ability to the monitor corrosion and wall thickness of these pipelines is paramount to reduce the risk of disastrous failure and increase reliability as well as safety. Optical frequency domain reflectometry (OFDR) can monitor distributed temperature and strain measurements along the fiber optic cable mounted on natural gas and product pipelines. This measurement technique can provide valuable information of pipe structural health conditions through hoop strain changes due to pipe wall thinning, and temperature changes due to gas leaks based on the Joule-Thomson effect. Since these pipelines are typically buried, the depth of the pipeline results in a potential loss in the sensing range where the fiber is not physically monitoring the hoop strain of the pipeline. A different configuration of the interferometer will allow for the distributed fiber measurement to start atop the pipeline itself. This configuration results in no loss of sensing range and maintains the sensing resolution of a typical interferometer for an OFDR measurement. The quasi-extended range distributed hoop strain measurements were demonstrated on an active transmission pipeline.

In the oil and gas, CO₂ sequestration, H₂ subsurface storage, and geothermal energy sectors, subsurface pH measurements are critical for monitoring the geochemical conditions and estimating potential corrosion rates of wellbore systems. Real-time pH measurements in these conditions are vital for detecting and predicting corrosion deterioration of wellbore components that may jeopardize the safety and continued operation of wellbore systems. Previous tests using metal oxide-based coatings (TiO₂) provided strong responses at elevated temperatures and moderate pressure stability but provided poor differentiation between acidic and alkaline solutions. Building off the pH responsiveness of the TiO₂ surface and known pH sensitivity of amine-based polymers, a coating based on the secondary amine polymer polyethyleneimine (PEI) was developed. As the polymer itself is highly water soluble and easily removed by aqueous solutions, the sensor coating was treated with a high temperature (500 °C) calcination procedure in air to convert it into a more stable oxidized coating capable of withstanding hot aqueous solutions without dissolving while retaining linear pH sensitivity from pH values between 2 and 11. The sensor performance was measured using optical transmission measurements in solutions of various pHs and using optical backscatter reflectometry for distributed pH sensing demonstration in wellbore-relevant pressures (up to 1,000psi) and temperatures (80 °C). A calibration curve with strong differentiation between acidic and alkaline pH was developed for both transmission-based and distributed pH measurements, using fixed wavelength transmission and integrated linear amplitude of backscattered light for distributed measurements.

Current ultrasonic acoustic NDE methods for long distance inspection in cylindrical structures are primarily focused on axisymmetric guided waves excitation. However, there are many occasions where the physical limitations imposed by the system to be inspected restrain the ability to utilize equipment capable of exciting those waves. This study explores the excitation of the flexural guided wave modes by a limited number of piezoelectric transducers for damage detection in hollow cylinders with limited surface access and large diameter. In addition, the use of distributed optical fiber system as the guided wave receptor is investigated as an alternative to piezoelectric transducers (PZT), as their capability to acquire spatial-temporal data synergizes with the complexity in a signal containing several flexural GW modes. More specifically, the study is conducted based on a numerical analysis of the guided waves excited by a 2 PZT configuration in a pipe available for experimental testing. The resulting flexural modes and its interaction with welds and local loss of material are analyzed in terms of the time series data of a local sensor in the surface, and the angular profile differences from a healthy case. A method based on the analytical solution of an infinite cylinder is introduced in preliminary stage to extract the behavior of the dominant modes from simulation and experimental results and used as a simulation-experiment similarity comparison. Finally, a simplified convolutional neural network (CNN) is trained to demonstrate feasibility of using flexural modes excited by limited actuators for damage detection. Overall, this study contributes to the development of a damage detection method applicable to cylindrical structures with dimensional and access limitations, by enhancing the understanding of how simultaneous several flexural modes interact with mechanical features, presenting an early-stage interpretable method to compare simulation and experimental fiber optic sensor data, and demonstrating the feasibility of using DAS like data for analyzing the structure.

Distributed fiber optic sensing is a cutting-edge technology that has found extensive applications in the monitoring of pipelines. Among them, distributed acoustic sensors, phase-sensitive optical time domain reflectometry (Φ-OTDR) is a versatile technology that can continuously detect external perturbations and provide spatial and real-time information along the kilometer lengths of sensing fiber. Considering the low backscattering level of standard single-mode fiber as fiber under test, a Rayleigh-enhanced optical fiber embedded within the tight-buffered cable is demonstrated in field testing. We analyzed the increased backscattering fiber cable's vibration performance to the conventional single-mode telecom fiber using a custom-built Φ-OTDR interrogator system. Thereafter, using a 4-inch steel pipeline with a flow rate of 5, 10, 15, and 20ft/s and a fixed pressure level of 1000psi, we field-tested the sensor system for monitoring natural gas pipeline acoustic vibrations. We also field-tested the Brillouin optical time domain analysis (BOTDA) system for pipeline hoop strain monitoring under various pressure conditions. The pilot-scale testing results presented in this study suggested that pipeline operators can accurately perform flow monitoring, leak detection, and pressure monitoring for pipeline integrity monitoring.

Structural Health Monitoring (SHM) of pipelines using nondestructive testing/evaluation (NDT/E) techniques is important particularly for the energy industries and for the oil/gas distribution which helps reduction in maintenance costs as well as increased service lifespan. Among various NDE techniques, ultrasonic guidedwaves (GWs) technique is popular for inspection and monitoring of pipes due to its advantages e.g., long-distance monitoring using a fixed sensor probe, full volumetric coverage, and inspection for invisible or inaccessible structure. Recently, performance and scope of the GWs method is explored using optical fiber sensing technology such as fiber Bragg gratings are demonstrated for many ultrasonic sensing applications. The optical fiber sensors bring the advantage of remote sensing, large acoustic bandwidth, and multiplexing capability of the sensors to extend the range of GWs based NDE method. This work describes the health monitoring of damaged pipeline structure in a nondestructive manner using alternative No-core fiber (NCF) based quasi-distributed fiber-optic acoustic sensor combined with ultrasonic GWs excitation. We set up two similar 6-inch carbon-steel pipes (16-ft long), one consists of various defects and the other is healthy without any defect for reference. The pipes are actively excited by employing different ultrasonic sources; (1) magnetostrictive collar (MR) to generate the axisymmetric (torsion) GWs and (2) conventional piezoelectric patches to generate the antisymmetric flexural waves on the exterior surface, and the characteristics of acoustic-ultrasonic signals are studied using NCF based multiplexed fiber-optic sensor. Fiber optic sensor is an inline multimode interferometer made by sandwiching a piece of NCF (~5cm) between the single mode fibers. The NCF sensor is remotely bonded at 45° w.r.t pipe axis on one end and has an ultrasonic sensing range of >600kHz. Finally, the measured acousto-ultrasonic signals for different ultrasonic sources are compared to those obtained by the numerical simulation or electrical-based sensor for the healthy and damaged test pipes. The proposed work presents useful insight for damage detection in pipes using an NCF-based quasi-distributed fiber-optic acoustic sensor combined with ultrasonic GWs excitation.

Fiber optic sensors show many advantages as compared to other alternatives for a wide range of energy applications spanning electrical grid, pipelines, and civil infrastructure monitoring amongst others. Multimode interference-based fiber optic sensor configuration is one device architecture that is being explored for a range of different analytes, and which is fabricated by sandwiching a section of multimode fiber between two single mode fibers. Fiber optic devices based on multimode interference (MMI) are easy to fabricate and offer attractive prospects for applications in the areas of optical communication and fiber lasers as well as sensing. When light is coupled from a single mode fiber to a multimode fiber (MMF), multiple modes supported by the MMF are excited and interfere with each other, giving rise to an interference pattern along the MMF length. At specific positions along the axis of the MMF, light is concentrated and forms replicas of the input field which are known as self-images, with the self-imaging condition providing a narrow-band interference feature as a function of wavelength that is also affected by a wide range of analytes for sensing purposes. The self-images formed are simulated here using commercial software COMSOL Multiphysics. Sensitivity to a range of different analytes including refractive index, strain and hydrogen gas is explored. Optical fiber sensors based upon multimode interferometer sensors are investigated as attractive sensing options for infrastructure monitoring applications.

Internal corrosion in natural gas transmission pipelines can occur through the condensation of water droplets onto the interior of the pipe. Use of optical fiber sensors (OFS) has been demonstrated to be very successful for distributed sensing of humidity and corrosion under pressurized natural gas pipeline conditions. In this work, the capability of the OFS for successful monitoring of humidity has been extended to monitor the humidity, CH₄, and CO₂ with different gas composition based on the strain produced along the single-mode fiber (SMF) sensor. This is enabled by absorption of H₂O/gases onto the commercially available polyacrylate coated jacketed portion of the fiber resulting in a change in strain. Under equilibrium, a differential microstrain was observed along the jacketed portion of the SMF with N₂, CH₄, and CO₂ at different relative humidity (RH) conditions, while the unjacketed portion of the fiber was used only for sensing pressure/temperature induced strain. In the case of N₂ at 800psig pressure, the observed microstrain (με) was approximately 80, 65, 50, and 35με at 100, 75.0, 46.8, and 23.4 RH%, respectively. Comparatively, a microstrain of approximately 95με was observed with 100% RH CH₄ which demonstrates that SMF produces a measurable CH₄ response alongside water. Similarly, the observed microstrain with CO₂ was approximately 105, 90, 85, 80, and 70με at 100, 75.0, 46.8, 23.4, and 0 RH%, respectively. The strain response of the SMF under various mixed gas composition and different RH conditions were also measured and made calibration curves accordingly. Linear regression and principal component analysis of these datasets provided deconvolution of the impact of strain from H₂O, N₂, CH₄, and CO₂. Additionally, modified OFS comprised of the Fe coated fiber section was employed to monitor corrosion based on the increase in backscattered intensity amplitude of the light being passed once corrosion of Fe occurs. The corrosion rates were studied by monitoring the rate at which the intensity of backscattered light amplitude attains a steady state value when complete corrosion of Fe with a specific coating thickness occurs.

Even though the dynamic response of the magnetic fluid is well investigated by magnetic means and known for their high frequency response in MHz to GHz range, the same has not been explored as much by optical means in the context of optical fiber sensor platform. The use of magnetic fluid as sensing materials in various fiber optic senor platform is limited to DC magnetic field sensing. In this work, we present the results of magnetic fluid functionalized multimode interferometric fiber optic sensors for their efficacy in measuring the dynamic current induced magnetic field and their limitations to AC frequency response imposed by optical spectrometer as interrogator unit, the response time of the sensing material and the sensor itself. The interferometric structure optimized for unique narrow linewidth spectral response, also known as “fourth self-imaging” in fiber optic platform has demonstrated response time to ~15ms enabling sensing of AC magnetic field and measured up to 10Hz sinusoidal H-field within the peak-to-peak applied H-field of 30Gauss. By further tailoring of magnetic nanoparticle’s concentration in ferrofluid, it is anticipated that the sensors frequency response can be pushed to higher frequencies. The study extends the versatility of these magnetic field sensitive materials for their applications to AC-current and magnetic field sensing.

The cost of conventional fiber optic interrogation system has been the limiting factor for its commercialization and market penetration into electrical asset monitoring where the room for capital investment on sensors is relatively small. The ability to deliver portable and cost-effective sensors without compromising their performances becomes critical. Here, we demonstrate the application and low-cost interrogation strategy for intensity-modulated evanescent wave fiber temperature sensor made of plasmonenabled thin films. Optical transmittance change resulting from the thermal damping of plasmonic absorption intensity is converted into analog voltage signals, then transmitted wirelessly through a set of commercial wireless hardware to enable remote monitoring capability. The temperature response is compared against a custom-designed intensity-based Fiber Bragg Grating (FBG) interrogator with Long Period Grating (LPG) edge filter, where its temperature and strain sensing performance of the intensity-based FBG interrogator is presented and discussed. Both sensors are deployed to monitor the dynamic thermal behavior of Li-ion polymer pouch cell under normal charging/discharging conditions. Finally, the initial design and implementation of an energy harvesting circuit that powers the low-cost wireless interrogator from a potential instrumented power conversion/storage device itself is also discussed.

A wide range of high-value applications, including power generation and chemical manufacturing, involve harsh chemical conditions and extreme temperatures. Options for in-situ monitoring of these processes are very limited, as traditional electronic sensing materials, packaging, and metallic interconnects rapidly degrade under such conditions. Even high-temperature stable electrochemical sensors require electrical feedthrough which may limit applicability. Silica optical fiber-based sensors provide a low cost and extremely rugged platform for applications up to 800 °C but tend to degrade over long time scales at higher temperatures. For higher temperature applications, single crystal optical fiber may be employed, limited only by the melting point of the material (e.g., approximately 2000 °C for sapphire). In this work, we discuss the implementation of evanescent field optical fiber sensors for distributed gas sensing of H₂, focusing on results using a Ni/Gd-doped CeO₂ nanocomposite sensing material for detection of low levels of H₂ (0-4%) at 700 °C. This approach utilizes sensors prepared using a low-cost, wet chemical deposition process, in conjunction with a custom-built interrogator system leveraging optical time domain reflectometry (OTDR). Using a specially designed dual-gas flow reactor system, the sensor is tested by establishing a controlled equilibrium gradient of gas concentration. Initial results shown using silica fiber provide a pathway for utilization with high-temperature stable single crystal optical fiber for operation at higher temperatures and higher levels of H₂ relevant for solid oxide fuel cell (SOFC) operating conditions.

This work introduces a groundbreaking Open-Ended Hollow Coaxial Cable Resonator (OE-HCCR) CO2 sensor and the OE-HCCR as a chemical sensing platform technology. This OE-HCCR chemical sensing platform has a displacement detection limit of 0.01nm, interpreted as less than one ppb change in sensing material permittivity values. The OE-HCCR sensor is employed for high-performance CO2 monitoring by integrating novel CO2-sensitive composite materials with the developed platform, marking the first instance of this application worldwide. This sensor has the potential to revolutionize CO2 monitoring in challenging industrial environments, which would contribute to global decarbonization efforts and assist with environmental preservation activities.

Early detection of gas influx (or a kick) into the wellbore during drilling and completion operations is crucial for preventing uncontrolled release of hydrocarbon incidents that could lead to loss of lives, properties, and environmental contamination. In this study, we demonstrate the application of deep learning to automatically detect gas influx signatures across a 5163ft-deep wellbore. Optical fiber-based distributed acoustic sensor (DAS) data from eight well-scale tests were analyzed to investigate the accuracy of the automatic kick detection algorithm for gas influx volumes ranging from 2 to 15 barrels, wellbore circulation rates of 0 to 200 gallons per minute, and gas injection methods through the tubing or an injection line. The deep learning model uses convolutional autoencoders and effectively captures the gas signature for the eight datasets analyzed, providing an overall accuracy of 81% on the blind testing data (based on the structural similarity index measure). The results demonstrate an automated approach for gas kick detection to improve the safety of energy operations.

Subsurface hydrogen storage is a cost-effective and environmentally friendly energy storage option for large-scale and long-term applications. Hydrogen is usually stored in subsurface storage facilities at high temperature under high pressure and high humidity. Monitoring hydrogen concentration in those harsh environments of the underground storage deposits is crucial to ensure the integrity of the hydrogen storage infrastructure and to enable reliable long-term operation. Thus, this study focuses on the development of optical fiber hydrogen sensors capable of monitoring hydrogen in harsh environments that are representative of underground storage conditions. The optical fiber hydrogen sensor developed in this study consists of a palladium-based sensing film with a protective polymer layer which can improve thermal, chemical, and mechanical stability of the optical fiber sensor. The impact of high temperature and high pressure under high humidity conditions on hydrogen sensing with the developed optical fiber sensor will be presented. The effect of cushion gases such as CO2 and CH4 on hydrogen sensing under those harsh conditions will also be discussed.