We report a dual-modal fiber-optic probe that integrates electrochemical quantification of hydrogen peroxide (H2O2) with co-localized fluorescent pH sensing for pH-indexed interpretation of the H2O2response. H2O2is a reactive oxygen species involved in oxidative stress, inflammation, and cellular signaling, and local pH modulates both its production and electrochemical response. Many electrochemical H2O2sensors exhibit pH-dependent sensitivity, creating ambiguity unless pH is measured and used for compensation, which is difficult in small, heterogeneous, or rapidly changing microenvironments. A three-electrode configuration-working (WE), counter (CE), and Ag/AgCl pseudo-reference (pRE) electrodes-is fabricated directly on the cylindrical surface of a 710-& micro;m-diameter optical fiber using femtosecond laser-induced graphene (FSLIG). The WE is functionalized with Prussian blue (PB) to enable selective, low-potential amperometric quantification of H2O2, while ratiometric pH sensing is implemented at the distal fiber tip using a fluorescent indicator. The on-fiber electrochemical electrodes and the tip-confined fluorescent coating operate independently while sampling the same adjacent solution volume, providing a concurrent pH readout for pH-indexed interpretation of the electrochemical signal. Electrochemical calibration quantified H2O2over 50-300 & micro;M, with an estimated limit of detection (S/N = 3) of 2.0-34.4 & micro;M across pH 2-8. Concurrent optical measurements provide pH determination from pH 3-8. Together, these capabilities enable real-time, pH-informed calibration/compensation for more reliable H2O2quantification when pH varies.

ABSTRACT Femtosecond (fs) laser inscription enables fiber Bragg gratings (FBGs) to be positioned beyond the fiber core, including within the cladding, thereby expanding control over modal interactions and directional sensing. These cladding fiber Bragg gratings (CLFBGs) extend FBG functionality through interactions with core, cladding, and evanescent fields. Their operation does not involve fundamentally different grating physics; rather, core‐mode, cladding‐mode, and coupled‐mode responses depend on power distribution, modal overlap with the laser‐inscribed refractive‐index modulation (RIM), grating position, fiber geometry, modal excitation, and interrogation configuration. This Perspective reviews advances in CLFBG fabrication, optical characteristics, sensing mechanisms, and applications. Cladding‐waveguide‐assisted, evanescent‐field‐coupled, and direct cladding‐mode‐excitation architectures are compared in terms of mechanisms, design trade‐offs, and performance, and distinguished from conventional core FBGs and tilted FBGs (TFBGs) regarding directional sensitivity, spectral complexity, environmental interaction, multiplexing, and application suitability. Capabilities in vectorial and multiparameter sensing, including curvature, bending direction, torsion, acceleration, temperature, and environmental monitoring, are discussed. Challenges including limited reflectivity, birefringence‐induced distortion, multiplexing constraints, fabrication repeatability, and long‐term stability are assessed alongside mitigation strategies involving plane‐by‐plane inscription, mode‐field‐engineered fibers, hybrid distributed interrogation, and data‐driven demodulation. This Perspective outlines pathways toward reliable, multifunctional, multiplexed, and distributed sensing in harsh environments.

Distributed, cost-effective sensing with sub-millimeter spatial resolution remains critical and yet challenging for a wide range of industries, including aerospace, biomedical engineering, and additive manufacturing. In this Letter, we propose and experimentally demonstrate a novel staggered contiguous cladding fiber Bragg grating (SC-CLFBGs)-array-based fully distributed sensing configuration. As a proof of concept, a spatial resolution of 800 μm over a total sensing length of 1.04 cm is realized based on an FBG array fabricated by femtosecond (fs) laser direct inscription. To the best of our knowledge, this marks the first demonstration of fully distributed FBG sensing with sub-millimeter resolution by eliminating the dark zones in conventional FBG array compatible with standard spectral interrogators. The experimental validation confirms the excellent performance and thermal resilience of the proposed FBG array at temperatures up to 710°C without any observed hysteresis, establishing its suitability for high-temperature environments. A maximum temperature sensitivity of 15.17 pm/°C was recorded. This approach opens up a cost-effective and straightforward route for realizing sub-millimeter spatial resolution with enhanced signal fidelity, making it highly promising for advanced sensing applications in demanding fields.

Single-crystal sapphire optical fibers have emerged as promising candidates for sensing in extreme environments where conventional silica fibers fail, owing to their exceptional thermal stability, high mechanical strength, and broad optical transparency up to ∼2040 °C. Recent progress has extended sapphire fiber sensing into diverse domains, including temperature, pressure, strain, and chemical detection, yet significant challenges remain, particularly the highly multimode nature of these fibers and the absence of a robust cladding suitable for harsh conditions. This review provides a comprehensive overview of recent advances in sapphire fiber–based sensing technologies, with a primary focus on interferometric configurations and sapphire fiber Bragg grating–based sensors, while also briefly discussing emerging approaches such as distributed Raman and evanescent-field–based sensing. Special emphasis is placed on innovative optical cladding strategies, including spinel and ceramic coatings, ion implantation, nanoporous structures, and advanced oxide systems, which are evaluated in terms of thermal stability, refractive index compatibility, and fabrication scalability. Key challenges such as strain sensing at ultra-high temperatures, long-term stability, and reliable integration are critically examined. The review concludes by outlining future research directions focused on developing durable cladding systems, refining fabrication methods, and integrating hybrid sensing modalities to enable deployable sapphire fiber sensors for aerospace, energy, and industrial applications in extreme environments. The objective is to consolidate current knowledge and guide ongoing efforts to realize deployable sapphire fiber sensors for harsh environments.

Metal-coated optical fibers are widely employed in sensing applications owing to their superior mechanical strength and corrosion resistance. However, their calibration at elevated temperatures is hindered by hysteresis, manifested as discrepancies between heating and cooling cycles, primarily caused by residual strain from mismatched thermal expansion coefficients (TECs) between the metal coating and silica cladding. This research introduces an optimal heat-treatment procedure aimed at minimizing the impact of the mismatch in TECs between the cladding and the coating materials that causes the residual strain in gold (Au) and copper (Cu) coated fibers for achieving reliable distributed high-temperature sensing up to 500 degrees C using optical frequency domain reflectometry (OFDR) technology. The treatment facilitates stress relaxation and microstructural modifications, including surface diffusion, grain growth, and oxidation (for Cu coatings), which collectively induce partial interfacial delamination and thereby suppress hysteresis. This work presents the first comprehensive experimental study to systematically investigate and demonstrate the mitigation of hysteresis through an optimized heat-treatment process and its underlying mechanisms in metal-coated fibers, supported by microstructural insights. The identified treatment range of 25 degrees C-300 degrees C achieves substantial reductions in residual strain, lowering hysteresis effects by approximately 90.2% in Cu-coated fibers and 86.6% in Au-coated fibers. Furthermore, Au-coated fibers exhibit a consistently lower degree of hysteresis than Cu-coated fibers under comparable thermal conditions. Heat treatment also enhances temperature sensitivity, with improvements of approximately 28.9% for Cu-coated fibers and 6.6% for Au-coated fibers. Posttreatment strain analysis confirms maximum sustainable strain limits of similar to 6000 mu epsilon for Au-coated fibers and similar to 12 000 mu epsilon for Cu-coated fibers. Both fiber types also demonstrated excellent thermal stability, maintaining consistent performance across three heating cycles between 25 degrees C and 300 degrees C over a 20-h period. Collectively, the findings not only advance the scientific understanding of residual strain relaxation mechanisms but also provide a practical route toward robust, precise, and reliable distributed sensing for demanding industrial environments such as the steel, oil, and gas sectors.

The development of photonic-based gas sensors using metal-organic frameworks (MOFs) and other microporous solids is often a multistep, complex process, typically involving MOF synthesis, purification, and attachment of microcrystals to an optical fiber end face. This study introduces a one-step method that integrates MOF synthesis and sensor head fabrication directly onto the fiber end face, forming an extrinsic Fabry-Perot interferometer (EFPI) with a thin film of MOF microcrystals. The resulting film, only 3-10-μm-thick, enhances sensor response by enabling rapid gas detection within seconds. Utilizing a pendant micro-droplet evaporation technique, this method forms a microporous MOF layer in situ, allowing unreacted molecular components to act as an adhesive that secures the MOF crystallites to the optical fiber, potentially also contributing to the film's adsorption properties. This process, demonstrated with the HKUST-1 MOF as a model system, optimizes both the fabrication speed and sensor response times by reducing the film formation process to 24 s under nitrogen and 90 s in ambient conditions. In situ Raman spectroscopy, X-ray diffraction (XRD), thermogravimetry (TGA), and energy dispersive X-ray spectroscopy (EDS) were used to validate the composition of the sensor head, confirming the presence of MOF crystallites as the primary sensing component within the EFPI film and characterizing additional film components that may enhance stability, selectivity, and response. This bottom-up approach holds significant promise for the scalable production of fiber-optic sensors that leverage MOF's gas adsorption properties.

Wearable sensors are increasingly being used as biosensors for health monitoring. Current wearable devices are large, heavy, invasive, skin irritants, or not continuous. Miniaturization was chosen to address these issues, using a femtosecond laser-conversion technique to fabricate miniaturized laser-induced graphene (LIG) sensor arrays on and encapsulated within a polyimide substrate. The femtosecond laser-converted conductive traces can have a size of 20 to 2 μm compared to the traditionally larger CO2 laser dimensions of around 300 to 100 μm. This marks a 93-98% decrease in trace size when using a femtosecond laser. This miniaturization allows for the ability to process temperature, electrocardiography (ECG), electromyography (EMG), and glucose data in the same space that would have been occupied by a single sensor. The femtosecond laser-converted graphene (FSLIG) electrodes were modified to function as glucose sensors, and comprehensive electrochemical analyses using cyclic voltammetry (CV) and chronoamperometry (CA) were performed. These tests confirmed the capability of the sensors to detect glucose levels, showing a stability of 96.14%. Encapsulation of FSLIG within polyimide was achieved for the first time, demonstrating the ability to nondestructively create FSLIG electrodes within existing materials, thereby protecting them from external environmental factors. The encapsulated FSLIG shows potential as a method to produce LIG-coated Cu traces for improved multilayered printed circuit boards or layered circuits with complex geometries in polyamide to reduce size and increase functionality. Even sterile probes for use inside the body or under dermis polyamide injections and subsequent FSLIG circuit tattoos are possible. This study demonstrates the novel miniaturization and encapsulation capabilities enabled by the femtosecond laser, developing next-generation wearable biosensors focusing on miniaturization, flexibility, continuous monitoring, multifunctionality, and comfort.

High-precision dynamic sensing is critical in fields, such as industrial automation, structural health monitoring, and environmental sensing, where real-time responses to minuscule changes can prevent system failures or optimize performance. In this work, we introduce and demonstrate a phase-variation coaxial cable resonator (CCR) as a highly sensitive sensor for dynamic sensing applications. As a proof of concept, a prototype device based on a custom-designed CCR is thoroughly investigated for dynamic displacement measurements, as displacement is a fundamental quantity essential to numerous applications. The sensor consists of two components: a static CCR device and a movable conducting plate. As the conducting plate is positioned closer to the open end of the CCR, the phase of the reflection coefficient varies due to changes in the fringing electric field, which is dependent on the distance between the plate and the CCR’s open end. Crucially, compared to conventional open-ended probes, the phase sensitivity of the CCR is significantly enhanced and can be finely tuned, owing to the resonant structure integrated into the coaxial line. Theoretical analysis and numerical simulations predict amplified phase sensitivity when the sensor operates at its resonance frequency. The amplification factor, determined by the resonance depth, indicates that deeper resonances yield higher phase sensitivity. A prototype sensor is fabricated and tested for static displacement detection based on phase measurements, confirming the theoretical predictions. Additionally, dynamic displacement measurements further demonstrate the CCR’s capacity for ultrasensitive dynamic sensing. The sensor’s potential application is extended to monitor variations in the dielectric constant of analytes, enabling high-precision analysis in chemical and environmental monitoring. The successful development of dynamic sensing with the CCR opens pathways to create various physical and chemical sensors, enhancing applications in fields requiring precise, real-time measurements.

The bottom anode in the direct current electric arc furnace (dc EAF) is critical for completing the electrical circuit necessary for sustaining the arc within the furnace. For pin-type bottom anodes, monitoring of the temperature of select pins instrumented with thermocouples (TCs) is performed to track bottom wear in EAF and inform the operator when the furnace should be removed from service. This work presents the results from a plant trial using distributed temperature monitoring of bottom anode pins in a 165-ton dc EAF over a two-month service period utilizing two optical fiber sensing techniques: fiber Bragg grating (FBG) and Rayleigh backscattering (RBS). The early detection of temperature anomalies along the length of the anode pin through distributed sensing enhances operational safety, providing a robust alternative to traditional TCs.

The reliable detection of carbon monoxide (CO) is of great importance, as its toxicity has resulted in serious threats to public health, the safety of industries, and environmental health. Conventional CO sensors based on metal oxide semiconductors typically require elevated operating temperatures and often suffer from poor selectivity. Metal-organic frameworks (MOFs), particularly nickel-based MOFs (Ni-MOFs), have gained attention as promising gas sensing materials owing to their large surface area, tunable porosity, and selective gas adsorption properties. However, their inherent sensitivity to humidity impairs electrical conductivity and sensing accuracy. In this study, a chemiresistive CO sensor was fabricated by modifying laser-induced graphene (LIG) electrodes with electrodeposited Ni-MOFs. The use of LIG was selected due to its high electrical conductivity, hierarchical porosity, and tunable surface chemistry, which collectively enhance gas adsorption and charge transfer. To address the humidity-related challenges of Ni-MOFs, four optimization strategies were systematically applied: (1) thermal treatment was used to control Ni-MOF crystallinity and adsorption behavior, (2) protective polymer coatings were introduced to limit moisture interference while maintaining gas permeability, (3) composite integration with carbon-based materials was conducted to improve structural stability and reaction kinetics, and (4) surface modification was performed through chemical treatments and organic ligand functionalization to increase selectivity and reduce cross-sensitivity. Sensor performance was evaluated under controlled CO concentrations and humidity levels, and improvements in sensitivity, stability, and durability were observed. These findings suggest that Ni-MOF/LIG-based chemiresistive sensors offer a promising and robust platform for reliable CO detection in real-world environmental and industrial monitoring applications.

This work presents a fiber-optic Raman probe specifically designed for real-time, in situ spectral measurements of high-temperature materials such as steelmaking slag, mold flux, and bioactive glass. To withstand extreme environments, the probe incorporates a customized external telescope that increases the working distance, allowing safe operation and reliable data collection at temperatures up to 1500 degrees C. The technique's versatility was demonstrated through tests on three high-melting-point samples, revealing significant temperature-dependent molecular structure changes. Raman spectra acquired at both room temperature and elevated temperatures showed key transformations, including the decomposition of carbonate phases and structural shifts in silicate network units (Q degrees, Q(1), Q(2), Q(3)). A deconvolution algorithm was employed to resolve overlapping spectral features, confirming the probe's ability to perform detailed compositional analyses of complex multicomponent melts. In bioactive glass studies, the system effectively monitored temperature-induced phosphate and silicate transitions, highlighting its potential for biomedical materials research. Simultaneously, the analysis of molten slag and flux provided valuable insights into polymerization states that are crucial for steel processing applications. Overall, this high-temperature fiber-optic Raman technique delivers a portable, flexible solution for monitoring the evolving chemistry and structural dynamics of materials under extreme conditions. Its capability for real-time, on-site analysis offers significant benefits for process optimization, quality control, and materials development in the glass, steel, and biomedical industries.

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 2 mu m 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 0.65 mm. 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 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 degrees 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 degrees 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.

In biochemistry, the absence of a compact, assembly-free pH sensor with high sensitivity and signal-to-noise ratio has been a persistent hurdle in achieving accurate pH measurements in real time, particularly in complex liquid environments. This manuscript introduces what we believe to be a novel solution in the form of a miniaturized pH sensor utilizing an assembly-free ball lens on a tapered multimode optical fiber (TMMF), offering the potential to revolutionize pH sensing in biochemical applications. A multimode optical fiber (MMF) was subjected to tapering processes, leading to the creation of an ultra-thin needle-like structure with a cross-sectional diameter of about 12.5 µm and a taper length of 3 mm. Subsequently, a ball lens possessing a diameter of 20 µm was fabricated at the apex of the taper. The resultant structure was coated utilizing the dip-coating technique, involving a composite mixture of epoxy and pH-sensitive dye, 2',7'-bis(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), thereby ensconcing the tapered ball lens with dye molecules for pH sensing. This study encompassed the fabrication and evaluation of six distinct fiber structures, incorporating the cleaved endface, the convex lens, and the ball lens structures to compare light focal lengths and propagation intensities. Computational simulations and numerical analyses were conducted to elucidate the encompassing light focal distances across the full array of lens configurations. The efficacy of the proposed pH sensor was subsequently assessed through its deployment within a complex liquid medium spanning a pH spectrum ranging from 6 to 8. Real-time data acquisition was performed with a fast response time of 0.5 seconds. A comparative analysis with a pH sensor predicated upon a single TMMF embedded with the fluorescent dye underscored the substantial signal enhancement achieved by the proposed system twice the fluorescence signal magnitude. The proposed assembly-free miniaturized pH sensor not only substantiates enhanced signal collection efficiency but also decisively addresses the persistent challenges of poor signal-to-noise ratio encountered within contemporary miniaturized pH probes.

This study reports for the first time, to the best of our knowledge, a real-time detection of ultralow-concentration chemical gases using fiber-optic technology, combining a miniaturized Fabry-Perot interferometer (FPI) with metal-organic frameworks (MOFs). The sensor consists of a short and thick-walled silica capillary segment spliced to a lead-in single-mode fiber (SMF), housing a tiny single crystal of HKUST-1 MOF, imparting chemoselectivity features. Ethanol and benzene gases were tested, resulting in a shift in the FPI interference signal. The sensor demonstrated high sensitivity, detecting ethanol gas concentrations (EGCs) with a sensitivity of 0.428 nm/ppm between 24.9 and 40.11 ppm and benzene gas concentrations (BGCs) with a sensitivity of 0.15 nm/ppm between 99 and 124 ppm. The selectivity study involved a combination of three ultralow concentrations of ethanol, benzene, and toluene gases, revealing an enhancement factor of 436% for benzene and 140% for toluene, attributed to the improved miscibility of these conjugated ring molecules with the alkane chains of the ethanol-modified HKUST-1. Experimental tests confirmed the sensor's viability, demonstrating significantly improved response time and spectral characteristics through crystal polishing, indicating its potential for quantifying and detecting chemical gases at ultralow concentrations. This technology may prevent energy resource losses, and the sensor's small size and robust construction make it applicable in confined and hazardous locations.

This research reports a distributed fiber optic high-temperature sensing system tailored for applications in the steel industry and various other sectors. Recent advancements in optical sensor technology have led to the exploration of sapphire crystal fibers as a solution for sensing in harsh environments. Utilizing a femtosecond (fs) laser, cascaded fiber Bragg gratings (FBGs) were meticulously fabricated within a multimode sapphire optical fiber. These FBGs endowed the system with distributed sensing capabilities and underwent rigorous testing under extreme temperatures, reaching up to 1800 °C. The study delves into the investigation of the FBG reflection spectrum, facilitated by the development of a sophisticated multimode demodulation system, which contributed to the attainment of precise temperature measurements with a performance accuracy of 99.9%. Demonstrating exceptional thermal stability, the sapphire FBGs endured temperatures of 1600 °C for a sustained duration of 22 h. Furthermore, this article explores the application of distributed temperature sensing employing multiple sapphire FBGs, showcasing their utility in temperature measurements related to molten steel studies.

The study investigates a refined heat treatment methodology tailored for commercial metal-coated optical fibers, with the objective of accurately reducing residual stress to prime them for use in distributed sensing applications at high temperatures.

This study reports the fabrication of a fourth-order line-by-line fiber Bragg grating (FBG) array using femtosecond laser inscription within a highly multimode coreless optical fiber, with a particular focus on achieving substantial multiplexing capabilities. An ultrafast annealing procedure is employed, resulting in an impressive enhancement of the FBG sensor's fringe visibility by approximately 13 dB, signifying a notable improvement of approximately similar to 4 dB. This substantial enhancement contributes to the long-term stability and performance of the multiplexed FBG array in extreme-temperature conditions. The systematic fabrication approach employed for the multiplexed FBG array guarantees a high signal-to-noise ratio (SNR) for each individual FBG within the array. This FBG array is intended for extreme-temperature applications, addressing limitations associated with traditional FBGs based on doped optical fibers, including SNR degradation and temperature-induced fringe drift. Testing at temperatures up to 1120 degrees C demonstrates the FBG array's stability without fluctuations in readings. Furthermore, it endures seven heat cycles, spanning from 500 degrees C to 1120 degrees C, over 60 h, exhibiting outstanding thermal stability. This highly multiplexed FBG array with an ultrafast annealing approach holds promise for extreme-temperature environments, such as steelmaking, where precise and reliable distributed temperature monitoring is imperative.

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 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 degrees C with sapphire crystalline fiber and 1100 degrees 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.