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Fiber-optic sensor

A fiber-optic sensor is a device that employs optical fibers as the sensing element to detect and measure variations in physical, chemical, or biological parameters by altering properties of transmitted light, such as intensity, phase, wavelength, or polarization.^[1]^[2] These sensors operate on the principle of total internal reflection, where light is confined within the fiber core and interacts with external stimuli through mechanisms like evanescent waves, gratings, or interferometry, leading to measurable changes in the light signal.^[1]^[2] Originating from advancements in optical fiber technology during the 1970s for telecommunications, fiber-optic sensing emerged prominently in the 1980s and has evolved over the past four decades, driven by innovations in materials and fabrication techniques.^[1]^[2] Key advantages include their compact size, electromagnetic interference immunity, ability to operate in harsh environments, and capacity for remote, distributed sensing over long distances.^[1]^[2] Common types encompass Fiber Bragg Gratings (FBGs), which reflect specific wavelengths altered by strain or temperature; long-period gratings (LPFGs) for coupling light modes sensitive to refractive index changes; interferometric sensors like Fabry-Pérot or Mach-Zehnder for phase-based detection; and surface plasmon resonance (SPR) configurations for high-sensitivity biochemical analysis.^[1]^[2] Applications span diverse fields, including structural health monitoring in civil engineering (e.g., strain detection in bridges with FBGs achieving resolutions down to 1 μɛ), biomedical diagnostics (e.g., glucose sensing via SPR with limits of detection around 0.17 mg/dL), environmental monitoring (e.g., gas detection like H₂ or NH₃), and industrial processes (e.g., pressure measurement in oil and gas pipelines).^[1]^[2] Despite their versatility, fiber-optic sensors face challenges such as temperature-strain crosstalk in grating-based systems, fabrication complexity for interferometric designs, and sensitivity to light source fluctuations in intensity-modulated types, though ongoing research in nanotechnology and hybrid materials continues to mitigate these limitations.^[1]^[2]

Fundamentals

Principles of Operation

Fiber-optic sensors are devices that employ optical fibers to transmit light from a source to a detector while sensing environmental changes through alterations in the light's properties, including intensity, phase, wavelength, and polarization.^[3] These sensors leverage the fiber's ability to guide light over long distances with minimal loss, enabling remote and distributed measurements immune to electromagnetic interference.^[1] Light propagation in optical fibers occurs via total internal reflection at the interface between the core and cladding, where the core's higher refractive index confines the light within the fiber.^[3] Fibers operate in single-mode or multimode configurations: single-mode fibers, with core diameters of 8–10 μm, support one propagation mode for low dispersion over long distances, while multimode fibers, with cores of 50 μm or larger, allow multiple modes but exhibit higher modal dispersion.^[1] Attenuation factors, such as material absorption, Rayleigh scattering, and waveguide imperfections, gradually reduce light intensity along the fiber, typically by approximately 0.2 dB/km in silica fibers at 1550 nm.^[4] Measurands like strain, temperature, and pressure modify the fiber's properties, such as its refractive index or physical geometry (e.g., length or diameter), thereby perturbing light propagation and detectable parameters.^[3] For intensity-based sensing, the output intensity can be approximated as I_{\text{out}} = I_{\text{in}} (1 - \alpha \Delta x) for small perturbations, where I_{\text{in}} is the input intensity, \alpha is the absorption coefficient influenced by the measurand, and \Delta x represents the effective path length change; this derives from the linear approximation of the Beer-Lambert law I_{\text{out}} = I_{\text{in}} e^{-\alpha \Delta x}.^[5] Fiber-optic sensors are configured as active or passive systems: active sensors require electrical power at the sensing location for internal light sources or modulators, whereas passive sensors operate without local power, relying solely on remote interrogation via the fiber.^[6] This distinction influences deployment in harsh environments, with passive designs favored for simplicity and safety.^[3]

Key Components

Fiber-optic sensors rely on several essential hardware components to enable the transmission, modulation, and detection of optical signals for sensing purposes. The core element is the optical fiber itself, which serves as the medium for light propagation. Optical fibers used in sensors are primarily categorized into silica-based and polymer types, each offering distinct properties suited to different applications. Silica fibers, made from fused silica (SiO₂) with optional dopants like germania for enhanced photosensitivity, are the most common due to their low optical loss (typically <0.2 dB/km at 1550 nm) and high mechanical strength.^[4] Single-mode silica fibers, featuring a small core diameter of 8-10 μm surrounded by a cladding of 125 μm, support a single propagation mode and are ideal for long-distance sensing with minimal modal dispersion.^[7] Polymer optical fibers (POFs), constructed from materials like polymethyl methacrylate, provide larger core diameters (up to 1 mm) for easier coupling and flexibility but exhibit higher attenuation (around 100-200 dB/km), making them suitable for short-range, cost-sensitive sensors.^[8] Specialty fibers, such as those doped with rare-earth elements or microstructured photonic crystal fibers, enhance sensitivity to environmental parameters by altering refractive index or introducing defects for specific interactions like Bragg gratings. Recent developments include hollow-core fibers achieving attenuation as low as 0.091 dB/km at 1550 nm (as of 2025), offering potential for even longer-distance sensing with reduced latency.^[9]^[10] Light sources provide the optical input signal, with selection depending on the required coherence, wavelength, and power stability. Light-emitting diodes (LEDs) serve as broadband sources (typically 20-100 nm bandwidth) for intensity-based sensors, offering low cost and simplicity but limited coherence length (<1 m).^[11] Semiconductor lasers, such as distributed feedback (DFB) lasers operating at 1550 nm, are preferred for low-loss transmission in silica fibers and interferometric sensors, providing narrow linewidths (<0.1 nm) and coherence lengths exceeding 10 km, with output powers up to 10 mW to ensure adequate signal strength.^[11] These wavelengths align with the minimum attenuation window of silica fibers, minimizing signal degradation over distance.^[4] Detectors convert the modulated optical signal back to electrical form, with performance governed by sensitivity and signal-to-noise ratio (SNR). Photodiodes, particularly InGaAs types for near-infrared wavelengths, are widely used due to their high quantum efficiency (>80% at 1550 nm) and fast response times (<1 ns), enabling detection of small phase or intensity changes.^[12] Spectrometers, employing array detectors like CCDs or CMOS sensors, analyze wavelength shifts in spectral sensors, achieving resolutions down to 1 pm but with trade-offs in SNR (typically 40-60 dB) influenced by integration time and noise sources like shot noise.^[13] High SNR is critical for resolving weak signals in noisy environments, often enhanced by avalanche photodiodes (APDs) that provide internal gain for improved detection limits.^[12] Interrogators and signal processing units are integral for extracting measurable data from the optical output, functioning to demodulate signals and convert them into usable electrical or digital formats. Interrogators employ techniques like wavelength-division multiplexing (WDM) or time-division multiplexing (TDM) to query multiple sensors along a single fiber, resolving parameters such as strain or temperature with resolutions down to 1 μɛ or 0.1°C.^[14] These units typically include analog-to-digital converters and digital signal processors to filter noise, perform calibration, and output processed data via interfaces like Ethernet, ensuring real-time monitoring without manual intervention.^[15] Connectors and packaging ensure reliable signal transmission and environmental protection, minimizing losses at interfaces. Standard connectors, such as FC or SC types with ceramic ferrules (2.5 mm or 1.25 mm diameter), facilitate detachable links with typical insertion losses of 0.2-0.3 dB per connection.^[16] For permanent, low-loss joints, fusion splicing uses electric arcs to fuse fiber ends, achieving losses below 0.1 dB and near-zero reflectance, essential for maintaining signal integrity in sensor arrays.^[17] Packaging involves protective sheathing, such as Kevlar-reinforced jackets or metallic armoring, to shield fibers from mechanical stress, moisture, and temperature extremes while preserving flexibility for deployment.^[9]

Classifications

Intrinsic Sensors

In intrinsic fiber-optic sensors, the optical fiber functions as both the waveguide and the sensing element, where external measurands directly interact with and perturb the propagating light within the fiber core or cladding, modulating properties such as phase, intensity, or polarization.^[1] This direct interaction enables high sensitivity to environmental changes without requiring light to exit the fiber until detection.^[3] A prominent example is the Mach-Zehnder interferometer configured within the fiber for strain sensing, where axial strain induces both length elongation and refractive index variation, resulting in a phase shift given by

\Delta \phi = \frac{2\pi n L}{\lambda} \left( \frac{\Delta n}{n} + \frac{\Delta L}{L} \right),

with n as the refractive index, L as the sensing length, \lambda as the wavelength, \Delta n/n from the strain-optic effect, and \Delta L/L as the applied strain \epsilon.^[18] This configuration achieves precise measurements by demodulating the interferometric output, often using a 3×3 coupler for phase extraction.^[18] Temperature sensing in intrinsic configurations exploits thermal expansion of the fiber and the thermo-optic coefficient, which shifts the Bragg wavelength in fiber Bragg gratings (FBGs) with sensitivities typically around 10 pm/°C.^[19] For pressure and acoustic detection, microbends introduced by external forces alter light coupling between modes, while evanescent waves in the cladding interact with surrounding pressure variations to modulate transmission loss.^[20] These sensors excel in distributed sensing applications, enabling continuous monitoring over distances up to several kilometers by leveraging backscattering phenomena like Rayleigh, Brillouin, or Raman, with spatial resolutions down to 1 m.^[21] In contrast to extrinsic sensors, which use the fiber solely for light transmission to an external transducer, intrinsic designs integrate the sensing directly into the fiber for enhanced compactness and immunity to electromagnetic interference.^[1]

Extrinsic Sensors

Extrinsic fiber-optic sensors employ optical fibers primarily as a means of delivering light to and collecting it from an external sensing element, where the actual modulation of the light occurs outside the fiber itself. In this configuration, the fiber acts as a conduit, transmitting unmodulated light from a source to a remote transducer—such as a diaphragm, piezoelectric element, or Fabry-Perot cavity attached at the fiber tip—and returning the modulated light for detection. This approach contrasts with systems where the fiber core directly participates in the sensing process, enabling the use of robust, non-optical transducers in locations inaccessible to traditional electronics.^[22]^[23] Common examples include fiber-linked piezoelectric transducers for vibration monitoring, where the fiber delivers light to a piezoelectric element that modulates intensity or phase based on mechanical stress, allowing remote detection of dynamic events like structural vibrations. Another prevalent type is the tip-mounted intensity modulator for displacement sensing, in which light reflects off a target surface—such as a moving diaphragm or reflective marker—altering the return signal's intensity proportional to the gap distance, often achieving resolutions on the order of micrometers. These sensors are particularly suited for measuring parameters like rotation, velocity, acceleration, torque, and twisting, where the external element provides the necessary mechanical or electrical transduction.^[22]^[3]^[24] Configurations often involve specialized fiber arrangements to optimize light delivery and collection. Bifurcated fibers, consisting of a Y-shaped bundle with separate input and output legs, facilitate reflection-based sensing by directing incident light toward a target and capturing the backscattered signal, commonly used in proximity or position detection with minimal cross-talk. Fiber bundles, comprising multiple fibers, enable imaging applications by transmitting light to an external lens or mirror assembly, allowing visualization in confined spaces. These setups leverage multimode fibers for higher light throughput, though they require careful alignment to maintain signal fidelity.^[25]^[26] A key advantage of extrinsic sensors lies in their ability to perform remote sensing in harsh environments, such as high-radiation zones or areas with intense electromagnetic interference (EMI), due to the dielectric nature of the fibers, which provides inherent immunity to EMI and radio-frequency interference (RFI) while withstanding temperatures up to several hundred degrees Celsius. This makes them ideal for applications in nuclear facilities, aerospace, or industrial processes where electrical sensors would fail. However, signal integrity can be compromised by losses from back-reflections at fiber interfaces, which are mitigated by angled cleaving of the fiber ends—typically at 8 degrees—to redirect reflected light away from the core, reducing return loss to below -60 dB without significantly increasing insertion loss.^[27]^[28]^[29]

Sensing Mechanisms

Interferometric Methods

Interferometric methods in fiber-optic sensors exploit phase differences arising from split and recombined light paths to achieve high sensitivity in detecting environmental perturbations such as strain, temperature, and vibration. The core principle involves measuring the optical path difference (OPD) induced by changes in the fiber's length or refractive index, which modulates the phase of the interfering beams. This enables resolutions as fine as sub-microstrain (e.g., <1 μɛ for strain measurements), far surpassing many non-interferometric techniques due to the coherent nature of light interference.^[30]^[31] Common configurations include the Michelson interferometer, where light is split into two arms with reflectors and recombined to detect OPD changes; the Sagnac interferometer, which uses counter-propagating beams in a fiber loop to sense rotation via the Sagnac effect; and ring resonators, which form a closed-loop cavity for enhanced phase accumulation. The phase shift Δφ resulting from a path length change ΔL is given by the equation:

\Delta \phi = \frac{2\pi \Delta L}{\lambda}

where λ is the wavelength of the light source; this fringe shift directly correlates with the measurand, allowing precise quantification. These types are often implemented in intrinsic sensor setups, where the sensing element is the fiber itself.^[30]^[32]^[33] To address phase ambiguity in coherent interferometry, white-light interferometry employs broadband sources like superluminescent diodes, producing low-coherence interference patterns with a central fringe corresponding to zero OPD for absolute measurements. By analyzing the channelled spectrum—often via autocorrelation of Gaussian envelopes—the absolute position or displacement can be determined over ranges up to 200 μm with errors as low as 6 nm, without unwrapping multiple 2π phases. This technique is particularly useful for quasi-static sensing where unambiguous readout is essential.^[34] Demodulation of the interferometric signal is critical for extracting the phase information; phase-generated carrier (PGC) methods modulate the input light with a high-frequency carrier via an electro-optic modulator, generating quadrature signals at harmonics that are processed to yield the measurand phase, robust against low-frequency drifts. Heterodyne techniques, involving frequency shifting of one beam (e.g., using acousto-optic modulators), produce beat signals for direct phase tracking, enabling dynamic range extensions for applications like ultrasound detection with sensitivities down to 4.2 × 10⁻⁴ pm wavelength shifts.^[35] In distributed sensing, optical frequency domain reflectometry (OFDR) leverages interferometric principles by sweeping a tunable laser and analyzing Rayleigh backscattering spectra along the fiber, achieving spatial resolutions as fine as 10 μm over sensing lengths up to ~100 m, with strain resolutions of ~1 μɛ and temperature resolutions of ~0.08 °C in optimized short-range setups; longer ranges exceeding 100 km are possible but with reduced spatial resolutions on the order of 10 cm to 1 m. Compensation for laser nonlinearity via techniques such as the non-uniform fast Fourier transform (NUFFT) ensures high-fidelity distributed profiles, making OFDR ideal for structural health monitoring in civil engineering and aerospace.^[31]

Intensity and Spectral Methods

Intensity modulation in fiber-optic sensors relies on direct measurement of changes in optical power transmitted through the fiber, induced by external perturbations such as bending, absorption, or scattering. These changes occur without the need for phase recombination, making the approach suitable for straightforward detection using photodetectors. For instance, in macrobend sensors, light leakage into the cladding increases as the bend radius decreases, resulting in a relative intensity loss approximated by \Delta I / I = k / R, where R is the bend radius and k is a constant dependent on fiber parameters like core radius and refractive index difference. This mechanism enables sensing of curvature, displacement, or force, with examples including polymer optical fiber configurations achieving sensitivities around 0.1668 dB/mm for displacement over 0–201 mm ranges.^[36]^[37]^[38] Spectral methods in fiber-optic sensing detect shifts in the wavelength of transmitted or reflected light spectra, often due to variations in the effective refractive index or optical path length caused by measurands. A key example is evanescent wave absorption, where the evanescent field extending from the fiber core interacts with the surrounding medium, leading to wavelength-dependent absorption that correlates with refractive index changes. These sensors, typically employing tapered or U-shaped fiber geometries to enhance evanescent field exposure, achieve resolutions as fine as $10^{-10} RIU in optimized designs incorporating materials like graphene. Such techniques are particularly effective for refractive index sensing in environmental or chemical monitoring, with absorption spectra analyzed in the infrared range for molecular identification.^[39]^[40] Fiber Bragg gratings (FBGs) represent a prominent spectral method, featuring periodic refractive index modulations in the fiber core that create narrow reflection peaks at the Bragg wavelength \lambda_B = 2 n_{eff} \Lambda, where n_{eff} is the effective refractive index and \Lambda is the grating period. External strain alters both \Lambda and n_{eff}, producing a wavelength shift given by \Delta \lambda = \lambda (1 - p_e) \varepsilon, with p_e the photoelastic coefficient (typically ~0.22) and \varepsilon the applied strain. This enables precise strain measurement, with sensitivities around 1.2 pm/µε at 1550 nm. FBGs can be integrated with extrinsic sensor tips for hybrid configurations, enhancing versatility in displacement or pressure detection.^[41]^[42] Interrogation systems for spectral methods, including FBGs, commonly employ tunable lasers to sweep wavelengths and monitor reflection or transmission peaks, or spectrometers for broadband spectral analysis and peak tracking. Tunable laser systems offer high resolution (e.g., <1 pm) and fast scanning rates, suitable for dynamic measurements, while spectrometer-based approaches provide simultaneous multiplexing of multiple gratings using wavelength-division techniques. These setups ensure accurate demodulation of shifts, with integrated photonic designs reducing size and cost for field applications.^[43]^[44]^[45] Compared to interferometric methods, intensity and spectral approaches exhibit lower sensitivity—typically limited by source fluctuations or resolution constraints—but offer simpler setups without requiring coherent light or complex phase demodulation, facilitating cost-effective and scalable deployments in industrial monitoring.^[46]

Specialized Sensors

Chemical Sensors

Fiber-optic chemical sensors detect molecular species through optical interactions between the guided light and analytes surrounding the fiber. A primary mechanism involves evanescent field absorption, where the evanescent wave penetrating the fiber's cladding interacts with chemical analytes, leading to attenuation of the transmitted light proportional to the analyte concentration.^[47] This absorption follows the Beer-Lambert law, expressed as A = \epsilon c l, where A is the absorbance, \epsilon is the molar absorptivity, c is the analyte concentration, and l is the effective path length of interaction.^[48] Alternatively, cladding-etched fibers expose the core directly to the analyte, enhancing interaction efficiency by increasing the overlap between the light field and the sensing region.^[49] Common examples include pH sensors that utilize indicator dyes immobilized on the fiber surface, where pH-induced color changes alter the dye's absorption spectrum, enabling ratiometric measurement across physiological ranges.^[50] For gas detection, such as carbon dioxide (CO₂), colorimetric indicators embedded in a polymer matrix on the fiber tip respond to CO₂ exposure by shifting absorption peaks, particularly observable around 850 nm, allowing for selective quantification.^[51] To achieve selectivity for specific analytes, surface functionalization techniques immobilize chemical receptors or dyes onto the fiber, often via sol-gel coatings or silane coupling agents, which bind target molecules while minimizing non-specific interactions.^[52] These modifications enable discrimination among similar chemical species, such as distinguishing CO₂ from other gases through pH-sensitive indicators.^[49] These sensors typically exhibit limits of detection in the parts-per-million (ppm) range for gaseous analytes, with some achieving sub-ppm sensitivity depending on the interaction length and indicator efficiency.^[53] Response times are generally less than 1 second for optimized designs, facilitating real-time monitoring.^[54] Integration with waveguides forms the basis of lab-on-fiber systems, where microstructured fibers or tapered sections incorporate fluidic channels and optical elements to enable multiplexed chemical analysis in compact platforms.^[55] This approach leverages spectral methods for multi-analyte detection while maintaining the fiber's remote sensing capabilities.^[56] Recent advances as of 2025 include the integration of metal-organic frameworks (MOFs), such as ZIF-8 combined with plasmonic nanostructures, into fiber-optic designs to achieve trace-level detection of gases and chemicals with enhanced selectivity and sensitivity.^[57]

Biosensors

Fiber-optic biosensors integrate biological recognition elements, such as antibodies or enzymes, with optical fibers to enable selective detection of biomolecules through mechanisms like fluorescence resonance energy transfer (FRET) or surface plasmon resonance (SPR) at the fiber tip. In FRET-based designs, a donor fluorophore attached to the fiber interacts with an acceptor linked to the bioreceptor, where binding-induced proximity changes energy transfer efficiency, altering the emission spectrum. SPR configurations, often using gold-coated fiber tips, detect refractive index shifts from biomolecular binding events at the metal-dielectric interface. Bioreceptors are immobilized on the fiber surface via covalent attachment or adsorption, ensuring stable interaction with target analytes while maintaining optical signal integrity.^[58]^[59] Prominent examples include immunosensors for pathogen detection, such as those employing evanescent wave fluorescence to identify SARS-CoV-2 antigens, where antibodies capture viral proteins on the fiber cladding, leading to localized fluorescence enhancement. Glucose biosensors typically feature enzyme-coated fibers, with glucose oxidase immobilized on the fiber core or cladding to catalyze glucose oxidation, producing detectable optical changes via oxygen-sensitive dyes or direct fluorescence quenching. These designs leverage chemical functionalization techniques, similar to those in chemical sensors, to anchor bioreceptors securely.^[60]^[61]^[62] Signal transduction in these biosensors relies on intensity variations arising from binding events, where analyte-bioreceptor interactions modulate light propagation, such as through fluorescence quenching or refractive index alterations, quantifiable in real-time. Affinity constants for antibody-antigen pairs in such systems typically range from nanomolar (K_d ~10^{-9} M), enabling high specificity and sensitivity for low-concentration targets like biomarkers.^[58]^[63] Miniaturization via tapered fiber geometries enhances evanescent field exposure, achieving single-molecule sensitivity by confining light to sub-micrometer scales and amplifying local binding signals. These tapered structures, often with waist diameters below 10 μm, facilitate integration into microfluidic or implantable devices for precise biomolecular assays.^[64]^[65] For in vivo applications, fiber-optic biosensors must undergo sterilization processes like autoclaving or gamma irradiation to eliminate contaminants without degrading optical or biological components. Biocompatibility is ensured through biopolymer coatings, such as hydrogels, to minimize immune responses and tissue adhesion, allowing safe implantation for continuous monitoring. Regulatory considerations, including ISO 10993 standards, guide material selection to support long-term physiological compatibility.^[59]^[66] As of 2025, advancements in wearable and implantable fiber-optic biosensors have enabled real-time monitoring of neural activity and neurological biomarkers, incorporating flexible materials and array designs for enhanced integration into biomedical devices.^[67]

Applications

Industrial and Environmental Monitoring

Fiber-optic sensors play a crucial role in structural health monitoring (SHM) of large-scale infrastructure, enabling the detection of strains, cracks, and deformations in real time over extended distances. Distributed fiber Bragg grating (FBG) arrays, which utilize quasi-distributed sensing along optical fibers, are particularly effective for monitoring bridges and pipelines, where they can span up to 10 km while providing high-resolution data on structural integrity under loads such as traffic or environmental stresses.^[68]^[69] These systems leverage the intrinsic distributed capabilities of fiber optics to embed sensors directly into concrete or composite materials, offering multiplexed measurements that enhance safety and reduce maintenance costs without the need for electrical power at each point.^[70]^[71] In the oil and gas industry, fiber-optic sensors are deployed downhole to measure temperature and pressure in harsh well environments, supporting efficient reservoir management and production optimization. These sensors, often based on FBG or Fabry-Pérot interferometry, operate reliably at temperatures up to 150°C and pressures up to 100 MPa, withstanding corrosive conditions and hydrogen exposure common in deep wells.^[72]^[73] For instance, distributed temperature sensing (DTS) and strain sensing systems provide continuous profiling along wellbores, aiding in the detection of fluid flows and zonal isolation during drilling and completion operations.^[74] Environmental monitoring benefits from fiber-optic sensors through applications like pipeline leak detection and water quality assessment, promoting sustainability and risk mitigation. Acoustic intrinsic sensors, such as distributed acoustic sensing (DAS) systems, detect leaks in pipelines by capturing vibrations from fluid escapes, enabling precise localization over kilometers with high sensitivity to micro-leaks.^[75]^[76] For water quality, chemical probes integrated with fiber optics, including evanescent wave sensors, monitor parameters like pH, turbidity, and pollutants in real time, offering corrosion-resistant deployment in aquatic environments.^[77]^[78] A notable case study emerged following the 2010 Deepwater Horizon oil spill, which underscored the need for robust subsea monitoring and accelerated the adoption of fiber-optic technologies in offshore operations. Post-incident deployments, such as advanced DTS and DAS systems on pipelines and risers, enabled real-time surveillance of pressure, temperature, and acoustic events to prevent similar environmental disasters, with NASA's spinoff technologies facilitating retrofits on legacy infrastructure.^[79]^[80] Integration with Internet of Things (IoT) platforms further enhances scalability in these applications, allowing multiplexed networks of over 100 sensors per fiber to form intelligent monitoring systems. FBG-based IoT setups transmit data wirelessly to cloud analytics for predictive maintenance, supporting large-scale deployments in industrial settings with minimal infrastructure.^[3]^[81]^[82]

Medical and Biomedical Uses

Fiber-optic sensors have found significant application in endoscopy for gastrointestinal diagnostics, particularly through pH and oxygen probes that enable minimally invasive assessment of mucosal conditions. These probes leverage optical properties such as fluorescence or phosphorescence to detect biochemical changes in real time. For instance, multicore fiber-optic optrodes, with diameters as small as 200 µm, integrate fluorescein-based indicators for pH sensing and palladium porphyrin complexes for oxygen measurement, achieving accuracies of ±0.02 pH units and 0.6 mg/L for oxygen, respectively, as demonstrated in ex vivo tissue models relevant to endoscopic procedures.^[83] Visible light spectroscopy using fiber-optic probes has also been employed during upper gastrointestinal endoscopy to measure mucosal oxygen saturation, aiding in the diagnosis of conditions like chronic mesenteric ischemia by correlating optical signals with microvascular oxygen tension.^[84] In glucose monitoring, fiber Bragg grating (FBG)-based sensors have been investigated for detecting refractive index changes induced by glucose interactions with functionalized coatings, with demonstrated sensitivity over physiological ranges (e.g., 0–20 mmol/L in lab settings), though commercial implantable systems for diabetic patients remain primarily electrochemical.^[62] Such systems address the need for long-term, non-invasive monitoring, though challenges like biofouling require periodic recalibration.^[85] Biosensing applications include catheter-tip pressure sensors for cardiovascular procedures, where fiber-optic designs offer high precision in confined vascular environments. Microfabricated extrinsic Fabry-Pérot interferometric sensors at the catheter tip achieve pressure resolutions down to 0.11 mmHg, enabling accurate hemodynamic monitoring during interventions like angioplasty, with minimal invasiveness compared to electrical alternatives.^[86] Recent advances in the 2020s have integrated fiber-optic sensors with optogenetics for neural sensing, combining optical fibers for light delivery with electrodes for electrophysiological recording to modulate and monitor neural activity with high spatiotemporal resolution. For example, polymer-based optrodes (80–250 µm diameter) enable simultaneous optogenetic stimulation and multi-site neural recording in brain tissue, as shown in rodent models for studying neural circuits. As of 2025, advances include AI-enhanced data processing for real-time neural monitoring.^[87] Regulatory aspects emphasize biocompatibility, with FDA approvals requiring adherence to ISO 10993 standards for cytotoxicity and sensitization testing of fiber-optic components. Polyethylene glycol (PEG) functionalization of sensor surfaces enhances hemocompatibility and reduces protein adsorption, facilitating approvals for implantable devices by improving long-term stability in vivo.^[88]

Advantages and Limitations

Benefits and Performance Characteristics

Fiber-optic sensors exhibit inherent immunity to electromagnetic interference (EMI) due to their electrical passivity, making them ideal for deployment in high-voltage environments and during magnetic resonance imaging (MRI) procedures where traditional electrical sensors would require extensive shielding or fail altogether.^[89] This non-conductive nature allows reliable operation in electrically noisy settings, such as power grids or medical imaging suites, without signal degradation from external fields.^[90] These sensors are exceptionally compact and lightweight, with standard optical fibers featuring cladding diameters as small as 125 μm, enabling integration into space-constrained structures like aircraft components or biomedical implants.^[91] Their design supports high-density multiplexing, accommodating up to 1,000 or more sensors along a single fiber, which facilitates distributed monitoring over extended lengths without adding significant weight—typically 0.1-1% that of conventional strain gauges.^[91] Performance metrics underscore their high sensitivity, with fiber Bragg grating (FBG) variants achieving strain resolutions of 1 μɛ and temperature resolutions of 0.1°C, enabling precise detection of subtle structural changes or thermal variations.^[92] Fabry-Pérot interferometric sensors further enhance this capability, offering strain resolutions down to 0.15 μɛ while maintaining the same temperature precision.^[92] Long-term stability is another key strength, with field-deployed systems demonstrating lifespans exceeding 20 years under harsh conditions, supported by the durable silica-based fibers that resist degradation over decades. In distributed sensing applications, fiber-optic systems prove cost-effective compared to arrays of discrete point sensors, as a single fiber can monitor kilometers of infrastructure at reduced installation and maintenance expenses, leveraging existing fiber networks for scalable deployment.^[21] This approach minimizes wiring complexity and operational costs, particularly in large-scale industrial or environmental setups where multiplexing offsets the initial outlay.^[93]

Challenges and Future Developments

Fiber-optic sensors face several inherent limitations that hinder their broader deployment. One primary challenge is mechanical fragility, as these sensors typically require a minimum bend radius exceeding 15 mm to prevent signal loss or fiber breakage, limiting their use in compact or flexible applications. Additionally, the high initial costs associated with interrogator systems, often ranging from $10,000 or more for multi-channel units, pose a barrier to widespread adoption compared to traditional electronic sensors. Calibration is another critical issue, as environmental factors such as temperature fluctuations can induce signal drift, necessitating frequent recalibration to maintain accuracy in long-term monitoring scenarios. Manufacturing challenges further complicate the scalability of specialized fiber-optic sensors. For chemical sensors, modifying the fiber cladding—such as through etching, coating, or doping with sensitive materials—remains difficult to achieve at scale, due to inconsistencies in material deposition and integration that affect reproducibility and cost-efficiency. Looking ahead, future developments aim to address these limitations through innovative integrations and enhancements. Hybrid photonic approaches, including silicon photonics platforms, are expected to mature by 2025, enabling more compact and efficient sensor interrogators by combining fiber optics with on-chip processing. AI-enhanced signal processing techniques are emerging to reduce noise and improve resolution, particularly in distributed sensing systems where environmental interferences are prevalent. Quantum-enhanced methods, such as those utilizing entangled photons, promise to boost sensitivities beyond classical limits, potentially achieving sub-wavelength precision in measurements. Key research gaps persist, including the lack of standardized protocols for biosensors, which complicates interoperability and regulatory approval in biomedical applications. Post-2020 advances in polymer optical fibers have improved flexibility and strain tolerance compared to silica-based counterparts, enabling wearable and implantable designs, though biocompatibility and long-term stability require further validation. These advancements could drive significant impacts, with projections indicating widespread adoption of fiber-optic sensors in smart cities by 2030 for real-time infrastructure monitoring, traffic management, and environmental sensing, supported by declining costs and integration with IoT networks.

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Dynamic Strain Measured by Mach-Zehnder Interferometric Optical ...
Thus, once the phase shift Δϕ of the Mach-Zehnder interferometer is demodulated, the strain of the host structure can be determined by utilizing Equation (3). 3 ...Missing: formula | Show results with:formula
[17]
[PDF] Characterization of Intrinsic Fiber Optic Temperature Sensors by In ...
A standard. FBG can have a temperature sensitivity of around 9-13 pm/℃. [11] The wavelength shift for sensor 1,. FBG 9, corresponds to a temperature drift of ...
[18]
[PDF] Fiber Optic Sensors
When an acoustic wave exerts pressure on the sensing fiber, the microbending device induces modal-phasing changes so that an output signal can be detected ...
[19]
Distributed optical fiber sensing: Review and perspective
Oct 11, 2019 · This work is focused on a review of three types of distributed optical fiber sensors which are based on Rayleigh, Brillouin, and Raman scattering, and use ...
[20]
Extrinsic Sensor - an overview | ScienceDirect Topics
Extrinsic sensors are defined as sensors that consist of two parts: a specific layer that interacts with a sample and a transducer that detects the ...
[21]
Fiber-optic Sensors - RP Photonics
Fiber-optic sensors are optical sensors based on fiber devices. They are often used for sensing temperature and/or mechanical stress.
[22]
Types of fiber optics sensor - BrainKart
Apr 1, 2017 · Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting. 3. Phase Modulated ...
[23]
Bifurcated Optical Fiber Sensor for Weight Measurement System
Having high sensitivity to short distances, bifurcated optical fiber (Y-sensor) is well suited not only for control applications as position sensors, but also ...
[24]
Non-Contact Vibration Sensor using Bifurcated Bundle Glass Fiber ...
A fiber optic vibration sensor is demonstrated using bifurcated bundle fiber based on the principle of extrinsic displacement sensor.
[25]
Fiber Optic Sensors: Advantages and Disadvantages
Immunity to EMI/RFI: Fiber optic sensors are not disturbed by Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI). · Suitable for Harsh ...
[26]
Fiber Optic Sensors for Harsh and High Radiation Environments in ...
The advantage of OFDR is that it has greater spatial resolution along the fiber than OTDR, which is at the meter level. In addition, OFDR also has a large ...
[27]
Cleaving of Fibers - RP Photonics
Substantial cleave angles may be required for suppressing effects of reflections. If the application is very sensitive to back-reflections, a large enough ...What is Fiber Cleaving? · Fiber Cleaving Tools · Judging the Cleaving Results
[28]
Principles of Fiber-Optic Interferometry - SpringerLink
Optical interferometers are well-known for their ability to make high-precision measurements of optical path difference (OPD) or changes that may be induced ...
[29]
Distributed Optical Fiber Sensors Based on Optical Frequency ...
Apr 3, 2018 · In this review, we will survey the key technologies for improving sensing range, spatial resolution and sensing performance in DOFS based on OFDR.
[30]
Fiber Optic Sensors Based on the Sagnac Interferometer and ...
This chapter begins with an introduction to the use of the Sagnac interferometer and passive ring resonator for rotation sensing.
[31]
Reflectometric and interferometric fiber optic sensor's principles and ...
There are four broad categories of interferometric sensor which are based on: Michelson interferometer, Fabry-Perot interferometer, Sagnac interferometer, and ...
[32]
A simple white-light fiber-optic interferometric sensing system for ...
White-light channelled spectrum interferometry offers very simple absolute position measurement based on a simple signal processing of the captured channelled ...
[33]
Modified phase-generated carrier demodulation of fiber-optic interferometric ultrasound sensors
### Phase-Generated Carrier (PGC) Demodulation for Fiber-Optic Interferometric Sensors
[34]
Advanced intensity-modulated fiber sensors for scalable sensing - NIH
This article presents a structured overview of the current state and development of intensity-modulated fiber optic sensors.
[35]
Simplified Formula of Bending Loss for Optical Fiber Sensors
Aug 5, 2025 · This simple formula considers various bending radii, number of turns, extra bending angle, and wavelength and has good agreement with ...Missing: ΔI/ k /
[36]
Intensity Modulation based U shaped Plastic Optical Fiber Refractive Index Sensor
- **Title**: Intensity Modulation based U-shaped Plastic Optical Fiber Refractive Index Sensor
[37]
https://www.researchgate.net/publication/243394624_Simplified_Formula_of_Bending_Loss_for_Optical_Fiber_Sensors
[38]
Evanescent Wave Absorption Based Fiber-Optic Sensor - SpringerLink
Evanescent wave absorption (EWA) based fiber-optic sensors have found widespread applications ranging from environmental sensing to biosensing.Missing: spectral | Show results with:spectral
[39]
Fiber Bragg Gratings – FBG, index modulation, filters ... - RP Photonics
where Λ is the grating period, λ is the vacuum wavelength, and n e f f is the effective refractive index of light in the fiber. Essentially, the condition means ...What is a Fiber Bragg Grating? · Apodized Gratings · Bragg Gratings in Multimode...
[40]
Fibre Bragg Grating Based Strain Sensors - PubMed Central - NIH
Δ λ B λ B = ( 1 − p e ) ⋅ ε, (16). where pe is the elasto-optic coefficient that links Δλ/λ with ΔL/L through the relation Δλ/λ = 0.79 ΔL/L [8]. The ...
[41]
Integrated Photonic Interrogators for Fiber-Optic Sensing Systems
Insufficient relevant content. The provided content (title and metadata) does not contain specific details on interrogation systems for fiber optic sensors using tunable lasers or spectrometers. Only the title "Integrated Photonic Interrogators for Fiber-Optic Sensing Systems" and a reference to IEEE Xplore are available, with no substantive text or data.
[42]
High-resolution spectrally-resolved fiber optic sensor interrogation ...
This paper presents a spectrally-resolved integration system suitable for the reading of Bragg grating, all-fiber Fabry-Perot, ...
[43]
Fiber-Optic Sensor Interrogation Based on a Widely Tunable ...
Aug 6, 2025 · We introduce and demonstrate a tunable laser measurement system based on a widely tunable monolithic laser diode (TML). It is capable of ...
[44]
Advanced Intensity-Modulated Fiber Sensors for Scalable Sensing
Oct 9, 2025 · An analysis with interferometric and Bragg grating-based sensors is presented to clarify the trade-offs in sensitivity, complexity, and cost.
[45]
Evaluation of an Evanescent Fiber Optic Chemical Sensor ... - CORE
Sensors based on this principal, known as evanescent fiber optic chemical sensors,. EFOCS, or fiber optic evanescent wave sensors, FEWS, hold promise for use in.
[46]
Evanescent wave absorption in U-bent fiber optic sensors—a ray ...
The evanescent wave-based absorbance (EWA) phenomenon in optical fibers allows real-time monitoring of the binding of chromophoric chemicals [8–10], biological ...
[47]
Review of Optical Fiber Optofluidic Chemical Sensors and Biosensors
Nov 23, 2023 · One common feature of these optical fibers is that they have a strong evanescent field in the light transmission and can interact with the ...
[48]
A fiber optic pH sensor with dye doped multi-layer sol-gel coatings
It makes use of at least one indicator dye, HI, to induce pH sensitive changes in absorption spectrum or colour. An indicator dye dissolved in a solution has ...
[49]
[PDF] Lightweight Fiber Optic Gas Sensor for Monitoring Regenerative ...
Dec 18, 1995 · Although the number of peaks remains the same in the 850 nm to 1400 nm region, the relative intensities of the peaks changed when the ...
[50]
[PDF] A Comprehensive Review on Optical Waveguide and Fiber-Based ...
Nov 17, 2022 · Evanescent field absorption-based gas sensors are only effective when the gas being measured exhibits the characteristic absorption line at the ...
[51]
Optical fiber sensors for water and air quality monitoring: a review
Mar 1, 2024 · high sensitivity, and fast response time, optical fiber sensors (OFSs) are widely used ... limit of detection was 0.001 ppm. Nazari et al.82 ...
[52]
Multi-Parameter Optical Fiber Sensing of Gaseous Ammonia and ...
The limit of detection and response time for CO2 and NH3 are 637 ppm and 0.15 ppm, 86 s and 83 s, respectively.
[53]
[PDF] Lab-on-Fiber Technology: a New Avenue for Optical Nanosensors
This paper reviews the strategies, the main achievements and related devices in the lab-on-fiber roadmap discussing perspectives and challenges that lie ahead.
[54]
[PDF] Review of Optical Fiber Optofluidic Chemical Sensors and Biosensors
Nov 23, 2023 · COFs emerge as a promising platform for optofluidic sensing applications, offering unique advantages in terms of the light-matter interaction,.
[55]
Recent Development in Optical Fiber Biosensors - PMC
Fiber optic biosensors are analytical devices in which a fiber optic device serves as a transduction element. The usual aim is to produce a signal that is ...
[56]
Optical fiber biosensors toward in vivo detection. - ScienceDirect
May 1, 2024 · In particular, we focus on optical fiber biosensors based on fluorescence, evanescent wave, plasmonics, interferometry, and Raman phenomenon.
[57]
Rapid and universal detection of SARS‐CoV‐2 and influenza ... - NIH
Accordingly, we developed a reusable dual‐channel optical fiber immunosensor (DOFIS), which utilized the evanescent wave‐sensing properties and tandem detection ...
[58]
SARS-CoV-2 Detection Using Optical Fiber Based Sensor Method
A novel plastic optical fiber (POF) U-shaped probe sensing method is presented for accurate detection of SARS-CoV-2, commonly known as the COVID-19 virus.
[59]
Fiber Optic Sensors: A Review for Glucose Measurement - PMC - NIH
Tapered plasmonic fiber optic coated with gold film for increased ... enzymatic glucose detection based on phenylboronic acid modified optical fibers.
[60]
Low-cost optical fiber multimode interference biosensor based on a ...
Here, we propose the glucose fiber sensors study coated with a thin film fabricated with PAH and GOD or called (PAH/GOD) via the LbL method; representing an ...
[61]
Tapered Optical Fibre Sensors: Current Trends and Future ... - MDPI
This review focuses on the use of tapered optical fibres as an environmental sensing platform. Tapered fibres allow access to the evanescent wave of the ...
[62]
Sensitive and Specific Protein Sensing Using Single-Mode Tapered ...
We examine and demonstrate a biosensor using single-mode tapered fiber that has been immobilized with biorecognition molecules to sense targeted proteins.<|separator|>
[63]
Challenges in Adapting Fibre Optic Sensors for Biomedical ... - NIH
Fibre optic sensors (FOSs) have developed as a transformative technology in healthcare, often offering unparalleled accuracy and sensitivity in monitoring ...
[64]
Research on distributed monitoring system of intelligent highway ...
This research proposes a system architecture consisting of weak fiber Bragg grating (wFBG), data link, edge computing region and client interface.<|control11|><|separator|>
[65]
Optical Fiber-Based Structural Health Monitoring - MDPI
Tunnels benefit from BOTDA's ability to capture distributed strain under geological stress, while bridges leverage FBG arrays for real-time load monitoring.
[66]
Recent Progress of Fiber-Optic Sensors for the Structural Health ...
Therefore, the multiplexed quasi-distributed FBG sensor is more suitable for monitoring crucial parts of large structures (such as pipelines, bridges, and dams) ...
[67]
Fiber Bragg grating (FBG)-based sensors: a review of technology ...
Dec 24, 2024 · FBG sensors are optical fiber sensors used in civil engineering for structural health monitoring, with high sensitivity and low cost, and are ...
[68]
Technologies - Precise Downhole Solutions
The RFT sensor is offered in various temperature and pressure ranges up to 315°C (600°F) and 150 MPa (21,750 psi). Specifically designed for thermal ...
[69]
Fiber-Optic Pressure Sensors: Recent Advances in Sensing ...
Oct 14, 2025 · [36] proposed a fiber Bragg grating temperature and pressure sensor based on a carbon fiber tube, which can work stably in 0~150 °C and 0~80 MPa ...
[70]
Optical Fibre-Based Sensors for Oil and Gas Applications - MDPI
This review paper covers a detailed review of different fibre-optic sensing technologies to identify a feasible sensing solution for the O&G industry.<|control11|><|separator|>
[71]
Optical fiber distributed acoustic sensing for the leakage diagnosis ...
Among all leak detection methodologies, acoustic signal-based approaches, characterized by their non-destructive, high accuracy, and efficiency, remain the most ...
[72]
Pipeline Micro Leakage Detection via Fiber Optic Acoustic Sensing ...
Fiber-optic distributed acoustic sensing (DAS) technology offers highly sensitive long-distance monitoring. This study evaluates DAS for detecting pinhole gas ...<|separator|>
[73]
Optical fiber sensors for water and air quality monitoring: a review
Optical fiber-sensing technology enables the monitoring of physical, chemical, biological, and other parameters by utilizing the propagation properties of light ...
[74]
Recent advances in fiber-optic evanescent wave sensors for ...
Fiber-optic evanescent wave (FOEW) sensors are promising in pollutant detection and evaluation of water quality because of their high resistance to corrosion.
[75]
Underwater Adhesives Retrofit Pipelines with Advanced Sensors
The company_s underwater adhesives allow it to retrofit older deepwater systems in order to measure pressure, temperature, strain, and flow properties.
[76]
Fiber-Optic Sensors: Playing Both Sides ... - Optics & Photonics News
They can be used as a tool to identify and classify critical inflows and other events over time; to monitor reservoir response to process changes; and to ...
[77]
Optical Fiber Sensors: High Resolution Fiber Optic Sensing
Choose between 8 simultaneously monitored fibers with over 2,000 equally spaced sensors per fiber in 100 hz mode, or 4 fibers in 200 hz mode; Software ...
[78]
Roles of Optical Fiber Sensors in the Internet of Things
Aug 4, 2025 · This study explores the roles, opportunities, and challenges of optical fiber sensors in Internet of Things adding specific properties like sensitivity and ...
[79]
Recent Advances in Biomedical Photonic Sensors - PubMed Central
In 2019, a flexible miniaturized optrode was reported using multicore optical fibres and novel chemical probes to sense pH and oxygen [79]. The sensing ...
[80]
Evaluation of endoscopic visible light spectroscopy: comparison with ...
Feb 28, 2019 · Visible light spectroscopy (VLS) is a technique used to measure the mucosal oxygen saturation during upper gastrointestinal endoscopy to ...
[81]
Challenges in Adapting Fiber Optic Sensors for Biomedical ...
Apr 4, 2025 · One significant application of FOSs is in continuous glucose monitoring (CGM), particularly for diabetes management. These sensors can measure ...
[82]
Precision-Microfabricated Fiber-Optic Probe for Intravascular Pressure and Temperature Sensing
**Summary of Catheter-Tip Fiber-Optic Pressure Sensors for Cardiovascular Procedures:**
[83]
Multimodal neural probes for combined optogenetics and ...
During optogenetic stimulation, visible light is emitted from the optical fiber in a conical pattern, then scattered and absorbed as it passes through the ...Missing: 2020s | Show results with:2020s
[84]
Challenges in Adapting Fibre Optic Sensors for Biomedical ... - MDPI
FDA (USA), FDA Medical Device Approval Process, Safety, efficacy, and reliability assessments ensure FOSs meet regulatory requirements before market approval.
[85]
Emerging Technologies for Fiber-Optic-Based Sensors in ...
Oct 16, 2024 · An extensive review on recent developments in seven fiber-optic-based sensing methodologies, namely, grating-based sensors, fiber-optic plasmonic sensors ( ...
[86]
Fiber Optic Sensors for Structural Health Monitoring of Air Platforms
Mar 25, 2011 · The resolution is dependent on the wavelength interrogator, which is currently up to 1 pm, corresponding to 1 µɛ for strain measurement and 0.1 ...
[87]
[PDF] Fiber Optic Sensing System (FOSS) Technology
• Multiplex 100s of sensors onto one optical fiber. • Fiber gratings are ... – 1000's of sensors on a single fiber (up to 80 feet per fiber). – No copper ...
[88]
https://www.mdpi.com/2079-6374/15/5/312
[89]
An Analysis of Cost-Effectiveness of Optical Technology - HBM
Optical technology can actually be cost-effective. Optical sensors, in particular FBG sensors, are also a good choice in terms of cost-effectiveness.