High temperature fiber sensor with high sensitivity based on core diameter mismatch

Sensitivity-enhanced strain sensor based on a shape-modulated multimode fiber

In this Letter, we demonstrate a sensitivity-enhanced strain sensor based on a shape-modulated multimode fiber (MMF). In contrast to conventional single-mode-multimode-single-mode (SMS) fiber structures, which typically contain a single cylindrical homogeneous MMF section, the shape of the MMF section in this investigation is modulated by lateral offset splicing of multiple MMF segments. Simulation results show that the designed shape-modulated MMF has a higher peak mechanical strain than that of a cylindrical MMF. Experimental results demonstrate that the strain sensitivity achieved by the shaped-modulated MMF-formed SMS fiber structure is as high as -55.63 pm/µε, which is 33 times higher than that for a cylindrical MMF-formed conventional SMS fiber structure at -1.65 pm/µε. This high sensitivity and low-fabrication cost SMS fiber sensor has the potential to be a promising candidate in precise strain measurement applications.

Highly-sensitive temperature sensor based on photopolymerized-waveguide embedded Mach-Zehnder interferometer

Biology, medicine, and chemistry all rely heavily on highly sensitive optical fiber temperature sensors. To the best of our knowledge, this research introduces a unique design framework for high-performance fiber temperature sensors that helps eliminate the all-fiber interferometers' sensitivity bottleneck. A section of photopolymerized waveguide is embedded in a typical Mach-Zehnder interferomenter framework with multimode fiber-single mode fiber-multimode fiber (MSM) structure. The thermal-optical coefficient (TOC) of the photopolymerized waveguide core, which is created via the fiber-end lithography technique, differs dramatically from that of the resin cladding. Due to the considerable TOC difference, the phase difference between the interfering beams significantly increases as the temperature changes. The fundamental variables affecting temperature sensitivity are conceptually explored and experimentally verified. The suggested device achieves a typical temperature sensitivity of 1.15 nm/ ∘C in the range of 30-100 ∘C, which is about 10 times as high as that of the all-fiber MSM sensors. The suggested designing framework offers a fresh thought for creating high-performing fiber optic temperature sensors.

Ultra-sensitive temperature and pressure sensor based on PDMS-based FPI and Vernier effect

An ultra-sensitive sensor, based on two Fabry-Perot interferometers (FPIs), has been realized for temperature and pressure sensing. A polydimethylsiloxane (PDMS)-based FPI₁ was used as a sensing cavity, and a closed capillary-based FPI₂ was used as a reference cavity for its insensitivity to both temperature and pressure. The two FPIs were connected in series to obtain a cascaded FPIs sensor, showing a clear spectral envelope. The temperature and pressure sensitivities of the proposed sensor reach up to 16.51 nm/°C and 100.18 nm/MPa, which are 25.4 and 21.6 times, respectively, larger than these of the PDMS-based FPI₁, showing a great Vernier effect.

Modal interference discrepancy and its application to a modified fiber Mach-Zehnder Vernier interferometer

In this paper, modal interference discrepancy in an all-fiber MZI is theoretically analyzed and experimentally verified. Theoretical analysis demonstrates that ambient refractive index (RI) response of core-cladding modal interference in an all-fiber MZI is blue-shift, while that of cladding-cladding modal interference is red-shift. Temperature response trends of the two kinds of modal interference are uniformly red-shift. The discrepancy is used to fabricate an improved Vernier sensor which is cascaded by two unit MZIs. One MZI is slightly core-offset fused to obtain core-cladding modal interference, and the other is obviously offset fused to get cladding-cladding modal interference. Ambient RI sensitivity of the cascaded sensor is improved with temperature cross-talk restrained. Ambient RI responses of the two unit MZIs are measured to be opposite, which are -54.009 nm/RIU (within RI range of 1.3362∼1.3811) for the slight and 142.581 nm/RIU for the obvious offset unit MZI. While, temperature response trends of them are consistent, which are 0.042 nm/°C for the slight and 0.025 nm/°C for the obvious offset unit MZI, respectively. For the cascaded Vernier sensor ambient RI sensitivity reaches -1788.160 nm/RIU, which is 33.1 and 12.5 folds improved over the two unit MZIs, respectively. Temperature sensitivity of the cascaded sensor is as low as 0.167 nm/°C and only causes a slight RI error of 9.339 × 10^-5 RIU/°C. Due to the simple structure, ease of fabrication, and low temperature cross-talk, the modal interference discrepancy-based Vernier sensor is believed to have potential application prospects in biochemical sensing fields.

Modal dispersion characteristics of LP0m modes in a step-index multimode fiber

A modal interferometer method (MIM) is applied to measure the differential mode delay (DMD) between the L P _0m modes traversing a step-index multimode fiber (SI-MMF). Only linearly polarized radial modes, i.e., L P _0m modes, are excited and transmitted in the SI-MMF by using a single-mode-multimode-single-mode (SMS) fiber structure. The measurement principle is based on investigating a transmitted spectrum through temporal decomposition by means of a Fourier transform. The Fourier-transform-based MIM provides simultaneous measurements of the DMD between the L P _0m modes. The wavelength dependence of the DMD is estimated experimentally in both the 1260-1360 nm and 1450-1625 nm telecommunication bands. The normalized frequency dependence of the DMD is also investigated theoretically. The result suggests that the 1260-1360 nm band is preferable to the 1450-1625 nm band for a mode-division multiplexing (MDM) transmission employing an SI-MMF in terms of realizing a smaller DMD.

Fiber structures and material science in optical fiber magnetic field sensors

Magnetic field sensing plays an important role in many fields of scientific research and engineering applications. Benefiting from the advantages of optical fibers, the optical fiber-based magnetic field sensors demonstrate characteristics of light weight, small size, remote controllability, reliable security, and wide dynamic ranges. This paper provides an overview of the basic principles, development, and applications of optical fiber magnetic field sensors. The sensing mechanisms of fiber grating, interferometric and evanescent field fiber are discussed in detail. Magnetic fluid materials, magneto-strictive materials, and magneto-optical materials used in optical fiber sensing systems are also introduced. The applications of optical fiber magnetic field sensors as current sensors, geomagnetic monitoring, and quasi-distributed magnetic sensors are presented. In addition, challenges and future development directions are analyzed.

Optical Fiber Sensors for High-Temperature Monitoring: A Review

High-temperature measurements above 1000 °C are critical in harsh environments such as aerospace, metallurgy, fossil fuel, and power production. Fiber-optic high-temperature sensors are gradually replacing traditional electronic sensors due to their small size, resistance to electromagnetic interference, remote detection, multiplexing, and distributed measurement advantages. This paper reviews the sensing principle, structural design, and temperature measurement performance of fiber-optic high-temperature sensors, as well as recent significant progress in the transition of sensing solutions from glass to crystal fiber. Finally, future prospects and challenges in developing fiber-optic high-temperature sensors are also discussed.

High Sensitivity Fiber Interferometric Strain Sensors Based on Elongated Fiber Abrupt Tapers

We demonstrate high-sensitivity fiber strain sensors based on an elongated abrupt taper. The fiber abrupt taper, with a tapered diameter ranging from 40–60 μm, was made by using a hydrogen microflame to break the waveguide adiabaticity so as to convert the fundamental mode into cladding modes. The abrupt taper was further uniformly tapered by using a normal moving flame with a torch diameter of 7 mm to elongate the tapered region until the tapered diameter was down to 2.5–5 μm. The excited high-order modes were confined to propagate along the cladding and then recombined at the rear edge of the fiber taper to produce interferences with extinction ratios of up to 16 dB. The tapered region was pulled outwardly to change the optical path difference (OPD) between modes to measure the tensile strain with all the interfering wavelengths blue-shifted. The measured best strain sensitivity was 116.21 pm/με and the coefficient of determination R2 of linear fitting exhibits high linearity. This strain sensor based on elongated abrupt taper is several times higher than that of most of the fiber strain sensors ever reported.

A Review of Sensitivity Enhancement in Interferometer-Based Fiber Sensors

Optical fiber sensors based on an interferometer structure play a significant role in monitoring physical, chemical, and biological parameters in natural environments. However, sensors with high-sensitivity measurement still present their own challenges. This paper deduces and summarizes the methods of sensitivity enhancement in interferometer based fiber optical sensors, including the derivation of the sensing principles, key characteristics, and recently-reported applications.The modal coupling interferometer is taken as an example to derive the five terms related to the sensitivity: (1) the wavelength-dependent difference of phase between two modes/arms ∂ϕd/∂λ, (2) the sensor length Lw,A, (3) refractive index difference between two modes/arms Δneff,A, (4) sensing parameter dependent length change α, and (5) sensing parameter dependent refractive index change γ. The research papers in the literature that modulate these terms to enhance the sensing sensitivity are reviewed in the paper.

Hydroxyethyl cellulose sensitized SMDMS structure with optical fiber relative humidity and temperature simultaneous measurement sensor

We have manufactured an intensity modulated optical fiber SMDMS sensor with hydroxyethyl cellulose (HEC) hydrogel coating for simultaneous measurement of RH and temperature. The SMDMS sensor was manufactured by splicing single-mode fiber (SMF), multi-mode fiber (MMF), dispersion compensation fiber (DCF), MMF, and SMF in sequence to form a structure of SMF + MMF + DCF + MMF + SMF (SMDMS). The cladding of MMFs and DCF were corroded by hydrofluoric acid (HF) and coated with HEC hydrogel to excite a strong evanescent field and increase the sensitivity of the SMDMS sensor. The adsorption of water molecules by HEC will cause a change in the effective refractive index of cladding mode, which will eventually change the intensity of the transmission spectrum. The experimental results indicate that the sensitivities are 0.507 dB/%RH and 0.345 dB/°C in the RH range of 30%-80% and temperature range of 10°C-50°C, respectively. At last, a dual-parameter measurement matrix is constructed based on the experimental results to achieve the simultaneous measurement of RH and temperature. The SMDMS sensor has the advantages of high sensitivity and good robustness, and has potential application prospects in daily life and other fields.

Temperature Sensor Based on Periodically Tapered Optical Fibers

In this paper, the fabrication and characterization of a temperature sensor based on periodically tapered optical fibers (PTOF) are presented. The relation between the geometry of the sensors and sensing ability was investigated in order to find the relatively simple structure of a sensor. Four types of PTOF structures with two, four, six and eight waists were manufactured with the fusion splicer. For each PTOF type, the theoretical free spectral range (FSR) was calculated and compared with measurements. The experiments were conducted for a temperature range of 20–70 °C. The results proved that the number of the tapered regions in PTOF is crucial, because some of the investigated structures did not exhibit the temperature response. The interference occurring inside the structures with two and four waists was found be too weak and, therefore, the transmission dip was hardly visible. We proved that sensors with a low number of tapered regions cannot be considered as a temperature sensor. Sufficiently more valuable results were obtained for the last two types of PTOF, where the sensor’s sensitivity was equal to 0.07 dB/°C with an excellent linear fitting (R² > 0.99). The transmission dip shift can be described by a linear function (R² > 0.97) with a slope α > 0.39 nm/°C.

Sensitivity-enhanced temperature sensor based on Mach-Zehnder interferometer coated with thermal-sensitive material

In this paper, a Mach-Zehnder interferometer temperature sensor with a single-mode fiber-thin-core fiber-single-mode fiber cascaded is designed. When the light is transmitted from the single-mode fiber to the thin-core fiber, different modes will be excited because of the mode-field mismatch. Optical power from the input fiber can be partly coupled to the cladding modes of the thin-core fiber, and the cladding modes of the thin-core fiber then re-coupled to the lead-out fiber, which constitutes the Mach-Zehnder interferometer in the output single-mode fiber. To improve the temperature sensitivity of the sensor, we coated the thermal-sensitive material (such as polymethoxane or ultraviolet glue) on the surface of the thin-core fiber. The experimental results show that the temperature sensitivity of the sensor coated with a polydimethylsiloxane film was increased from 32.0 to 90.0 pm/°C, about three times, compared to the uncoated thin-core fiber sensor. When the surface of the thin-core fiber is coated with an ultraviolet glue film, the temperature sensitivity of the sensor was increased from 32.0 to 166.8 pm/°C, about six times. The designed sensor, with compact structure, simple production, strong robustness, and high sensitivity, has a wide application prospect in industrial production and national defense technology.

Simply Fabricated Inexpensive Dual-Polymer-Coated Fabry-Perot Interferometer-Based Temperature Sensors with High Sensitivity

We designed simply fabricated, highly sensitive, and cost-effective dual-polymer-coated Fabry–Perot interferometer (DFPI)-based temperature sensors by employing thermosensitive polymers and non-thermosensitive polymers, as well as different two successive dip-coating techniques (stepwise dip coating and polymer mixture coating). Seven sensors were fabricated using different polymer combinations for performance optimization. The experiments demonstrated that the stepwise dip-coated dual thermosensitive polymer sensors exhibited the highest sensitivity (2142.5 pm °C⁻¹ for poly(methyl methacrylate)-polycarbonate (PMMA_PC) and 785.5 pm °C⁻¹ for poly(methyl methacrylate)- polystyrene (PMMA_PS)). Conversely, the polymer-mixture-coated sensors yielded low sensitivities (339.5 pm °C⁻¹ for the poly(methyl methacrylate)-polycarbonate mixture (PMMA_PC mixture) and 233.5 pm °C⁻¹ for the poly(methyl methacrylate)-polystyrene mixture (PMMA_PS mixture). Thus, the coating method, polymer selection, and thin air-bubble-free coating are crucial for high-sensitivity DFPI-based sensors. Furthermore, the DFPI-based sensors yielded stable readouts, based on three measurements. Our comprehensive results confirm the effectiveness, reproducibility, stability, fast response, feasibility, and accuracy of temperature measurements using the proposed sensors. The excellent performance and simplicity of our proposed sensors are promising for biomedical, biochemical, and physical applications.

High-sensitivity transverse-load and high-temperature sensor based on the cascaded Vernier effect

In this paper, we demonstrate a novel, to the best of our knowledge, transverse-load and high-temperature sensor based on the cascaded Vernier effect. Two Fabry-Perot interferometers fabricated by a piece of hollow-core fiber (HCF) and a piece of polarization-maintaining photonic crystal fiber (PM-PCF) are connected by a long part of single-mode fiber with a length of 1 m, and play the roles of transverse-load sensor and high-temperature sensor, respectively. The sensitivity of not only the transverse load but also that of temperature can be enhanced by the Vernier effect. The sensitivity of the transverse load is raised by 7.7 times to 5.84 nm/N, and the temperature sensitivities increased by 5.5 and 5.9 times to -0.0689nm/^∘C and -0.1038nm/^∘C within the temperature range of 50-400°C to 400-900°C. Moreover, both the HCF cavity and PM-PCF cavity can be split and combined flexibly. Hence, such a sensor could have great potential in sensing applications.

Single core-offset Mach-Zehnder interferometer coated with PVA for simultaneous measurement of relative humidity and temperature

A single core-offset Mach-Zehnder interferometer (MZI) coated with polyvinyl alcohol (PVA) for simultaneous measurement of relative humidity (RH) and temperature is proposed in this paper. The sensing structure is fabricated by splicing dispersion compensating fiber (DCF) and no-core fiber (NCF) and splicing two single-mode fibers (SMF) at both ends, where the core-offset is located at the splicing of SMF and DCF. A part of the cladding of DCF is etched to excite the high-order cladding mode (LP₁₀), and PVA is coated on the etched area. The refractive index of PVA varies due to the adsorption of water molecules. Therefore, when the ambient relative humidity and temperature change, the change of MZI phase difference causes the wavelength of the resonant dip to shift. The experimental results indicate that the proposed sensor has a sensitivity of 0.256 nm/RH% for RH range of 30%-95%, and a sensitivity of 0.153 nm/℃ for temperature range of 20℃-80℃, respectively. The simultaneous measurement of RH and temperature can be achieved by demodulating the sensitivity coefficient matrix. The proposed sensor has the characteristics of good repeatability, high sensitivity, and good stability, which make it potentially applications for the detection of RH and temperature measurement.

Ammonia Gas Sensor Based on Graphene Oxide-Coated Mach-Zehnder Interferometer with Hybrid Fiber Structure

A graphene oxide-coated in-fiber Mach-Zehnder interferometer (MZI) formed with a multimode fiber-thin core fiber-multimode fiber (MMF-TCF-MMF) is proposed and experimentally demonstrated for ammonia gas (NH₃) sensing. The MZI structure is composed of two segments of MMF of length 2 mm, with a flame-tapered TCF between them as the sensing arm. The MMFs act as mode couplers to split and recombine light owing to the core diameter mismatch with the other fibers. A tapered TCF is formed by the flame melting taper method, resulting in evanescent wave leakage. A layer of graphene oxide (GO) is applied to the tapered region of the TCF to achieve gas adsorption. The sensor operates on the principle of changing the effective refractive index of the cladding mode of a fiber through changing the conductivity of the GO coating by adsorbed NH₃ molecules, which gives rise to a phase shift and shows as the resonant dip shifts in the transmission spectrum. So the concentration of the ammonia gas can be obtained by measuring the dip shift. A wavelength-shift sensitivity of 4.97 pm/ppm with a linear fit coefficient of 98.9% is achieved for ammonia gas concentrations in the range of 0 to 151 ppm. In addition, we performed a repetitive dynamic response test on the sensor by charging/releasing NH₃ at concentration of 200 ppm and a relative humidity test in a relative humidity range of 35% to 70%, which demonstrates the reusability and stability of the sensor.

Iterative normalized cross-correlation method for absolute optical path difference demodulation of dual interferometers

It is still a challenge to realize the absolute optical path difference (OPD) demodulation of multi-interference systems with a narrow spectral interval and small OPD interval. In this paper, an iterative normalized cross-correlation algorithm is firstly proposed for demodulating the multiple absolute OPDs of a dual-interference system and applied to optical fiber sensing system. By constructing a template function in combined form, the optimal solutions of its components and OPDs are solved iteratively based on the reconstruction matrix method and cross-correlation algorithm, respectively. The simulation and experiment show that the demodulation accuracies near the OPDs of 560 µm and 660 µm are both up to 5 nm in different spectral intervals from 45 to 80 nm. The simulation results show that all demodulation precisions at the spectral interval of 55 nm do not exceed 4 nm when the OPD changes in the range of 650-670 µm. Besides, the experimental verification shows the temperature accuracy (0.125 °C) with 95% confidence of T-distribution is very close to the control accuracy (0.1 °C). The proposed algorithm can improve the multiplexing capability of optical fiber sensor system and reduce its cost.

Ultra-compact silicon-microcap based improved Michelson interferometer high-temperature sensor

An ultra-short high-temperature fiber-optic sensor based on a silicon-microcap created by a single-mode fiber (SMF) and simple fusion splicing technology is proposed and experimentally demonstrated. A section of the SMF with a silicon-microcap at one end is connected to the "peanut" structure to build the microcap-based optical fiber improved Michelson interferometer (MI). The optimal discharge parameters of microcap and length of SMF has been investigated to achieve the best extinction ratio of 6.61 dB. The size of this microcap-based improved MI sensor is 560 µm and about 18 times shorter compared to the current fiber tip interferometers (about 10 mm). Meanwhile, it showed good robustness during the two heating-cooling cycles and the duration period stability test at 900 °C. This microcap-based improved MI sensor with the smaller size, simple fabrication, low cost, high reliability, and good linearity within a large dynamic range is beneficial to practical temperature measurement and massive production.

Hollow-Core Fiber-Tip Interferometric High-Temperature Sensor Operating at 1100 °C with High Linearity

Over decades, fiber-optic temperature sensors based on conventional single-mode fibers (SMF) have been demonstrated with either high linearity and stability in a limited temperature region or poor linearity and thermal hysteresis in a high-temperature measurement range. For high-temperature measurements, isothermal annealing is typically necessary for the fiber-optic sensors, aiming at releasing the residual stress, eliminating the thermal hysteresis and, thus, improving the high-temperature measurement linearity and stability. In this article, an annealing-free fiber-optic high-temperature (1100 °C) sensor based on a diaphragm-free hollow-core fiber (HCF) Fabry-Perot interferometer (FPI) is proposed and experimentally demonstrated. The proposed sensor exhibits an excellent thermal stability and linearity (R² > 0.99 in a 100-1100 °C range) without the need for high-temperature annealing. The proposed sensor is extremely simple in preparation, and the annealing-free property can reduce the cost of sensor production significantly, which is promising in mass production and industry applications.

Vibration Sensor Based on Hollow Biconical Fiber

A hollow biconical fiber is proposed and experimentally demonstrated for vibration sensing. It is fabricated by creating an air micro-cavity in single-mode fiber, followed by tapering it. Experimental results show that the device is highly sensitive to bending with a sensitivity of 21.30 dB/m⁻¹. When it is exposed to vibration, its transmission loss is modulated periodically, then based on the measured transmission, the vibration frequency can be demodulated accurately. The acoustic vibration testing results show that the proposed device can detect and demodulate the exciting acoustic frequency accurately and distinguish its sound intensity, and the maximum signal to noise ratio (SNR) achieves up to 59 dB. Moreover, cantilever beam testing proves its performance reliable. Additionally, the sensing head has the advantages of a lightweight, compact size (with a total length of less than 250 μm), and insensitivity of temperature. All these features indicate the proposed sensor has a promising potential in the engineering field.

Dynamic Characterisation of Fibre-Optic Temperature Sensors for Physiological Monitoring

Fast, miniature temperature sensors are required for various biomedical applications. Fibre-optics are particularly suited to minimally invasive procedures, and many types of fibre-optic temperature sensors have been demonstrated. In applications where rapidly varying temperatures are present, a fast and well-known response time is important; however, in many cases, the dynamic behaviour of the sensor is not well-known. In this article, we investigate the dynamic response of a polymer-based interferometric temperature sensor, using both an experimental technique employing optical heating with a pulsed laser, and a computational heat transfer model based on the finite element method. Our results show that the sensor has a time constant on the order of milliseconds and a -6 dB bandwidth of up to 178 Hz, indicating its suitability for applications such as flow measurement by thermal techniques, photothermal spectroscopy, and monitoring of thermal treatments.

Sensitivity enhanced temperature sensor with serial tapered two-mode fibers based on the Vernier effect

A sensitivity enhanced temperature sensor with cascaded tapered two-mode fibers (TTMFs) based on the Vernier effect is proposed and experimentally demonstrated. It is confirmed that series connection exhibits higher extinction ratio than parallel one both by theory and experiments, which provides guidance for related experiments. In experiments, two TTMFs have the same single-mode fiber-TTMF-single-mode fiber configuration, while the free spectral ranges (FSRs) are chosen with slightly difference by modifying the parameters in the tapering process. Experimental results show that the proposed temperature sensor possesses sensitivity of -3.348 nm/°C in temperature measurement range from 25 °C to 60°C, 11.3 times sensitivity enhancement in comparison with single TTMF. Benefiting from advantages of high temperature sensitivity, simplicity of manufacture and long distance sensing, this novel sensitivity enhanced temperature sensor can be applied to various particular fields, such as oil wells, coal mines and so on.

UV Sensor Based on Fiber Bragg Grating Covered with Graphene Oxide Embedded in Composite Materials

Polymer-matrix composites degrade under the influence of UV radiation in the range of the 290-400 nm band. The degradation of polymer-matrix composites exposed to UV radiation is characterized by extensive aging of the epoxy matrix, resulting in deterioration of their mechanical properties. Glass fibers/epoxy resin composites were made by an out-of-autoclave method whereas a fiber optic sensor was placed between different layers of laminates. In our work, we used a fiber Bragg grating sensor covered with graphene oxide and embedded in a polymer matrix composite to monitor UV radiation intensity. Measurements of UV radiation may allow monitoring the aging process of individual components of the polymer composite. In order to estimate the number of microcracks of epoxy resin, microstructure observations were carried out using a scanning electron microscope.

In-situ dual-channel surface plasmon resonance fiber sensor for temperature-compensated detection of glucose concentration

External temperature variations inevitably affect the accuracy of surface plasmon resonance (SPR) biosensors. To that end, we propose an ultra-compact label-free dual-channel SPR fiber sensor (DSPRFS) that can simultaneously measure the glucose concentration and ambient temperature in real-time. The proposed sensor is based on a unique dual-channel structure fabricated by etching a side-hole fiber (SHF), and has significantly higher spatial sensitivity than traditional SPR biosensors. After coating with silver and zinc oxide films, one channel was filled with polydimethylsiloxane (PDMS) to sense the ambient temperature, and the other channel was immobilized with glucose oxidase (GOx) enzyme for glucose sensing. The proposed sensor is analyzed theoretically, fabricated and characterized. Glucose concentration sensitivity and temperature sensitivity of the manufactured sensor sample were found to be as high as 6.156 nm/mMand -1.604 nm/°C with limits of detection (LOD) of 16.24 µM and 0.06 °C, respectively. The proposed sensor has an extremely compact structure, enables temperature compensation, and is suitable for in-situ monitoring and high-precision sensing of glucose and other biological analytes.

A Low-Cost High-Temperature Sensor Based on Long-Period Fiber/Microfiber Gratings by Local Fictive Temperature Modification

A high temperature-sensitive long-period fiber grating (LPFG) sensor fabricated by the local fictive temperature modification is proposed and demonstrated. High-frequency CO2 laser pulses scan standard single-mode fiber (SMF), and the modification zones extended to the core of SMF. Experimental results demonstrate that the LPFG temperature sensors with 600 μm grating period and 32 period numbers offer the average sensitivity of 0.084 nm/C in the temperature range of room temperature (RM) to 875°C. The LPFGs fabricated here show exponential change in terms of the spectral wavelength shift versus changes in temperature. In addition, the maximum temperature sensitivity of 0.37 nm/C is achieved by employing long-period microfiber grating (LPMFG), fabricated by the microheater brushing technique and the local fictive temperature modification. LPMFG sensor exhibits better temperature characteristics due to a thinner diameter.

Residual thickness enhanced core-removed D-shaped single-mode fiber and its application for VOC evaporation monitoring

A core-removed D-shaped structure with different residual thickness (RT) was manufactured on a single mode silica fiber (SMF) to enhance the sensitivity by using of ultra-precise polishing technology. With six different RTs ranging from ∼55 µm to ∼28 µm, the RT enhancement effect in a D-shaped SMF was researched in detail. The influence of the RT on its transmission spectra was investigated both theoretically and experimentally. Considering a compromise between the multimode interference efficiency and optical power loss, an optimum RT value of 34.09 µm was achieved. The obtained refractive index (RI) sensitivity was 10243 nm/RIU in the RI range of 1.430-1.444, corresponding to a RI resolution of 1.9×10^-6 RIU. A high-performance all-fiber sensor was developed to monitor the evaporation process volatile organic compounds (VOCs) based on the RT-enhanced D-shaped SMF. As proof of concept, a 2-hour continuous monitoring was carried to monitor the chloroform and alcohol mixture. As a result, the evaporation of alcohol and chloroform were clearly identified and monitored. The developed RT-enhanced D-shaped fiber sensor provides an alternative way for chemical process monitoring and industrial applications.

Stimuli-Responsive Microarray Films for Real-Time Sensing of Surrounding Media, Temperature, and Solution Properties via Diffraction Patterns

Stimuli-responsive polymers have attracted increasing attention over the years due to their ability to alter physiochemical properties upon external stimuli. However, many stimuli-responsive polymer-based sensors require specialized and expensive equipment, which limits their applications. Here an inexpensive and portable sensing platform of novel microarray films made of stimuli-responsive polymers is introduced for the real-time sensing of various environmental changes. When illuminated by laser light, microarray films generate diffraction patterns that can reflect and magnify variations of the periodical microstructure induced by surrounding invisible parameters in real time. Stimuli-responsive polyelectrolyte complexes are structured into micropillar arrays to monitor the pH variation and the presence of calcium ions based on reversible swelling/shrinking behaviors of the polymers. A pH hysteretic effect of the selected polyelectrolyte pair is determined and explained. Furthermore, polycaprolactone microchamber arrays are fabricated and display a thermal-driven structural change, which is exploited for photonic threshold temperature detection. Experimentally observed diffraction patterns are additionally compared with rigorous coupled-wave analysis simulations that prove that induced diffraction pattern alterations are solely caused by geometrical microstructure changes. Microarray-based diffraction patterns are a novel sensing platform with versatile sensing capabilities that will likely pave the way for the use of microarray structures as photonic sensors.

All-optical intensity fluctuation magnification using Kerr effect

We present a new all-optical method for the magnification of small-intensity fluctuations using the nonlinear Kerr effect. A fluctuation of interest is impressed onto a sinusoidally modulated optical signals (SMOS) and spectral sidebands are generated as the SMOS experiences self-phase modulation in a nonlinear medium. Magnification of these temporal variation is obtained by filtering one of the sidebands. For small fluctuations, the amount of magnification obtained is proportional to (2m + 1), with m being the sideband order. This technique enhances fiber-based point sensor capabilities by bringing signals originally too small to be detected into the detection range of photodetectors.

Enhancing Temperature Sensitivity of the Fabry-Perot Interferometer Sensor with Optimization of the Coating Thickness of Polystyrene

The exploration of novel polymers for temperature sensing with high sensitivity has attracted tremendous research interest. Hence, we report a polystyrene-coated optical fiber temperature sensor with high sensitivity. To enhance the temperature sensitivity, flat, thin, smooth, and air bubble-free polystyrene was coated on the edge surface of a single-mode optical fiber, where the coating thickness was varied based on the solution concentration. Three thicknesses of the polystyrene layer were obtained as 2.0, 4.1, and 8.0 μm. The temperature sensor with 2.0 μm thick polystyrene exhibited the highest temperature sensitivity of 439.89 pm °C^-1 in the temperature range of 25-100 °C. This could be attributed to the very uniform and thin coating of polystyrene, along with the reasonable coefficient of thermal expansion and thermo-optic coefficient of polystyrene. Overall, the experimental results proved the effectiveness of the proposed polystyrene-coated temperature sensor for accurate temperature measurement.

Broadband single-polarization optical fiber based on surface plasmon resonance

We present, to the best of our knowledge, a new scheme of broadband single-polarization optical fiber with high extinction ratio based on surface plasmon resonance (SPR). The double-hole optical fiber with a Ge-doped core is modified by integrating the stacks of conductive and dielectric layers to support SPR. The strong couplings between the guided modes and surface plasmon mode can bring about serious polarization loss of TM mode, while supporting the efficient transmission of TE polarization in broadband. The achievable extinction ratio can be more than 25 dB covering a wide telecom band of 1.17-1.42 µm in short fiber lengths=2.5mm. Meanwhile, the insertion loss of the fiber is less than 0.25 dB. The modified SPR fiber shows promising application in high-quality fiber-integrated polarizers.

Organic field-effect transistor-based flexible sensors

Flexible transistors are the next generation sensing technology, due to multiparametric analysis, reduced complexity, biocompatibility, lightweight with tunable optoelectronic properties. We summarize multitude of applications realized with OFETs.

Optical fiber temperature sensor based on a Mach-Zehnder interferometer with single-mode-thin-core-single-mode fiber structure

A Mach-Zehnder interferometer for measurement of temperature is proposed and experimentally demonstrated, which consists of two sections of single mode fiber (SMF) and a section of thin core fiber spliced between the two SMFs. The two welding areas are heated and stretched to improve the split and recombination of light. The wavelength of the resonant dip will shift when temperature varies due to the thermo-optic and thermal expansion effect. The experimental results show that a temperature sensitivity of 65 pm/°C with a linear correlation coefficient of 0.996 can be achieved in a temperature range from 25 °C to 80 °C. Due to its ease of manufacture, low cost, and high sensitivity, the fiber optic temperature sensor is suitable for temperature measurement applications.

Addressing Challenges in Fabricating Reflection-Based Fiber Optic Interferometers

Fabrication of multimode fiber optic interferometers requires accurate control of certain parameters to obtain reproducible results. This paper evaluates the consequences of practical challenges in fabricating reflection-based, fiber optic interferometers by the use of theory and experiments. A guided-mode propagation approach is used to investigate the effect of the end-face cleave angle and the accuracy of the splice in core-mismatched fiber optic sensors. Cleave angles from high-end fiber cleavers give differences in optical path lengths approaching the wavelength close to the circumference of the fiber, and the core-mismatched splice decides the ensemble of cladding modes excited. This investigation shows that the cleave angle may significantly alter the spectrum, whereas the splice is more robust. It is found that the interferometric visibility can be decreased by up to 70% for cleave angles typically obtained. An offset splice may reduce the visibility, but for offsets experienced experimentally the effect is negligible. An angled splice is found not to affect the visibility but causes a lower overall intensity in the spectrum. The sensitivity to the interferometer length is estimated to 60 nm/mm, which means that a 17 µm difference in length will shift the spectrum 1 nm. Comparisons to experimental results indicate that the spliced region also plays a significant role in the resulting spectrum.

Highly Sensitive Temperature and Humidity Sensor Based on Carbon Nanotube-Assisted Mismatched Single-Mode Fiber Structure

Here we report on a miniaturized optical interferometer in one fiber based on two mismatched nodes. The all-fiber structure shows stable performance of temperature and humidity sensing. For temperature sensing in large ranges, from 40 to 100 °C, the sensor has a sensitivity of 0.24 dB/°C, and the adjusted R-squared value of fitting result reaches 0.99461 which shows a reliable sensing result. With carbon nanotubes coating the surface of the fiber, the temperature sensitivity is enhanced from 0.24561 to 1.65282 dB/°C in a small region, and the performance of humidity sensing becomes more linear and applicable. The adjusted R-squared value of the linear fitting line for humidity sensing shows a dramatic increase from 0.71731 to 0.92278 after carbon nanotube coating, and the humidity sensitivity presents 0.02571 nm/%RH.

High Temperature (Up to 950 °C) Sensor Based on Micro Taper In-Line Fiber Mach–Zehnder Interferometer

A high temperature (up to 950 °C) sensor was proposed and demonstrated based on a micro taper in-line fiber Mach–Zehnder interferometer (MZI) structure. The fiber MZI structure comprises a single mode fiber (SMF) with two micro tapers along its longitudinal direction. An annealing at 1000 °C was applied to the fiber sensor to stabilize the temperature measurement. The experimental results showed that the sensitivity was 0.114 nm/°C and 0.116 nm/°C for the heating and cooling cycles, respectively, and, after two days, the sensor still had a sensitivity of 0.11 nm/°C, showing a good stability of the sensor. A probe-type fiber MZI was designed by cutting the sandwiched SMF, which has good linear temperature responses of 0.113 nm/°C over a large temperature range from 89 to 950 °C. The probe-type fiber MZI temperature sensor was independent to the surrounding refractive index (RI) and immune to strain. The developed sensor has a wide application prospect in the fields of high temperature hot gas flow, as well as oil and gas field development.

Linear multimode interference fiber temperature sensor using the liquid in glass thermometer principle

A temperature sensor based on a multimode interference thermometer is designed and fabricated. The operation mechanism is based on the thermal expansion of a specific volume of ethylene glycol contained in a glass bulb that is connected to a capillary of the same material, with a no-core fiber (NCF) inserted and centered into the capillary tube. As the temperature is increased, the liquid is expanded, and the NCF is gradually covered by the liquid, resulting in a peak wavelength shift that is correlated to the temperature variations. A sensitivity of 0.4447 nm/°C and highly linear response with an R² of 0.99962 are obtained. The advantage of this configuration is that the sensing temperature range can be adjusted by changing either the inner diameter of the capillary tube or the bulb volume. We can also measure negative temperatures by simply modifying the freezing point of the liquid, which demonstrates the viability of the sensor for many applications.

High Sensitivity Refractometer Based on a Tapered-Single Mode-No Core-Single Mode Fiber Structure

We have proposed a novel tapered-single mode-no core-single mode (TSNS) fiber refractometer based on multimode interference. The TSNS structure exhibits a high contrast ratio (>15 dB) and a uniform interference fringe. The influence of different lengths and diameters of the TSNS on the refractive index unit (RIU) sensitivity was investigated. The experimental investigations indicated a maximum sensitivity of 1517.28 nm/RIU for a refractive index of 1.417 and low-temperature sensitivity (<10 pm/°C). The experimental and simulation results are also in good agreement.

Tapered Fiber-Optic Mach-Zehnder Interferometer for Ultra-High Sensitivity Measurement of Refractive Index

A Mach-Zehnder interferometer (MZI) based fiberoptic refractive index (RI) sensor is constructed by uniformly tapering standard single mode fiber (SMF) for RI measurement. A custom flame-based tapering machine is used to fabricate microfiber MZI sensors directly from SMFs. The fabricated MZI device does not require any splicing of fibers and shows excellent RI sensitivity. The sensor with a cladding diameter of 35.5 µm and length of 20 mm exhibits RI sensitivity of 415 nm/RIU for RI range of 1.332 to 1.384, 1103 nm/RIU for RI range of 1.384 to 1.4204 and 4234 nm/RIU for RI range of 1.4204 to 1.4408, respectively. The sensor reveals a temperature sensitivity of 0.0097 nm/°C, which is relatively low in comparison to its ultra-high RI sensitivity. The proposed inexpensive and highly sensitive optical fiber RI sensors have numerous applications in chemical and biochemical sensing fields.

Hydrogen sulfide gas sensor based on copper/graphene oxide coated multi-node thin-core fiber interferometer

A hydrogen sulfide gas sensor based on a copper/graphene oxide (Cu/GO) coated multi-point thin-core fiber Mach-Zehnder interferometer is proposed and experimentally demonstrated. The single-mode fiber (SMF) is sandwiched between the thin-core-fiber-1 (TCF-1) and thin-core-fiber-2 (TCF-2), and the SMF-TCF-1-SMF-TCF-2-SMF Mach-Zehnder interferometer is obtained. In order to detect the concentration of hydrogen sulfide, Cu/GO composite sensitive film was coated on the outside surface of two thin-core fibers. When the composite film absorbs the gases, it leads to a change of the effective refractive index of the cladding and causes the regular shift of dip wavelength. The result indicates that the thickness of the sensitive film is 1.6 μm. With the increase of concentration of hydrogen sulfide, the transmission spectra appear blueshift in the range of 0-60 ppm H₂S. The linearity of 0.9884 and sensitivity of 4.83 pm/ppm are achieved. In addition, the dynamic response time and recovery time of the hydrogen sulfide sensor are about 32 s and 52 s, respectively. This sensor has the advantages of the small size, simple structure, and easy manufacture, and it is suitable for the detection of trace hydrogen sulfide.

Review of Fiber Optic Sensors for Structural Fire Engineering

Reliable and accurate measurements of temperature and strain in structures subjected to fire can be difficult to obtain using traditional sensing technologies based on electrical signals. Fiber optic sensors, which are based on light signals, solve many of the problems of monitoring structures in high temperature environments; however, they present their own challenges. This paper, which is intended for structural engineers new to fiber optic sensors, reviews various fiber optic sensors that have been used to make measurements in structure fires, including the sensing principles, fabrication, key characteristics, and recently-reported applications. Three categories of fiber optic sensors are reviewed: Grating-based sensors, interferometer sensors, and distributed sensors.

In-line reflective Mach-Zehnder interferometer based on a tilted in-fiber beam splitter

A fiber in-line reflective Mach-Zehnder interferometer is proposed and experimentally demonstrated, which is based on a tilted in-fiber beam splitter inscribed by a femtosecond laser. The beam splitter splits the incident light beam into two parts; one is directed to the fiber cladding-air interface, where it experiences total internal reflection and is directed to the fiber core. The other part of the light beam keeps traveling in the fiber core. The two parts of the light beams are recombined in the fiber core to generate interference before being reflected at the fiber cut end face. The device proposed is compact in size, robust in mechanical strength, easy in fabrication, and can be used for environmental refractive index sensing in a convenient manner.

Optical Fiber Magnetic Field Sensors Based on Magnetic Fluid: A Review

Magnetic field sensing is an important issue for many application areas, such as in the military, industry and navigation. The current sensors used to monitor this parameter can be susceptible to electromagnetic interferences, however due to their advantages over the traditional sensors, the optical fiber devices could be an excellent alternative. Furthermore, magnetic fluid (MF) is a new type of functional material which possesses outstanding properties, including Faraday effect, birefringence, tunable refractive index and field dependent transmission. In this paper, the optical fiber magnetic field sensors using MF as sensing element are reviewed. Due to the extensive literature, only the most used sensing configurations are addressed and discussed, which include optical fiber grating, interferometry, surface plasmon resonance (SPR) and other schemes involving tailored (etched, tapered and U-shaped) fibers.

Surface-Plasmon-Resonance-Based Optical-Fiber Micro-Displacement Sensor with Temperature Compensation

Micro-displacement measurements play a crucial role in many industrial applications. Aiming to address the defects of existing optical-fiber displacement sensors, such as low sensitivity and temperature interference, we propose and demonstrate a novel surface plasmon resonance (SPR)-based optical-fiber micro-displacement sensor with temperature compensation. The sensor consists of a displacement-sensing region (DSR) and a temperature-sensing region (TSR). We employed a graded-index multimode fiber (GI-MMF) to fabricate the DSR and a hetero-core structure fiber to fabricate the TSR. For the DSR, we employed a single-mode fiber (SMF) to change the radial position of the incident beam as displacement. The resonance angle in the DSR is highly sensitive to displacement; thus, the resonance wavelength of the DSR shifts. For the TSR, we employed polydimethylsiloxane (PDMS) as a temperature-sensitive medium, whose refractive index is highly sensitive to temperature; thus, the resonance wavelength of the TSR shifts. The displacement and temperature detection ranges are 0–25 μm and 20–60 °C; the displacement and temperature sensitivities of the DSR are 4.24 nm/μm and −0.19 nm/°C, and those of the TSR are 0.46 nm/μm and −2.485 nm/°C, respectively. Finally, by means of a sensing matrix, the temperature compensation was realized.

High Temperature High Sensitivity Multipoint Sensing System Based on Three Cascade Mach⁻Zehnder Interferometers

A temperature multipoint sensing system based on three cascade Mach⁻Zehnder interferometers (MZIs) is introduced. The MZIs with different lengths are fabricated based on waist-enlarged fiber bitapers. The fast Fourier transformation is applied to the overlapping transmission spectrum and the corresponding interference spectra can be obtained via the cascaded frequency spectrum based on the inverse Fourier transformation. By analyzing the drift of interference spectra, the temperature response sensitivities of 0.063 nm/°C, 0.071 nm/°C, and 0.059 nm/°C in different furnaces can be detected from room temperature up to 1000 °C, and the temperature response at different regions can be measured through the sensitivity matrix equation. These results demonstrate feasibility of multipoint measurement, which also support that the temperature sensing system provides new solution to the MZI cascade problem.

Femtosecond laser inscribed straight waveguide in no-core fiber for in-line Mach-Zehnder interferometer construction

A new fiber in-line Mach-Zehnder interferometer based on a straight waveguide along the central axis of the no-core fiber sandwiched between single mode fibers is fabricated by a femtosecond laser. The device can be used for high temperature sensing with a sensitivity of -278.86  pm/°C and for bending sensing with a sensitivity of 0.28  nm/m^-1. The high mechanical strength, simple fabrication, and precisely controlled free spectral range make the device attractive for potential applications in high temperature monitoring.

Few-mode multicore fiber enabled integrated Mach-Zehnder interferometers for temperature and strain discrimination

We proposed and experimentally demonstrated paralleled Mach-Zehnder interferometers (MZIs) in few-mode multicore fiber (FM-MCF) for temperature and strain discriminative sensing. A section of FM-MCF is sandwich-spliced between two single-mode multicore fiber (SM-MCF) with a rotational offset. The arbitrarily controlled angular misalignment generates intentional intermodal interferences in outer cores of the FM-MCF thus multiple MZI structures are implemented. Experimental results show that the temperature sensitivities are 105.8 pm/°C and 223.6 pm/°C for two outer cores, strain sensitivity is 13.96 pm/με for the outer core 1 and 11.7 pm/με for the outer core 2, respectively. Due to the low condition number of the cross coefficient matrix dependent on the temperature and strain response indexes, the temperature-strain cross sensitivity can be efficiently eliminated. In addition, the structure's fabrication process is simple, cost effective, and repeatable. The sensing structure can be applied to a wide range of measurements and is expected to develop potentials by building a higher dimensional matrix with more cores.

A Novel Strain Sensor with Large Measurement Range Based on All Fiber Mach-Zehnder Interferometer

We have proposed a high sensitive photonic crystal fiber (PCF) strain sensor based on the Mach-Zehnder interferometer (MZI). The sensing head is formed by all-fiber in-line single mode-multimode-photonic-crystal-single mode fiber (SMPS) structure, using only the splicing method. Such a strain sensor exhibited a high sensitivity of −2.21 pm/με within a large measurement range of up to 5000 με and a large fringe visibility of up to 24 dB. Moreover, it was found that the strain sensitivity was weekly dependent of the length of PCF or MMF. In addition, the sensor exhibited the advantages of simplicity of fabrication, high sensitivity and larger fringe visibility.

Fabry-Perot Interferometric High-Temperature Sensing Up to 1200 °C Based on a Silica Glass Photonic Crystal Fiber

A Fabry-Perot interferometric sensor for temperature measurement was fabricated based on a silica glass solid-core photonic crystal fiber with a central air-bore. By splicing a stub of photonic crystal fiber to a standard single-mode fiber, an intrinsic Fabry-Perot cavity was formed inside the photonic crystal fiber. Sensing experiment results show that the sensor can work stably for a consecutive 24 h under temperatures up to 1100 °C, and the short-term operation temperature can reach as high as 1200 °C (<30 min). In the measurement range of 300–1200 °C, the temperature sensitivity of the peak wavelength shift can reach as high as 15.61 pm/°C, with a linearity of 99.76%. The presented interferometric sensor is compact in size and possesses advantages such as an extended working range and high sensitivity, showing promising application prospects.

All-Fiber Laser Curvature Sensor Using an In-Fiber Modal Interferometer Based on a Double Clad Fiber and a Multimode Fiber Structure

An all-fiber curvature laser sensor by using a novel modal interference in-fiber structure is proposed and experimentally demonstrated. The in-fiber device, fabricated by fusion splicing of multimode fiber and double-clad fiber segments, is used as wavelength filter as well as the sensing element. By including a multimode fiber in an ordinary modal interference structure based on a double-clad fiber, the fringe visibility of the filter transmission spectrum is significantly increased. By using the modal interferometer as a curvature sensitive wavelength filter within a ring cavity erbium-doped fiber laser, the spectral quality factor Q is considerably increased. The results demonstrate the reliability of the proposed curvature laser sensor with advantages of robustness, ease of fabrication, low cost, repeatability on the fabrication process and simple operation.