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An optical fiber relative humidity (RH) sensor based on tapered no-core fiber (TNCF) coated with Ti 3 C 2 T x MXene is proposed. The fluoride etching method is used to prepare Ti 3 C 2 T x , which is used as the humidity sensitive material with a multi-layer structure, large specific surface area and rich hydrophilic groups, and optical deposition method is used to integrate Ti 3 C 2 T x with TNCF. The diameter of TNCF between two sections of SMF is reduced to 11.6 μ m to enhance RH sensitivity. The refractive index (RI) of Ti 3 C 2 T x changes as a result of variations in RH, and this ultimately causes a shift in the transmission spectrum. The sensitivity of the device is −77 pm/%RH in the range of 33 %RH to 75 %RH. In the region of 75 %RH to 91 %RH, the sensitivity increases to 685 pm/%RH, the linear correlation coefficients of the two regions are 0.9521 and 0.9940, respectively. On average, the response time is approximately 3.5 s and the recovery time is about 12.7 s. The sensor may be utilized in possible domains, including biological, chemical, and food processing. Its benefits include a simple construction, cheap cost, high reversibility, and stability. [ABSTRACT FROM AUTHOR]

A highly sensitivity balloon-like fiber interferometer based on ethanol coating is presented in this paper. The Mach–Zehnder interferometer is formed by bending a single-mode fiber to a balloon-like structure and nested in the Teflon tube. Then, an ethanol solution was filled into the tube of the balloon-like fiber interferometer by the capillary effect. Due to the high sensitivity of the refractive index (RI) of ethanol solutions to temperature, when the external temperature varies, the optical path difference changes. The change in temperature can be detected by the shift in the interference spectrum. Limited by the size of the balloon-like structure, three kinds of these structures with different sensitive lengths were prepared to select the best parameters. The sensitive lengths were 10, 15 and 20 mm, respectively, and the RI detection performance of each structure in 10~26% NaCl solutions was investigated experimentally. The results show that when the sensitive length is 20 mm, the RI sensitivity of the sensor is the highest, which is 212.88 nm/RIU. Ultimately, the sensitive length filled with ethanol is 20 mm. The experimental results show that the temperature sensitivity of the structure is 1.145 nm/°C in the range of 28.1 °C~35 °C, which is 10.3 times higher than that of an unfilled balloon-like structure (0.111 nm/°C). The system has the advantages of low cost and easy fabrication, which can potentially be used in high-precision temperature monitoring processes. [ABSTRACT FROM AUTHOR]

In this research, we proposed and numerically investigated a D-shaped dual-core photonic crystal fiber (PCF) refractive index (RI) sensor based on surface plasmon resonance (SPR), and a plasma material indium tin oxide (ITO) is deposited on the polished surface of a D-shaped PCF to produce the SPR effect. The proposed PCF sensor incorporates a dual-core structure and innovatively introduces an elliptical air hole, which increases the coupling of the SPR effect, resulting in a notable improvement in sensor performance. This enhancement enables the sensor to efficiently detect variations in the RI of the analyte. Utilizing the finite element method (FEM), we analyzed the influencing characteristics of sensor performance, meticulously investigating and optimizing various structural parameters. In the RI sensing range spanning from 1.18 to 1.33, the designed PCF sensor exhibits an outstanding maximum wavelength sensitivity of 29,300 nm/RIU, along with the highest achievable amplitude sensitivity of 261.01 RIU−1. Upon analyzing the light signal modulated by the sensor, the findings unequivocally demonstrate that the proposed sensor displays outstanding sensitivity to changes in the analyte's RI via the SPR effect. This responsiveness enables effective and precise real-time detection. The proposed sensor is characterized by exceptional sensing performance, strong economic feasibility, and low manufacturing intricacy. Consequently, the proposed sensor can be used to detect medical oxygen, anesthetics, methane gas, glucose, fluorine-containing organics, etc., and it holds a diverse array of promising applications spanning industrial detection, biochemical analysis, and medical diagnostics. [ABSTRACT FROM AUTHOR]

An ultrahigh sensitivity surface plasmon resonance sensor based on double-side polished photonic crystal fiber is designed, and the gold film with a zigzag cross section is used as the plasmonic material. We study the refractive index sensing characteristics of the proposed sensor by numerical simulation. The results show that compared with the flat gold film structure, the zigzag gold film structure effectively improves the sensing performance, and the sensitivity and resolution of the sensor are significantly enhanced. The maximum wavelength sensitivity and resolution of the proposed sensor can reach 20,000 nm/RIU and 5.00 × 10−6 RIU, respectively. The working wavelength range of the sensor is between 590 and 1200 nm, and the refractive index range that can be used for detection is 1.350 RIU-1.400 RIU. This can be applied to precise detection of extracellular vesicles, breast cancer cell, and red blood cells. The proposed sensor has significant advantages such as stable structure, ultrahigh sensitivity, fast response, small size, and saving noble metal. It has great application potential in biomedical testing and medical equipment. [ABSTRACT FROM AUTHOR]

A highly sensitive D-shaped microfluidic channel (MFC) incorporated in a single-mode fiber (SMF) equipped with a tungsten trioxide ( W O 3 ) layer coated with gold (Au) or silver (Ag) on the base of the MFC is designed and investigated utilizing a finite element method (FEM) to obtain the sensing performance of the optical fiber surface plasmon resonance (SPR) structure. The proposed sensor is appropriate for both bimetallic layers and analytes. With the sizeable cladding diameter of SMF, the fluidic channel size can be changed following demand, making fabrication more accessible and applicable. The hollow D-section above the core deposited a bimetallic structure of silver (Ag) or gold (Au) as plasmonic materials, followed by a tungsten trioxide ( W O 3 ) nanolayer. This optical fiber biochemical monitors the changes in the refractive index (RI) by measuring the transmission spectral shifts of the fiber at their resonance wavelengths. The W O 3 layer protects the plasmonic material from oxidation and effectively enhances the surface plasmon wave (SPW) while interacting with free electrons. Upon achieving the optimum structural parameters, the numerical investigation results showed that the maximum wavelength sensitivity of 6.0 μ m//RIU and 5.0 μ m//RIU for Au/ W O 3 and Ag/ W O 3 were obtained, with the coefficient of performance of 50.12 R I U - 1 and 134.84 R I U - 1 , respectively, for ultra-wide sensing ranges from 1.28 to 1.38. Also, the simulation outcomes revealed that the tungsten trioxide-coated with Ag or Au layers significantly enhanced the sensor. Finally, the numerical results of this study showed that the proposed sensor has substantial possibilities for large sensing RI applications such as biology and chemistry analytes, and it is also valid for other lower RI analyte sensing requirements as a consequence of its features such as high sensitivity, simple design, cost-effective design, and promising linear sensing performance. [ABSTRACT FROM AUTHOR]

A novel photonic crystal fiber (PCF) sensor for refractive index detection based on polydimethylsiloxane (PDMS) is presented in this research, as well as designs for single-channel and dual-channel structures for this PDMS-PCF sensor. The proposed structures can be used to develop sensors with biocompatible polymers. The performance of the single-channel PDMS-PCF sensor was studied, and it was found that adjusting parameters such as pore diameter, lattice constant, distance between the D-shaped structure and the fiber core, and the radius of gold nanoparticles can optimize the sensor's performance. The findings indicate that the detection range of the single-channel photonic crystal is 1.21–1.27. The maximum wavelength sensitivity is 10,000 nm/RIU with a resolution of 1 × 10 − 5 RIU, which is gained when the refractive index is set to 1.27. Based on the results of the single-channel PCF, a dual-channel PDMS-PCF sensor is designed. The refractive index detection range of the proposed sensor is 1.2–1.28. The proposed sensor has a maximum wavelength sensitivity of 13,000 nm/RIU and a maximum resolution of 7.69 × 10 − 6 RIU at a refractive index of 1.28. The designed PDMS-PCF holds tremendous potential for applications in the analysis and detection of substances in the human body in the future. [ABSTRACT FROM AUTHOR]

We propose a spectral filter based on a plastic optical fiber with micro-holes as a low-cost, robust, and highly reproducible spectral filter. The spectral filter is explored for two configurations: a fiber extended in a straight line and a fiber optic loop mirror scheme configuration. The transmission traces indicate a spectral blue shift in peak transmission, at 587 nm, 567 nm, 556 nm, and 536 nm for zero, one, two, and three holes in the fiber, respectively. The filter exhibits a bandpass period of approximately 120 nm. Additionally, we conduct a comparison of the transmission with holes separated by distances of 1 cm and 500 μm. The results demonstrate that the distance between holes does not alter the spectral transmission of the filter. In the case of the fiber loop mirror configuration, we observe that the bandpass can be adjusted, suggesting the presence of multimode interference. Exploring variations in the refractive index within the holes by filling them with glucose solutions at various concentrations, we determine that the filtering band and spectral shape remain unaltered, ensuring the stable and robust operation of our spectral filter. [ABSTRACT FROM AUTHOR]

A new type of human immunoglobulin G (IgG) sensors based on the surface plasmon resonance (SPR) in the low refractive index (RI) plastic optical fiber (POF) and an antibody immobilization method is presented. A 50-nm-thick gold film was formed on the polished D-shaped fiber surface by magnetron sputtering. The RI response of the POF sensor is 30 049.61 nm/RIU, which is 26.5 times higher than that of single mode fiber (SMF) SPR sensors. The proposed SPR biosensor can be developed by simple and rapid modification of the gold film with 11-mercapto undecanoic acid (MUA). Upon immobilization of the goat anti-human IgG antibody, the resonance wavelength shifts by 11.2 nm. The sensor can be used to specifically detect and quantify the human IgG at concentrations down to 245.4 ng/mL with the sensitivity of 1.327 7 nm per µg/mL, which offers an enhancement of 12.5-fold compared to that of the conventional SMF based SPR sensors. The proposed device may find the potential applications in the case of use at the point of care. [ABSTRACT FROM AUTHOR]

A dual-channel surface plasmon resonance (SPR) sensor, based upon a multimode-single mode-multimode-single mode- multimode (MSMSM) fiber structure, is reported for the simultaneous measurement of temperature and refractive index. One channel is coated with an indium tin oxide-silver layer for refractive index sensing, and the channel is coated with Au and cyclohexane layer for temperature sensing. The experiments show that the sensor provides high sensitivity with values of 3379 nm/RIU across the refractive index range from 1.333 to 1.3699 and −3.163 nm/°C from 40 °C to 90 °C. The cross-sensitivity problem is resolved by the dual-wavelength matrix method. Moreover, the proposed sensor has good stability, low cost, and a compact structure and may be employed in biology, chemistry, and environmental testing. [ABSTRACT FROM AUTHOR]

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As the focus tilts toward online detection methodologies for transformer oil aging, bypassing challenges associated with traditional offline methods, such as sample contamination and misinterpretation, fiber optic sensors are gaining traction due to their compact nature, cost-effectiveness, and resilience to electromagnetic disturbances that are typical in high-voltage environments. This study delves into the sensitivity analysis of intensity-modulated plastic optical fiber sensors. The investigation encompasses key determinants such as the influence of optical source wavelengths, noise response dynamics, ramifications of varying sensing lengths, and repeatability assessments. Our findings highlight that elongating sensing length detrimentally affects both linearity response and repeatability, largely attributed to a diminished resistance to noise. Additionally, the choice of the optical source wavelength proved to be a critical variable in assessing sensor sensitivity. [ABSTRACT FROM AUTHOR]

Here, we improved the performance characteristics of surface plasmon resonance (SPR)-based optical sensors by using new two-dimensional materials in the structure of these sensors. We studied three types of two-dimensional materials: graphene, molybdenum disulfide (MoS2), and black phosphorus (BP). We also applied the angular interrogation method and employed numerical studies within MATLAB simulations. We investigated the characteristics of a simple configuration, including the type of prism, the thickness of the gold layer, and the number of layers of two-dimensional materials. We compared reflectance, sensitivity, detection accuracy (DA), and figure of merit (FoM) for various materials. We also studied the sensitivity of the sensor in relation to the refractive index of the sensing medium. We observed that the sensitivity reaches a peak at specific refractive index values. The best value for sensitivity is measured at 9778.9 RIU−1, with a detection accuracy of 32 deg−1 and FoM = 15,375 (deg·RIU)−1 for a black phosphorus-based structure. One distinctive feature of this study is investigating the effects of changing the resolution in simulations, which is considered one of the main factors in obtaining more accurate answers and achieving higher sensitivity. [ABSTRACT FROM AUTHOR]

A negative curvature hollow core fiber (NCHCF) refractive index (RI) sensor based on localized surface plasmon resonance (LSPR) is proposed. The gold nanowires are deposited in four cladding tubes in the y-direction, with the core region serving as the analyte channel. The full vector finite element method (FEM) is used to analyze the influence of various structural parameters on this sensor, and the structural parameters are optimized and selected. Ultimately, an average sensitivity of 9356.59 nm/RIU is accomplished within a spacious refractive index detection scope of RI = 1.28–1.43. The sensor attained a maximum sensitivity of 10,220 nm/RIU at RI = 1.36. In the bargain, there is an excellent linear correlation between the resonance wavelength and the refractive index of the analyte, with a value of 0.99901 and a factor of merit (FOM) range of 119.9563–155.9432 RIU−1, achieving a resolution of 10−6 RIU. The sensor has potential applications in various fields such as environmental protection, food safety, and medical diagnostics due to its high sensitivity, spacious detection scope, and positive linear response. [ABSTRACT FROM AUTHOR]

In this study, a sensor probe based on surface plasmon resonance (SPR) concept is proposed and examined for the detection of tuberculosis. The sensor probe includes a D-shaped photonic crystal fiber (PCF) micro-elliptical channel, gold (Au), and 2D materials like MOS2 and MXene. The finite element method (FEM) is utilized to compute the confinement loss and performance parameters for a healthy and tuberculosis-infected biomolecules. The effect of MoS2 on gold is to strengthen the coupling between the core mode and the surface plasmon polariton (SPP) mode of the fiber. In addition, the monolayer of MXene is functionalized as a bio-recognition element (BRE) above the MoS2 layer. The influence of inserting MoS2 and MXene on the sensor probe is examined in depth. Using MoS2 and MXene layers, the maximum wavelength sensitivity of the PCF structure is increased to 45,000 nm/RIU. Maximum figure of merit (FOM) for the sensor is 265 RIU−1, and it has a high refractive index (RI) resolution of 2.22 × 10−6 RIU. The acquired results indicate that the proposed probe has superior sensing capabilities compared to the biosensors used in recent investigations. Finally, the proposed 2D material-based D-shaped PCF sensor probe with functionalized MXene has a high value of sensitivity and FOM, making it a good platform for detecting tuberculosis-infected biomolecules having refractive index between 1.343 and 1.351 RIU. [ABSTRACT FROM AUTHOR]

Fiber optic surface plasmon resonance (SPR) sensors are widely used in sensing applications in the fields of physics, chemistry and biology. This paper presents a numerical simulation of a D-shaped fiber optic SPR based refractive index sensor designed with a silver, gold, molybdenum disulfide, and graphene coated structure with and without gratings, for the first time. A FEM-based software is used to carry out the simulation studies. Aiming to obtain the highest performance, gold/silver layers are coated with MoS2 and graphene. Sensor parameters are computed and optimum values are discovered for these parameters; grating periods, gap size between gratings, residual cladding thickness, gratings numbers, and thickness of coating layers. 11,775 nm /RIU as the maximum sensitivity in the range of 1.33–1.4 RI is reached. Results demonstrate that the proposed multilayer and grated structure yields a 98.7% higher sensitivity value with reference to the simulation of a regular D-shaped Ag-coated fiber SPR sensor, which makes it promising for chemical, biochemical, and other applications of the future with the delicate sensing ability. [ABSTRACT FROM AUTHOR]

An optical fiber sensor for the simultaneous measurement of microdisplacement and temperature based on balloon-shaped single-mode fibers cascaded with a fiber Bragg grating with two core-offset joints is proposed. The interference between the core mode and cladding mode is caused by the stimulation of the cladding mode by the core-offset joints' structure. The cladding of the core has a distinct refractive index, which causes optical path differences and interference. The balloon-shaped structure realizes mode selection by bending. As the displacement increases, the radius of the balloon-shaped interferometer changes, resulting in a change in the interference fringes of the interferometer, while the Bragg wavelength of the fiber grating remains unchanged. Temperature changes will cause the interference fringes of the interferometer and the Bragg wavelength of the fiber grating to shift. The proposed optical fiber sensor allows for the simultaneous measurement of microdisplacement and temperature. The results of the experiment indicate that the sensitivity of the interferometer to microdisplacement is 0.306 nm/µm in the sensing range of 0 to 200 μm and that the temperature sensitivity is 0.165 nm/°C, respectively. The proposed curvature sensor has the advantages of a compact structure, extensive spectrum of dynamic measurement, high sensitivity, and simple preparation, and has a wide range of potential applications in the fields of structural safety monitoring, aviation industry, and resource exploration. [ABSTRACT FROM AUTHOR]

A surface plasmon resonance-based arc-shaped photonic crystal fiber high-sensitivity refractive index (RI) sensor is proposed. An open arc-shaped analyte channel is produced at the top of the fiber to facilitate RI detection of the analyte, and a gold film is coated inside the arc-shaped structure to stimulate mode coupling. The performance of the sensor is analyzed by using the finite element method (FEM). The results have demonstrated that the sensor can detect a sensing range of 1.35-1.42 with maximum RI sensitivity of 24900 nm/RIU and resolution of 4:01×10-6 RIU. Furthermore, the highest figure of merit (FOM) of 661.71 RIU-1 is obtained. Additionally, the effects of air hole size and air hole distance on sensitivity are investigated. Finally, the proposed sensor characterizes great potential in biomedical, chemical, and other fields due to its excellent performance. [ABSTRACT FROM AUTHOR]

We proposed an active differential intensity (DI) fiber coupled plasmon waveguide resonance (CPWR) sensor based on the mode competition effect in the C-band, aiming to enhance the sensitivity. The sensing head is a fiber probe with a sensing layer of ITO/Au/ITO/TiO2 film, enabling the excitation of CPWR in the C-band. The narrow CPWR spectrum allows DI interrogation by tracking the intensity of light at two wavelengths. The fiber probe is inserted into a dual-wavelength fiber laser to adjust the intra-cavity loss at the two lasing wavelengths. By using the differential modulation of the reflectivity from the refractive index variations, the mode competition effect is triggered. The powers at two lasing wavelengths change oppositely, enlarging the power difference and then resulting in enhanced sensitivity. The average sensitivity is up to 5702 dB/RIUs, which is 10 times higher than that of conventional DI interrogation. The sensitivity enhancement mechanism based on the mode competition effect provides a new technical approach for enhancing the sensitivity of DI SPR sensors. [ABSTRACT FROM AUTHOR]

A structure for simultaneous measurement of refractive index (RI) and temperature (T) is proposed based on photonic crystal (PhC) beam structures. The sensor structure consists of three parallel 1D PhC cavities, in which the middle cavity 1 has a wide linewidth, and cavity 2 and cavity 3 on both sides have a narrow linewidth. By changing the RI of the analyte and the T of the ambient, we explored the linear relationship between the two parameters and resonant wavelengths, respectively. For cavity 2 with air mode, the RI and T sensitivities are 377.2 nm/RIU and 101 pm/K, respectively. For cavity 3 with dielectric mode, the RI and T sensitivities are 194 nm/RIU and 261 pm/K, respectively. In addition, we also discussed the crosstalk between two adjacent cavities at different RI and T, and all crosstalk values are lower than −7 dB and −6 dB for RI and T, respectively. Compared with previous sensor structures, the proposed sensor structure could be used in simultaneous RI and T sensing with higher sensitivity and lower crosstalk values. [ABSTRACT FROM AUTHOR]

Detection of air bubbles in fluidic channels plays a fundamental role in all that medical equipment where liquids flow inside patients' blood vessels or bodies. In this work, we propose a multi-parameter sensing system for simultaneous recognition of the fluid, on the basis of its refractive index and of the air bubble transit. The selected optofluidic platform has been designed and studied to be integrated into automatic pumps for the administration of commercial liquid. The sensor includes a laser beam that crosses twice a plastic cuvette, provided with a back mirror, and a position-sensitive detector. The identification of fluids is carried out by measuring the displacement of the output beam on the detector active surface and the detection of single air bubbles can be performed with the same instrumental scheme, exploiting a specific signal analysis. When a bubble, traveling along the cuvette, crosses the readout light beam, radiation is strongly scattered and a characteristic fingerprint shape of the photo-detected signals versus time is clearly observed. Experimental testing proves that air bubbles can be successfully detected and counted. Their traveling speed can be estimated while simultaneously monitoring the refractive index of the fluid. [ABSTRACT FROM AUTHOR]

In this study, a new approach using optical fiber sensor based on a single fiber Bragg grating (FBG) is proposed to overcome the cost and complexity limitations. The reflected frequency of the half-etched region is shifted by utilizing a half-etched uniform FBG as the sensing element. The proposed sensor is evaluated using the refractive index measurement technique, and its validity is demonstrated by subjecting it to temperature variations. A sensitivity of − 524.48 MHz/°C is exhibited by the proposed sensor over a dynamic range of about 25 °C to 70 °C. It is demonstrated that the proposed sensor is highly sensitive, cost-effective, and reduces the complexity of the manufacturing and measurement process. This novel approach has significant potential for applications in various fields, including biomedical sensing, environmental monitoring, and industrial process control. [ABSTRACT FROM AUTHOR]

Optical biosensors based on grating-coupled surface plasmon resonance (GCSPR) technology are an important research topic in the field of bio-photonics. This paper presents a high-performance and high-sensitivity nanostructured bimetallic GCSPR sensor based on two-dimensional materials. When designing the sensor, the sensitivity, full width at half peak (FWHM) and dip strength of the absorption peak (DS) were considered comprehensively, and the comprehensive evaluation parameter FOM+ is defined by making improvements on the basis of figure of merit (FOM). The performance of the sensor can be judged more comprehensively. The performance of the sensor was further improved by optimizing the structure of the sensor. An ultra-thin gold layer was added on the surface of the silver-based GCSPR sensor, which solves the problem of the easy oxidation of silver metal. We tried to coat graphene oxide two-dimensional nanomaterials on the surface of the bimetallic sensor, and the sensitivity and FOM+ of the sensor reached 350 deg/RIU and 473.23, respectively. This is a great improvement compared with the GCSPR sensor in a previous study, and it can be improved at least 74.7%. This sensor can measure a variety of biological molecules and biological cells with high sensitivity and performance by detecting the change in the refractive index of the solution to be measured. [ABSTRACT FROM AUTHOR]

This study fabricated an ultra-high refractive index (RI) sensor based on tapered no-core fiber (NCF) involving a simple inexpensive process. A splice section of NCF in the middle of single mode fiber was tapered to small diameters. The sensor was sensitive to the surrounding RI with a large measurement range of 1.3330–1.4437. The RI sensitivity differed with varying wavelengths, with a value of 41 916 nm/RIU at approximately 1550 nm, for the RI ranges of 1.4407–1.4437. It yielded a low temperature sensitivity of 8 pm °C−1, which indicates an ultra-low temperature cross-sensitivity. The proposed fiber optic RI sensor can be used in many fields such as medicine and biochemical applications. [ABSTRACT FROM AUTHOR]

Polymer-based surface plasmon resonance (SPR) sensors can be used to realize simple, small-size, disposable, and low-cost biosensors for application in several fields, e.g., healthcare. The performance of SPR sensors based on optical waveguides can be changed by tuning several parameters, such as the dimensions and the shape of the waveguides, the refractive index of the core, and the metal nanofilms used to excite the SPR phenomenon. In this work, in order to develop, experimentally test, and compare several polymer-based plasmonic sensors, realized by using waveguides with different core refractive indices, optical adhesives and 3D printed blocks with a trench inside have been used. In particular, the sensors are realized by filling the blocks' trenches (with two plastic optical fibers located at the end of these) with different UV-cured optical adhesives and then covering them with the same bilayer to excite the SPR phenomenon. The developed SPR sensors have been characterized by numerical and experimental results. Finally, in order to propose photonic solutions for healthcare, a comparative analysis has been reported to choose the best sensor configuration useful for developing low-cost biosensors. [ABSTRACT FROM AUTHOR]

A multi-resonance plasmonic temperature and refractive index (RI) sensor is developed based on a metal–insulator-metal (MIM) waveguide with two trapezoidal-shaped cavities and an elliptical island inside. Different dielectrics including ethanol, chloroform, carbon disulfide, glycerin, and polydimethylsiloxane (PDMS) have been utilized for the temperature investigation in the range of − 50 to 70 °C. Results revealed the maximum temperature sensitivity (TS) of 1.058 nm/°C and the maximum temperature figure of merit (FoM) (FoMT) of 0.0141 1/°C which correspond to the carbon disulfide and PDMS in the first resonance (FR), respectively. The RI-sensitivity (RIS) of this sensor at the first and second resonance (SR) is 1328.57 nm per RI unit (RIU) and 735.71 nm/RIU as well as its FoM which is 36 1/RIU and 31.31 1/RIU, respectively. This paper confirms the high capabilities for measuring the temperature and RI in a wide spectral range of wavelengths (600 to 3000 nm), and multi-resonance sensing. This reveals significant potential in optical and photonic circuit applications such as different quantities detection, including hemoglobin and cancer. [ABSTRACT FROM AUTHOR]

A D-shaped photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor is proposed to measure the refractive index in visible and near-infrared regions. Compared with traditional PCF, the use of D-shaped PCF can avoid filling materials and coatings in the hole. The plasma material of sensors is silver, and graphene is coated on the surface of the silver film. Graphene layer can not only prevent the oxidation of the silver but also improve the performance of the sensor. Then the influence of structural parameters on the sensor performance is studied by the finite element method (FEM). Results show that the performance of the sensor is not sensitive to the structural parameters, which makes the sensor has a good manufacturing tolerance. Moreover, when the refractive index (RI) of the analyte is 1.33 ~ 1.41, the maximum wavelength sensitivity is 13,750 nm/RIU, and the corresponding RI resolution is 7.27 × 10−6 RIU−1. In addition, the maximum figure of merit is 201 RIU−1. The proposed sensor is advanced with high sensitivity, simple structure and big fabrication tolerance, which could become a competitive candidate for water pollution monitoring, medical diagnosis, biochemical analysis, and detection fields. [ABSTRACT FROM AUTHOR]

Surface plasmon resonance (SPR)-based sensors have attracted huge interest due to their ability to provide real-time sensing of a wide range of quantities, while still boasting high sensitivity, low cost, and electromagnetic immunity. Among various types of these sensors, waveguide-based sensors show great promise in terms of integrability into more complex circuits while still maintaining similar sensitivity and versatility as their stand-alone counterparts. This paper presents the results of investigation based on design and simulation of optimal parameters for multimode SPR-based waveguide sensor sensitive to any commonly used liquid analytes. The sensitivity for a wider range of analytes is achieved by utilisation of SiON material platform because refractive index adjustability of SiON offers a way to tailor sensor's sensitivity to exact applications and analytes. One of the research goals in this work was to show that multi-mode waveguides may be suitable for such sensors with comparable sensitivity and higher transmission rates as single-mode waveguides. To showcase design possibilities, two variations with the same design and material's refractive index adjustment only were investigated, and their optimal parameters have been found. Sensitivity for all commonly used liquid analytes was validated, with sensitivity rising to 1400 nm/RIU for label-free refractive index sensing, even without the use of any sensitivity-enhancing structures. [ABSTRACT FROM AUTHOR]

In this study, a new temperature sensor with high sensitivity was achieved by four-layer Ge and B co-doped long-period fiber grating (LPFG) based on the mode coupling principle. By analyzing the mode conversion, the influence of the surrounding refractive index (SRI), the thickness and the refractive index of the film on the sensitivity of the sensor is studied. When 10 nm-thick titanium dioxide (TiO2) film is coated on the surface of the bare LPFG, the refractive index sensitivity of the sensor can be initially improved. Packaging PC452 UV-curable adhesive with a high-thermoluminescence coefficient for temperature sensitization can realize high-sensitivity temperature sensing and meet the requirements of ocean temperature detection. Finally, the effects of salt and protein attachment on the sensitivity are analyzed, which provides a reference for the subsequent application. The sensitivity of 3.8 nm/°C in the range of 5–30 °C was achieved for this new sensor, and the resolution is about 0.00026 °C, which is over 20 times higher than ordinary temperature sensors. This new sensor meets the accuracy and range of general ocean temperature measurements and could be used in various marine monitoring and environmental protection applications. [ABSTRACT FROM AUTHOR]

This paper reports a double-sided polished microstructured fiber based on the refractive index that simultaneously measures the refractive index and temperature. Two polished planes were introduced into the cladding. A gold film and polydimethylsiloxane are deposited on one side of the plane for temperature measurements, and graphene layers are coated on the surface of the silver film on the other side for refractive index measurements. The finite element method is used to characterize the sensing characteristics of the sensor. The results show that when the liquid refractive index is from 1.36 to 1.4 and the temperature from 70 °C to 110 °C, the maximum sensitivity of the sensor is 15,000 nm/RIU and 8.8 nm/°C, respectively. The double-sided polished structure facilitates the development of multi-parameter measurement sensors for various applications. [ABSTRACT FROM AUTHOR]

For the quick identification of diverse types of cancer/malignant cells in the human body, a new hollow-core optical waveguide based on Photonic Crystal Fiber (PCF) is proposed and numerically studied. The refractive index (RI) differs between normal and cancerous cells, and it is through this distinction that the other crucial optical parameters are assessed. The proposed cancer cell biosensor's guiding characteristics are examined in the COMSOL Multiphysics v5.5 environment. The Finite Element Method (FEM) framework is used to quantify the display of the suggested fiber biosensor. Extremely fine mesh elements are additionally added to guarantee the highest simulation accuracy. The simulation results on the suggested sensor model achieve a very high relative sensitivity of 99.9277%, 99.9243%, 99.9302%, 99.9314%, 99.9257% and 99.9169%, a low effective material loss of 8.55 × 10 − 5 cm − 1 , 8.96 × 10 − 5 cm − 1 , 8.24 × 10 − 5 cm − 1 , 8.09 × 10 − 5 cm − 1 , 8.79 × 10 − 5 cm − 1 , and 9.88 × 10 − 5 cm − 1 for adrenal gland cancer, blood cancer, breast cancer type-1, breast cancer type-2, cervical cancer, and skin cancer, respectively, at a 3.0 THz frequency regime. A very low confinement loss of 6.1 × 10 − 10 dB/cm is also indicated by the simulation findings for all of the cancer cases that were mentioned. The straightforward PCF structure of the proposed biosensor offers a high likelihood of implementation when used in conjunction with these conventional performance indexes. So, it appears that this biosensor will create new opportunities for the identification and diagnosis of various cancer cells. [ABSTRACT FROM AUTHOR]

In this study, a dual−core D-shaped photonic crystal fiber (PCF) surface plasmon resonance sensor coated with gold grating is designed and analyzed using the finite-element method (FEM). The surface plasmon resonance (SPR) effect between the fiber core modes and surface plasmon polariton (Spp) modes is used to measure the analyte refractive index. The effects of the PCF structure parameters (polishing depths, large holes, and small holes) and grating parameters (grating heights, grating periods, and grating duty) are discussed, and a two-feature interrogation method that combines wavelength and intensity interrogations is introduced to enhance the resolution. The results show that the grating and dual-core play important roles in enhancing the sensor properties. The proposed sensor achieves an average wavelength sensitivity of 994.5 nm/RIU when the analyte refractive index increases from 1.33 to 1.37. Furthermore, a maximum amplitude sensitivity of 181.049 RIU−1 is obtained. The two-feature interrogation is determined to have a resolution of 2.03 × 10−6 RIU, which is better than the wavelength and amplitude interrogations. The proposed sensor has a good sensing performance and is highly suitable for practical applications. [ABSTRACT FROM AUTHOR]

In this paper, an optical fiber Fabry–Pérot (FP) microfluidic sensor based on the capillary fiber (CF) and side illumination method is designed. The hybrid FP cavity (HFP) is naturally formed by the inner air hole and silica wall of CF which is side illuminated by another single mode fiber (SMF). The CF acts as a naturally microfluidic channel, which can be served as a potential microfluidic solution concentration sensor. Moreover, the FP cavity formed by silica wall is insensitive to ambient solution refractive index but sensitive to the temperature. Thus, the HFP sensor can simultaneously measure microfluidic refractive index (RI) and temperature by cross-sensitivity matrix method. Three sensors with different inner air hole diameters were selected to fabricate and characterize the sensing performance. The interference spectra corresponding to each cavity length can be separated from each amplitude peak in the FFT spectra with a proper bandpass filter. Experimental results indicate that the proposed sensor with excellent sensing performance of temperature compensation is low-cost and easy to build, which is suitable for in situ monitoring and high-precision sensing of drug concentration and the optical constants of micro-specimens in the biomedical and biochemical fields. [ABSTRACT FROM AUTHOR]

This study describes the basic principles of optical waveguide refractive index sensing and the various design structures of refractive index sensors. These waveguides generate different optical resonances, which cause changes in the sensing refractive index and temperature and are subsequently used to detect the concentration in the analyses. First, the structural characteristics and performance indices of the microring sensor and interferometer are studied based on the refractive index of the optical waveguide. Second, the principle and sensing detection mechanism of the two types of refractive index sensing employed in these sensors are analyzed. Then, the two sensors are classified and discussed from the perspective of the waveguide materials and structures, as well as the substances to be measured. Simultaneously, performance indicators such as sensitivity and detection range are compared and summarized. The comparison results show that there is a compromise between the sensitivity and quality factor of the optical waveguide refractive index sensor. Finally, applications of refractive index sensing in the biochemical field for material detection are discussed, showing that the optical waveguide refractive index sensor has significant advantages over other types of biochemical optical sensors. [ABSTRACT FROM AUTHOR]

In recent years, the D-shaped photonic-crystal-fiber localized surface-plasmon-resonance (LSPR) sensor is a research hot spot, especially for liquid refractive-index sensors. In this paper, we propose a doublemeasurement-accuracy LSPR sensor. In numerical simulations, when the refractive index of the analyte changes from 1.33 to 1.4, we obtain the maximum sensitivity equal to 15,500 nm/RIU. At refractive index values of the analyte equal to 1.38 and 1.39, there are two resonance peaks, and the wavelength sensitivity of the second peak is 25,000 nm/RIU, which has a good prospect in specific liquid detection. [ABSTRACT FROM AUTHOR]

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The present study proposes a photonic crystal fiber sensor based on a plasmon resonance. A thin gold metal layer was used as a plasmonic‐active material to produce the surface plasmons to detect changes in the refractive index (RI) around it. The performance of the proposed sensor is evaluated by the finite‐element method for various refractive indices. The sensitivity of this sensor ranges from 3740 to 51227 nm/RIU for the RI range of 1.36–1.42 in the wavelength range of 700–1900 nm. Within this range, we observed the highest resolution, the best FOM, and amplitude sensitivity with values of 1.95 × 10−6 RIU, 883.224 RIU−1, and 2328.59 RIU−1, respectively. This sensor has a simple structure and a better performance as compared with available sensors. The present study opens a new window to studies in the fields of biological materials, chemical materials, and the environment. [ABSTRACT FROM AUTHOR]

In this work, a surface plasmon resonance (SPR) multiparameter sensor for simultaneous determination of refractive index and temperature was manufactured through a novel and low-cost approach. Monitoring these parameters is useful when biosensors are developed by exploiting SPR phenomena. A polymer planar optical structure was realized via inkjet 3D printing, by using photo-curable resins having tailored refractive index for device's core and cladding, respectively. The multiparameter sensor was fully designed, manufactured, and experimentally tested to check the numerical analyses run on a preliminary phase. In such a way, a temperature resolution equal to about 0.5 °C and a refractive index resolution equal to about 2 × 10−4 RIU (refractive index unit) were obtained. Next, even a quality control analysis of the 3D printed surface was carried out by following a novel approach that relies on the profile monitoring technique, with the aim to evaluate the suitability of the design and the geometric accuracy control. In addition, thanks to the cost analysis performed through a properly model, it was proved that the multiparameter sensor designed, manufactured, and tested satisfies the low-cost requirements, being the estimated cost ~ 23 €, which is an absolutely competitive cost if compared with other traditional sensors. In the end, even the performance of the sensor in terms of bulk sensitivity (equal to about 900 nm/RIU) resulted to be higher than similar devices already presented in the state-of-the-art, thus proving the validity of the developed SPR multiparameter sensor both in economic and performance terms. [ABSTRACT FROM AUTHOR]

A novel refractive index (RI) sensor based on a D-shaped single-mode-graded-index multimode-single-mode fiber (D-SGIMS) structure is presented and investigated in detail. A CO2 laser was used to process the SGIMS structure, which produced a strong evanescent field. The structural parameters were determined via simulations, and the experimental results were consistent with those of the theoretical analysis. The transmission spectra and RI sensitivity of the fabricated sensors with different lengths were compared via experiments, the results of which indicated that the maximum RI sensitivity was 811.00 nm/RIU in the range 1.333–1.419. Therefore, the proposed D-SGIMS structure can provide high-precision measurement capabilities and can be applied in the field of biochemistry. [ABSTRACT FROM AUTHOR]

An all fiber Mach–Zehnder interferometric (MZI) sensor for measuring temperature and refractive index (RI) is proposed and experimentally demonstrated. The proposed sensor is fabricated with a coreless–few mode–coreless fiber structure. Variations in ambient temperature and RI cause changes in phase differences between the fundamental mode and the higher-order modes, leading to shifts in interference spectrum. The two resonance dips shifts within the wavelength spectrum are used to investigate the temperature and RI characteristics of the sensor. Experimental results show that the two dips have different responses to temperature and RI, indicating that the sensor can realize simultaneous measurement of temperature and RI; the maximum sensing sensitivities are 0.0739 nm/°C and −25.29 nm/RIU, respectively. The proposed sensor has potential applications in physical, biological, and chemical sensing owing to its low cost, good linearity, and simple fabrication. [ABSTRACT FROM AUTHOR]

In this work, a double-slab structure is proposed, in which one slab is made of near-zero-real-part-of-impedance metamaterial (NZRPIM), and the other slab is made of ordinary transparent material. We shall demonstrate that, when a beam of electromagnetic wave is normally incident into the double-slab structure, the transmission may be enhanced significantly due to existence of a cross term induced by coherent superposition between forward and backward waves arising in the NZRPIM slab, thus the corresponding transmission peak may be strongly affected by the other slab and fluid arising between the two slabs, which is instructive significance for the design and application of high sensitivity refractive index sensor based on electromagnetic wave intensity detection. Our work may provide a new possible way to design and manufacture sensors by using NZRPIM. [ABSTRACT FROM AUTHOR]

This research focuses on the side-polished Single-Mode Optical Fibers (SMOF) as refractive index (RI) sensor utilizing properties of Surface Plasmon Resonance. The SMOF with cladding stripped off shows a D-shaped optical fiber with high sensitivity functionalization. Here we show silver protected by graphene oxide (GO) as viable candidates. A few layers of GO on top of silver thin layer were applied as a material to overcome silver coating degradation process. This silver/GO sensor was characterized using various RIs of analyte likely deionized water and alcohol solutions. Sensor with GO has shown fast response and high sensitivity in respect of RI, where sensitivity increases in range from 1.30 to 1.34 RIU (refractive index unit). This device shows highest sensitivity of up to 833.33 nm/ RIU. The polishing process using SMOF stops at few microns above the core of the fiber with the drop of output power fixed to be 1 dB as an indication to standardize the amount of cladding been removed. The thickness of 30 nm Ag was deposited using electron beam evaporation method on D-shaped optical fiber with subsequent protected layer of GO in solutions form. The use of COMSOL Multiphysics has also been carried out to observe numerical findings. The response of the GO has shown a sensitivity improvement which proves new promising approaches for the newly developed sensors. [ABSTRACT FROM AUTHOR]

Herein a gold nanosphere (AuNS)-coated wavelength-mode localized surface plasmon resonance (LSPR) fiber sensor was fabricated by a simple and time-saving electrostatic self-assembly method using poly(allylamine hydrochloride). Based on the localized enhanced coupling effect between AuNSs, the LSPR spectrums of the AuNS monolayer with good dispersity and high density exhibited a favourable capability for refractive index (RI) measurement. Based on the results obtained from the optimization for AuNS distribution, sensing length, and RI range, the best RI sensitivity of the fiber modified by 100 nm AuNS reached up to about 2975 nm/RIU, with the surrounding RI range from 1.3322 to 1.3664. Using an 80 nm AuNS-modified fiber sensor, the RI sensitivity of 3953 nm/RIU was achieved, with the RI range increased from 1.3744 to 1.3911. The effect of sensing length to RI sensitivity was proven to be negligible. Furthermore, the linear relationship between the RI sensitivity and plasma resonance frequency of the bulk metal, which was dependent on the interparticle plasmon coupling effect, was quantified. Additionally, the resonance peak was tuned from 539.18 nm to 820.48 nm by different sizes of AuNSs-coated fiber sensors at a RI of 1.3322, which means the spectrum was extended from VIS to NIR. It has enormous potential in hypersensitive biochemistry detection at VIS and NIR ranges. [ABSTRACT FROM AUTHOR]

We demonstrated the fabrication of long-period fiber gratings (LPFGs) in the tapered single-mode fiber by CO 2 laser. The phase-matching curves of the LPFGs inscribed in the tapered fiber with different waist diameters have been studied theoretically and experimentally. When the waist diameter of the tapered fiber decreased from 90 μm to 22 μm, the slope of the phase-matching curve changed dramatically. The refractive index sensitivity can be increased by inscribing the LPFGs in the tapered fiber around the dispersion turning point. A maximum refractive index sensitivity of 21702.78 nm/RIU has been achieved in the range of 1.445–1.457 for the LPFG inscribed in the tapered fiber with a waist diameter of 50 μm. This kind of LPFGs inscribing in the tapered fiber may be a good candidate for the development of refractive index based chemical and biological sensors. • The phase-matching curves of the LPFGs inscribed in the tapered fiber with different waist diameters from 90 μm to 22 μm have been studied. • The maximum SRI sensitivity of 21702.78 nm/RIU in the range of 1.445–1.457 was achieved by the LPFG with a DTP. • The waist length of the tapered fiber can reach 60 mm, enabling the fabrication of tapered fiber LPFGs with high repeatability. [ABSTRACT FROM AUTHOR]

Monitoring volatile organic compounds (VOCs), the majority of which are toxic and carcinogenic and pose serious threats to human health and the environment, provides diverse environmental data from industrial and biological processes. In this study, a sensor for the detection of refractive index (RI) and N,N-dimethylformamide (DMF) vapors was developed by electrospinning polyacrylonitrile (PAN) and Ti 3 C 2 MXene/TiO 2 hybrid materials onto a U-shaped evanescent optical fiber (UOFEs). UOFEs coated with PAN/Ti 3 C 2 MXene/TiO 2 films exhibited a 7-fold increase in RI sensitivity, with a maximum RI response value of 540 %/RI unit when compared to bare fibers. The UOFEs exhibited a limit of detection for DMF in the gas of 5.33 ppm at 25 °C. The proposed sensor works on the principle that DMF, which has a high RI, binds to PAN, which is rich in binding sites, via high compatibility and strong dipole–dipole interactions, increasing the RI of the films. • PAN/Ti 3 C 2 MXene/TiO 2 -coated fibers offer a 7-fold increase in RI sensitivity. • Electrospun polymer films exhibit a high concentration of gas adsorption sites. • DMF affinity for PAN enhances film RI and allows for selective detection. • VOC sensing with a fiber-optic sensor exhibits exceptional reproducibility. [ABSTRACT FROM AUTHOR]

In this study, we proposed and validated a temperature fiber sensor based on optical signal capture Mach-Zehnder Interferometer (OSC-MZI) with high-precision temperature measurement capabilities. By creating three air cavities on a continuously offset-spliced no-core fiber (NCF), the optical signal experiences reflection and divergence within these cavities, which enhances the optical path of propagating light. Gradually, filling these NCF cavities with polymethyl methacrylate (PMMA) microspheres to increase the light coupling efficiency and as a sensing unit. Due to the directly contact between the optical fiber waveguide and PMMA, the state of incident lights is easily influenced by the refractive index (RI) variations of PMMA. When the environmental temperature changes, RI of the PMMA with high thermal-optic coefficient shows marked change, which finally leads to the obvious wavelength shift of interference dips in the transmission spectrum. Experimental results show that MZI coated with the polymer materials achieves a temperature sensitivity of up to 2.04 nm/°C within the temperature range of 25–50 °C. Furthermore, as an interference-based MZI sensor, its total length is only 4.75 mm. The proposed OSC-MZI provides a new approach for combining optical fiber sensors with polymer materials. [Display omitted] • A high sensitive temperature sensor is achieved by offset-spliced and point coating method. • The highest temperature sensitivity achieves 2.08 nm/°C within the temperature range of 25–50 °C. • The total length of the fiber sensing structure is only 4.75 mm. • Simple structure, easy fabrication, high mechanical strength, good repeatability. [ABSTRACT FROM AUTHOR]

A new concave shaped high refractive index plasmonic sensor with a micro-channel is proposed in this work, which comprises an analyte channel in the core hole. The sensor is elaborately designed to reduce the interference effect from the metal coating. Furthermore, the impact of the proposed structure on the sensitivity is also investigated by engineering the geometric parameters using the finite element method. We select gold as the plasmonic material in this theoretical study because it is widely used to fabricate plasmonic and metamaterial devices due to its chemical stability and compatibility. According to wavelength interrogation technique, simulations results show that this sensor can obtain maximal wavelength sensitivity of 10,050 nm/refractive index unit. In view of the excellent indicators of this device, it has important development potential in chemical and biological research fields. [ABSTRACT FROM AUTHOR]

In this work, highly sensitive D-shaped photonic crystal fiber (PCF) sensor with plasmonic pyramid grating is introduced and analyzed. The sensing mechanism depends on the coupling between the fundamental core mode and the surface plasmon mode around the plasmonic pyramid grating. The resonance frequency depends on the analyte refractive index (RI) which can be employed to detect the analyte RI changes. The geometrical parameters of the proposed PCF and gold pyramid grating are studied to enhance the RI sensor sensitivity. The simulation results are obtained using full vectorial finite element method. The suggested sensor has higher sensitivity of 5200 nm/RIU than 3340 nm/RIU of the conventional grating design through the studied RI range from 1.355 to 1.385. Therefore, the reported RI sensor can be used for basal cancer cell detection with normal and cancerous cells' refractive indices of 1.36 and 1.38, respectively. Additionally, linear performance is achieved using the reported design. [ABSTRACT FROM AUTHOR]