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AbstractThis article introduces a novel high-sensitivity pressure and temperature sensor utilizing a tapered no-core fiber (NCF) structure created by splicing the NCF with a single-mode fiber (SMF) to form a “SMF-NCF-SMF” structure, tapering the NCF, and applying a polydimethylsiloxane (PDMS) film on the tapered NCF surface. The experimental results show that the device with a diameter of 10 μm has a pressure sensitivity of 86.63 nm/MPa within one to two atmospheres. Furthermore, the temperature sensitivity is −1.97 nm/°C from 30 °C to 60 °C. The sensor exhibits excellent stability and repeatability, and the PDMS film enhances the mechanical strength of the device, indicating promising potential for high-sensitivity pressure and temperature applications. [ABSTRACT FROM AUTHOR]

AbstractWe investigate refractive index (RI) sensors using no-core fibers with diameters of 250 µm, 125 µm, and 62.5 µm. Experiments show that RI sensors with sizes of 62.5 µm and 250 µm are more sensitive than those with a diameter of 125 µm. The maximum RI sensitivity was 4886 nm/RIU. The RI sensor has a high sensitivity, a wide measurement range, and is insensitive to temperature, making it well-suited for a variety of practical applications. [ABSTRACT FROM AUTHOR]

Using the no-core fiber (NCF)–seven-core fiber (SCF) interference structure, two curvature sensors of Michelson interference type and Mach–Zehnder interference (MZI) type are proposed, respectively. The curvature sensor based on Michelson interference shows wavelength-modulation characteristics, the sensitivity is about −22.76 nm/m−1 with a linearity of 0.9823, the temperature sensitivity is only 0.054 nm/°C, and the effect of temperature on curvature can be negligible. The MZI sensor based on an enlarged taper-embedded cascaded structure is an intensity-modulated sensor. The sensitivities are −63.6271 dB m/m−1 and 93.3293 dBm/m−1 for the forward and reverse curvature, respectively, and the linearities are 0.9987 and 0.9930, respectively. But the strain sensitivity (8.357 × 10−4 dBm/με) of the MZI sensor is so tiny, which can avoid the strain cross effect. The two sensors can be used in the detection of the curvature at different (temperature/strain) conditions. [ABSTRACT FROM AUTHOR]

Tunable fiber lasers have the advantages of good beam quality, high integration, and adjustable output wavelength, and they are widely used in fields such as optical fiber communication and optical fiber sensing. The fiber filter is one of the key components of tunable fiber lasers. Among the various filters currently used, multimode interference filters have the advantages of simple structure, convenient implementation, flexible tuning methods, and convenient spectral range design. The structures of multimode interference filters based on multimode fibers, no-core fibers, multi-core fibers, tapered fibers, and other special fibers are introduced in this paper. The working principles and tuning methods are analyzed and the research progress of tunable fiber lasers based on these filters is summarized. Finally, the development trend of tunable fiber lasers based on multimode interference filters is discussed. The rapid development and applications of multimode interference filters can help improve the performance of continuous and pulse lasers as well as promote the practicality of tunable fiber lasers. [ABSTRACT FROM AUTHOR]

In this study, we developed a glucose fiber sensor incorporating heterodyne interferometry to measure the phase difference produced by the chemical reaction between glucose and glucose oxidase (GOx). Both theoretical and experimental results showed that the amount of phase variation is inversely proportional to glucose concentration. The proposed method provided a linear measurement range of the glucose concentration from 10 mg/dL to 550 mg/dL. The experimental results indicated that the sensitivity is proportional to the length of the enzymatic glucose sensor, and the optimum resolution can be obtained at a sensor length of 3 cm. The optimum resolution of the proposed method is better than 0.6 mg/dL. Moreover, the proposed sensor demonstrates good repeatability and reliability. The average relative standard deviation (RSD) is better than 10% and satisfied the minimum requirement for point-of-care devices. [ABSTRACT FROM AUTHOR]

A polydimethylsiloxane (PDMS) coated fiber-optic sensor based on the Mach–Zehnder interference is presented. The sensor is composed of multimode fiber, thin-core fiber, and no-core fiber. Owing to the different cladding and core sizes of the three fibers, the Mach–Zehnder interference and strong external evanescent wave are generated. To improve the temperature sensitivity, we introduce the PDMS with high thermal expansion coefficient and high negative thermo-optic coefficient into the sensor. Results show that the temperature sensitivity of the original fiber-optic sensor is 0.0177 dB/℃ in the range of 40–80℃. After coating PDMS, the temperature sensitivity of the proposed sensor is improved to 0.0356 dB/℃. At the same time, the heating and cooling experiments and the long-time temperature testing are performed to demonstrate the repeatability and stability of the sensor. Owing to its excellent performances, the proposed sensor exhibits a great potential in the temperature sensing application. [ABSTRACT FROM AUTHOR]

In this work, a set of titanium nitrides thin-films was synthesized with the technique of reactive RF and DC magnetron-sputtering. To demonstrate the versatility and effectiveness of the deposition technique, thin films were deposited onto different fiber structures varying the deposition parameters for optical applications as saturable absorbers in passively q-switched fiber lasers and as lossy mode resonance fiber refractometers. After deposition, optical and electronical properties of samples were characterized by UV–Vis and XPS spectroscopies, respectively. Samples presented coexisting phases of Ti nitride and oxide, where the nitride phase was non-stoichiometric metallic-rich, with a band gap in the range of Eg = 3.4–3.7 eV. For all samples, glass substrates were used as templates, and on top of them, optical fibers were mounted to be covered with their respective titanium compounds. [ABSTRACT FROM AUTHOR]

The measurement of pH has received great attention in diverse fields, such as clinical diagnostics, environmental protection, and food safety. Optical fiber sensors are widely used for pH sensing because of their great advantages. In this work, an optical fiber pH sensor is fabricated, by combining the merits of the multimode interference configuration and pH-sensitive polyaniline/polyacrylic acid (PAni/PAA) coatings, which was successfully in situ deposited on the no-core fiber (NCF) by the layer-by-layer (LBL) self-assembly method. The sensors' performance was experimentally characterized when used for pH detection. It has a high sensitivity of 0.985 nm/pH and a great linear response in a universal pH range of 2–12. The response time and recovery time is measured to be less than 10 s. In addition, its temperature sensitivity is tested to be about 0.01 nm/°C with a low temperature crosstalk effect, which makes it promising for detecting pH in the liquid phase with temperature variation. The sensors also demonstrated easy fabrication, good stability, and repeatability, which are adapted to pH detection in most practical applications. [ABSTRACT FROM AUTHOR]

Tunable fiber lasers have the advantages of good beam quality, high integration, and adjustable output wavelength, and they are widely used in fields such as optical fiber communication and optical fiber sensing. The fiber filter is one of the key components of tunable fiber lasers. Among the various filters currently used, multimode interference filters have the advantages of simple structure, convenient implementation, flexible tuning methods, and convenient spectral range design. The structures of multimode interference filters based on multimode fibers, no-core fibers, multi-core fibers, tapered fibers, and other special fibers are introduced in this paper. The working principles and tuning methods are analyzed and the research progress of tunable fiber lasers based on these filters is summarized. Finally, the development trend of tunable fiber lasers based on multimode interference filters is discussed. The rapid development and applications of multimode interference filters can help improve the performance of continuous and pulse lasers as well as promote the practicality of tunable fiber lasers.

A Mach-Zehnder interference fiber-optic sensor for simultaneous measurement of temperature and refractive index (RI) is proposed and fabricated. In the sensing structure, no-core fiber (NCF) is cascaded with fiber Bragg grating (FBG), two enlarged tapers between NCF and the single-mode fiber serve as the light coupler, and the shrinked taper between two NCFs is used to enhance the interference effect. The different diameter influence of the shrinked taper on the sensing performance of RI and temperature has been evaluated, and the optimum diameter has been obtained. By monitoring the wavelength shift of FBG and the interference dip in the transmission spectra, the simultaneous measurement of temperature and RI is achieved. The maximum sensitivities of the attenuation peak to RI and temperature are 201.7157 nm/RIU and 87.4 pm/°C, respectively. The sensitivities of FBG to RI and temperature are 80.6 pm/RIU and 13.5 pm/°C, respectively. The transformation matrix has also been given. Own to the simple fabrication and high sensitivity, the sensor has the potential application in simultaneous measurement of multiple parameters.

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The label-free detection of SARS-CoV-2 spike protein is demonstrated by using slightly tapered no-core fiber (ST-NCF) functionalized with ACE2. In the fabricated sensor head, abrupt changes in the mode-field diameter at the interfaces between single-mode fiber and no-core fiber excite multi-guided modes and facilitate multi-mode interference (MMI). Its slightly tapered region causes the MMI to be more sensitive to the refractive index (RI) modulation of the surrounding medium. The transmission minimum of the MMI spectrum was selected as a sensor indicator. The sensor surface was functionalized with ACE2 bioreceptors through the pretreatment process. The ACE2-immobilized ST-NCF sensor head was exposed to the samples of SARS-CoV-2 spike protein with concentrations ranging from 1 to 104 ng/mL. With increasing sample concentration, we observed that the indicator dip moved towards a longer wavelength region. The observed spectral shifts are attributed to localized RI modulations at the sensor surface, which are induced by selective bioaffinity binding between ACE2 and SARS-CoV-2 spike protein. Also, we confirmed the capability of the sensor head as an effective and simple optical probe for detecting antigen protein samples by applying saliva solution used as a measurement buffer. Moreover, we compared its detection sensitivity to SARS-CoV-2 and MERS-CoV spike protein to examine its cross-reactivity. In particular, we proved the reproducibility of the bioassay protocol adopted here by employing the ST-NCF sensor head reconstructed with ACE2. Our ST-NCF transducer is expected to be beneficially utilized as a low-cost and portable biosensing platform for the rapid detection of SARS-CoV-2 spike protein. [ABSTRACT FROM AUTHOR]

Simultaneously increasing the sensitivity and dynamic range of an optical fiber sensor is desired and yet challenging. In this article, we demonstrate an optical fiber curvature sensor based on a no-core fiber (NCF) cascaded with a hollow-core fiber (HCF), realizing simultaneously high sensitivity and a large dynamic range with the assistance of machine learning analysis. The sensor is fabricated by simply fusion splicing a section of NCF and HCF to two single-mode fibers (SMFs), forming the SMF-NCF-HCF-SMF hybrid structure. It is shown that the multimode interference in the NCF can increase the sensitivity of the device for curvature measurements, compared to the conventional SMF-HCF-SMF structure. However, the enhanced sensitivity poses a limitation on the dynamic range of the proposed curvature sensor. We propose the use of machine learning to analyze the measured spectra of the sensor device, achieving one-to-one mapping between the measured raw spectrum and the exerted curvature on the sensor, and thereby the issue of the limited dynamic range is resolved. The proposed strategy, enabling the co-existence of high sensitivity and a large dynamic range, is a highly generalizable technique and can be extended to other optical fiber sensors for measuring other physical, chemical, and biological quantities in different applications. [ABSTRACT FROM AUTHOR]

Liquid-level sensors are required in modern industrial and medical fields. Optical liquid-level sensors can solve the safety problems of traditional electrical sensors, which have attracted extensive attention in both academia and industry. We propose a distributed liquid-level sensor based on optical frequency domain reflectometry and with no-core fiber. The sensing mechanism uses optical frequency domain reflectometry to capture the strong reflection of the evanescent field of the no-core fiber at the liquid–air interface. The experimental results show that the proposed method can achieve a high resolution of 0.1 mm, stability of ±15 μm, a relatively large measurement range of 175 mm, and a high signal-to-noise ratio of 30 dB. The sensing length can be extended to 1.25 m with a weakened signal-to-noise ratio of 10 dB. The proposed method has broad development prospects in the field of intelligent industry and extreme environments. [ABSTRACT FROM AUTHOR]

In this study, we developed a glucose fiber sensor incorporating heterodyne interferometry to measure the phase difference produced by the chemical reaction between glucose and glucose oxidase (GOx). Both theoretical and experimental results showed that the amount of phase variation is inversely proportional to glucose concentration. The proposed method provided a linear measurement range of the glucose concentration from 10 mg/dL to 550 mg/dL. The experimental results indicated that the sensitivity is proportional to the length of the enzymatic glucose sensor, and the optimum resolution can be obtained at a sensor length of 3 cm. The optimum resolution of the proposed method is better than 0.6 mg/dL. Moreover, the proposed sensor demonstrates good repeatability and reliability. The average relative standard deviation (RSD) is better than 10% and satisfied the minimum requirement for point-of-care devices.

Keywords: -Fe2O3/ZrO2; fluoride-ion sensor; Michelson interferometer; no-core fiber; thin-core fiber EN -Fe2O3/ZrO2 fluoride-ion sensor Michelson interferometer no-core fiber thin-core fiber 269 278 10 03/11/22 20220301 NES 220301 1 Introduction In recent years, more attention has been paid to the study of anions in solutions because of the important applications in the fields of biology, environment, and industrial production [[1]]. The Michelson interference intensity can be expressed as [[16]], HT ht Graph (1)in which I I i SB core sb and I I i SB cladding sb are, respectively, the propagation intensities of core mode and cladding mode in TCF. Fiber-optic Michelson interferometer based on -Fe2O3/ZrO2 sensing membrane and its application in trace fluoride-ion detection. [Extracted from the article]

A novel fiber-optic surface plasmon resonance (SPR) hydrogen sulfide gas sensor based on silver/3-aminpropyltriethoxysilane/cuprous oxide modified graphene sheets (Ag/APTES/ ${\text{Cu}}_{x}\text{O}$ (${x}=1$ , 2)-MGS) composite membrane coated no-core fiber (NCF) was proposed and fabricated. The NCF is sandwiched between two segments of multi-mode fiber (MMF), forming the fiber-optic sensing unit with MMF1-NCF-MMF2 structure. The comparative experiment of sensing performance of two sensors coated respectively by Ag/APTES and Ag/APTES/ ${\text{Cu}}_{x}\text{O}$ (${x}=1$ , 2)-MGS film is investigated in detail. The results show that the sensor with Ag/APTES/ ${\text{Cu}}_{x}\text{O}$ (${x}=1$ , 2)-MGS film has better sensing performance for hydrogen sulfide in the concentration range of 10–60 ppm. After adsorption of hydrogen sulfide, the resonance wavelength of the sensor with Ag/APTES/ ${\text{Cu}}_{x}\text{O}$ (${x}=1$ , 2)-MGS film shows the red shift, and the total shift is about 132.642 nm. In the concentration range of 10–40 ppm, the sensitivity is 3385.67 pm/ppm, the detection limit is 0.56±0.12 ppm, and the response time is 27 s. The sensor has good temperature, humidity, time stability and excellent selectivity, and can be potentially used in the trace hydrogen sulfide detection. [ABSTRACT FROM AUTHOR]

A microfiber interferometer based on a single-mode tapered-no-core single-mode (STNCS) fiber structure coated with a thin layer of graphene oxide (GO) was proposed and demonstrated as a relative humidity (RH) sensor. The STNCS fiber structure has a strong evanescent field, which can sensitively response to changes in the surrounding refractive index. The moisture-sensitive material GO was used to form the film on the no-core fiber by a dip impregnation method. The effects of GO concentration and the tapered waist diameter on RH sensitivity were studied experimentally. The experimental results showed that a thinner waist diameter provides higher RH sensitivity. When the concentration of GO solution is 0.01 mg/mL, the STNCS fiber structure with a waist diameter of 2.9 μm performs better in RH sensing; a high sensitivity of 0.461 nm/%RH in the range of 40-98% was achieved. The sensor has good stability with wavelength fluctuations of ±0.009 nm and ±0.008 nm over 60 mins at RHs of 50% and 80%, respectively. The proposed fiber optic humidity sensor has a high sensitivity over a wide measurement range and offers good stability showing promising potential application in the field of RH sensing.

A ring-shaped structure based on polymer coated no-core fiber (NCF) is presented for simultaneous temperature and pressure measurements. The sensing structure is fabricated by splicing the partially coated NCF between single-mode fibers and bending it into a loop. The anti-resonance effect is formed in the NCF and the resonant dips have wavelength shift responses to the temperature and displacement variations. It should be noted that the increase of temperature leads to a decrease in the polymer coating refractive index and changes the displacement measurement sensitivity, thus, the compensation algorithm is required to resolve this cross-sensitivity problem. Combining the ring-shaped structure with the elastic diaphragm encapsulation can convert seawater pressure into structural deformation displacement to achieve pressure measurement. The experimental and simulation results show that this sensor offers a high temperature sensitivity of −5.098 nm/°C with the resolution of $2\times 10^{-4}$ °C and a pressure sensitivity of −2.368 nm/MPa with the resolution of $0.4\times 10^{-3}$ MPa. In addition, the sensing performances could be changed by adjusting the NCF size and the ring-shaped structure diameter, which enables this sensing structure to have a broader application potential for seawater parameter measurement. [ABSTRACT FROM AUTHOR]

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A novel liquid refractive index sensor based on the connected single-mode fiber (SMF), no-core fiber (NCF), four-core fiber (FCF), and silver mirror (SM) to form an SMF–NCF–FCF–SM Michelson probe structure is proposed and fabricated. The change of light field in the probe structure has been simulated by the light-beam propagation method. The theoretical results show that light is excited in the NCF and couples into the cores and cladding of FCF at the junction of NCF and FCF. The interference fringes are generated between the cladding modes and core modes of FCF. The sensitivities of the probe in NaCl, sucrose, and glycerol are 171.75 dB/RIU, 121.41 dB/RIU, and 207.50 dB/RIU, respectively. The temperature sensitivity is 0.05 nm/°C, and the intensity change of temperature (≤0.046 dB/°C) is very small and has little effect on the liquid refractive index. Thus, the cross-sensitivity of temperature for the liquid refractive index can be removed. The proposed probe structure has the advantages of easy fabrication, good stability, and linear response, having potential application in the liquid refractive index monitoring environments. [ABSTRACT FROM AUTHOR]

The measurement of pH has received great attention in diverse fields, such as clinical diagnostics, environmental protection, and food safety. Optical fiber sensors are widely used for pH sensing because of their great advantages. In this work, an optical fiber pH sensor is fabricated, by combining the merits of the multimode interference configuration and pH-sensitive polyaniline/polyacrylic acid (PAni/PAA) coatings, which was successfully in situ deposited on the no-core fiber (NCF) by the layer-by-layer (LBL) self-assembly method. The sensors’ performance was experimentally characterized when used for pH detection. It has a high sensitivity of 0.985 nm/pH and a great linear response in a universal pH range of 2–12. The response time and recovery time is measured to be less than 10 s. In addition, its temperature sensitivity is tested to be about 0.01 nm/°C with a low temperature crosstalk effect, which makes it promising for detecting pH in the liquid phase with temperature variation. The sensors also demonstrated easy fabrication, good stability, and repeatability, which are adapted to pH detection in most practical applications.

In this work, we thoroughly evaluate different types of lateral-offset Mach-Zehnder interferometers (MZIs) to search for the optimized design. Three different structures (SNS, SMNMS, and SNNNS. S: single-mode fiber; N: no-core fiber; M: multi-mode fiber) are proposed and their performances are compared numerically. Besides, for the SNNNS sensor, the effect of different no-core fiber (NCF) diameters on the sensor performance is discussed. The numerical analyses demonstrate that the SNNNS sensor with NCF diameter of $61.5{\mu m}$ shows the best performance among all the sensors. The experimental results confirm that, when compared with the SNS sensor, the SNNNS sensor is insensitive to the lateral-offset distance. It ensures an excellent interference performance within a large offset range, which facilitates the sensor fabrication. The optimized design of the sensor device shows the potential integration applications in the ultra-low concentration biomolecule detection field. [ABSTRACT FROM AUTHOR]

A singlemode-multimode-singlemode (SMS) fiber structure consists of a short section of multimode fiber fusion- spliced between two SMS fibers. The mechanism underpinning the operation of an SMS fiber structure is multimode interference and associated self-imaging. SMS structures can be used in a variety of optical fiber systems but are most commonly used as sensors for a variety of parameters, ranging from macro-world measurands such as temperature, strain, vibration, flow rate, RI and humidity to the micro-world with measurands such as proteins, pathogens, DNA and specific molecules. While traditional SMS structures employ a short section of standard multimode fiber, a large number of structures have been investigated and demonstrated over the last decade involving the replacement of the multimode fiber section with alternatives such as a hollow core fiber or a tapered fiber. The objective of replacing the multimode fiber has most often been to allow sensing of different measurands or to improve sensitivity. In this paper, several different categories of SMS fiber structures, including traditional SMS, modified SMS and tapered SMS fiber structures are discussed with some theoretical underpinning and reviews of a wide variety of sensing examples and recent advances. The paper then summarizes and compares the performances of a variety of sensors which have been published under a number of headings. The paper concludes by considering the challenges faced by SMS based sensing schemes in terms of their deployment in real world applications and discusses possible future developments of SMS fiber sensors. [ABSTRACT FROM AUTHOR]

A simple and cost-efficient fiber sensor based on multi-mode interference and core diameter mismatch technologies is proposed to detect the refractive index (RI) in this paper. The refractometer is fabricated by embedding a section of no-core fiber between a section of multimode fiber and dispersion-compensating fiber. Since the power distribution in the fiber will change with the effective refractive index of the cladding, the extinction ratio can be used to characterize the RI sensitivity. The experimental results show that a linear response is obtained with a RI sensitivity of 392.585 dB/RIU in the range of 1.333–1.374. Featured with high sensitivity, simple construction, easy fabrication, low cost and good repeatability, the novel refractometer has potential applications in practical chemical detecting and portable biological sensing. • A refractometer based on multi-mode interference and core diameter mismatch technologies is proposed. • The sensor head merely includes a MMF, NCF, and DCF orderly through automatic splicing without any special processing. • Utilizing extinction ratio demodulation, a RI sensitivity of 392.585 dB/RIU is achieved from 1.3330 to 1.3740, 5-7 times higher than untreated sensors. • Featured with high sensitivity, simple construction, easy fabrication, low cost and good repeatability, it holds potential for practical detection. [ABSTRACT FROM AUTHOR]

Liquid-level sensors are required in modern industrial and medical fields. Optical liquid-level sensors can solve the safety problems of traditional electrical sensors, which have attracted extensive attention in both academia and industry. We propose a distributed liquid-level sensor based on optical frequency domain reflectometry and with no-core fiber. The sensing mechanism uses optical frequency domain reflectometry to capture the strong reflection of the evanescent field of the no-core fiber at the liquid–air interface. The experimental results show that the proposed method can achieve a high resolution of 0.1 mm, stability of ±15 μm, a relatively large measurement range of 175 mm, and a high signal-to-noise ratio of 30 dB. The sensing length can be extended to 1.25 m with a weakened signal-to-noise ratio of 10 dB. The proposed method has broad development prospects in the field of intelligent industry and extreme environments.

In this paper, the design and simulation of a new optical fiber-based relative humidity (RH) sensor formed by spherical hydrogel droplets over the no-core fiber is stablished. The introduced sensor is capable of detecting humidity changes based on Evanescent waves and splice losses. Refractive index of PEGDMA hydrogel sphere changes with variation of humidity which can cause intensity changes in fiber. The sensor structure includes a no-core fiber in the length of 4 mm connected to two multimode fibers which performs sensing by three hydrogel spheres with diameter of 1 mm. The sensor operation wavelength is adjusted at 631nm. The results show that the designed structure has linear response for humidity in the range of 20 to 95%RH. In the meantime, the obtained sensitivity is about 0.13643 mW /% RH. The simulated sensor has some useful advantages in addition to having some associated with the two primary sensors including the simplicity of the structure and the sensor's function which can be explained by reduction of the length of sensing portion and increase in the sensitivity and range of humidity detection.

We demonstrated temperature-insensitive, label-free detection of SARS-CoV-2 spike protein (SSP) by harnessing the complementary refractive index and temperature dependence of multi-mode interference (MMI) created by a no-core fiber (NCF) and phase-matched resonance generated by a long-period fiber grating (LPFG). To combine MMI and grating resonance, primarily sensitive to the surrounding medium refractive index (SMRI) and ambient temperature, respectively, a fiber-optic transducer was fabricated by splicing an NCF segment with an LPFG inscribed on double-clad fiber. The transducer was functionalized with human ACE2 receptors to selectively capture SSP. The functionalized sensor head exhibited high SSP selectivity, with overall average wavelength displacements of ∼253.33 and ∼160.00 pm in PBS and saliva, respectively, for SSP with concentrations ranging from 1 to 104 ng/mL. These spectral shifts are associated with localized SMRI modulations on the sensor surface induced by specific binding between SSP and ACE2. We also examined the cross-reactivity of the sensor head for MERS-CoV spike protein to confirm its SSP specificity. Moreover, we proved the capability of temperature-independent SSP detection and ambient temperature measurement by scrutinizing the temperature effect on the sensor performance. Our functionalized fiber transducer showed great promise as a temperature-insensitive and portable platform for rapid SSP detection. [Display omitted] • Temperature-insensitive detection of SARS-CoV-2 spike protein (SSP) was achieved. • We combined fiber multi-mode interference with grating resonance for the detection. • A no-core fiber spliced with a long-period fiber grating was used as a sensor head. • The affinity between ACE2 receptors and SSP was studied in PBS and saliva. • SSP specificity was confirmed by cross-reactivity tests for MERS-CoV spike protein. • We also confirmed ambient temperature readout capability during the SSP detection. [ABSTRACT FROM AUTHOR]

Highlights • Sensing structure based on no-core fiber cascaded with fiber Bragg gratings. • Temperature self-compensative magnetic field sensing. • All-fiber magnetic field sensing based on differential intensity interrogation. • Low temperature cross-sensitivity. Abstract A kind of fiber-optic magnetic field sensing structure based on no-core fiber (NCF) cascaded with fiber Bragg gratings (FBGs) is proposed. The NCF is sandwiched between two pieces of singlemode fibers to form singlemode-no-core-singlemode (SNS) interferometric fiber structure, which is cladded with the magnetic field sensitive material (i.e. magnetic fluid). Measurement is done by monitoring the differential intensity of the signal at the two Bragg peak wavelengths, which increases the magnetic field sensitivity and can decrease the cost of the pragmatic sensing system. Due to the similar thermal effect of NCF and FBG, the temperature cross-sensitivity is greatly reduced. The achieved magnetic field sensitivity is 0.1774 dB/mT at 0–6 mT and 1.9296 dB/mT at 6–18 mT. The temperature sensitivity is only 0.0623 dB/ °C in the range of 21–35 °C. The corresponding device does not necessitate intensity-stabilized broadband light source as well. [ABSTRACT FROM AUTHOR]

By coating graphene oxide (GO) onto the surface no-core fiber (NCF), we designed a single-mode no-core single-mode (SNS) fiber Step-Index Distribution sucrose sensor. With wavelength demodulation and the beam propagation method (BPM), the sensor without a GO coating was studied in the low RI range of 1.33~1.389, and the high RI range of 1.389~1.4185. The experiments show that the RI sensitivity of the sensor respectively reaches 132.9 nm/RIU and 292.22 nm/RIU. Both the numerical simulation and the experiments are highly consistent with the theoretical analysis results. Especially, having coated GO on the NCF for sensitization, a high sensitivity was achieved for the response to sucrose concentration solutions. The sensor’s RI sensitivity was increased from 132.9 nm/RIU up to 1348.67 nm/RIU in the ultra-narrow range of 1.33 to 1.3385. This result provides a theoretical and experimental basis for the enrichment and development of sensor detection with a low threshold sucrose concentration.

• A MZI sensor for simultaneous measurement of three parameters is developed. The MZI sensor is spliced by SMF-NCF-TCF-SMF. • The maximum sensitivities of temperature, strain and curvature are 248.7 pm/℃, −2 pm/με and −24.784 nm/m−1 respectively. • The sensitivity matrix of three resonance wavelengths is constructed and realizing three-parameters simultaneous measurement. A Mach-Zehnder interferometer (MZI) sensor for simultaneous measurement of three parameters is developed. The sensor structure is spliced by single-mode fiber, no-core fiber, thin-core fiber (TCF) and single-mode fiber in turn, where the sensing element is TCF. Due to the different core diameters of three types of optical fibers, the light in the transmission process excites a number of higher-order modes and interacts to form a mode interferometer. The interference spectrum produces many resonance peaks that are sensitive to different parameters. We select three resonant peaks sensitive to temperature, strain and bending for measuring temperature, strain and curvature. The experimental results display that the resonant wavelength of the sensor transmission spectrum drifts linearly as a function of the environmental parameters, with a maximum sensitivity of 248.7 pm/℃ for temperature, −2 pm/με for strain and −24.784 nm/m−1 for curvature. Selecting three resonance peaks to construct a measurement matrix realizes three-parameters simultaneous measurement and eliminates the cross-sensitivity among the three physical quantities. In addition, the proposed sensor possesses appealing characteristics of facile manufacturing process, compact size, high sensitivity, good linear fitting, robustness and low cost, and the sensor has a broad application prospect in the field of multi-parameter simultaneous measurement. In short, only one MZI in the optical fiber can realize the simultaneous monitoring of temperature, strain and bending, which provides a new solution for creating a compact multi-parameter sensor for structural health monitoring. [ABSTRACT FROM AUTHOR]

A novel refractive index fiber laser sensor based on a fiber Bragg grating (FBG) integrated with a section of no-core fiber (NCF) is proposed and experimentally demonstrated. The oscillating wavelength of the fiber laser is only decided by the central wavelength of the FBG that is insensitive to the surrounding refractive index (SRI). However, the output power of the fiber laser varies with the SRI because the laser output is filtered by the edge of the bandpass filter (BPF) formed by the multimode interference (MMI) effect in the NCF, which is sensitive to the SRI. By measuring the variation of the output power, a cost-effective power detection RI sensor could be realized. The fiber laser sensor has a linear relationship with SRI and a sensitivity of 113.73 dB/RIU in the RI range of 1.333-1.4076.

Highlights • A highly sensitive liquid level monitoring based on discrete wavelength sampling method is proposed and demonstrated. • The proposed method can archive the highly sensitive resolution of 0.5 mm and the large measurement range of 50 cm. • It is the first time to apply wavelength sampling method to liquid level measurement based on SNS structure. • The sensor has the ability to discriminate different RI of liquid. Abstract The liquid level fiber sensor with a measurement range of about 50 cm is fabricated through a kind of single-no-core-single mode (SNS) structure. It is found that the change of liquid level will only induce the wavelength shifts of the whole interference spectrum. The liquid level monitoring with sub-millimeter resolution can be achieved by the proposed discrete wavelength sampling method. In the experiment, the transmit interference spectrum is sampled by a tunable laser with a step of 0.5 nm, then constructed by cubic spline interpolation algorithm. The wavelength shifts can be obtained through a minimum variance demodulation algorithm. Experiment results exhibit an excellent linear response (R2 = 0.99993) and the high sensitivity response to different liquid refractive index (RI). And the sensitivity of the sensor is 23.38 pm/mm with the liquid RI of 1.3505. In particular, the sensing resolution of the liquid level can reach 0.5 mm. This anti-electromagnetic interference technique with such high resolution and large sensing range can find the promising applications of monitoring the left fuel in the aircraft tanker, the surface evaporation in the water resource cycle, etc. [ABSTRACT FROM AUTHOR]

The light-beam expansion effect of a no-core fiber on the focusing performance of an ultra-small gradient-index fiber probe is investigated with a view to optimizing the optical performance of such probes. By taking the variable relationship between the focusing performance (including the working distance and the focusing spot size) of the probe and the length of the no-core fiber as the criterion, the effective beam expansion length of the no-core fiber in the ultra-small gradient-index fiber probe is calculated based on the basic properties of the Gaussian beam. Verification and analysis are done by numerical calculations and experimental measurements, respectively. The obtained results show that the working distance of an ultra-small gradient-index fiber probe can be increased effectively by adding a no-core fiber; however, this will lead to increasing the focusing spot size. For the parameters of the fiber probe studied here, the effective beam expansion length of the no-core fiber spacer is less than 0.357 mm. [ABSTRACT FROM AUTHOR]

An optical fiber sensor based on the single-mode/no-core/single-mode (SNS) core-offset technology along with a Faraday rotator mirror structure has been proposed and experimentally demonstrated. A transverse optical field distribution of self-imaging has been simulated and experimental parameters have been selected under theoretical guidance. Results of the experiments demonstrate that the temperature sensitivity of the sensor is 0.0551 nm/°C for temperatures between 25 and 80 °C, and the correlation coefficient is 0.99582. The concentration sensitivity of the device for sucrose and glucose solutions was found to be as high as 12.5416 and 6.02248 nm/(g/ml), respectively. Curves demonstrating a linear fit between wavelength shift and solution concentration for three different heavy metal solutions have also been derived on the basis of experimental results. The proposed fiber-optic sensor design provides valuable guidance for the measurement of concentration and temperature. [ABSTRACT FROM AUTHOR]

We demonstrate a whispering-gallery-mode (WGM) microsphere resonator on a no-core fiber (NCF) tip. The device is fabricated by fusion splicing of a single-mode fiber (SMF) with a short section of NCF, and followed by gluing a microsphere on the arc end of NCF. The incident light propagates from the SMF core to the microsphere via the NCF, and when the angle between the SMF-NCF axis and the center of microsphere is appropriate, WGM is excited. The device is simple in structure, easy in fabrication, and of low cost, and can be used for temperature sensing with a good sensitivity of 21.34 pm/°C. [ABSTRACT FROM AUTHOR]

A fiber laser refractometer based on single-mode-no-core-single-mode (SNS) structure cascaded with a FBG is proposed and experimentally demonstrated. The output wavelength of the fiber laser keeps constant because the oscillating wavelength is only determined by the central wavelength of the FBG which is insensitive to the surrounding refractive index (SRI). However, the output power is sensitive to the SRI because the intracavity loss of the fiber laser varies with the SRI. A cost-effective power detection refractometer with reflective operation can be realized through measuring the variation of the fiber laser’s output power. The refractometer has a sensitivity of 195.52 dB/RIU and 365.52 dB/RIU in the RI range of 1.3330–1.3687 and 1.3687–1.4135, respectively. Moreover, the refractometer can also be used for temperature measurement through discriminating the output wavelength of the fiber laser. [ABSTRACT FROM AUTHOR]

A novel dual-point reflective fiber-optic refractometer based on multimode interference (MMI) effect and fiber Bragg grating (FBG) reflection is proposed and experimentally demonstrated, which adopts parallel structure. Each point of the refractometer consists of a single mode-no core-single mode fiber (SNS) structure cascaded with a FBG. Assisted by the reflection of FBG, refractive index (RI) measurement can be achieved by monitoring the peak power variation of the reflected FBG spectrum. By selecting different length of the no core fiber and center wavelength of the FBG, independent dual-point refractometer is easily realized. Experiment results show that the refractometer has a nonlinear relationship between the surrounding refractive index (SRI) and the peak power of the reflected FBG spectrum in the RI range of 1.3330–1.4086. Linear relationship can be approximately obtained by dividing the measuring range into 1.3330–1.3611 and 1.3764–1.4086. In the RI range of 1.3764–1.4086, the two sensing points have higher RI sensitivities of 319.34 dB/RIU and 211.84 dB/RIU, respectively. [ABSTRACT FROM AUTHOR]

A differential intensity-modulated refractive index (RI) sensor consisting of a no-core fiber (NCF) filter, a circulator and two fiber Bragg gratings (FBGs) is proposed and demonstrated. A section of the NCF is sandwiched between two parts of single mode fibers (SMFs) to form a band-pass filter. The Bragg wavelengths of the FBGs are chosen at the two edges of the filter, respectively. The peak wavelength of the NCF filter has a red-shift with the increase of the surrounding refractive index (SRI) while the Bragg wavelengths have no change, which results in the variation of the difference of the two FBGs reflective intensities, thus the differential intensity modulation to the SRI can be accomplished. Compared with directly connecting the NCF filter and the FBGs, this sensing structure can increase the output power so as to improve the measuring resolution. The experimental results show that the RI sensitivities are −99.191 dB/RIU and −139.958 dB/RIU at the range of 1.3329–1.3781 and 1.3781–1.401, respectively. In addition, the disturbance from the light source fluctuation and temperature cross sensitivity can be minimized effectively, which has great potential in actual applications. [ABSTRACT FROM AUTHOR]

In the present study we report the fabrication and characterization of no-core fiber sensor (NCFS) using smart hydrogel coating for pH measurement. The no-core fiber (NCF) is stubbed between two single-mode fibers with SMA connector before immobilizing of smart hydrogel. The wavelength interrogation technique is used to calculate the sensitivity of the proposed sensor. The result shows a high sensitivity of 1.94 nm/pH for a wide range of pH values varied from 3 to 10 with a good linear response. In addition to high sensitivity, the fabricated sensor provides a fast response time with a good stability, repeatability and reproducibility. [ABSTRACT FROM AUTHOR]

• The sensing probe of UTNCF can greatly improve the sensitivity of the sensor because of its strong evanescent field. The probe is achieved by bending the no-core fiber (NCF) to the minimum diameter of 20 μm and bending to 60°. So it is simple to operate, low in cost and has good stability. • The salinity sensitivity of 0.69 nm/% in the salinity range of 0–15 % is achieved, which is higher than that of other sensors with singlemode-multimode-singlemode (SMS) structure owing to the strong evanescent field of UTNCF. The temperature and salinity are measured simultaneously by using matrix transfer method. An all fiber seawater salinity and temperature sensor based on U-shaped tapered no-core fiber (UTNCF) is proposed for the first time. The sensing structure is composed of a piece of UTNCF spliced between two sections of single mode fiber (SMF). The seawater with different salinity and temperature is used as the cladding of UTNCF. Because the dips of transmission spectra have different sensitivities to the surrounding seawater salinity and temperature, the measurement of salinity and temperature of the seawater can be realized. The sensitivities of optical fiber salinity sensors based on no-core-fiber (NCF), tapered NCF (TNCF), and UTNCF are measured. On this basis, UTNCF is selected as the probe of salinity and temperature sensor. The temperature response of the sensor based on UTNCF is also measured. The salinity sensitivity of 0.69 nm/% in the salinity range of 0–15 % and the temperature sensitivity of −0.11 nm/°C in the temperature range of 19.2−35.3°C are achieved. The problem of cross sensitivity between temperature and salinity is solved by using matrix transfer method. The sensor is simple to operate, low in cost and has good stability. [ABSTRACT FROM AUTHOR]

A relative humidity (RH) sensor based on single-mode-no-core-single-mode fiber (SNCS) structure is proposed and experimentally demonstrated. The agarose gel is coated on the no-core fiber (NCF) as the cladding, and multimode interference (MMI) occurs in the SNCS structure. The transmission spectrum of the sensor is modulated at different ambient relative humidities due to the tunable refractive index property of the agarose gel film. The relative humidity can be measured by the wavelength shift and intensity variation of the dip in the transmission spectra. The humidity response of the sensors, coated with different concentrations and coating numbers of the agarose solution, were experimentally investigated. The wavelength and intensity sensitivity is obtained as -149 pm/%RH and -0.075 dB/%RH in the range of 30% RH to 75% RH, respectively. The rise and fall time is tested to be 4.8 s and 7.1 s, respectively. The proposed sensor has a great potential in real-time RH monitoring. [ABSTRACT FROM AUTHOR]

A differential intensity modulation refractometer based on single-mode-no-core-single-mode (SNS) structure cascaded with two fiber Bragg gratings (FBGs) is proposed. The SNS structure acts as a band pass fiber (BPF), and the Bragg wavelengths of the two FBGs located at each edge of the BPF. Because the interference spectrum of the SNS is sensitive to refractive index (RI) but FBG is not, the reflective powers of the FBGs are modulated by the surrounding refractive index (SRI) and have contrary changing trends. The reflective intensity difference of the FBGs is used to describe the change of the SRI, which not only increases the RI sensitivity but diminishes the influence of the input light fluctuation as well. In addition, as the SNS and the FBGs have approximate temperature sensitivities, this sensor can accomplish temperature self-compensation. The experiments show that the RI sensitivity of this sensor is about –199.6 dB/RIU in the range of 1.3326–1.3702 and –355.5 dB/RIU in the range of 1.3702–1.4066, which is greater than the traditional SNS-FBG structure. However, its temperature sensitivity is only 0.0148 dB/°C. When the input light varies 7 mW, the shift of the intensity difference is only 0.28 dB, which is much smaller than the intensity shift of the single FBG. [ABSTRACT FROM PUBLISHER]

The present work describes the fabrication and characterization of an optical fiber pH sensor using a sol--gel technique. The sensing head configuration is incorporated using a short section of no-core fiber, coated with tetraethyl orthosilicate and spliced at the end of a single mode fiber with a bulge. Different types of indicators (bromophenol blue, cresol red, and chlorophenol red) were used to achieve a wide pH range from 2 to 13. High sensitivities of the fabricated device were found to be 1.02 and -0.93 nm/pH for acidic and alkaline solutions, respectively. From the characterization results, it was noted that there is an impact of ionic strength and an effect of the temperature of liquid on the response characteristic, which is an advantage of the existing device over the other pH sensors. The fabricated sensor exhibited good reflection spectrum, indicating a blueshift in resonance wavelength for alkaline solutions and a redshift for acidic solutions. [ABSTRACT FROM AUTHOR]

We describe and demonstrate a liquid level sensor by parallel connection of two liquid level sensors that consists of single-mode-no-core-single-mode fiber structure followed by a fiber Bragg grating. The parallel connection can extend measurement range of liquid level. The structure parameters are optimized and the performance of the sensor is experimentally demonstrated. Results show that the sensor can achieve a linear sensitivity of 0.196 dB/mm within the liquid level range of 0–59 mm and 0.233 dB/mm within 59–117 mm. It has great potential in industrial applications due to its simple structure and extended measurement range, also it can be applied in multi-point measurement. [ABSTRACT FROM PUBLISHER]

A multipoint fiber optic sensor based on two cascaded multimode interferometer (MMI) and fiber Bragg grating (FBG) structures is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The MMI is fabricated by splicing a section of no-core fiber (NCF) with two single-mode fibers. The suitable NCF lengths of 19.1 and 38.8 mm are selected by simulations to achieve wavelength division multiplexing. The two MMIs are sensitive to RI and temperature with the maximal RI sensitivities of 429.42228 and 399.20718 nm/RIU in the range of 1.333 to 1.419 and the temperature sensitivities of 10.05 and 10.22 pm/°C in the range of 26.4°C to 100°C, respectively. However, the FBGs are only sensitive to the latter with the sensitivities of 10.4 and 10.73 pm/°C. Therefore, dual-parameter measurement is obtained and cross-sensitivity issue can be solved. The distance between the two sensing heads is up to 12 km, which demonstrates the feasibility of long-distance measurement. During measurement, there is no mutual interference to each sensing head. The experimental results show that the average errors of RI are 7.61 × 10-4 RIU and 6.81 × 10-4 RIU and the average errors of temperature are 0.017°C and 0.012°C, respectively. This sensor exhibits the advantages of high RI sensitivity, dual-parameter and long-distance measurement, low cost, and easy and repeatable fabrication. [ABSTRACT FROM AUTHOR]

We report an experimental investigation on liquid sealed no-core (NCF) fiber based on modal interferometer and applied for temperature sensing applications. The sensor fabrication was very simple and inexpensive because that requires only a splice section of NCF in the middle of single mode fiber (SMF). After fabrication, the interferometer was packed into a liquid-sealed capillary tube and then the sensing measurements were carried out by detecting wavelength sweeping. An experimental demonstration shows that the fabricated interferometer exhibits a high linear response with sensing range and can be easily controlled by tuning the refractive index of the sealed liquid. The highest temperature sensitivity is found to be 201.17 pm/°C. [ABSTRACT FROM AUTHOR]

A silicone rubber-coated Mach-Zehnder interferometer (MZI) is proposed and applied to temperature measurement. The MZI is fabricated by splicing single mode fiber between a short section of no-core fiber (NCF) and the ultra-abrupt taper region. The sensing length of MZI is coated with liquid silicone rubber to enhance the temperature sensitivity. Here, NCF is used to excite the higher order cladding mode, the ultra-abrupt taper region acts as a optical fiber coupler, and the silicone rubber coating on sensing length is used as solid cladding material instead of liquid. The enhancement of the sensitivity of a device is due to the high refractive index (1.42) and thermo-optic coefficient (-1.4 × 10-4/°C) of silicone rubber as compared to liquid cladding temperature sensors. The experiment was performed for both coated and uncoated MZI and the results were compared. The MZI exhibits a high temperature sensitivity of 253.75 and 121.26 pm/°C for coated and uncoated sensing probes, respectively, in the temperature range from 30°C to 75°C. [ABSTRACT FROM AUTHOR]