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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]

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]

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]

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 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]

• 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]

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]

An optical fiber sensor for simultaneous measurement of liquid level and temperature is proposed and experimentally demonstrated. The sensor is formed by the integration of a no-core fiber (NCF) with a fiber Bragg grating (FBG).When the liquid level is changed, the interference fringe of the NCF would shift while the Bragg wavelength of the FBG remains the same. On the other hand, the interference fringe of the NCF and the Bragg wavelength of the FBG would shift simultaneously with the variation of temperature. Thus the liquid level sensor with dynamic temperature compensation can be easily achieved by using the temperature sensing property of FBG. For 10 pm wavelength resolution, the resolution of the sensor is 0.046 cm and 1.1 °C in liquid level and temperature, respectively. The fabrication of the proposed sensor is very simple and it is characterized by the dynamic temperature compensation, which makes it desirable in liquid level measurement. [ABSTRACT FROM AUTHOR]

The combination of optical fibers with nanostructured films deposition offers potential opportunities and perspectives for the realization of novel optical sensors. A refractive index (RI) sensor based on ZnO as outer layer coating over no-core fiber (NCF) sensing region is proposed and experimentally demonstrated here. Different morphologies of ZnO nanoparticles have been achieved via hydrothermal method by controlling the growth time, temperature, and concentration of reacting solution. The proposed sensor detects power variation of transmitted light propagating through the fiber caused by a change in RI of the surrounding medium. Few basic material characterizations like FESEM, UV–Vis, PL spectroscopy, and XRD's were done to confirm the formation of ZnO nanolayer. The superior sensing performance are obtained for vertically oriented ZnO nanorods among the various nanostructures. It may be attributed due to the large and uniform surface area, better crystallinity, higher average dispersion relation, and excellent adhesion of analyte molecules. The most heightened average sensitivity of 799.42%/RIU with stable results is found for a wide RI range of 1.33–1.40 which could be applied in multipurpose chemical, physical, and biomedical sensing applications. [ABSTRACT FROM AUTHOR]

An optical fiber magnetic field sensor based on the no-core fiber (NCF) and magnetic fluid (MF) is proposed and demonstrated. By using the temperature sensing property of fiber Bragg grating (FBG), the magnetic field sensor with dynamic temperature compensation can be achieved. The transmittance of the NCF that is sealed in a capillary with MF is highly sensitive to the surrounding magnetic field. Experimental results show that the sensor has the magnetic field sensitivity of 9.2 pm/mT ranging from 0 to 9 mT and the temperature sensitivity of 11.9 pm/°C ranging from 15 to 45 °C. The prototype might be attractive due to its low fabrication cost and simple configuration, which are desirable features in magnetic field measurement. [ABSTRACT FROM AUTHOR]

A fiber optic sensor capable of measuring strain and temperature simultaneously is proposed and demonstrated. The key component is a no-core fiber (NCF) followed by a fiber Bragg grating (FBG), which helps to resolve the crosstalk issue. The interference fringe of the NCF and the Bragg wavelength of the FBG would shift with the variation of strain and temperature. Since the NCF and the FBG have different sensitivity responses to strain and temperature, it is possible to measure strain and temperature simultaneously by simply monitoring the wavelength shifts. The experimental results show that the strain sensitivity and the temperature sensitivity of the sensor are 1.19 pm/με and 12.8 pm/°C, respectively. The prototype might be attractive due to its low fabrication cost and simple configuration, which are desirable features in dual-parameter measurement. [ABSTRACT FROM AUTHOR]

Refractive index and temperature are two basic properties of matter. Therefore, we propose a dual-parameter sensor based on no-core fiber and surface plasmon resonance (SPR). It can simultaneously measure temperature and refractive index. The sensor structure comprises no-core fiber, metal silver, and polydimethylsiloxane (PDMS). Metallic silver is used to excite the SPR effect, and PDMS is a suitable temperature-sensitive material. The silver film is deposited on the bottom of no-core fiber for refractive index measurement. Silver film and PDMS are deposited on the top of no-core fiber for temperature measurement. Numerical results show that refractive index and temperature can be reflected in the same spectrum, and the two parameters do not interfere with each other. This phenomenon improves the efficiency of measurement. In addition, the maximum spectral sensitivities of the sensors are 5200 nm/RIU and 7.2 nm/℃, respectively. The results also show that the sensor's performance is not sensitive to the change in structural parameters, which makes the sensor easy to prepare. Dual parameter measurement and simple structure make the sensor unique. It has good application prospects in biochemical detection, environmental monitoring, medical sensing, and other fields. [ABSTRACT FROM AUTHOR]

Abstract: In this work, improved self-imaging for optical fiber is proposed. For the first time, a precise formula of reimaging distance involving normalized frequency and cladding refractive index is given. By using our proposal, we can estimate the reimaging distance and evaluate the impact of cladding refractive index precisely and effectively. This method can be used to analyze multi-mode fiber interferences, especially no-core fiber interference. [Copyright &y& Elsevier]

• An optical fiber formic acid gas working at room temperature was proposed. • Sensing performance for formic acid gas was experimentally demonstrated. • MoS2 nanosheets contribute an excellent selectivity. Molybdenum disulfide (MoS 2) nanosheets were prepared by the method of liquid phase ultrasonic separation. It is coated on the surface of single mode-no-core-single mode (SNS) fiber structure by drip coating method, which realizes the detection of formic acid gas under room temperature. The corresponding sensing performance has been discussed in the concentration range of 0–250 ppm and 0–1100 ppm, respectively. The good selectivity, repeatability and response/recovery characteristics have been experimentally demonstrated and used for exploring an effect method to greatly improve the sensitivity of the formic acid gas sensor. This passive optical fiber probe has the promising potential application in medical and industrial production process for determining the formic acid concentration at room temperature. [ABSTRACT FROM AUTHOR]

A reflective typed fiber probe has been proposed to determine the ethanol concentration at room temperature. The sensing performance has been enhanced by modification of ZnO nanosheets. The probe structure is designed by cascaded-splicing a section of 5 mm no-core fiber with a single-mode fiber, where the high-order modal light is reflected on the surface of the no-core fiber and the low-order optical modes are independent on the environment. The two kinds of light modes are taken as the detection and reference optical signal, respectively. The concentration of ethanol gas was determined by analyzing the intensity change of interference spectrum. The ethanol sensitivity is effectively improved and reaches 6.2 × 10−4 dBm/ppm in the concentration of 0–200 ppm due to the introduction of ZnO nanosheets. Furthermore, the sensing performance is seriously dependent on the reasonable selection of the polymer, as well as the optimization of film coating process. • A compact reflected NCF/SMFs ethanol probe was proposed. • Ethanol sensitivity of 6.2 × 10−4 dBm/ppm in 0–150 ppm. • Post-treatment process and time were compared and analyzed. [ABSTRACT FROM AUTHOR]

Highlights: [•] An optical fiber structure with a section of no-core fiber for measuring pH is presented. [•] Precursor TEOS and pH-sensitive indicators are used to form the pH coating on the no-core fiber by dip-coating. [•] The structure with 2cm no-core fiber has a high pH sensitivity of 0.6dBm/pH at 1550nm and a better linearly response from pH 1 to 13. [Copyright &y& Elsevier]

Abstract: A novel and simple optical fiber structure with a section of no-core fiber for measuring relative humidity (RH) is presented. Moisture-sensitive materials HEC/PVDF are used to form the hydrogel coating on the no-core fiber by dip impregnation method. Under different humidity conditions, the humidity induced refractive index changes on the outside coating of no-core fiber will lead to the variations of the optical output power. The different lengths of no-core fiber are tested at the two wavelengths of 1310nm and 1550nm, respectively. The results show that the structure with 2cm no-core fiber has a higher RH sensitivity of 0.196dB/%RH at 1310nm and a better linearly response when the RH is lower than 75%. Furthermore, the optical output powers are measured with the rise and drop of RH, and the four times repetition are also recorded, which all demonstrate the good stability and repeatability of our proposed structure. [Copyright &y& Elsevier]

[Display omitted] • The proposed sensor solves the problem of cross-sensitivity to a certain extent. • The sensor adopts a symmetrical structure to ensure that it is independent of access direction. • There is a linear relationship between amplitude variation of spatial frequency spectrum and strain or temperature variation. • The indicators of strain and temperature sensitivity can be used as one of the evaluation methods of sensor performance. An optical fiber sensor based on no-core fiber (NCF) and two-core fiber(TCF) for simultaneous measurement of strain and temperature is proposed. The sensor is based on a hybrid structure of NCF-TCF-NCF-TCF-NCF. The proposed sensor solves the problem of cross-sensitivity to a certain extent. The sensitivities of strain are -5.46 p m / μ ε and -3.36 p m / μ ε in the range from 0 to 600 μ ε. The sensitivities of temperature are 48.6 p m / ℃ and 25.1 p m / ℃ in the ranging from 35 to 60 ℃ , respectively. By analyzing the amplitude of spatial frequency spectrum, the sensitivities of stain and temperature are 0.00025 a. u. / μ ε and 0.0118 a. u. / ℃. [ABSTRACT FROM AUTHOR]

A simple configuration of fiber-optic temperature sensor based on multimode interference (MMI) effect has been proposed and demonstrated. The MMI structure sensor is based on a thinner no-core fiber and is fabricated by using a multimode fiber without cladding, also known as no-core fiber. In the experiment, the peak wavelength response of the sensor exhibits an excellent linear response within the measured temperature intervals. It is found that the peak wavelength shifts responding to temperature change is about 38.7 pm/°C, much bigger than the temperature sensors based on fiber Bragg grating in traditional multimode fibers, whose wavelength shift is about 11.5 pm/°C. [ABSTRACT FROM AUTHOR]

In this paper, an open-cavity Mach-Zehnder interferometer (MZI) based on no-core fiber (NCF) for salinity measurement in seawater is proposed and experimentally demonstrated. The sensor is fabricated by splicing a short section of NCF in a large offset between two sections of NCF. Therefore, when the input light is transmitted to the offset fusion point, it is divided into two parts, one part is transmitted in the offset NCF and the other part is transmitted in the solution under detection. Since the interaction between transmitted light and solution is large, high RI sensitivity can be realized. In the experiment, the sensitivity of salinity and temperature of the sensor are measured about 3.444 nm/% and 0.798 nm/℃ respectively by tracking the shift of interference dips. The seawater salinity sensor demonstrated in this paper has the advantages of corrosion resistance, low cost, high sensitivity and compact size, which would play an important role in marine environment monitoring or other micro-sample detection in the biological field. [ABSTRACT FROM AUTHOR]

A novel optical fiber refractive index (RI) and strain sensor based on the combination of single-mode fiber (SMF), thin-core fiber (TCF), and no-core fiber (NCF) was proposed. Due to the inter-mode interference caused by the mode mismatch, the sensor is sensitive to the RI of the environment but insensitive to temperature. The experimental results show that in the RI range of 1.3333–1.3794, the sensitivity of dip A (1543 nm) and dip B (1565 nm) are 101.9446 nm/RIU and 109.9009 nm/RIU, respectively. In the temperature range of 15–85 ℃, the maximum wavelength shifts are only -3.5 pm/℃ and -4.9 pm/℃ respecitvely for dip A and dip B, indicating that the sensor can eliminate the cross-sensitivity of temperature to the RI. The sensor was used to detect alcohol, sucrose, NaCl and glycerin solution with the same RI and there is no obvious dip shift of the interference spectra. This sensor has the advantages of high RI sensitivities, strong anti-temperature interference ability, compact structure and has the potential application in the health and security monitoring. [ABSTRACT FROM AUTHOR]

A novel optical fiber refractive index (RI) and strain sensor based on the combination of single-mode fiber (SMF), thin-core fiber (TCF), and no-core fiber (NCF) was proposed. Due to the inter-mode interference caused by the mode mismatch, the sensor is sensitive to the RI of the environment but insensitive to temperature. The experimental results show that in the RI range of 1.3333–1.3794, the sensitivity of dip A (1543 nm) and dip B (1565 nm) are 101.9446 nm/RIU and 109.9009 nm/RIU, respectively. In the temperature range of 15–85 ℃, the maximum wavelength shifts are only -3.5 pm/℃ and -4.9 pm/℃ respecitvely for dip A and dip B, indicating that the sensor can eliminate the cross-sensitivity of temperature to the RI. The sensor was used to detect alcohol, sucrose, NaCl and glycerin solution with the same RI and there is no obvious dip shift of the interference spectra. This sensor has the advantages of high RI sensitivities, strong anti-temperature interference ability, compact structure and has the potential application in the health and security monitoring. [ABSTRACT FROM AUTHOR]

The structural diagram (a) and transmission respond (b) of MNM structure sensor [34]. The schematic diagram of MNM structure and transmission respond of surrounding RI are shown in Fig. 1. The resonant wavelength shifts to long-wavelength direction and relative transmitted power changes simultaneously while increasing surrounding RI from 1.3 to 1.42. And then, Y. Li et al further achieved the measurement relative error of 0.1 % merely by slightly reducing the NCF diameter [35].This kind of MNM structure has the advantages of high reliability and preferable practicability due to the simple fusion fabrication process. The unique structural characteristic of no-core fiber (NCF) have extended the sensing applications based on multimode interference self-imaging, evanescent wave absorption and materials coating, which are widely used in the measurement of various physical, chemical and biological parameters. An overview of the NCF-based sensing applications is spread out with specific emphasis on measurement methods, structural improvements and the combination of sensitive materials. Different prototypes of NCF-based sensors are meticulously outlined. First, the single NCF-based sensors with different analysis principles are reviewed, and the advantages of structural improvement including fiber taper, lateral-offset fusing and internal waveguide writing are compared. Second, the cascading of NCF with fiber gratings or other structures to realize multi-parameter measurement and widen the application field are also discussed. Then, NCF combines sensitive materials to measure physical/chemical parameters or coats metal film to excite surface plasmon resonance are addressed, and representative examples of the latest research progress are presented to confirm the potential value of NCF-based sensors in biosensing. What's more, the sensing characteristics of applying NCF structure in different ways are also introduced. In a whole, this article detailedly summarizes the sensing applications, concludes the challenges and points out the research direction in the development of NCF-based sensors. [ABSTRACT FROM AUTHOR]

• A SSNSS-based salinity sensing probe with high sensitivity of up to −3.42 nm/‰ (−16,310 nm/RIU) was proposed. • It has good fringe visibility and linearity which can reach more than 15 dB and 0.999 respectively. • It also has compact structure, good stability, simple manufacture and low cost. A compact, stable, low cost and high sensitivity fiber salinity sensor based on single mode fiber-single mode fiber-no-core fiber-single mode fiber-single mode fiber (SSNSS) structure is proposed and demonstrated. It is proved by experiments and simulations that the sensor has high sensitivity to external refractive index (RI) changes, so as to achieve the purpose of gauging the seawater salinity. When the RI of surrounding media in sensing area changes, the absolute value of sensitivity for the refractive index measurement can be above 16,000 nm/refractive index unit (RIU) that is higher than other fiber salinity sensors as we known. In the experimental study, a fiber salinity sensor with the sensitivity of up to -3.42 nm/‰ was achieved when the salinity varied between 0‰ and 40.001‰, and the sensor showed a good linear response. [ABSTRACT FROM AUTHOR]

A simple cascaded structure composed of all-fiber Mach-Zehnder interferometer (MZI) and polarization-maintaining fiber (PMF) is proposed and experimentally demonstrated for high resolution refractive index measurement with resistance to temperature crosstalk. • A low cost structure for RI and temperature detection is proposed and investigated. • The lateral offset of MZI based on TNCF greatly enhances the RI sensitivity. • Temperature crosstalk in RI detection of MZI is eliminated by cascading with PMF. • The RI and temperature resolutions are the highest to the best of our knowledge. A simple cascaded structure composed of all-fiber Mach-Zehnder interferometer (MZI) and polarization-maintaining fiber (PMF) is proposed and experimentally demonstrated for simultaneous refractive index (RI) and temperature measurement. By splicing a section of thin no-core fiber (TNCF) between two conventional no-core fibers (NCFs) with large lateral offset, a part of the light in the MZI structure can interact directly with the external environment to obtain a high RI sensitivity. A Panda-type PMF is cascaded with the MZI to eliminate temperature crosstalk in the RI measurement. The experimental results show that the RI sensitivity of MZI is as high as −13862 nm/RIU, and the temperature sensitivities of MZI and PMF are −0.4 nm/℃ and 1.47 nm/℃, respectively. With the help of a sensitivity matrix, the change of RI and temperature can finally be demodulated. The RI and temperature resolutions of the cascaded structure can achieve 1.83 × 10−6 RIU and 0.014 ℃ respectively, to the best of our knowledge, which are the highest among all fiber structures without special materials coating or filling for simultaneous measurement. This structure presents outstanding advantages of easy fabrication, low cost and less sample consumption, which has great potential for integrated biochemical analysis systems. [ABSTRACT FROM AUTHOR]