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• This study is the first to show that a DCSHF-based interferometric temperature sensor is more sensitive than common multimode interferometer. • Alcohol-filled DCSHF shows a much greater enhancement in temperature sensitivity compared to ordinary hollow optical fibers. • A microsphere is innovatively inserted into alcohol-filled DCSHF to significantly enhance temperature sensitivity. In this paper, a temperature sensing structure comprising single-mode fiber (SMF) and dual-core single-side hole fiber (DCSHF) is introduced and demonstrated to enhance temperature sensing performance based on modal interference. The biased air hole of DCSHF minimizes temperature-induced variations in the effective refractive index of the core mode, resulting in high sensitivity of temperature sensing. For further enhancement, alcohol with a high thermo-optical coefficient is introduced into the air hole, while barium titanate microsphere is innovatively inserted as coupling factor to enhance mode coupling efficiency. The experimental results prove that the maximum sensitivity of this sensor can reach up to −5.57 nm/°C. The sensor offers advantages such as simple preparation, high sensitivity, excellent repeatability, rapid response, and remarkable stability, suggesting wide-ranging potential applications in temperature sensing. [ABSTRACT FROM AUTHOR]

We propose a bidirectional flowmeter based on specialty fiber interferometry by controlled mode excitation for measuring the frequency of the vortices induced by a bluff body. The proposed arrangement of the cylindrical bluff body with a slit and the interferometric system based on modal excitation allows the bidirectional flowmeter to determine the flow velocities in either direction precisely and effectively. We have encapsulated the specialty fiber interferometer in a protective metal casing, which not only allows the interferometer to respond to the vortex frequency but also provides mechanical stability to the whole flowmeter so at to operate in a harsh environment. Experiments are conducted by introducing the modal interferometer in the slit of the cylindrical bluff body at various flow rates ranging from Reynolds number (Re) = 7200 to 56 600. A linear response is observed between the flow velocity and the vortex frequency from both directions. Thus, this work provides a reconfigurable bidirectional flowmeter, which is sensitive even at low flow rates and demonstrates a large operating range. [ABSTRACT FROM AUTHOR]

This paper presents all-optical tunable multi-channel mode conversion based on a Norland Optical Adhesive (NOA)-coated modal interferometer constructed by two-mode fiber. The modal interferometer consists of a tapered two-mode fiber (TTMF) sandwiched between two pieces of single-mode fibers. The TTMF provides a uniform modal interferometer with LP 01 and LP 11 modes as well as a strong evanescent field on its surface. The NOA is illuminated by a 405 nm laser which changes the refractive index of the NOA, thus the output spectrum of the modal interferometer can be controlled. Due to the strong light-matter interaction between NOA and the evanescent field around the TTMF, a large wavelength shift of the output spectrum can be observed under external pump light excitation. The experimental results show the transmission spectrum exhibits a blue shift with increasing pump power. A wavelength variation rate of 289 pm/mW is obtained under external pump light excitation. The interference spectrum has multiple dips where the input LP 01 mode is converted to the LP 11 mode. Thus, the modal interferometer acts as a tunable multi-channel mode converter. The proposed device might have potential applications in all-optical controllable devices and mode division multiplexing systems, etc. • The modal interferometer exhibits a wavelength variation rate of 289 pm/mW under external pump light excitation. • The modal interferometer acts as a tunable multi-channel mode converter. • The tapered two-mode fiber provides a uniform modal interferometer with LP 01 and LP 11 modes. [ABSTRACT FROM AUTHOR]

In this work, an optical fiber vacuum sensor based on a single-mode multimode single-mode (SMS) structure coated with polydimethylsiloxane (PDMS) is studied. The SMS structure generates an interferometric pattern based on multimode interference. The structure is dip-coated with a layer of PDMS, whose optical properties change when it is subjected to varying vacuum pressure. Different strategies are applied in an attempt to improve the final performance of the sensor, such as decreasing the diameter of the fiber and modifying the properties of the coating by modifying the proportion of solvent. Decreasing the diameter of the optical fiber and using toluene as a solvent are both proved to be successful strategies for increasing the sensitivity of the sensor. The devices are studied in the $1\times 10^{-3}$ –10 mbar range with a maximum wavelength shift of 12 nm, leading to a maximum sensitivity of 35 nm/mbar. The simplicity of the fabrication process, which can be applied to more sensitive structures, suggests that PDMS may be a good choice for the development of optical fiber vacuum sensors. [ABSTRACT FROM AUTHOR]

This report describes a simple and very sensitive modal interferometer (modalmetric) opto‐acoustic detector primarily thought for digital acoustic communications. The modalmetric device was made from a single‐mode fiber spliced with a sensitive multimode fiber (s‐MMF) strand. A nonlinear increase of the phase‐shift and a higher effective sensitivity of the device were observed when the interaction length of the s‐MMF in contact with the surface receiving acoustic signals was increased from 5 to 25 cm and also when the s‐MMF was bent—scaling and bend effects. A sensitivity increase of 14.2 dB was measured for very weak acoustic signals caused by at least three interfering modes. All tests in the proof‐of‐principle basis were performed at 42.9 kHz carrier frequency and 1550 nm wavelength. [ABSTRACT FROM AUTHOR]

The basic structure of fiber‐based modalmetric devices consists of a single‐mode fiber (SMF) spliced to a sensitive multimode fiber (s‐MMF) strand. The aim of this article, is to show and compare the sensitivity measurement results amongst of distinct implementations of lumped multimode interference fiber‐optic reflecting (R‐MMI) and transmitting (T‐MMI) modalmetric detectors, and a fiber Bragg grating (FBG) device, each one working as uncalibrated detector of ultrasound acoustic‐signals. All experiments were performed at 42.9 kHz carrier frequency and most of them were at 1551.5 nm wavelength. In all cases, the interaction length of the s‐MMF for a single traveling was kept at 50 mm and the fiber ends of R‐MMI samples were not mirrored. The sensitivity was observed to increase when: the modalmetric device was used instead of the FBG; the core diameter of s‐MMF was increased from 50 μm to 62.5 μm; the SMF/s‐MMF connection was angularly misaligned from 0° to 8°; the feeding optical power level was increased; the interaction length of s‐MMF was doubled and the wavelength probe was reduced from 1551.5 nm to 1520 nm. The dependence of the modalmetric devices' transmission with the length of the s‐MMF and wavelength was numerically calculated. It was concluded that modalmetric devices are highly sensitive and can be further improved yet, but have a stochastic nature in their present implementation. [ABSTRACT FROM AUTHOR]

A tunable erbium-doped fiber laser (EDFL) is proposed and demonstrated experimentally. It is based on a tunable fiber filter using the pump induced refractive index change characteristics of the Er/Yb codoped phosphosilicate fiber (EYDF) based modal interferometer (MI). The laser achieves a tunability of 2.39 nm and 1.60 nm, with the wavelength linearly tuned from 1534.05 nm to 1536.44 nm and from 1554.36 nm to 1555.96 nm by changing the pump power injected into the MI. Its side mode suppression ratios are 30 dB and 33 dB, respectively. The stability of the tunable EDFL is getting better with the continuous improvement of the power of pump 1, and the dual-wavelength power fluctuations are less than 0.22 dB and 0.12 dB within 20 min at 250 mW of pump 1, respectively. [ABSTRACT FROM AUTHOR]

Abstract We present a study of the feasibility of transforming the multicore fiber in a supermode-interference based bending sensor into an active stage. By compensating the antisymmetric supermode losses for high curvatures, fringe visibility is improved and the sensor operating range can be extended. For a given multicore fibre length and Er3+-ion concentration sensor calibration curves allow assessing the bending radius by measuring the visibility as a function of the input pump power. The Er3+-ion concentration, available input pump power and multicore fiber length set the minimum bending radius that could be measured. Highlights • Active MCF allow to extend bending sensor range. • The design parameters are studied and determined. • Practical bending sensor visibility curves are included. [ABSTRACT FROM AUTHOR]

A magnetic field sensor with high precision based on a dual-loop optoelectronic oscillator (OEO) in series with a compact in-line modal interferometer (MI) is designed and experimentally demonstrated. According to the sensing principles of the OEO-based sensor, the mapping relationship between the wavelength sensitivity of the designed MI and the microwave frequency sensitivity of the OEO is established, revealing that the frequency sensitivity of the OEO enlarges with the increment in the wavelength sensitivity. Given to this, we design a kind of high-sensitivity fiber magnetic field probe comprised of a tapered two-mode fiber (TMF) coated with magnetic fluid (MF), which is sandwiched between two segments of single-mode fiber (SMF) with unilateral core-offset. Compared to the bilateral core-offset, the unilateral core-offset reduces the mode mismatching loss further. At the same time, it can make the cladding mode have high intensity and interference spectrum generate large contrast. The above treatment ensures high sensing performance of the proposed OEO-based sensor. In the experiment, the in-line MI with a core-offset distance of 5 µm and a waist diameter of ∼9.2 µm is prepared. Firstly, we use Fourier phase unwrapping method to characterize the magnetic field in order to overcome the extraction problem of inaccurate dip wavelength due to the raw interference spectra with irregular burrs, showing maximum wavelength sensitivity of 691.5 pm/mT (655.4 pm/mT) for the light wave vector parallel (perpendicular) to the magnetic field. In order to enhance the measurement accuracy further, by means of the designed dual-loop OEO with high signal-to-noise ratio (SNR), we directly put the designed in-line MI into the above OEO system, obtaining the frequency sensitivity of − 1.508 kHz/mT (−1.429 kHz/mT) in the range of 6.69–8.35 mT (1.96–3.17 mT) for the light wave vector parallel (perpendicular) to the magnetic field, corresponding to the measurement error of within ± 0.089 mT (± 0.01 mT). The designed device can have potential in the fields of high-accuracy magnetic field measurement in a confined space. [Display omitted] • A compact in-line modal interferometer based on the tapered TMF spliced to the SMFs with a unilateral core-offset is prepared. The above treatment ensures the fiber magnetic field sensing probe with high contrast, high sensitivity and low mode mismatching loss. • Utilizing Fourier phase unwrapping method characterizes the magnetic field in wavelength interrogation mode, which overcomes the inaccurate extraction problem of the dip wavelength due to raw interference spectra with irregular burrs. • In order to improve measurement accuracy further, we connect the above fiber magnetic field sensing probe to the designed dual-loop OEO with high signal-to-noise ratio, which realizes a minimum measurement error of within ±0.01 mT, settling large measurement error problem due to the interference spectrum with poor quality. [ABSTRACT FROM AUTHOR]

A novel optical fiber magnetic field sensor based on D-shaped fiber modal interferometer and magnetic fluid is proposed and experimentally investigated. Thanks to the etched cladding of D-shaped fiber, the resulting interference is strongly influenced by the surrounding magnetic fluid, which leads to a high magnetic field sensitivity. Simultaneous measurement of the magnetic field and temperature was realized by monitoring the wavelength shift of the two resonance dips at the same time. In the experiment, the magnetic field and temperature sensitivity can reach 99.68 pm/Oe and −77.49 pm/°C, respectively. The proposed magnetic field sensor based on D-shaped fiber is featured with high sensitivity, low cost and simple configuration. [ABSTRACT FROM AUTHOR]

The paper presents the properties of a strain sensor, which was made using the micro hole collapse method and operates in the configuration of a Mach-Zehnder modal interferometer with a PM-1550-01 polarization maintaining photonic crystal fibre. The sensor's transfer curve was determined analytically. Its strain sensitivity, determined from measurements, decreases slightly with increasing wavelength and is in a range from -2.01 to -2.23 pm/με in the wavelength range 1520-1580 nm. Based on the Fourier analysis of the wavelength spectrum of the constructed sensor, the difference of the group refractive indices of the core and the cladding of the photonic crystal fibre was determined, which are in a range from 7.45·10-3 to 1.01·10-2. The temperature sensitivity of the sensor, determined on the basis of measurements performed in a range from 23 to 60◦C, is positive and equals 5.9 pm/K. [ABSTRACT FROM AUTHOR]

In this paper, a novel in-line modal interferometer for refractive index (RI) sensing is proposed and experimentally fabricated by cascading single-taper and multimode-double- cladding-multimode (MDM) fiber structure. Owing to evanescent field in taper area, the ultra-sensitive and linear intensity-responses to the varied surrounding RI are gained in both single- and double-pass structures. Moreover, the crosstalk from temperature can be effectively discriminated and compensated by means of the RI-free nature of MDM. The experimental results show that the RI sensitivities in single- and double-pass structures, respectively, reach 516.02 and 965.46 dB/RIU (RIU: refractive index unit), both with the slight wavelength shift (~0.2 nm). The temperature responses with respect to wavelength and intensity are 68.9 pm°C−1/0.103 dB°C−1 (single-pass structure) and 103 pm°C−1/0.082 dB·°C−1 (double-pass structure). So the calculated cross-sensitivity of intensity is constrained within 8.49 × 10−5 RIU/°C. In addition, our sensor presents high measurement-stability (~0.99) and low repeatability error (

This work implements and demonstrates an interferometric transducer based on a trimodal optical waveguide concept. The readout signal is generated from the interference between the fundamental and second-order modes propagating on a straight polymer waveguide. Intuitively, the higher the mode order, the larger the fraction of power (evanescent field) propagating outside the waveguide core, hence the higher the sensitivity that can be achieved when interfering against the strongly confined fundamental mode. The device is fabricated using the polymer SU-8 over a SiO2 substrate and shows a free spectral range of 20.2 nm and signal visibility of 5.7 dB, reaching a sensitivity to temperature variations of 0.0586 dB/°C. The results indicate that the proposed interferometer is a promising candidate for highly sensitive, compact and low-cost photonic transducer for implementation in different types of sensing applications, among these, point-of-care.

The performance of a fiber-based modal interferometer as lateral stress sensor has been analyzed, both for static and periodic forces applied on it. The central fiber of the interferometer is a photonic crystal fiber. Forces are applied on it perpendicular to its axis, so that they squeeze it. In static situations, changes in the transmission spectrum of the interferometer are studied as a function of the charges applied. Measurements with several interferometers have been carried out in order to analyze the influence of its length and of its splices’ transmission on the device operation, looking for optimization of its linearity and sensibility. The effect of periodic charges, as an emulation of vibrations, has also been studied. The analysis is centered on the frequency dependence of the response. In linear regime (small enough periodic charges), the results obtained are satisfactorily explained by treating the central fiber of the interferometer as a mechanical resonator whose vibration modes coincide with the ones of a cylinder with clamped ends. In nonlinear regime, period doubling and other anharmonic behaviors have been observed. [ABSTRACT FROM AUTHOR]

A novel coreless-D-shape-coreless (CDC) fiber structure for liquid-level sensing is proposed and experimentally investigated. The D-shape fiber with cladding partially etched is sensitive to the ambient refractive index change and the CDC structure transmission dip wavelength shifts related to the length of D-shape fiber immersed in the liquid, so liquid level is measured by monitoring the dip wavelength. The sensitivities for three different liquids with the RIs of 1.333, 1.355, and 1.377 are 191.89, 208.11, and 213.80 pm/mm, respectively. The temperature influence is also studied. The sensor exhibits a low-temperature cross-sensitivity of −0.128 mm/°C. The proposed modal interferometer based on the D-shape fiber with high sensitivity, low cost, and easy fabrication is suitable to liquid-level sensing applications. [ABSTRACT FROM PUBLISHER]

A wide range of applications such as health, human comfort, agriculture, food processing and storage, and electronic manufacturing, among others, require fast and accurate measurement of humidity. Sensors based on optical fibers present several advantages over electronic sensors and great research efforts have been made in recent years in this field. The present paper reports the current trends of optical fiber humidity sensors. The evolution of optical structures developed towards humidity sensing, as well as the novel materials used for this purpose, will be analyzed. Well-known optical structures, such as long-period fiber gratings or fiber Bragg gratings, are still being studied towards an enhancement of their sensitivity. Sensors based on lossy mode resonances constitute a platform that combines high sensitivity with low complexity, both in terms of their fabrication process and the equipment required. Novel structures, such as resonators, are being studied in order to improve the resolution of humidity sensors. Moreover, recent research on polymer optical fibers suggests that the sensitivity of this kind of sensor has not yet reached its limit. Therefore, there is still room for improvement in terms of sensitivity and resolution. [ABSTRACT FROM AUTHOR]

Regenerated Fibre Bragg Gratings have the potential for high-temperature monitoring. In this paper, the inscription of Fibre Bragg Gratings (FBGs) and the later regeneration process to obtain Regenerated Fiber Bragg Gratings (RFBGs) in high-birefringence optical fiber is reported. The obtained RFBGs show two Bragg resonances corresponding to the slow and fast axis that are characterized in temperature terms. As the temperature increases the separation between the two Bragg resonances is reduced, which can be used for low cost interrogation. The proposed interrogation setup is based in the use of optical filters in order to convert the wavelength shift of each of the Bragg resonances into optical power changes. The design of the optical filters is also studied in this article. In first place, the ideal filter is calculated using a recursive method and defining the boundary conditions. This ideal filter linearizes the output of the interrogation setup but is limited by the large wavelength shift of the RFBG with temperature and the maximum attenuation. The response of modal interferometers as optical filters is also analyzed. They can be easily tuned shifting the optical spectrum. The output of the proposed interrogation scheme is simulated in these conditions improving the sensitivity.

A sensing head based on a hollow-core photonic crystal fiber for in-reflection measurement of micro-displacements is presented. The sensing structure takes advantage of the multimodal behavior of a short segment of hollow-core photonic crystal fiber in-reflection, being spliced to a single mode fiber at its other end. A modal interferometer is obtained when the sensing head is close to a mirror, through which displacement is measured.

We report a temperature-insensitive microdisplacement sensor with a locally bent microfiber taper interferometer. The microfiber taper waist diameter can be optimized to minimize the spectral shift of the sensor owing to the environmental temperature change. With a ~ 1.92-μm-diameter microfiber taper, a bimodal fiber interferometer is proposed and experimentally fabricated. The transmission spectrum shows substantially small temperature dependence matching well with the theoretical estimation. The transmission spectrum is red-shifted in response to microdisplacement with a high sensitivity of 102 pm/μm but without requirement for temperature compensation.

A novel strain sensor based on all-fiber modal interferometer is presented and experimentally demonstrated. It is formed by splicing a section of polarization-maintaining fiber (PMF) between two waist-enlarge fusion tapers. When the strain increased from 0 μe to 1764 με, the sensitivities of sensors with lengths of 34 mm, 40 mm and 47 mm are −1.36 pm/με, −1.22 pm/με, −1.06 pm/με, respectively. The absolute value of sensitivity will increase with the decrease of the length. The temperature response of the sensor is also discussed. The sensor has merits of simple fabrication, low cost, and good robustness. [ABSTRACT FROM AUTHOR]

• A novel method for displacement sensing with 100 nm resolution is demonstrated. • Extrinsic Fabry-Perot consist of a fiber tip and a hybrid structure. • The lead-in fiber tip was made with a 7-cores fiber spliced to a single-mode fiber. • The supermode interferometer enhances the Fabry-Perot interferometer's reflectance. • An envelope is exhibited when optical lengths of the two cavities are almost equal. Nano-displacement sensing based on an extrinsic hybrid fiber Fabry-Perot interferometer is proposed and demonstrated. The lead-in fiber tip of such an interferometer consists of a strongly-coupled multicore fiber section fusion spliced to a single-mode fiber. The referred lead-in fiber tip is placed in front of a microscope slide, whose rear surface is coated with a high reflecting layer. The gap between the end-face of the fiber tip and the layer is composed of an air cavity in series with a glass one. Light exiting from the lead-in fiber tip is partially reflected at the fiber-air and air-glass interfaces and the reflecting layer generating three beams that are recoupled into the multicore fiber and combined with supermode interference. By making the optical path length of the air cavity slightly different from the glass one, it is possible to generate an envelope in the interference spectra with a larger period. Thus, by tracking the shift of such an envelope, displacements of 0.47 nm can be resolved. The nano-displacement sensing approach reported here is easy to implement; moreover, the sensitivity, resolution, and dynamic range can be reconfigured by an appropriate selection of the glass cavity. [ABSTRACT FROM AUTHOR]

ABSTRACT An all-fiber temperature sensor based on two peanut-shape structures modal interferometer is demonstrated. The results show that a high temperature sensitivity of 63.7 pm/°C is acquired for the 30 mm sensor, and the strain sensitivity is as low as −0.86 pm/με. The cross sensitivity is only 0.013°C/με, which is suitable for temperature sensing. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2841-2844, 2015 [ABSTRACT FROM AUTHOR]

We demonstrate an arc discharge fabrication method for a high extinction-ratio (ER) single mode fiber (SMF) based dual thin-taper modal interferometer (MI) and investigate the performance of the MI as thermal and refractive index (RI) sensors. With the optimized discharge parameters of a commercial fusion splicer and a simple mechanical taper setup, the MI with about 24 dB ER could be achieved if fabricating two thin-tapers with 20-µm waist diameter on the common SMF. It can be used as a highly sensitive thermal sensor with a thermal sensitivity of −72.8 pm/°C. However, for the high ER MI, the propagation position of the cladding mode is more close to that of core mode in the MI. Hence, its sensitivity to ambient RI variation is not very high, about 10.299 nm/RIU. If using it for dual parameter measurement, its thermal and RI measurement resolutions are ±1.1 °C and ±0.009, respectively, and are 6.6 and 2.4 times bigger than those of the other fiber sensors. [ABSTRACT FROM AUTHOR]

A compact fiber-optic accelerometer based on a modal interferometer, which is fabricated by misaligned splicing of a short section of a thin-core fiber between two sections of a standard single-mode fiber, is demonstrated experimentally. A spectrum analysis method is used to detect an acceleration signal rapidly. The experimental results show that the thin-core fiber-based fiber-optic accelerometer has a minimum detectable acceleration of 3.3×10-3g (g - gravitational acceleration), and a wide frequency response range from 10 to 1200 Hz. Moreover, the proposed accelerometer exhibits the advantages of low cost, simple structure and easy fabrication. [ABSTRACT FROM AUTHOR]

We demonstrate a simple, robust, and highly sensitive temperature sensor based on the resonant excitation of whispering gallery modes in a coated single-mode fiber (SMF) loop that was created by means of bending a standard coated SMF into a loop with a radius of 5 mm. Clear interference fringes were observed in the transmission spectrum of the coated-SMF loop. The resonant wavelength exhibits a linear temperature response at the temperature range from −60° to 10 °C, and a quadratic temperature response at the temperature range from 10° to 140 °C. Moreover, the coated-SMF loop exhibits an ultrahigh sensitivity of up to −5.22 nm/°C at 120 °C, and the sensitivity (i.e., −4.01 nm/°C at 80 °C) is two orders of magnitude higher than that of an uncoated-SMF loop (i.e., 0.04 nm/°C at 80 °C). Hence, such a coated-SMF loop could be used to develop a temperature sensor with a large measurement range from −60° to 140 °C. [ABSTRACT FROM PUBLISHER]

A cantilever fiber-optic accelerometer using modal interferometer, with a large fringe visibility of up to 30 dB, is proposed and demonstrated. In the system, an electrical testing replaces optical-spectrum detection to achieve fast acceleration measurement of the cantilever fiber-optic accelerometer, and the performance of the cantilever fiber-optic accelerometer is studied experimentally. Experimental results show that the minimum detectable acceleration is 1.36 mg, and the resonance frequency is 520 Hz. Meanwhile, the angular dependence of the fiber accelerometer is also analyzed in the article and the result shows that the output intensity difference is up to 18 dB for different angles. This fiber-optic accelerometer can be used to detect underwater acoustic and seismic wave. [ABSTRACT FROM PUBLISHER]

An optical fiber inclinometer applying modal interference based on an abrupt tapering cascading a peanut-shape configuration is proposed. The abrupt taper splits part of the core mode into cladding modes and the interference occurs when the peanut-shape recombines the cladding modes and core mode together. Four inclinometers with different lengths have been fabricated and characterized, and the high sensitivities of −1.76 and −4.49 nm/° are obtained in the angle range of 0°–3° and 3°–6.66°, respectively. [ABSTRACT FROM PUBLISHER]

An optical fiber curvature sensor based on bulge-taper splicing structures modal interferometer is proposed and experimentally demonstrated. The sensor is fabricated by a length of 10 mm single mode fiber (SMF) which is spliced between two SMFs with bulge-taper splicing. The results show that the sensor is sensitive to curvature with a sensitivity coefficient of −31.821 nm/m −1 in the range of 4.29–4.98 m −1 and −17.051 nm/m −1 in the range of 4.98–5.59 m −1 . The proposed sensor also shows the temperature sensitivity of 72 pm/°C, and the curvature error caused by temperature is only 0.0023 m −1 /°C in the range of 4.29–4.98 m −1 and 0.0042 m −1 /°C in the range of 4.98–5.59 m −1 . This sensor exhibits the advantages of compactness, robustness, low cost, all-SMF structure, easy and repeated fabrication, and high sensitivity. [ABSTRACT FROM AUTHOR]

A simple fiber-optic sensor based on a fiber Bragg grating (FBG) embedded in a fiber modal interferometer (MI) for simultaneous measurements of temperature and strain is proposed and experimentally demonstrated. The fiber MI is constructed by splicing two sections of the no-core fiber (NCF) between the single-mode fibers. Due to the different responses of the NCF-based MI and the FBG to the same temperature and strain, the discrimination between temperature and strain can be easily achieved. For a 0.01-nm wavelength resolution, the resolution of the sensor is 0.216°C and 6.75 µε in temperature and strain, respectively. [ABSTRACT FROM AUTHOR]

A thinned fiber taper (TFT) with high refractive index (RI) sensitivity has been fabricated by a fusion splicer. The TFT has a total length of about 1.3 mm and a waist diameter of less than 10 μm. Simulations of the TFT have been implemented and the RI experiments show a remarkable sensitivity. The highest RI sensitivities of 6360 nm/RI unit (RIU) and 3850 nm/RIU in the RI ranges of 1.409-1.423 and 1.333-1.353, respectively, have been obtained on two TFTs with different taper waist diameters. The RI dynamic ranges of the TFTs also have been discussed. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1054-1057, 2014 [ABSTRACT FROM AUTHOR]

Highlights: [•] We report a novel Ampere force based photonic crystal fiber (PCF) magnetic field sensor. [•] The sensor does not have the intrinsic magnetic saturation and hysteresis drawbacks. [•] The sensor has advantages of simple fabrication, low cost, high sensitivity and good repeatability. [•] The qualitative theory is explained and the experiment results are analyzed well. [•] The sensor can even detect the geomagnetic field if we increase the applied electrical current to 1A. [Copyright &y& Elsevier]

An ultrasensitive temperature sensor based on a thin-core fiber modal interferometer (TCFMI) is proposed and demonstrated in this paper. After its middle is etched to a thin residual diameter, the TCFMI is sealed in a capillary tube filled with refractive index (RI) liquid. The thermo-optic effect of RI liquid turns the TCFMI into an ultrasensitive temperature sensor with sensitivity up to -22.23 nm/°C at 3.2 °C. The sensitivity is over 1000-fold that of the bare TCFMI in air. This proposed temperature sensor is also simple, cost-effective, and suitable for a variety of applications. [ABSTRACT FROM PUBLISHER]

Highlights: [•] A strain-insensitive high temperature fiber sensor is proposed. [•] The sensor is based on a fiber modal interferometer. [•] A piece of small-core photosensitive fiber is used in the sensor. [•] The sensor owns the highest temperature sensitivity. [•] The temperature to strain cross sensitivity of the sensor is low. [ABSTRACT FROM AUTHOR]

Highlights: [•] A novel optical liquid level sensor based on polarization-maintaining fiber (PMF) modal interferometer with waist enlarge splicing is proposed. [•] The waist enlarge splicing can make the sensor manufacturing simple, structural stability. [•] The sensitivities for the liquids with indices of 1.335, 1.363 and 1.382 are 0.279nm/mm, 0.437nm/mm and 0.516nm/mm respectively. [•] The higher sensitivity is acquired for the higher refractive index liquid. [Copyright &y& Elsevier]

A modal interferometric fiber-optic displacement sensor based on the balloon-like structure is investigated experimentally. When the bending radius of the balloon-like structure is reduced to 3.0 mm, the displacement sensitivity could reach up to 71.49 nm/mm. Moreover, the evolution process of the transmission spectra in a wide dynamic displacement range exceeding 22 mm is analyzed, which can provide guiding significance for sensor designing. The proposed displacement sensor is extremely low-cost, robust, and compact. [Display omitted] • An extremely simple and low-cost method for displacement measurement. • The dynamic range of displacement change could reach up to 22 mm. • The displacement sensitivity could be 71.49 nm/mm when the sensor size is reduced to ~ 6.0 mm. [ABSTRACT FROM AUTHOR]

Highlights: [•] A novel curvature sensor based on polarization-maintaining fiber interferometer with waist enlarge splicing is proposed. [•] The sensitivity of −10.634nm/m−1 and 10.152nm/m−1 were acquired by concave and convex bending with the range of 0–2.054m−1 [•] The PMF-based sensor has a higher sensitivity than the SMF-based one. [•] The length of PMF is shorter, the sensitivity of the sensor is higher. [•] The waist-enlarge splicing can make sensor stable. [ABSTRACT FROM AUTHOR]

Regenerated Fibre Bragg Gratings have the potential for high-temperature monitoring. In this paper, the inscription of Fibre Bragg Gratings (FBGs) and the later regeneration process to obtain Regenerated Fiber Bragg Gratings (RFBGs) in high-birefringence optical fiber is reported. The obtained RFBGs show two Bragg resonances corresponding to the slow and fast axis that are characterized in temperature terms. As the temperature increases the separation between the two Bragg resonances is reduced, which can be used for low cost interrogation. The proposed interrogation setup is based in the use of optical filters in order to convert the wavelength shift of each of the Bragg resonances into optical power changes. The design of the optical filters is also studied in this article. In first place, the ideal filter is calculated using a recursive method and defining the boundary conditions. This ideal filter linearizes the output of the interrogation setup but is limited by the large wavelength shift of the RFBG with temperature and the maximum attenuation. The response of modal interferometers as optical filters is also analyzed. They can be easily tuned shifting the optical spectrum. The output of the proposed interrogation scheme is simulated in these conditions improving the sensitivity. [ABSTRACT FROM AUTHOR]

An optical in-fiber modal interferometer-based volume strain sensor for earthquake prediction is proposed and experimentally demonstrated. The sensing element is formed by wrapping a multimode-singlemode-multimode fiber structure onto a polyurethane hollow column. Due to the modal interference between the excited guided modes in the fiber, strong interference pattern could be observed in the transmission spectrum. Theoretical analysis verifies that the resonant wavelength shifts as a result of the volume strain variation caused by the column deformation with square root relationship. Sensitivity >3.93~pm/\mu\varepsilon within the volume strain ranging from 0 to 1300 \mu\varepsilon is also experimentally demonstrated. By taking the response of bidirectional change of volume strain and the sluggish character of the employed sensing material into consideration, the sensing system presents good repeatability and stability. [ABSTRACT FROM PUBLISHER]

A novel fiber optic hybrid device for simultaneously measuring temperature and relative humidity (RH) is proposed and demonstrated in this paper. The device is composed of a fiber Bragg grating (FBG) and a reflection type photonic crystal fiber interferometer (PCFI) infiltrated with humidity sensitive material Agarose. The Agarose infiltrated PCFI is used to monitor the RH and an in-line FBG is used to monitor the temperature. The temperature is measured by detecting the shift in the Bragg wavelength and the humidity is measured by monitoring the change in the optical power of the reflected spectrum. The sensor shows a temperature sensitivity of 9.8 pm/^\circC and an optical power variation of >7~dB for an RH change of 75%, with less cross sensitivity. [ABSTRACT FROM PUBLISHER]

A compact fiber sensor based on fiber Bragg grating (FBG) embedded in a fiber modal interferometer (MI) for simultaneous measurement of strain and temperature is proposed. Because the total length of the sensor is 12 mm and can be further reduced, the spatial resolution of the sensor is improved. Due to the different responses of the fiber MI and the FBG to strain and temperature, the simultaneous measurement at the same point is achieved. For 0.01-nm wavelength resolution, the resolution of the sensor is 5.96 \mu\varepsilon and 0.31^\circC in strain and temperature, respectively. [ABSTRACT FROM PUBLISHER]

A simple fiber sensor capable of simultaneous measurement of liquid level and refractive index (RI) is proposed and experimentally demonstrated. The sensing head is an all-fiber modal interferometer manufactured by splicing an uncoated single-mode fiber with two short sections of multimode fiber. The interference pattern experiences blue shift along with an increase of axial strain and surrounding RI. Owing to the participation of multiple cladding modes with different sensitivities, the height and RI of the liquid could be simultaneously measured by monitoring two dips of the transmission spectrum. Experimental results show that the liquid level and RI sensitivities of the two dips are 245.7 pm/mm, −38 nm/RI unit (RIU), and 223.7 pm/mm, −62 nm/RIU, respectively. The approach has distinctive advantages of easy fabrication, low cost, and high sensitivity for liquid level detection with the capability of distinguishing the RI variation simultaneously. [ABSTRACT FROM AUTHOR]

A sensing head based on a hollow-core photonic crystal fiber for in-reflection measurement of micro-displacements is presented. The sensing structure takes advantage of the multimodal behavior of a short segment of hollow-core photonic crystal fiber in-reflection, being spliced to a single mode fiber at its other end. A modal interferometer is obtained when the sensing head is close to a mirror, through which displacement is measured. [ABSTRACT FROM AUTHOR]

We report a detailed study of a miniature optical relative humidity (RH) sensor based on a polymer-infiltrated photonic crystal fiber interferometer. The sensor shows a high sensitivity to RH variations with a change in its reflected power of about 12 dB for a humidity change of 84%RH. The sensor has the advantages of a very compact length of 1 mm, and an end-type probe configuration makes it suitable for monitoring humidity in microenvironments. The repeatability, long-term stability, measurement accuracy, and temperature dependence of the sensor are studied in this paper. The observed low thermal sensitivity of the sensor suggests that temperature compensation may not be needed if it is used in normal environments. The response time of the sensor is found to be 400 ms for a change in RH of \sim 30\%RH. The fast response time suggests that the sensor can potentially be used as a human breath rate monitor in a clinical situation. [ABSTRACT FROM PUBLISHER]

A temperature sensor based on modal interference in a hollow-core photonic bandgap fiber (HC-PBGF) is presented. The modal interferometer is fabricated by splicing the two ends of a 12-mm HC-PBGF to the standard single-mode fibers. The airholes in the core and cladding of the HC-PBGF are completely collapsed during the fusion splicing, and the length of the collapsed area of about 300 μm is observed. The transmission spectra under different temperatures are measured. The sensitivity of 31.8 pm/°C is achieved in the range of 25 °C-60 °C. The influence of the length of HC-PBGF on the sensitivity is also investigated. [ABSTRACT FROM PUBLISHER]

A novel method for dew detection based on photonic crystal fiber (PCF) interferometer that operates in reflection mode is presented in this paper. The fabrication of the sensor head is simple since it only involves cleaving and fusion splicing. The sensor shows good sensitivity to dew formation with a large wavelength peak shift of the interference pattern at the onset of dew formation. The device's response to ambient humidity and temperature are also studied and reported in this paper. From our experiment it is also concluded that by attaching a thermoelectric cooler with temperature feedback, the sensor head demonstrated can be used as a miniature dew point hygrometer. [ABSTRACT FROM PUBLISHER]

Abstract: An in-fiber modal interferometer is demonstrated by splicing a section of homemade dual-concentric-core photonic crystal fiber (DCCPCF) and two segments of single-mode fibers (SMFs) with collapsing the air holes in the splice regions. By analyzing the transmission spectra and the spatial frequency spectra of the interferometers, it''s observed that the main interfering modes are the high-order ring modes, which are different from the previously reported interferometers using the single-, few- or multi-mode fibers. The influences of the lengths of DCCPCF and the first collapsed region on the interferometers were investigated experimentally. The strain, temperature and refractive index characteristics of the interferometer were analyzed. [Copyright &y& Elsevier]