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[EN]In general, a sensor is used to monitor a single parameter only, and in many cases, a reference sensor is necessary to compensate the effect of temperature. Here, we demonstrate that a single supermode interferometer is capable of monitoring two parameters simultaneously. Said interferometer was fabricated with a segment of strongly coupled multicore fiber fusion spliced at the end of a standard single mode fiber. The free end of the multicore fiber was flat, thus, it behaved as a low reflectivity mirror whose reflection depended on the external refractive index. The reflection spectrum of our supermode interferometer consisted of well-defined periodic maxima and minima whose values and position varied when the interferometer was exposed to refractive index and temperature changes. In the Fourier domain, the changes of the interference pattern can be decoded easily. We demonstrate that the supermode interferometer here proposed can be useful to measure the thermo-optic coefficient of a sample. An important advantage of the device reported here is that the length of the multicore fiber is not determinant on the performance of the sensor. In addition, the device can be reused multiple times. This work was supported in part by the Ministerio de Economia y Competitividad (Spain) and the European Regional Development Fund under Grants PGC2018-101997-B-I00 and RTI2018-094669-B-C31, and in part by the Departamento de Educacion del GobiernoVasco, underGrant IT933-16.

The use of optical fibers with coupled cores is proposed for sensing applications. The interrogation of coupled-core fiber sensors is simple, fast and inexpensive as the parameter being sensed can be detected as intensity changes.

A fiber optic strain sensor, based on the Vernier effect, was constructed using two parallel MZIs. The strain sensitivity of one MZI was 0.84 pm/µε, using this configuration the sensitivity increased up to 13.00 pm/µε.

A small size and compactly packaged optical sensor for high-temperature measurements is reported. The sensor consists of a short piece of multi-core fiber (MCF) spliced to the distal end of a single-mode fiber. The packaging consists of an inner ceramic shield that prevents bending, curvature, and vibration effects on the MCF, and an outer metallic shield that protects the device against impacts. The interaction between specific supermodes excited in the MCF creates an interference pattern that shifts linearly with the temperature. The sensor was calibrated in the range from 200 to 1000 °C and a K-type thermocouple was used as a reference. The average temperature sensitivity was found to be 24.8 pm/°C with a response time of 15 s. Our results indicate that our MCF interferometric thermometer is as accurate as an electronic one with the advantage that it is passive. Therefore, we believe that the proposed sensor is suitable for industrial applications.

Bare and gold-coated tilted fiber Bragg gratings (TFBGs) can nowadays be considered as a mature technology for volume and surface refractometric sensing, respectively. As for other technologies, a continuous effort is made towards the production of even more sensitive sensors, thereby enabling a high-resolution screening of the surroundings and the possible detection of rare events. To this aim, we study in this work the development of TFBG refractometers in 4-core fibers. In particular, we show that the refractometric sensitivity of the cut-off mode can reach 100 nm/RIU for a bare grating. Using another demodulation method, a tenfold sensitivity increase is obtained when tracking the extremum of the SPR (surface plasmon resonance) envelope for a gold-coated TFBG configuration. Immobilization of DNA probes was performed as a proof-of-concept to assess the high surface sensitivity of the device.

In this Letter, a long-range optical fiber displacement sensor based on an extrinsic Fabry–Perot interferometer (EFPI) built with a strongly coupled multicore fiber (SCMCF) is proposed and demonstrated. To fabricate the device, 9.2 mm of SCMCF was spliced to a conventional single-mode fiber (SMF). The sensor reflection spectrum is affected by super-mode interference in the SCMCF and the interference produced by the EFPI. Displacement of the SMF-SCMCF tip with respect to a reflecting surface produces quantifiable changes in the amplitude and period of the interference pattern in the reflection spectrum. Since the multicore fiber is an efficient light collecting area, sufficient signal intensity can be obtained for displacements of several centimeters. By analyzing the interference pattern in the Fourier domain, it was possible to measure displacements up to 50 mm with a resolution of approximately 500 nm. To our knowledge, this is the first time that a multicore fiber has been used to build a displacement sensor. The dynamic measurement range is at least seven times larger than that achieved with an EFPI built with a conventional SMF. Moreover, the SMF-SCMCF tip is robust and easy to fabricate and replicate.

We report an ultrasensitive and selective plasmonic sensor for the detection of biomolecules, metal ions and small molecules. The sensor is comprised of low-power light-emitting diode, a multimode optical fiber coupler, a miniature spectrometer and multimode optical fibers with their facet coated with gold nanoparticles. It monitors the nanospectroscopic absorption changes of the plasmon resonance spectrum of the gold nanoparticles. The integration of these sensors for real time, on-line and multiplexing monitoring into microfluidics platforms is straightforward, and may be applied to many different fields, from environmental monitoring to cell biology studies.

Fibre optic technology is rapidly evolving, driven mainly by telecommunication and sensing applications. Excellent reliability of the manufacturing processes and low cost have drawn ever increasing attention to fibre-based sensors, e.g. for studying mechanical response/limitations of aerospace composite structures. Here, a vector bending and orientation distinguishing curvature sensor, based on asymmetric coupled multi-core fibre, is proposed and experimentally demonstrated. By optimising the mode coupling effect of a seven core multi-core fibre, we have achieved a sensitivity of - 1.4 nm/° as a vector bending sensor and - 17.5 nm/m

The use of two phase shifted supermode interferometers is proposed for precision optical fiber sensing. The advantage of such interferometers for sensing include sensitivity enhancement, easy implementation, simple interrogation, and compactness. Some sensors are demonstrated.

A compact and highly sensitive interferometric vibration sensor fabricated with two segments of asymmetric multicore fiber rotated 180º with respect to each other is presented. The sensor performs well for frequencies below 1 Hz.

We propose and demonstrate an extrinsic Fabry-Perot interferometer build with a strongly coupled multicore fiber. The interferometer can be used for measuring distances or any other parameter that induces changes in the cavity length.

[EN] A novel temperature-insensitive optical curvature sensor has been proposed and demonstrated. The sensor is fabricated by inscribing fiber Bragg gratings with short lengths into a piece of strongly coupled multicore fiber (SCMCF) and spliced to the conventional single-mode fiber. Due to the two supermodes being supported by the SCMCF, two resonance peaks, along with a deep notch between them, were observed in the reflection spectrum. The experimental results show that the depth of the notch changes with the curvature with a sensitivity up to 15.9dB/m-1 in a lower curvature range. Besides, thanks to the unique property of the proposed sensor, the notch depth barely changes with temperature. Based on the intensity demodulation of the notch depth, the temperature-insensitive curvature sensor is achieved with the cross sensitivity between the temperature, and the curvature is as low as 0.001m-1/¿C., National Natural Science Foundation of China (62071395); the 111 Project (B18045); Sichuan Province Science and Technology Support Program (2020YJ0329); Universitat Politècnica de València (PAID-01-18); Ministerio de Economía y Competitividad (PGC2018-101997-B-I00, TEC2017-88029-R).

We propose and demonstrate a compact and simple vector bending sensor capable of distinguishing any direction and amplitude with high accuracy. The sensor consists of a short segment of asymmetric multicore fiber (MCF) fusion spliced to a standard single mode fiber. The reflection spectrum of such a structure shifts and shrinks in specific manners depending on the direction in which the MCF is bent. By monitoring simultaneously wavelength shift and light power variations, the amplitude and bend direction of the MCF can be unmistakably measured in any orientation, from 0 degrees to 360 degrees. The bending sensor proposed here is highly sensitive even for small bending angles (below 1 degrees). This work was funded in part by the Fondo Europeo de Desarrollo Regional (FEDER), in part by the Ministerio de Ciencia, Innovacion y Universidades-under projects RTC2019-007194-4, RTI2018-0944669-B-C31 and PGC2018-101997-B-and in part by the Gobierno Vasco/Eusko Jaurlaritza IT933-16, ELKARTEK KK-2019/00101 (mu 4Indust), and ELKARTEK KK-2019/00051 (SMARTRESNAK). The work of Josu Amorebieta is funded by a PhD fellowship from the University of the Basque Country UPV/EHU

Fibre optic technology is rapidly evolving, driven mainly by telecommunication and sensing applications. Excellent reliability of the manufacturing processes and low cost have drawn ever increasing attention to fibre-based sensors, e.g. for studying mechanical response/limitations of aerospace composite structures. Here, a vector bending and orientation distinguishing curvature sensor, based on asymmetric coupled multi-core fibre, is proposed and experimentally demonstrated. By optimising the mode coupling effect of a seven core multi-core fibre, we have achieved a sensitivity of -1.4nm/° as a vector bending sensor and -17.5nm/m-1 as a curvature sensor. These are the highest sensitivities reported so far, to the best of our knowledge. In addition, our sensor offers several advantages such as repeatability of fabrication, wide operating range and small size and weight which benefit its sensing applications. This work has been funded in part by the Fondo Europeo de Desarrollo Regional (FEDER), in part by the Min-isterio de Ciencia, Innovación y Universidades under projects RTI2018-094669-B-C31 and GC2018-101997-B-I00, in part by the GobiernoVasco/Eusko Jaurlaritza IT933-16, ELKARTEK KK-2019/00101 (μ4Indust) and ELKARTEK KK-2019/00051 (SMARTRESNAK) and in part by NE/S012877/1 and EP/S013776/1. We also are grateful to Aernnova for allowing us to test our sensor in a real environment.

The facet of optical fibers coated with nanostructures enable the development of ultraminiature and sensitive (bio)chemical sensors. The reported sensors until now lack of specificity and the fabrication methods offer poor reproducibility. Here, we demonstrate that by transforming the facet of conventional multimode optical fibers onto plasmon resonance energy transfer (PRET) antenna surfaces the specificity issues may be overcome. To do so, a low cost chemical approach was developed to immobilize gold nanoparticles on the optical fiber facet in a reproducible and controlled manner. Our nanosensors are highly selective as PRET is a nanospectroscopic effect that only occurs when the resonant wavelength of the nanoparticles matches that of the target parameter. As an example, we demonstrate the selective detection of picomolar concentrations of copper ions in water. Our sensor is 1,000 times more sensitive than state of the art technologies. An additional advantage of our nanosensors is their simple interrogation; it comprises of a low-power light emitting diode, a multimode optical fiber coupler, and a miniature spectrometer. We believe that the PRET-based fiber optic platform reported here may pave the way of the development of a new generation of ultra-miniature, portable, and hypersensitive and selective (bio)chemical sensors. Gobierno de España, Ministerio de Economia y Competitividad, with Grant No. BIO2016-80417-P. European Union funds: DNASURF (H2020-MSCA-RISE-778001). Departamento de Educación del Gobierno Vasco Grant No. IT1271-19. Fondo Europeo de Desarrollo Regional (FEDER) and the Ministerio de Economia y Competitividad (Spain) under projects PGC2018-101997-B-I00 and RTI2018-094669-B-C31. Departamento de Educación del Gobierno Vasco Grant No. IT933‐16. The authors thank for technical and human support provided by electronic microscopy and material microanalysis services from Advanced Research Facilities (SGIker) of the University of the Basque Country UPV/EHU.

We report on microscopic polymer spherical caps bonded onto the cleaved end of a single-mode fiber (SMF) for fast, accurate, and sensitive temperature sensing. The microcaps were fabricated by dispensing sub nano-liter amounts of UV-curable polymer onto the end face of a standard SMF that was cleaved at a small angle. A reflection from the SMF-polymer interface combined with a reflection from the polymer-external medium interface gives rise to a well-defined interference pattern. The high thermal expansion coefficient of the polymer used to build the micro-caps allowed us to achieve temperature sensitivity up to 270 pm/°C, resolution of 0.04 °C, and thermal response time of around 2.5 s. The simple fabrication process and the broad operating wavelength (from 800 to 1600 nm, approximately) of our devices along with the diversity of polymers currently available, make the concept and approach proposed here appealing for diverse sensing applications.

In this study, we report on a microscopic refractometer built with off-center polymer spherical caps bonded on the end facet of a standard single-mode fiber (SMF). To achieve such microcaps, sub nanoliter amounts of UV-curable polymer were deposited onto the tip of SMFs that were cleaved with small angles. The working mechanism of our devices is explained as the interference of two Gaussian beams. High-contrast (visibility) interference patterns were achieved by optimizing the size of the polymer microcaps. The microrefractometers introduced here can operate over a broad wavelength range from 800 to 1600 nm approximately. The measuring refractive index range of our devices goes from 1 up to the index of the polymer. The refractive index sensitivity is high (∼10−4) over the whole measuring range. Temperature and refractive index cause two distinct effects on the interference patterns. Thus, our microrefractometers can be temperature self-compensated. Owing to the microscopic size of the refractometers, they can be ideal candidates to monitor refractive index in small spaces such as microfluidic channels. Other microsensors can be developed with suitable polymers or other materials that can be bonded on optical fiber tips.

A simple contactless optical fiber interferometric sensor for real-time measurement of low-level of water content in ethanol is proposed and demonstrated. The sensing head consists of a multi-cavity Fabry–Perot interferometer formed with a fiber tip in close contact with a quartz spectrophotometer cell. Ethanol–water solutions introduced into the cell cavity produce a change in the optical path length of one cavity of the interferometer. The increment of water content induces refractive index changes, but more important the absorption of the light increases. Both effects produce a significant change in the interference spectra that can be easily monitored and quantified in the Fourier domain. We demonstrate that our technique can detect water content (0%–60%) in ethanol with a detection limit as low as 0.5%.

[EN] In this paper, we report on a multicore fiber-based (MCF) temperature sensor that operates in a wide thermal range and that is robustly packaged to withstand harsh environments. To develop the sensor, the fundamentals concerning the effect of temperature on such fibers have been analyzed in detail to predict the most temperature sensitive MCF geometry. Thanks to it, the device, which operates in reflection mode and consists of a short segment of strongly coupled MCF fusion spliced to a standard single mode fiber, shows higher sensitivity than other devices with identical configuration. Regarding its packaging, it consists of an inner ceramic and two outer metallic tubes to provide rigidity and protection against impacts or dirt. The device was calibrated for a thermal range from -25 degrees C to 900 degrees C and a K-type thermocouple was used as reference. Our results suggest that the manufactured optical thermometer is as accurate as the electronic one, reaching a sensitivity up to 29.426 pm/degrees C with the advantage of being passive, compact and easy to fabricate and interrogate. Therefore, we believe this device is appealing for industrial applications that require highly sensitive temperature sensing in very demanding environments, and that the analysis included in this work could be analogously applied to develop sensitivity-optimized devices for other parameters of interest. Ministerio de Economia y Competitividad; Ministerio de Ciencia, Innovacion y Universidades; European Regional Development Fund (PGC2018-101997-B-I00 and RTI2018-094669-B-C31) ; Gobierno Vasco/Eusko Jaurlaritza (IT933-16) ; ELKARTEK KK-2019/00101 (mu 4Indust) and ELKARTEK KK-2019/00051 (SMARTRESNAK) . The work of Josu Amorebieta is funded by a PhD fellowship from the University of the Basque Country UPV/EHU. The work of Angel Ortega-Gomez is funded by a PhD fellowship from the MINECO (Ministerio de Economia y Empresa de Espana) .

In order to improve the sensitivity of an interferometric humidity sensor based on a photonic crystal fiber with a sputtered SnO 2 nanocoating, a study of the effect of its thickness on the sensitivity is presented in this paper. Sensors with coatings of different thickness were performed by applying distinct sputtering times in order to obtain an optimal thickness: the resulting nanofilms ranged from 470 to 1800 nm. Sensors were tested increasing the relative humidity from 20% to 90%, finding that the thickness was a key parameter which has to be optimized at nanometer scale to get the best sensitivity. The study points that there is an optimal thickness and higher or lower thicknesses worsen the sensitivity of the sensor. The optimal sensor showed a humidity resolution of 0.067%HR, a wavelength shift of 67 nm and a negligible sensitivity with the temperature.

In this paper, an optical fiber interferometric humidity sensor is presented. The device consists of 1 cm-long segment of photonic crystal fiber (PCF) spliced to standard single mode fibers (SMFs), forming an interferometer: the two collapsed interfaces between PCF and SMF segments produce the excitation and recombination of core and cladding modes. The latter interact with a poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) polymeric nanocoating deposited on the PCF by the well-established layer-by-layer nano assembly (LbL) technique. Humidity modifies the index of the polymeric nanolayer which in turns alters the cladding modes along the PCF segment and causes a detectable shift to the interference pattern. A study of different nanocoting thicknesses is presented in order to obtain the best possible sensibility for the sensor. Furthermore, the interrogation of the humidity sensor is presented not only by the conventional study of the spectrum shift amplitude, but also making use of the Fast Fourier Transform (FFT), which yields a linearization of the device response. The sensor here presented is reproducible, can resolve 0.074% of relative humidity (RH) and operates in the 20–95% RH range. Moreover, it exhibits response time of 0.3 s, a negligible cross sensitivity to temperature as well as long term stability.

[EN] A compact optical fiber sensor by embedding fiber Bragg gratings (FBGs) in strongly coupled multicore fiber (SCMCF) is proposed for multipoint and multiparameter sensing. To build the device, two FBGs with different peak wavelengths were inscribed in a segment of SCMCF. Then one end of the SCMCF was fusion spliced to a single-mode fiber (SMF) and the other end of the SCMCF was cleaved. In the SMF-SCMCF structure, two supermodes are excited, as a result, the reflection spectrum exhibits a sinusoidal pattern with two sharp peaks. The wavelength position of the FBGs and the supermode coupler can be extracted simultaneously. Two distinct FBGs inscribed in different positions of the SCMCF were used to demonstrate quasi-distributed multipoint sensing in the proposed structure. To the best of our knowledge, this is the first demonstration of an optical fiber sensor that combines FBGs with SCMCF. The sensor here proposed has the advantage of compact size, low-cost, good mechanical strength and ease of interrogation., This work was supported in part by National Key Research and Development Program of China (2019YFB2203204), National Natural Science Foundation of China (61922069, 61775185), the "111" Plan (B18045), Sichuan Science and Technology Program (2020YJ0329), and the Spanish Ministry of Economy and Competitiveness under the project DIMENSION TEC2017 88029-R. J.V. acknowledges funding from the Fondo Europeo de Desarrollo Regional (FEDER) and the Ministerio de Economia y Competitividad (Spain) under project PGC2018-101997-B-I00. Document