Plasmonic sensors based on tilted Bragg gratings in multicore optical fibers.

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.

Long-range multicore optical fiber displacement sensor.

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.

Temperature-insensitive curvature sensor based on Bragg gratings written in strongly coupled multicore fiber.

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.

Interferometer based on strongly coupled multi-core optical fiber for accurate vibration sensing.

We report on the use of a simple interferometer built with strongly-coupled core optical fiber for accurate vibration sensing. Our multi-core fiber (MCF) is designed to mode match a standard single-mode optical fiber (SMF). The interferometer consists of a low insertion loss SMF-MCF-SMF structure where only two super-modes interfere. The polymer coating of the MCF was structured and the interferometer was sandwiched between a flat piece and a V-groove. In this manner our device is highly sensitive to force with sensitivity reaching -4225 pm/N. To make the MCF interferometer sensitive to vibrations the flat piece was allowed to move, thus, its periodic movements exert cyclic localized pressure on the MCF which makes the interference pattern to shift periodically. Our sensors can be used to monitor vibrations in a broad frequency range with the advantage that the measurements are unaffected by temperature changes.

Miniature multicore optical fiber vibration sensor.

We demonstrate a compact and versatile interferometric vibration sensor that operates in reflection mode. To build the device, a short segment of symmetric strongly coupled multicore optical fiber (MCF) is fusion spliced to a single-mode optical fiber (SMF). One end of the MCF segment is cleaved and placed in a cantilever position. Due to the SMF-MCF configuration, only two supermodes are excited in the MCF. Vibrations induce cyclic bending of the MCF cantilever which results in periodic oscillations of the reflected interference spectrum. In our device, the MCF itself is the inertial mass. The frequency range where our device is sensitive can be easily tailored from a few hertz to several kilohertz through the cantilever dimensions.

Optical fiber temperature sensor based on a microcavity with polymer overlay.

An ultracompact, cost-effective, and highly accurate fiber optic temperature sensor is proposed and demonstrated. The sensing head consists of Fabry-Perot microcavity formed by an internal mirror made of a thin titanium dioxide (TiO2) film and a microscopic segment of single-mode fiber covered with Poly(dimethylsiloxane) (PDMS). Due to the high thermo-optic coefficient of PDMS the reflectance of the fiber-PDMS interface varies strongly with temperature which in turn modifies the amplitude of the interference pattern. To quantify the changes of the latter we monitored the visibility of the interference pattern and analyzed it by means of the fast Fourier transform. Our sensor exhibits linear response, high sensitivity, and response time of 14 seconds. We believe that the microscopic dimensions along with the performance of the sensor here presented makes it appealing for sensing temperature in PDMS microfluidic circuits or in biological applications.

Ultrasensitive vector bending sensor based on multicore optical fiber.

In this Letter, we demonstrate a compellingly simple directional bending sensor based on multicore optical fibers (MCF). The device operates in reflection mode and consists of a short segment of a three-core MCF that is fusion spliced at the distal end of a standard single mode optical fiber. The asymmetry of our MCF along with the high sensitivity of the supermodes of the MCF make the small bending on the MCF induce drastic changes in the supermodes, their excitation, and, consequently, on the reflected spectrum. Our MCF bending sensor was found to be highly sensitive (4094 pm/deg) to small bending angles. Moreover, it is capable of distinguishing multiple bending orientations.

Single tapered fiber tip for simultaneous measurements of thickness, refractive index and distance to a sample.

We demonstrate the capability of an air cavity Fabry-Perot interferometer (FPI), built with a tapered lead-in fiber tip, to measure three parameters simultaneously, distance, group refractive index and thickness of transparent samples introduced in the cavity. Tapering the lead-in fiber enhances the light coupling back efficiency, therefore is possible to enlarge the air cavity without a significant deterioration of the fringe visibility. Fourier transformation, used to analyze the reflected optical spectrum of our FPI, simplify the calculus to determine the position, thickness and refractive index. Samples made of 7 different glasses; fused silica, BK7, BalF5, SF2, BaF51, SF15, and glass slides were used to test our FPI. Each sample was measured nine times and the results for position, thickness and refractive index showed differences of ± 0.7%, ± 0.1%, and ± 0.16% respectively. The evolution of thickness and refractive index of a block of polydimethylsiloxane (PDMS) elastomer due to temperature changes in the range of 25°C to 90°C were also measured. The coefficients of the thermal expansion and thermo-optic estimated were α = 4.71x10(-4)/°C and dn/dT = -4.66 x10(-4) RIU/°C, respectively.

Photonic crystal fiber interferometric vector bending sensor.

A compact and highly sensitive interferometric bending sensor (inclinometer) capable of distinguishing the bending or inclination orientation is demonstrated. The device operates in reflection mode and consists of a short segment of photonic crystal fiber (PCF) inserted in conventional single-mode optical fiber (SMF). A microscopic collapsed zone in the PCF-SMF junction allows the excitation and recombination of core modes, hence, to build a mode interferometer. Bending on the device induces asymmetric refractive index changes in the PCF core as well as losses. As a result, the effective indices and intensities of the interfering modes are altered, which makes the interference pattern shift and shrink. The asymmetric index changes in the PCF make our device capable of distinguishing the bending orientation. The sensitivity of our sensor is up to 1225 pm/degree and it can be used to monitor small bending angles (±2°). We believe that the attributes of our sensor make it appealing in a number of applications.

Nano-displacement sensor based on photonic crystal fiber modal interferometer.

A stable nano-displacement sensor based on large mode area photonic crystal fiber (PCF) modal interferometer is presented. The compact setup requires simple splicing of a small piece of PCF with a single mode fiber (SMF). The excitation and recombination of modes is carried out in a single splice. The use of a reflecting target creates an extra cavity that discretizes the interference pattern of the mode interferometer, boosting the displacement resolution to nanometer level. The proposed modal interferometric based displacement sensor is highly stable and shows sensitivity of 32 pm/nm.

Optical Fiber Sensors for Aircraft Structural Health Monitoring.

Aircraft structures require periodic and scheduled inspection and maintenance operations due to their special operating conditions and the principles of design employed to develop them. Therefore, structural health monitoring has a great potential to reduce the costs related to these operations. Optical fiber sensors applied to the monitoring of aircraft structures provide some advantages over traditional sensors. Several practical applications for structures and engines we have been working on are reported in this article. Fiber Bragg gratings have been analyzed in detail, because they have proved to constitute the most promising technology in this field, and two different alternatives for strain measurements are also described. With regard to engine condition evaluation, we present some results obtained with a reflected intensity-modulated optical fiber sensor for tip clearance and tip timing measurements in a turbine assembled in a wind tunnel.

Locally pressed photonic crystal fiber interferometer for multiparameter sensing.

A mode interferometer consisting of a short section of photonic crystal fiber (PCF) fusion spliced to a standard single-mode optical fiber with localized perturbations is proposed for multiparameter sensing. In this sensing configuration, the parameter being sensed changes the visibility (an absolute parameter) of the interference pattern and also causes a shift (a relative parameter) to the interference pattern. To achieve this dual effect, a portion of the PCF is squeezed on localized regions with a serrated mechanical piece. In this manner, we introduce attenuation losses and effective refractive index changes to the interfering modes, hence, visibility changes and a shift to the interference pattern. Our device is suitable for monitoring diverse physical parameters, such as weight, lateral force, pressure, load, etc., with the advantage that compensation to temperature or power fluctuations is not required. Moreover, the sensor sensitivity can be adjusted in a simple manner.

Dual-Mode Comb Plasmonic Optical Fiber Sensing.

Surface plasmon (SP) excitation in metal-coated tilted fiber Bragg gratings (TFBGs) has been a focal point for highly sensitive surface biosensing. Previous efforts focused on uniform metal layer deposition around the TFBG cross section and temperature self-compensation with the Bragg mode, requiring both careful control of the core-guided light polarization and interrogation over most of the C + L bands. To circumvent these two important practical limitations, we studied and developed an original platform based on partially coated TFBGs. The partial metal layer enables the generation of dual-comb resonances, encompassing highly sensitive (TM/EH mode families) and highly insensitive (TE/HE mode families) components in unpolarized transmission spectra. The interleaved comb of insensitive modes acts as wavelength and power references within the same spectral region as the SP-active modes. Despite reduced fabrication and measurement complexity, refractometric accuracy is not compromised through statistical averaging over seven individual resonances within a narrowband window of 10 nm. Consequently, measuring spectra over 60 nm is no longer needed to compensate for small temperature or power fluctuations. This sensing platform brings the following important practical assets: (1) a simpler fabrication process, (2) no need for polarization control, (3) limited bandwidth interrogation, and (4) maintained refractometric accuracy, which makes it a true game changer in the ever-growing plasmonic sensing domain.

High-visibility photonic crystal fiber interferometer as multifunctional sensor.

A photonic crystal fiber (PCF) interferometer that exhibits record fringe contrast (~40 dB) is demonstrated along with its sensing applications. The device operates in reflection mode and consists of a centimeter-long segment of properly selected PCF fusion spliced to single mode optical fibers. Two identical collapsed zones in the PCF combined with its modal properties allow high-visibility interference patterns. The interferometer is suitable for refractometric and liquid level sensing. The measuring refractive index range goes from 1.33 to 1.43 and the maximum resolution is ~1.6 × 10(-5).

Coupled-core fiber Bragg gratings for low-cost sensing.

Sensors based on Bragg gratings inscribed in conventional single mode fibers are expensive due to the need of a sophisticated, but low-speed, interrogation system. As an alternative to overcome this issue, in this work, it is proposed and demonstrated the use of coupled-core optical fiber Bragg gratings. It was found that the relative reflectivity from such gratings changed when the coupled-core fiber was subjected to point or periodic bending. This feature makes the interrogation of such gratings simple, fast, and cost-effective. The reflectivity changes of the gratings are attributed to the properties of the supermodes supported by the coupled-core fiber. As potential applications of the referred gratings, intensity-modulated vector bending and vibration sensing are demonstrated. We believe that the results reported here can pave the way to the development of many inexpensive sensors. Besides, coupled-core fiber Bragg gratings may expand the use of grating technology in other areas.

A method for the controllable fabrication of optical fiber-based localized surface plasmon resonance sensors.

Optical fiber-based Localized Surface Plasmon Resonance (OF-LSPR) biosensors have emerged as an ultra-sensitive miniaturized tool for a great variety of applications. Their fabrication by the chemical immobilization of gold nanoparticles (AuNPs) on the optic fiber end face is a simple and versatile method. However, it can render poor reproducibility given the number of parameters that influence the binding of the AuNPs. In order to develop a method to obtain OF-LSPR sensors with high reproducibility, we studied the effect that factors such as temperature, AuNPs concentration, fiber core size and time of immersion had on the number and aggregation of AuNPs on the surface of the fibers and their resonance signal. Our method consisted in controlling the deposition of a determined AuNPs density on the tip of the fiber by measuring its LSPR signal (or plasmonic signal, Sp) in real-time. Sensors created thus were used to measure changes in the refractive index of their surroundings and the results showed that, as the number of AuNPs on the probes increased, the changes in the Sp maximum values were ever lower but the wavelength shifts were higher. These results highlighted the relevance of controlling the relationship between the sensor composition and its performance.

Twin-core fiber sensor integrated in laser cavity.

In this work, we report on a twin-core fiber sensor system that provides improved spectral efficiency, allows for multiplexing and gives low level of crosstalk. Pieces of the referred strongly coupled multicore fiber are used as sensors in a laser cavity incorporating a pulsed semiconductor optical amplifier (SOA). Each sensor has its unique cavity length and can be addressed individually by electrically matching the periodic gating of the SOA to the sensor's cavity roundtrip time. The interrogator acts as a laser and provides a narrow spectrum with high signal-to-noise ratio. Furthermore, it allows distinguishing the response of individual sensors even in the case of overlapping spectra. Potentially, the number of interrogated sensors can be increased significantly, which is an appealing feature for multipoint sensing.

Photonic crystal fiber sensor array based on modes overlapping.

An alternative method to build point and sensor array based on photonic crystal fibers (PCFs) is presented. A short length (in the 9-12 mm range) of properly selected index-guiding PCF is fusion spliced between conventional single mode fibers. By selective excitation and overlapping of specific modes in the PCF we make the transmission spectra of the sensors to exhibit a single and narrow notch. The notch position changes with external perturbation which allows sensing diverse parameters. The well-defined single notch, the extinction ratio exceeding 30 dB and the low overall insertion loss allow placing the sensors in series. This makes the implementation of sensor networks possible.

Fabry-Perot interferometers built by photonic crystal fiber pressurization during fusion splicing.

We report on a microscopic Fabry-Perot interferometer whose cavity is a bubble trapped inside an optical fiber. The microcavity is formed by pressuring a photonic crystal fiber (PCF) with large voids during fusion splicing with a conventional single-mode fiber. The technique allows achieving high repeatability and full control over the cavity size and shape. It was found that the size of the PCF voids contributes to control the cavity size independently of the pressure in the PCF. Our devices exhibit a record fringe contrast of 30 dB (visibility of 0.999) due to the ellipsoidal cavity whose surfaces compensate for the diffraction of the reflected beam. The strain sensitivity of the interferometers is higher when the cavities are ellipsoidal than when they are spherical.

Compact omnidirectional multicore fiber-based vector bending sensor.

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° to 360°. The bending sensor proposed here is highly sensitive even for small bending angles (below 1°).