All-fiber broadband spectral acousto-optic modulation of a tubular-lattice hollow-core optical fiber.

We demonstrate a broadband acousto-optic notch filter based on a tubular-lattice hollow-core fiber for the first time to our knowledge. The guided optical modes are modulated by acoustically induced dynamic long-period gratings along the fiber. The device is fabricated employing a short interaction length (7.7 cm) and low drive voltages (10 V). Modulated spectral bands with 20 nm half-width and maximum depths greater than 60% are achieved. The resonant notch wavelength is tuned from 743 to 1355 nm (612 nm span) by changing the frequency of the electrical signal. The results indicate a broader tuning range compared to previous studies using standard and hollow-core fibers. It further reveals unique properties for reconfigurable spectral filters and fiber lasers, pointing to the fast switching and highly efficient modulation of all-fiber photonic devices.

Agar-based optical sensors for electric current measurements.

Biodegradable optical waveguides are breakthrough technologies to light delivery and sensing in biomedical and environmental applications. Agar emerges as an edible, soft, low-cost, and renewable alternative to traditional biopolymers, presenting remarkable optical and mechanical characteristics. Previous works introduced agar-made optical fibers for chemical measurements based on their inherent response to humidity and surrounding concentration. Therefore, we propose, for the first time, an all-optical, biodegradable electric current sensor. As flowing charges heat the agar matrix and modulate its refractive index, we connect the optical device to a DC voltage source using pin headers and excite the agar sample with coherent light to project spatiotemporally deviating speckle fields. Experiments proceeded with spheres and no-core fibers comprising 2 wt% agar/water. Once the increasing current stimulates the speckles' motion, we acquire such images with a camera and evaluate their correlation coefficients, yielding exponential decay-like functions whose time constants provide the input amperage. Furthermore, the light granules follow the polarization of the applied voltage drop, providing visual information about the current direction. The results indicate a maximum resolution of [Formula: see text]0.4 [Formula: see text]A for electrical stimuli [Formula: see text] 100 [Formula: see text]A, which fulfills the requirements for bioelectrical signal assessment.

Graphene woven fabric-polydimethylsiloxane piezoresistive films for smart multi-stimuli responses.

The outstanding properties of graphene, including its electromechanical property, could be engineered for wearable electronic sensor platforms. The tubular graphene weaved into a mesh or graphene woven fabrics (GWF) has been reported as one of the most sensitive materials for deformation detection, as well as a promising temperature sensor. Herein, we present the performance of our developed flexible, stretchable, and multiple sensitive sensors fabricated from GWF embedded in polydimethylsiloxane (PDMS) film substrate. The GWF/PDMS sensor shows a pressure sensitivity of 0.0142 kPa-1 in a wide linearity range of 0-20 kPa, an outstanding Gauge factor (GF) of 582 at a strain of 6.2 %, and a very high positive sensitivity of 0.0238 °C-1 in the temperature range of 25-80 °C. A practical application as a high sensitivity air pressure sensor able to measure low pressures (in the range of Pa to kPa) was also demonstrated. This sensor platform having desirable performance characteristics is an excellent candidate for wearable devices in the healthcare sector.

Synthesis of Carbon Nanodots from Sugarcane Syrup, and Their Incorporation into a Hydrogel-Based Composite to Fabricate Innovative Fluorescent Microstructured Polymer Optical Fibers.

Carbon nanodots (CNDs) are interesting materials due to their intrinsic fluorescence, electron-transfer properties, and low toxicity. Here, we report a sustainable, cheap, and scalable methodology to obtain CNDs from sugarcane syrup using a domestic microwave oven. The CNDs were characterized by infrared spectroscopy, dynamic light scattering, atomic force microscopy, absorption, and emission spectroscopies. The CNDs have 3 nm in diameter with low polydispersity and are fluorescent. A fluorescent hydrogel-CNDs composite was obtained using gelatin polypeptide as the polymeric matrix. The new hydrogel-CNDs composite was incorporated in the cavities of a double-clad optical fiber using an innovative approach that resulted in a microstructured polymer optical fiber with intrinsic fluorescence. This work shows a promising alternative for the fabrication of fluorescent materials since the CNDs synthesis is sustainable and environmentally friendly. These CNDs might substitute the rare-earth and other heavy metals of high cost and toxicity, which are usually incorporated in double-clad fibers for applications on lasers, amplifiers, and spectroscopy.

Machine learning for sensing with a multimode exposed core fiber specklegram sensor.

Fiber specklegram sensors (FSSs) traditionally use statistical methods to analyze specklegrams obtained from fibers for sensing purposes, but can suffer from limitations such as vulnerability to noise and lack of dynamic range. In this paper we demonstrate that deep learning improves the analysis of specklegrams for sensing, which we show here for both air temperature and water immersion length measurements. Two deep neural networks (DNNs); a convolutional neural network and a multi-layer perceptron network, are used and compared to a traditional correlation technique on data obtained from a multimode fiber exposed-core fiber. The ability for the DNNs to be trained against a random noise source such as specklegram translations is also demonstrated.

Terahertz optical fibers [Invited].

Lying between optical and microwave ranges, the terahertz band in the electromagnetic spectrum is attracting increased attention. Optical fibers are essential for developing the full potential of complex terahertz systems. In this manuscript, we review the optimal materials, the guiding mechanisms, the fabrication methodologies, the characterization methods and the applications of such terahertz waveguides. We examine various optical fiber types including tube fibers, solid core fiber, hollow-core photonic bandgap, anti-resonant fibers, porous-core fibers, metamaterial-based fibers, and their guiding mechanisms. The optimal materials for terahertz applications are discussed. The past and present trends of fabrication methods, including drilling, stacking, extrusion and 3D printing, are elaborated. Fiber characterization methods including different optics for terahertz time-domain spectroscopy (THz-TDS) setups are reviewed and application areas including short-distance data transmission, imaging, sensing, and spectroscopy are discussed.

Multimode exposed core fiber specklegram sensor.

The inception of photonic crystal fibers (PCFs) allowed for unprecedented tailoring of waveguide properties for specialty sensing probes. Exposed core microstructured fibers (ECFs) represent a natural evolution of the PCF design for practical liquid and gas sensing. Until now, to the best of our knowledge, only single-mode or few-modes ECFs have been explored. In this Letter, we demonstrate a highly multimode ECF with a lateral access that extends throughout the whole length of the fiber. The ECF is operated as a fiber specklegram sensor for assessing properties of fluids and interrogated using a simple and low-cost setup. The probe exhibits a refractive index resolution and sensitivity of at least 4.6×10-4 refractive index units (RIUs) and -10.97RIU-1, respectively. A maximum temperature resolution up to 0.017°C with a -0.20∘C-1 temperature sensitivity over the 23°C-28°C range and a liquid level sensing resolution up to 0.12 mm with -0.015mm-1 sensitivity over the 0.0-50.0 mm bathed the length range in water.

Ultra-simplified Single-Step Fabrication of Microstructured Optical Fiber.

Manufacturing optical fibers with a microstructured cross-section relies on the production of a fiber preform in a multiple-stage procedure, and drawing of the preform to fiber. These processes encompass the use of several dedicated and sophisticated equipment, including a fiber drawing tower. Here we demonstrate the use of a commercial table-top low-cost filament extruder to produce optical fibers with complex microstructure in a single step - from the pellets of the optical material directly to the final fiber. The process does not include the use of an optical fiber drawing tower and is time, electrical power, and floor space efficient. Different fiber geometries (hexagonal-lattice solid core, suspended core and hollow core) were successfully fabricated and their geometries evaluated. Air guidance in a wavelength range where the fiber material is opaque was shown in the hollow core fiber.

Agarose-based structured optical fibre.

Biocompatible and resorbable optical fibres emerge as promising technologies for in vivo applications like imaging, light delivery for phototherapy and optogenetics, and localised drug-delivery, as well as for biochemical sensing, wherein the probe can be implanted and then completely absorbed by the organism. Biodegradable waveguides based on glasses, hydrogels, and silk have been reported, but most of these devices rely on complex fabrication procedures. In this sense, this paper proposes a novel structured optical fibre made of agarose, a transparent, edible material used in culture media and tissue engineering. The fibre is obtained by pouring food-grade agar into a mould with stacked rods, forming a solid core surrounded by air holes in which the refractive index and fibre geometry can be tailored by choosing the agarose solution composition and mould design, respectively. Besides exhibiting practical transmittance at 633 nm in relation to other hydrogel waveguides, the fibre is also validated for chemical sensing either by detecting volume changes due to agar swelling/dehydration or modulating the transmitted light by inserting fluids into the air holes. Therefore, the proposed agarose-based structured optical fibre is an easy-to-fabricate, versatile technology with possible applications for medical imaging and in vivo biochemical sensing.

Reusable polymer optical fiber strain sensor with memory capability based on ABS crazing.

A reusable memory capable polymer optical fiber (POF) strain sensor is reported. The fiber consists of an acrylonitrile butadiene styrene (ABS) core and polymethylmethacrylate cladding. The memory capability is derived from stress whitening due to crazing of the ABS core, which can be reversed by heating the fiber close to the ABS glass transition temperature. The probe was characterized under transverse compressive load, macrobending, and tensile loading. Testing shows that the optical properties of the fiber can be reversed to near pristine ABS conditions after thermal recovery and that the POF can be used on the accurate assessment of indentation, flexural and tensile loading, static or cyclical, even after removal of the load. The reusability of the proposed sensor combined with a deeper understanding of the memory mechanisms in POFs are of great interest for the development of new large-strain sensors for modern applications.

Biomechanical behaviour of bulk-fill resin composites in class II restorations.

The aim of this study was to evaluate the biomechanical properties expressed by shrinkage stress, cuspal strain, fracture strength and failure mode in molars with large class II mesio-occlusal-distal restorations. Sixty-four human caries-free third molars were selected and distributed randomly into four groups: Z100 restorative material (Z100), Tetric N-Ceram Bulk-Fill (TNC), Filtek Bulk-Fill (FBF) and Aura Ultra Universal (ABF). The bulk-fill materials were inserted in one singular bulk increment and the conventional composite resin in three ones. Polymerisation shrinkage stress was evaluated by optical Fibre Bragg Gratings (FBG) sensors (n = 6). The cuspal deformation was measured using an extensometer during three moments: restorative procedure, axial compressive loading and at fracture (n = 10). The fracture strength was evaluated on a universal machine. The failure mode was analysed by Scanning Electron Microscopy (SEM). Data were analysed using one-way ANOVA tests with Tukey's posthoc test (α = 5%). Data of the failure mode were submitted to a likelihood ratio chi-square test. Z100 presented the highest mean value for the shrinkage stress (p < 0.05) by FBG evaluation, whereas bulk-fill resin groups presented low polymerisation stress mean value, especially the TNC (p < 0.05). The cuspal deformation test showed that Z100 presented a significant difference mean value compared to the other groups (p < 0.01) during the restoration and compressive axial force; however, load until the fracture presented a difference only between TNC and FBF (p < 0.05). The fracture strength of TNC was statistically different from Z100 (p < 0.01). The failure mode was not statistically different in all the groups (p > 0.05). Bulk-fill composites promoted less polymerisation shrinkage stress than conventional microhybrid composite during and after the light curing process in class II posterior resin composite restorations.

Mid-IR Hollow-core microstructured fiber drawn from a 3D printed PETG preform.

Mid-infrared (mid-IR) optical fibers have long attracted great interest due to their wide range of applications in security, biology and chemical sensing. Traditionally, research was directed towards materials with low absorption in the mid-IR region, such as chalcogenides, which are difficult to manipulate and often contain highly toxic elements. In this paper, we demonstrate a Polyethylene Terephthalate Glycol (PETG) hollow-core fiber (HCF) with guiding properties in the mid-IR. Guiding is provided by the fiber geometry, as PETG exhibits a material attenuation 2 orders of magnitude larger than the HCF propagation loss. The structured plastic fiber preforms were fabricated using commercial 3D printing technology and then drawn using a conventional fiber drawing tower. The final PETG fiber outer diameter was 466 µm with a hollow-core diameter of 225 µm. Thermal imaging at the fiber facet performed within the wavelength range 3.5-5 µm clearly indicates air guidance in the fiber hollow-core.

Simplifying the design of microstructured optical fibre pressure sensors.

In this paper, we propose a way to simplify the design of microstructured optical fibres with high sensitivity to applied pressure. The use of a capillary fibre with an embedded core allows the exploration of the pressure-induced material birefringence due to the capillary wall displacements and the photoelastic effect. An analytical description of pressure-induced material birefringence is provided, and fibre modal characteristics are explored through numerical simulations. Moreover, a capillary fibre with an embedded core is fabricated and used to probe pressure variations. Even though the embedded-core fibre has a non-optimized structure, measurements showed a pressure sensitivity of (1.04 ± 0.01) nm/bar, which compares well with more complex, specially designed fibre geometries reported in the literature. These results demonstrate that this geometry enables a novel route towards the simplification of microstructured fibre-based pressure sensors.

Integration of bow-tie plasmonic nano-antennas on tapered fibers.

In this article, a new and flexible approach to control the electric field enhancement of bow-tie nano-antennas by integrating them on the lateral of a tapered optical fiber is proposed. The device is driven by a Q-switched laser and the performance of a fabricated nano-antenna in a quartz slide is tested by a Surface Enhanced Raman Scattering (SERS) experiment. A refractive index sensing experiment is also performed and a sensitivity of (240 ± 30) nm/RIU is found in the 1.33-1.35 index range.

Highly sensitive temperature sensor using a Sagnac loop interferometer based on a side-hole photonic crystal fiber filled with metal.

A highly sensitive temperature sensor based on an all-fiber Sagnac loop interferometer combined with metal-filled side-hole photonic crystal fiber (PCF) is proposed and demonstrated. PCFs containing two side holes filled with metal offer a structure that can be modified to create a change in the birefringence of the fiber by the expansion of the filler metal. Bismuth and indium were used to examine the effect of filler metal on the temperature sensitivity of the fiber-optic temperature sensor. It was found from measurements that a very high temperature sensitivity of -9.0 nm/°C could be achieved with the indium-filled side-hole PCF. The experimental results are compared to numerical simulations with good agreement. It is shown that the high temperature sensitivity of the sensor is attributed to the fiber microstructure, which has a significant influence on the modulation of the birefringence caused by the expansion of the metal-filled holes.

Characterizing Slow Photochemical Reaction Kinetics by Enhanced Sampling of Rare Events with Capillary Optical Fibers and Kramers' Theory.

Characterization of slow chemical reactions is essential for assessing catalytic efficiency in chemistry and biology. Traditionally, chemical reaction rates are obtained from population relaxation kinetics measurements and the Arrhenius equation. Unfortunately, it is difficult to use this approach to characterize reactions wherein concentrations change slowly. Thus, it is interesting to see whether a dynamical view of chemical reactions may be used to obtain the reaction rates of slow processes. In the present work, we perform Brownian dynamics simulations of an asymmetric double-well potential to investigate how enhanced sampling of barrier crossing at transition states improves the characterization of reaction rate constants. We then present the design of a liquid-filled capillary optical fiber-based fluorescence spectrometer, which, like rare events, is also based on Poissonian statistics. We use the instrument to characterize the slow photochemical degradation kinetics of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) in o-dichlorobenzene. We have employed in situ optical microscopy measurements and electrodynamics simulations to characterize the excitation beam profile inside a liquid-filled capillary fiber. We compare the cuvette and capillary fiber sample holders and show that the MEH-PPV fluorescence line shape is independent of the sample holder, as expected. We characterize the photochemical degradation kinetics of MEH-PPV in o-dichlorobenzene solutions placed in the cuvette versus that in the capillary fiber. We observe small and slow changes in the time-dependent fluorescence spectra when the degradation reaction is performed in the cuvette. On the other hand, we are able to characterize reactant-concentration decay and product-concentration buildup from the time-dependent fluorescence spectra recorded during photochemical degradation of MEH-PPV performed inside the capillary optical fiber. Ultrafast optically heterodyne-detected optical Kerr effect spectroscopy and multimode Brownian oscillator analysis provide further insights into the role of bath oscillator modes of friction in the mechanism of MEH-PPV photochemical degradation. Overall, the work presented herein shows that slow photochemical degradation kinetics of MEH-PPV can be successfully and efficiently assessed in the capillary fiber fluorescence spectrometer.

Morphology dependent polymeric capillary optical resonator hydrostatic pressure sensor.

A hydrostatic pressure sensor based on morphology dependent resonances in a polymeric tube is presented. By internal pressurization, normal tensions will increase the device's size and shrink its wall thickness, inducing a shift in the resonant wavelengths of the resonator. Numerical simulations indicate that there are two modal regimes of sensitivity and a maximum achievable sensitivity, related to the device's geometry, constitutive material and analysed mode order. A sensitivity as high as 0.36 ± 0.01 nm/bar has been experimentally found for a 1.8mm diameter PMMA tube with wall thickness of 80µm.

Analysis of immersed silica optical microfiber knot resonator and its application as a moisture sensor.

An embedded silica optical microfiber knot resonator humidity sensor is presented. As silica has a poor response to environmental humidity changes, a surrounding layer of Nafion is used as a transducer. Spectral characterization and also a procedure to determine the coupling and total loss coefficients are presented. Sensitivity as high as (0.29±0.01) nm/% relative humidity has been noticed. Possible issues that emerge from the use of Nafion such as bulk swelling, refractive index hysteresis, as well as a saturation process, are discussed.

Integrated polarizers based on tapered highly birefringent photonic crystal fibers.

This paper proposes and demonstrates the creation of sections with a high polarization dependent loss (PDL) in a commercial highly birefringent (polarization maintaining) photonic crystal fiber (PCF), via tapering with pressure applied to the holes. The tapers had a 1-cm-long uniform section with a 66% scale reduction, in which the original microstructure aspect ratio was kept by the pressure application. The resulting waveguides show polarizing action across the entire tested wavelength range, 1510-1600 nm, with a peak PDL of 35.3 dB/cm (c.f. ~1 dB/cm for a typical commercial polarizing fiber). The resulting structure, as well as its production, is extremely simple, and enable a small section with a high PDL to be obtained in a polarization maintaining PCF, meaning that the polarization axes in the polarizing and polarization maintaining sections are automatically aligned.

Photonic-crystal fiber-based pressure sensor for dual environment monitoring.

In this paper the development of a side-hole photonic-crystal fiber (SH-PCF) pressure sensor for dual environment monitoring is reported. SH-PCF properties (phase and group birefringence, sensitivity to pressure variations) are measured and compared to simulated data. In order to probe two environments, two sections of the SH-PCF with different lengths are spliced and set in a Solc filter-like configuration. This setup allows obtaining the individual responses of the first and second fiber independently, which is useful for a space-multiplexed measurement. As the employed fiber is sensitive to pressure variations, we report the use of this configuration for dual environment pressure sensing.