This work provides a method that combines graphene oxide coating and self-image theory to improve the sensitivity of optical sensors. The sensor is designed specifically to measure the amount of glucose present quantitatively in aqueous solutions that replicate the range of glucose concentrations found in human saliva. COMSOL Multiphysics 6.0 was used to simulate the self-imaging phenomenon using a coreless silica fiber (CSF). For high-quality self-imaging, the second and fourth self-imaging points are usually preferred because of their higher coupling efficiency, which increases the sensor sensitivity. However, managing the fourth self-image is more difficult because it calls for a longer CSF length. As a result, the first and second self-image points were the focus of the simulation in this work. After the simulation, using the Layerby- Layer method, the sensor was constructed to a length that matched the second self-image point (29.12 mm) and coated with an 80 μm/mL graphene oxide layer. When comparing uncoated and graphene oxide-covered sensors to measure glucose in liquids ranging from 25 to 200 mg/dL, one bilayer of polyethyleneimine/graphene demonstrated an eight-fold improvement in sensitivity. The final sensor, built on graphene oxide, showed stability with a low standard deviation of 0.6 pm/min. It also showed sensitivity at 10.403 ± 0.004 pm/(mg/dL) with a limit of detection of 9.15 mg/dL.
Spectral imaging is a powerful technology that uses spatially referenced spectral signatures to create informative visual maps of sample surfaces that can reveal more than what conventional RGB-visual images can show. Indeed, different spectroscopy modalities can provide different information about the same sample: for instance, Laser- Induced Breakdown Spectroscopy (LIBS) imaging can detect the presence of specific elements on the surface, while Raman imaging can identify the molecular structures and compositions of the sample, both of which have potential applications in various industrial processes, from quality control to material sorting. In the path from science to technology, the increasing accessibility to such solutions and the strong market pull have opened a window of opportunity for innovative multimodal imaging solutions, where information from distinct sources is set to be combined in order to enhance the capabilities of the single modality system. However, the practical implementation of multimodal spectral imaging is still a challenge, despite its theoretical potential, and as such, it is yet to be achieved.
In this work, we will go over multimodal spectral knowledge distillation, a disruptive approach to multimodal spectral imaging techniques that tries to explore the combination of two techniques to capitalize on their individual strengths. In specific, this approach allows us to utilize one technique as an autonomous supervisor for the other, leveraging the higher degree of knowledge and interpretability of one of the techniques to increase the performance and transparency of the other. We present some example scenarios with LIBS and HSI and Raman spectroscopy and LIBS, discussing the impact of this new approach for scientific and technological applications.
The generation of short pulses in fiber lasers using saturable absorbers made of graphene oxide (GO), focusing on film thickness, was studied and optimized. The saturable absorber comprised a GO thin film deposited onto a single-mode fiber using the spray coating technique. Water-dispersed GO with a concentration of 4 mg/mL, characterized by a high proportion of monolayer flakes, was employed. This thin film was integrated into a cavity ring laser featuring an erbiumdoped fiber amplifier (EDFA), resulting in a fiber laser emitting at a central emission wavelength of approximately 1564 nm and having a total cavity length of approximately 120 m. By controlling intracavity polarization, short-pulsed light was generated through mode-locking, Q switching, or a combination of both regimes.
This work presents a comprehensive characterization of the cavity ring laser operating under the mode-locking regime. It encompasses an analysis of the spectral behavior, focusing on the evolution of the Kelly’s sidebands with increasing pump power, as well as an assessment of its temporal stability. Moreover, the effects of the aging of the saturable absorber material were studied after a time period of 6 months after the fabrication. It was observed that the general characteristics of spectral signal of the laser were maintained, with long-term stability.
Hollow microsphere fiber sensors are Fabry-Perot interferometers (FPI) that can be used for lateral loading, temperature, and refractive index sensing. In this work, graphene oxide (GO) is explored as a tunable platform for enhancing the spectral properties of hollow microsphere fiber sensors. GO offers similar mechanical and optical properties as graphene, with the advantage of a wider range of deposition methods and a lower cost. The influence of multilayer coatings of polyethylenimine (PEI) and GO, achieved with the layer-by-layer technique, on the reflectivity of the outer surface, and hence, on the spectrum of the FPI for maximum of 30 bilayers was studied. The obtained results revealed a change of the microsphere outer surface reflectivity and also of visibility of the reflected spectrum when varying the number of bilayers. A maximum signal amplitude of 3.9 dB was attained for the 13th bilayer, allowing to conclude that PEI/GO multilayer coatings can be used for enhancing desired properties of the three-wave FPI for different sensing applications.
In this work, 3D printing is explored as a solution for fast prototyping of optical fiber sensors with applications in power transformers. Two different sensing structures were evaluated using finite element method (FEM) analysis and were fabricated using 3D printing. The printed structures are composed by acrylonitrile butadiene styrene (ABS), a common thermoplastic polymer used in 3D printing. Attaching a fiber Bragg grating (FBG) to each structure, frequency measurements were successfully obtained for values between 20 and 250 Hz.
Power transformers are at the core of power transmission systems. The occurrence of system failure in power transformers can lead to damage of adjacent equipment and cause service disruptions. Structural and electrical integrity assessment in real time is of utter importance. Conventional techniques, typically electrical sensors or chemical analysis, present major drawbacks for real-time measurements due to high electromagnetic interference or for being time-consuming. Optical fiber sensors can be used in power transformers, as they are compact and immune to electromagnetic interferences. In this work, an optical fiber sensor composed by 2 fiber Bragg gratings, attached in a cantilever structure was explored. The prototype was developed with a 3D printer using a typical filament (ABS) that enable a fast and low-cost prototyping. The response of the sensor to vibration was tested using two different vibration axes for frequencies between 10 and 500 Hz. Oil compatibility was also studied using thermal aging and electrical tests. The studies shown that ABS is compatible with the power transformer mineral oil, but the high working temperatures may lead to material creeping, resulting in permanent structural deformation.
Fusion splicing technique was explored for the fabrication of two sensing structures based on hollow microsphere Fabry- Perot cavity. The first sensor proposed was fabricated with a hollow microsphere tip, working as a probe sensor. This structure was studied for lateral load pressure, yielding a 1.56 ± 0.01 nm/N sensitivity. The second sensing structure relied on an in-line hollow microsphere, which allowed the detection of lateral load, with a sensitivity of 2.62 ± 0.02 nm/N. Furthermore, the proposed structure enabled strain sensing, with a sensitivity of 4.66 ± 0.03 pm/με. The two sensing structures were subjected to temperature, presenting low thermal cross-sensitivity.
A sensor based on Fabry-Perot interferometry with a hollow microsphere cavity embedded in a 3D printed structure is proposed. The sensor was tested for lateral loading and temperature, showing promising results. By imprintring the sensor on the structure, the dynamic range of application is severely increased enabling the application of the sensor in harsh environments.
A Fabry-Perot based sensor with two coupled hollow microspheres is presented. The sensor was fabricated using fusion splicing techniques, enabling a low-cost, highly reproducible, production. The coupling of the two microspheres gives rise to a highly sensitive strain sensor, reaching a sensitivity of 4.07 pm/με.The all-silica composition leads to a low thermal sensitivity, making the proposed structure suitable applications in environments with varying external conditions.
A curvature sensor based on a Fabry-Perot interferometer is proposed. A capillary tube of silica is fusion spliced
between two single mode fibers, producing a Fabry-Perot cavity. The light propagates in air, when passing through
the capillary tube. Two different cavities are subjected to curvature and temperature. The cavity with shorter length
shows insensitivity to both measurands. The larger cavity shows two operating regions for curvature measurement,
where a linear response is shown, with a maximum sensitivity of 18.77pm/m-1 for the high curvature radius range.
When subjected to temperature, the sensing head produces a similar response for different curvature radius, with a
sensitivity of 0.87pm/°C.
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