Nélia Alberto

Optical Sensors Based on Plastic Fibers

The recent advances of polymer technology allowed the introduction of plastic optical fiber in sensor design. The advantages of optical metrology with plastic optical fiber have attracted the attention of the scientific community, as they allow the development of low-cost or cost competitive systems compared with conventional technologies. In this paper, the current state of the art of plastic optical fiber technology will be reviewed, namely its main characteristics and sensing advantages. Several measurement techniques will be described, with a strong focus on interrogation approaches based on intensity variation in transmission and reflection. The potential applications involving structural health monitoring, medicine, environment and the biological and chemical area are also presented.

Analytical Analysis of Side-Polished Plastic Optical Fiber as Curvature and Refractive Index Sensor

The modeling of a side-polished plastic optical fiber as a sensor of two distinct physical parameters is presented. A comprehensive analytical study is performed using a geometric optic approach. Different details are taken into account, such as the geometric description of the sensor, the intensity profile of the emitter, and the possibility of a multireflection for a light ray at the sensitive area. The good agreement between the experimental and theoretical results validates the developed analytical model.

Three-parameter optical fiber sensor based on a tilted fiber Bragg grating

A three-parameter optical sensor based on a tilted fiber Bragg grating is proposed. Through the monitoring of the wavelength shift of the core mode resonance and the ghost mode resonance, it is possible to discriminate strain and temperature. In addition, the refractive index can be determined by calculating the normalized transmission spectrum area. With the current approach, resolutions of up to 5.7×10−4, 4 μe, and 3.1 °C were achieved, for refractive index, strain, and temperature, respectively. The developed sensor can be an important tool in several areas of engineering, namely, biomedical, biological, and environmental sensing.

Optical Fiber Microcavity Strain Sensors Produced by the Catastrophic Fuse Effect

We present an innovative and cost effective approach to produced sensors based on optical fiber microcavities. The proposed microcavities were manufactured by splicing a standard optical fiber with recycled optical fibers destroyed by the catastrophic fuse effect, yielding strain sensors with sensitivity up to 2.56 pm·με-1. The feasibility of this solution employing recycled optical fibers was demonstrated, presenting an economical solution for sensing purposes, when compared with cavities produced using complex methods. We also show, for the first time, that the sensitivity of these microcavities Fabry-Perot interferometers sensors depends on the cavity volume.

Feasibility studies of Bragg probe for noninvasive carotid pulse waveform assessment.

Abstract. The arterial stiffness evaluation is largely reported as an independent predictor of cardiovascular diseases. The central pulse waveform can provide important data about arterial health and has been studied in patients with several pathologies, such as diabetes mellitus, coronary artery disease and hypertension. The implementation and feasibility studies of a fiber Bragg grating probe for noninvasive monitoring of the carotid pulse are described based on fiber Bragg grating technology. Assessment tests were carried out in carotids of different volunteers and it was possible to detect the carotid pulse waveform in all subjects. In one of the subjects, the sensor was also tested in terms of repeatability. Although further tests will be required for clinical investigation, the first studies suggest that the developed sensor can be a valid alternative to electromechanical tonometers.

Optical Sensors Based on Fiber Bragg Gratings for Structural Health Monitoring

In this work we review the structural health monitoring techniques based on fiber Bragg gratings. The working principle of the fiber Bragg gratings sensors and the most common techniques to inscribe and interrogate these sensors are described. Several implemented examples are also presented, like the deformation monitoring of one historical building with reduced visual impact, the unidirectional acceleration measurements in a metallic bridge structure and the bidirectional acceleration monitoring in a 50 m mobile telecom tower. Finally, the implementation of an automated remote structural health monitoring system design to operate with optical sensors in a highway bridge is described.

Liquid Hydrostatic Pressure Optical Sensor Based on Micro-Cavity Produced by the Catastrophic Fuse Effect

We propose an optical fiber hydrostatic pressure sensor based on micro-cavities generated by the fiber fuse effect. The presented sensor is manufactured through the recycling of optical fiber destroyed by the fiber fuse effect, being, therefore, a cost-effective solution, when compared with other similar micro-cavity-based solutions. The developed sensor was characterized for pressures up to 20 kPa, showing a linear sensitivity coefficient of 0.47 ± 0.03 nm · kPa-1, for pressure values below 8 kPa. Furthermore, we propose a new theoretical model to describe the behavior of the microcavities embedded in optical fibers. This allows us to solve the discrepancies, already identified by other authors, between the experimental results and the ones attained with the flat mirrors Fabry-Perot model. By this way, we were able to describe the sensor response, within the full dynamic range.

In the trail of a new bio-sensor for measuring strain in bone: osteoblastic biocompatibility.

Fibre Bragg Grating (FBG) is an optical sensor recorded within the core of a standard optical fibre, which responds faithfully to strain and temperature. FBG sensors are a promising alternative to other sensing methodologies to assess bone mechanics in vivo. However, response of bone cells/bone tissue to FBGs and its sensing capability in this environment have not been recorded yet. The present study addressed these issues in long-term human osteoblastic cell cultures. Results showed that osteoblastic cells were able to adhere and proliferate over the fibre and, also, the protective polymer coating. RT-PCR analysis showed the expression of Col I, ALP, BMP-2, M-CSF, RANKL and OPG. In addition, cultures presented high ALP activity and the formation of a calcium phosphate mineralized extracellular matrix. Cell behavior over the fibre without and with the coating polymer was similar to that found in cultures grown in standard tissue culture plates (control). In addition to the excellent osteoblastic cytocompatibility, FBGs maintained the physical integrity and functionality, as its sensing capability was not affected through the culture period. Results suggest the possibility of in vivo osseointegration of the optical fibre/FBGs anticipating a variety of applications in bone mechanical dynamics.

Insole optical fiber Bragg grating sensors network for dynamic vertical force monitoring

Abstract. In an era of unprecedented progress in technology and increase in population age, continuous and close monitoring of elder citizens and patients is becoming more of a necessity than a luxury. Contributing toward this field and enhancing the life quality of elder citizens and patients with disabilities, this work presents the design and implementation of a noninvasive platform and insole fiber Bragg grating sensors network to monitor the vertical ground reaction forces distribution induced in the foot plantar surface during gait and body center of mass displacements. The acquired measurements are a reliable indication of the accuracy and consistency of the proposed solution in monitoring and mapping the vertical forces active on the foot plantar sole, with a sensitivity up to 11.06  pm/N. The acquired measurements can be used to infer the foot structure and health condition, in addition to anomalies related to spine function and other pathologies (e.g., related to diabetes); also its application in rehabilitation robotics field can dramatically reduce the computational burden of exoskeletons’ control strategy. The proposed technology has the advantages of optical fiber sensing (robustness, noninvasiveness, accuracy, and electromagnetic insensitivity) to surpass all drawbacks verified in traditionally used sensing systems (fragility, instability, and inconsistent feedback).

Internal and External Temperature Monitoring of a Li-Ion Battery with Fiber Bragg Grating Sensors

The integration of fiber Bragg grating (FBG) sensors in lithium-ion cells for in-situ and in-operando temperature monitoring is presented herein. The measuring of internal and external temperature variations was performed through four FBG sensors during galvanostatic cycling at C-rates ranging from 1C to 8C. The FBG sensors were placed both outside and inside the cell, located in the center of the electrochemically active area and at the tab-electrode connection. The internal sensors recorded temperature variations of 4.0 ± 0.1 °C at 5C and 4.7 ± 0.1 °C at 8C at the center of the active area, and 3.9 ± 0.1 °C at 5C and 4.0 ± 0.1 °C at 8C at the tab-electrode connection, respectively. This study is intended to contribute to detection of a temperature gradient in real time inside a cell, which can determine possible damage in the battery performance when it operates under normal and abnormal operating conditions, as well as to demonstrate the technical feasibility of the integration of in-operando microsensors inside Li-ion cells.