Lúcia Bilro

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.

Bragg gratings in a few mode microstructured polymer optical fiber in less than 30 seconds

We report the inscription of a Bragg grating in an undoped polymethylmethacrylate based microstructured fiber in a time record. The fiber has been irradiated with a 248 nm ultraviolet radiation, through the phase mask technique using low fluence and low repetition rate. The experimental conditions were chosen to modify the core refractive index of the fiber at the incubation regime and avoiding polymer ablation. The peak reflection of the Bragg grating was centered in the infrared region with 20 dB reflection and 0.16 nm bandwidth. These spectral properties are well attractive for sensors and communications applications.

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.

Acousto-Optic Effect in Microstructured Polymer Fiber Bragg Gratings: Simulation and Experimental Overview

A fine control of the microstructured polymer fiber Bragg grating spectrum properties, such as maximum reflected power and 3-dB bandwidth, through acousto-optic modulation is presented. For simulation purposes, the device is modelled as a single structure, comprising a silica horn and a fiber Bragg grating. For similar sized structures a good correlation between the numerical results and the experimental data is obtained, allowing the strain field to be completely characterized along the whole structure. It is also shown that the microstructured polymer fiber Bragg grating requires less effort from the piezoelectric actuator to produce modification in the grating spectrum when compared with a silica fiber Bragg grating. This technique has potential to be applied on tunable optical filters and tunable cavities for photonic applications.

A reliable low-cost wireless and wearable gait monitoring system based on a plastic optical fibre sensor

A wearable and wireless system designed to evaluate quantitatively the human gait is presented. It allows knee sagittal motion monitoring over long distances and periods with a portable and low-cost package. It is based on the measurement of transmittance changes when a side-polished plastic optical fibre is bent. Four voluntary healthy subjects, on five different days, were tested in order to assess inter-day and inter-subject reliability. Results have shown that this technique is reliable, allows a one-time calibration and is suitable in the diagnosis and rehabilitation of knee injuries or for monitoring the performance of competitive athletes. Environmental testing was accomplished in order to study the influence of different temperatures and humidity conditions.

Design and performance assessment of a plastic optical fibre-based sensor for measuring water turbidity

A turbidity sensor based on a plastic optical fibre is presented. The sensor is based on transmission and 90° scattering variations with the total suspended particles in a solution. Transmitted and scattered output signals were characterized and evaluated for different configurations for a large range of clay concentrations. The developed system, in comparison with the OBS-3+ standard system, is more robust, of low cost and has a user-friendly design. A good correlation between the systems was accomplished.

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.

Simultaneous measurement of strain, temperature and refractive index based on multimode interference, fiber tapering and fiber Bragg gratings

We report the development of an optical fiber sensor capable of simultaneously measuring strain, temperature and refractive index. The sensor is based on the combination of two fiber Bragg gratings written in a standard single-mode fiber, one in an untapered region and another in a tapered region, spliced to a no-core fiber. The possibility of simultaneously measuring three parameters relies on the different sensitivity responses of each part of the sensor. The results have shown the possibility of measuring three parameters simultaneously with a resolution of 3.77 μe, 1.36 °C and 5 × 10−4, respectively for strain, temperature and refractive index. On top of the multiparameter ability, the simple production and combination of all the parts involved on this optical-fiber-based sensor is an attractive feature for several sensing applications.

Turbidity sensor for determination of concentration, ash presence and particle diameter of sediment suspensions

The present work addresses the need for low-cost turbidity sensors felt in the context of an ongoing research project on enhanced soil erosion following wildfire in Portugal. To this end, a system based on plastic optical fibre was developed and tested, including against a commercially-available system. The performance of the sensor was tested using artificially-created samples with a wide range of concentration of three types of very distinct materials, six particle size classes of ashes and real runoff samples collected at the slope and catchment scale in a recently burned area.