In this work, fiber optic sensor networks were developed to be integrated in commercially available lithium-ion batteries (cylindrical and prismatic) and pre-fabricated batteries in a laboratory environment (pouch cells), with the objective of monitoring in situ, operando and in real time, the internal and external variations of temperature and strain, under different environmental conditions and different charge and discharge rates. To this end, and in order to show the improved performance of fiber optic sensors in relation to the electronic ones, typically used in this type of application, their response time and resolution were compared. An improvement of 28% of the response time and a better resolution are attained with fiber optic sensors. Monitorization studies of the temperature and strain variations using fiber Bragg gratings (FBGs) in the cylindrical configuration have been made, as well as temperature and bi-directional strain variations in the prismatic configuration, under normal or abusive operating conditions, using the FBG method strain free. When the batteries were subjected to abusive operating conditions, it was evident that greater temperature and strain variations occur, being promoted by the rapid transport of lithium ions between the positive and negative electrodes. Due to the thermal expansion of the materials that compose the battery, its internal structure is an important parameter to consider and that can influence its behavior in terms of expansion and contraction. In order to monitor the thermal performance of lithium-ion prismatic batteries in different environmental conditions, studies were performed in which the battery operated at different discharge rates over different conditions of temperature and relative humidity, in order to simulate the performance of the battery in three distinct climates: cold, temperate, and dry. From these studies, the poor performance of this type of batteries in the cold climate, and consequent lower thermal performance was verified. A network of 37 FBG sensors has also been used to monitor the interfaces of a pack of 3 lithium polymer batteries connected in series. It was possible to perform a spatial and temporal thermal mapping under different discharge rates, and to identify areas that are more susceptible to the appearance of hot spots and that are capable of endanger its normal functioning. Hotter zones were detected near the current collectors, due to the higher density of lithium ions in this region. For the first time, the simultaneous discrimination of internal temperature and strain variations in lithium-ion batteries in the pouch cell configuration was carried out, through the incorporation of hybrid sensors, which combine the operational characteristics of the Fabry-Perot and FBG sensors. The evolution of the strain and temperature signals was followed by the proposed sensors and the largest strain variations were detected at the beginning of the discharge process, in the bottom position of the pouch cell. With the work developed in this Thesis, it is concluded that the integration of optical fiber sensors into lithium-ion batteries contributes to a better internal and external knowledge of the thermal performance and volume variations under different operating conditions. This might improve the safety conditions and optimize the design of the next generation of lithium-ion batteries.

This study developed an algorithm running on the cloud that makes data process and diagnoses broken rotor bar faults in three-phase induction motors (TIMs), by analyzing stator dynamic deformation using fibre Bragg gratings (FBGs) as sensors. This method can diagnose mechanical faults (misalignment, imbalance) and electrical faults (fractures or cracks in rotor rings or bars). FBG-based sensors were used due to their high multiplexing capability, electromagnetic radiation immunity, and long-distance operation. Tests were conducted on a small-scale induction motor (3 HP) coupled to a generator to simulate load and the generator supplied by the grid. Were used 1 healthy rotor and one rotor with a broken bar fault running at two load conditions, 75% and 100%. The algorithm successfully identified broken bar faults in two frequency regions: around the mechanical rotational frequency of the rotor 28.06 Hz and 31.014 Hz operating at 75%; 25.325 Hz and 32.995 Hz operating at 100% and approximately twice the electrical frequency supply, 118.455 Hz and 123.942 Hz operating at 75%; 117.237 Hz and 124.683 Hz operating at 100%. The system showed high sensitivity, a good signal-to-noise ratio, and advantages over conventional methods for detecting broken bar faults in induction motors.

In this work we proposed a relative humidity (RH) sensor based on a Bragg grating written in an optical fiber, associated with a coating of organo-silica hybrid material prepared by the sol-gel method. The organo-silica-based coating has a strong adhesion to the optical fiber and its expansion is reversibly affected by the change in the RH values (15.0–95.0%) of the surrounding environment, allowing an increased sensitivity (22.2 pm/%RH) and durability due to the presence of a siliceous-based inorganic component. The developed sensor was tested in a real structure health monitoring essay, in which the RH inside two concrete blocks with different porosity values was measured over 1 year. The results demonstrated the potential of the proposed optical sensor in the monitoring of civil engineering structures.

In this paper we summarize the research activities at the Instituto de Telecomunicações—Pólo de Aveiro and University of Aveiro, in the field of fiber Bragg grating based sensors and their applications in dynamic measurements for Structural Health Monitoring of slender structures such as towers. In this work we describe the implementation of an optical biaxial accelerometer based on fiber Bragg gratings inscribed on optical fibers. The proof-of-concept was done with the dynamic monitoring of a reinforced concrete structure and a slender metallic telecommunication tower. Those structures were found to be suitable to demonstrate the feasibility of FBG accelerometers to obtain the structures’ natural frequencies, which are the key parameters in Structural Health Monitoring and in the calibration of numerical models used to simulate the structure behavior.

Over the past 18 months, we have performed hundreds of temperature characterizations of fiber Bragg gratings inscribed in different germanium-doped silica glass fibers. Under experimental conditions, the main conclusions are as follows: the temperature dependence of the “temperature gauge factor” or the normalized temperature sensitivity, KT, was found to be quadratic in the −50–200 °C range, while it may be considered linear for the −20–100 °C range; KT values at 20 °C vary from 5.176 × 10−6 K−1, for a B/Ge co-doped fiber up to 6.724 × 10−6 K−1, for a highly Ge-doped fiber; KT does not depend on the hydrogen-loading process or the gratings coupling strength; KT is essentially independent of wavelength in the 1500–1600 nm range, its value being accurately determined with a relative error ~0.2%; based on the accurate value of KT = 6.165 × 10−6 K−1, at 20 °C, obtained for gratings inscribed in the SMF-28 fiber, we calculated a value of 19.4 × 10−6 K−1 for the thermo-optic coefficient of bulk germanium glass; and gratings produced by femtosecond-laser radiation and UV-laser radiation exhibit comparable values of KT. The previous achievements allow, by having knowledge of KT for a single grating, the accurate determination of the temperature dependence of the Bragg wavelength for any other grating inscribed in the same fiber; the presented methodology enables one to determine the “unknown” gratings’ temperature sensitivity, typically with an error of 0.01 pm/°C, being, therefore, very useful in research labs and computer simulations. Thus, expressions for the temperature dependence of KT for gratings inscribed in several fibers are given, as well as an expression for KT as a function of the effective refractive index. We have also fully analyzed the potential sources of error in KT determination.

The use of wearable systems for monitoring vital parameters has gained wide popularity in several medical fields. The focus of the present study is the experimental assessment of a smart textile based on 12 fiber Bragg grating sensors for breathing monitoring and thoraco‐abdominal motion pattern analysis. The feasibility of the smart textile for monitoring several temporal respiratory parameters (ie, breath‐by‐breath respiratory period, breathing frequency, duration of inspiratory and expiratory phases), volume variations of the whole chest wall and of its compartments is performed on 8 healthy male volunteers. Values gathered by the textile are compared to the data obtained by a motion analysis system, used as the reference instrument. Good agreement between the 2 systems on both respiratory period (bias of 0.01 seconds), breathing frequency (bias of −0.02 breaths/min) and tidal volume (bias of 0.09 L) values is demonstrated. Smart textile shows good performance in the monitoring of thoraco‐abdominal pattern and its variation, as well.

An alternative technique that uses a flame-brush at high temperature to enhance UV light photosensitivity in an optical fiber is described. An extreme low-cost air aspirated propane-butane mini-torch is used, which produces a lower temperature flame than the one in the flame-brush original technique. It is shown that this change in the previous technique is also capable of improving photosensitivity and allowing the fiber Bragg grating imprinting process to be accelerated. Since the flame-brush photosensitivity enhancement process is designed to operate in an automated fiber taper rig, the process was evaluated in optical fiber tapers with different diameters. In this case, changes in photosensitivity are observed in the tapers in addition to the intrinsic photosensitivity occurring in the pristine fiber without being tapered.

This paper reports an effective method of packaging a fiber Bragg grating (FBG) for the simultaneous measurement of temperature and strain. The technique consists of embedding two fiber Bragg grating sensors inside a polymeric material with different geometrical characteristics. The mechanical and thermal characterizations of the optical transducer were performed. Then a matrix equation used measurements of the wavelength shifts from the two sensors and yielded information about the temperature and strain coefficients. The preliminary results demonstrate the feasibility of the encapsulation technique allowing measurements of temperature and strain in smart structures and harsh environments. The experimental procedure provides robustness to the sensor and the matrix equation approach has the potential to determine simultaneously the strain and temperature coefficients.