[show abstract][hide abstract] ABSTRACT: Partial discharges are a major source of insulation failure in electric power transformers. Their detection can be achieved through the associated acoustic emissions, and this work reports on the investigation of a fibre laser sensor based on a phase-shifted chirped fibre grating for acoustic emission detection in the power transformer environment. The performance of the sensing head is characterized and compared for different surrounding media: air, water and oil.
[show abstract][hide abstract] ABSTRACT: A sensing configuration based on an intensity optical fibre sensor for temperature measurement is reported. Two sensing heads, with identical geometrical configuration, connected in series are implemented. Each sensing head is placed between two fibre Bragg gratings (FBGs), being able to provide a self-referenced measurement, and thus eliminate errors that may arise from undesired intensity fluctuations in the configuration. The first FBG, placed before the aluminium tube, acts as the reference FBG, while the second FBG, placed after the aluminium tube, acts as the signal FBG. It is observed that the amplitude of the signal FBG decays when temperature increases, due to the increase of the ferrules' gap and as result of the material thermal expansion. The temperature response has a behaviour that corresponds to a polynomial fit of third order. The crosstalk between the two sensing heads in series is analysed. The temperature sensitivities obtained in the intervals regions of [36, 48.5] 1C and [64, 85] 1C are 2.67 Â 10 À 3 1C À 1 and 1.74 Â 10 À 4 1C À 1 , respectively. Ten sensing heads with this configuration can be multiplexed in series network topology. & 2012 Elsevier B.V. All rights reserved.
[show abstract][hide abstract] ABSTRACT: Fiber optic modal interferometry has been around as a sensing concept
since the outcome of fiber optic sensing. Initially supported by the
utilization of standard Hi-Bi fibres associated to polarimetric modal
interference, later this sensing approach evolved to modal interference
based on spatial modes propagating in the core, on spatial modes
propagating in the core and in the cladding with coupling performed by
fibre devices such as long period gratings and tapers, and more recently
on several types of modes propagating in photonic crystal fibers. This
paper will address fiber optic sensing based on modal interferometry,
and configurations of different type researched in last years will be
presented and their performance compared.
[show abstract][hide abstract] ABSTRACT: In this work, an innovative fully integrated monitoring infrastructure based on optical fibre sensors was developed and implemented.
In the framework of the research project named PROTEU [Tecnologias Avançadas para a Monitorização de Sistemas Estuarinos e
Costeiros (PDCTM/P/MAR/15275/1999)], an 11km optical fibre cable with Bragg sensors each 500m was installed from the lagoon
mouth to Vouga river, along the bed of the Espinheiro channel, allowing the real-time measurement of water temperature at
each sensor location. The results of this project are currently feeding several studies concerning Ria de Aveiro and the surrounding
area and are crucial for a continuous assessment and management of the environmental conditions. Meanwhile, a fibre optic
sensing system for simultaneous measurement of temperature and salinity based on fibre Bragg grating (FBG) technology was
also developed. In the following sections, a complete description of the fabrication process, as well as theoretical and experimental
results regarding this particular sensing system, are addressed. Earlier in situ local measurements, as well as the latest
remote monitoring and data processing scheme, are described. The developed technology is now being exploited by FiberSensing,
an INESC Porto spin-off company devoted to the development of optical fibre Bragg grating-based sensor systems for advanced
monitoring applications. The main markets of the company are the ones of structural health monitoring in civil and geotechnical
engineering, energy production and distribution, and environment.
[show abstract][hide abstract] ABSTRACT: In this work, two all-fiber interferometric configurations based on suspended core fibers (SCF) are investigated. A Fabry–Pérot cavity (FPC) made of SCF spliced in-between segments of single-mode and hollow-core fiber is proposed. The interferometric signals are generated by the refractive-index mismatches between the two fibers in the splice region and at the end of the suspended-core fiber. An alternative sensing head configuration formed by the insertion of a length of SCF as a birefringence element in a Sagnac loop interferometer is also demonstrated. In this structure, the interferometric signals are generated by interfering two counter propagating beams with different polarization states which propagate through a length of SCF as a birefringence element. The sensitivity to pressure and temperature was determined for both configurations. The results show that the pressure sensitivities are −4.68×10−5nm/psi and 0.032nm/psi for FPC and Sagnac loop interferometers, respectively. The temperature sensitivity of both structures has been obtained and the results have been discussed.
[show abstract][hide abstract] ABSTRACT: This work describes a fiber optic sensing structure that is sensitive to curvature, and features a low temperature- and strain cross-sensitivity. It is based on multimode interference, and relies on a singlemode–step index multimode–singlemode fiber structure. It was observed that the transmitted optical power in such a layout is highly sensitive to the wavelength of operation, and to the length of the multimode fiber. The optical spectrum exhibits two dominant loss bands, at wavelengths that have similar responses both to temperature and strain, but different responses to curvature. Based on this result, an interrogation approach is proposed that permits substantial sensitivity to curvature (8.7 ± 0.1 nm m) and residual sensitivities to temperature and strain (0.3 ± 0.1 pm °C−1 and (−0.06 ± 0.01) × 10−6 m m−1, respectively). The beam-propagation method was employed for modeling the propagation of light along the optical fiber sensing device proposed.
Measurement Science and Technology 06/2011; 22(8):085201. · 1.44 Impact Factor
[show abstract][hide abstract] ABSTRACT: Fiber structures based on the combination of abrupt tapers and fiber Bragg gratings are studied. Two situations are explored—in one, the taper is fabricated in the fiber region with a fiber Bragg grating; in the other, the taper is first fabricated followed by the fiber Bragg grating. It is shown that the first device presents the properties of a Fabry-Perot cavity and the other of a phase-shifted Bragg grating, where the phase shift is associated to the tapered fiber region. The sensing characteristics of these structures are studied, and it is shown that the temperature sensitivities are similar but with observable different responses to strain.
Fiber and Integrated Optics - FIBER INTEGRATED OPT. 01/2011; 30(1):9-28.
[show abstract][hide abstract] ABSTRACT: In this paper, we demonstrate the interrogation of fiber optic intensity sensors by using the combination of the frequency-modulated continuous wave concept with the spectral selective reflectivity of fiber Bragg gratings. Thus, we multiplex these kinds of sensors with this technique having simultaneously a referenced system. The basis of this dual functionality is described and results are presented for the case of interrogation of two multiplexed intensity sensors. Their evaluation permits to establish the conditions to address a sensor network of this type. Also, it is proposed a strategy to implement this sensing approach without the requirement of using optical fiber delay lines in the sensor heads.
[show abstract][hide abstract] ABSTRACT: Acoustic emission monitoring is often used in the diagnosis of electrical and mechanical incipient faults in high-voltage apparatus. Partial discharges are a source of failure in power transformers, and the differentiation from other sources of acoustic emissions is of the utmost importance. This paper reports the development of a new sensor concept-mandrel-based fiber-optic sensor-for the detection of incipient faults in oil-filled power transformers, taking direct measurements inside a transformer. These sensors can be placed in the inner surface of the transformer tank wall, not affecting the insulation integrity of the structure, and improving fault detection and location. The applicability of these acoustic sensors in air, water, and oil is investigated and the paper presents the promising results obtained, which will enable the industrial development of practical solutions.
IEEE Transactions on Power Delivery 11/2010; · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: Fibre optic modal interferometry has been around for long as a sensing concept. Initially mainly supported on the utilization of standard Hi-Bi fibres associated to polarimetric modal interferometry, later this sensing approach evolved to modal interference based on spatial modes propagating in the core, and on spatial modes propagating in the core and in the cladding, with coupling performed by fibre devices such as long period gratings and tapers. More recently the outcome of Photonic Crystal Fibres (PCF) originated a burst of activity around the concept of modal interferometry for sensing. The reasons for that viewed in a historic perspective are presented in this work.
[show abstract][hide abstract] ABSTRACT: In this work the concept of long period based optical fibre sensors with the broadband light illumination generated just after the sensing structure is presented. This new approach allows the interrogation in transmission of the sensing head while integrated in a reflective configuration, which means the LPG sensor is seen in transmission by the optical source but in reflection by the measurement system. Also, it is shown that with this illumination layout the optical power balance is more favorable when compared with the standard configurations, allowing better sensor performances particularly when the sensing head is located far away from the photodetection and processing unit. This is demonstrated for the case of the LPG structure applied to measure strain and using ratiometric interrogation based on the readout of the optical power reflected by two fibre Bragg gratings spectrally located in each side of the LPG resonance.
[show abstract][hide abstract] ABSTRACT: Acoustic emission monitoring is often used in the diagnosis of electrical and mechanical incipient faults in high voltage apparatus. Partial discharges are a major source of insulation failure in power transformers and the differentiation from other sources of acoustic emissions is of the utmost importance. Also, it is important to give an indication of the PD source location in order to obtain a useful diagnosis. This paper reports the developments in partial discharges source location through the associated acoustic emissions using three different algorithms.
[show abstract][hide abstract] ABSTRACT: This work describes a fibre optic sensing structure that is sensitive to curvature, and features a low temperature- and strain cross-sensitivity. It is based on multimode interference, and relies on a single mode -- step index multimode -- single mode fibre configuration. It was observed that the transmitted optical power in such layout becomes highly sensitive to the wavelength of operation and to the length of the multimode fibre. The optical spectrum entertains two dominant loss bands, at wavelengths that have similar responses both to temperature and strain, and different responses to curvature. Based on this result, an interrogation approach is proposed that permits substantial sensitivity to curvature (8.7+/-0.1 nm.m) and residual sensitivities to temperature and strain (0.3+/-0.1 pm/°C and -0.06+/-0.01 pm/muε respectively).
[show abstract][hide abstract] ABSTRACT: In this work, two all-fibre interferometric configurations based on suspended core fibres (SCF) are investigated. A Fabry-Pérot cavity made of SCF spliced in-between segments of single-mode and hollow-core fiber is proposed. An alternative sensing head configuration formed by the insertion of a length of SCF as a birefringence element in a Sagnac interferometer is also demonstrated. The sensitivity to pressure and temperature was determined for both configurations.
[show abstract][hide abstract] ABSTRACT: Optical fiber sensors are increasingly used for monitoring purposes, but flexible smart structures based in this type of technology have many industrial applications. This paper explores a new approach for integrating optical fiber sensors in flexible substrates that can be mounted in host structures to monitor. This approach combines two well establish components, Fiber Bragg grating (FBG) sensors and flexible skin-foils. A three-layer foil construction based on the spread-coating process was defined, in which the fiber was embedded in the middle layer. Such disposition ensured protection to the optical fiber element without reducing the sensitivity to external stimulus. The functional prototypes were subject to thermal and mechanical tests, in which its performance was evaluated. The smart structure behaves linearly to temperature cycles by 0.01 nm/°C and is able to withstand high strain cycles without affecting the measurement characteristics. The obtained results validated this approach. In addition, the flexibility of the explored method allows custom fiber layouts, finishing patterns and colors, enabling this way a range of possible application fields.
[show abstract][hide abstract] ABSTRACT: A novel miniature fiber Bragg grating-based temperature probe is presented. The sensor design integrates a u-shape lossless taper, thus offering the advantages of a terminal temperature probe while enabling effective serial multiplexing. We report on the experimental validation of the temperature probe design, demonstrating lossless operation and effective elimination of strain cross-sensitivity.
Measurement Science and Technology 07/2010; 21(9):094002. · 1.44 Impact Factor
[show abstract][hide abstract] ABSTRACT: A novel design for a miniature fiber Bragg grating-based temperature probe is presented. The sensor integrates a u-shaped lossless taper thus offering the advantages of a terminal temperature probe free from of strain cross-sensitivity and showing installation flexibility while enabling effective serial multiplexing. The experimental validation of the probe design is reported, being demonstrated lossless operation and effective elimination of strain cross-sensitivity up to 400°C.
[show abstract][hide abstract] ABSTRACT: Hollow-core photonic crystal glass fibers have a high potential for gas sensing applications, since large light-gas interaction lengths can be effectively attained. Nevertheless, in order to enhance effective diffusion of gas into the hollow-core fiber, multi-coupling gaps are often needed, which raise coupling loss issues that must be evaluated prior to the development of practical systems. In this paper, a study on the coupling losses dependence on lateral and axial gap misalignment for single-mode fiber and two different types of hollow-core photonic crystal glass fibers is carried out. In addition, an experimental technique on splicing these glass fibers is also described and some results are presented showing that low splice losses can be obtained with high reproducibility.
[show abstract][hide abstract] ABSTRACT: We report on the use of erbium doped fiber (EDF) amplification to enhance a frequency modulated continuous wave (FMCW) technique for referencing optical intensity sensors located between two Bragg grating structures. The experiment combines the concept of FMCW with the spectrally selective mirror properties of Bragg gratings to interrogate with referencing properties intensity based sensors. The interrogation system without amplification yields a sensor resolution of around 0.078 dB. When the EDF amplifier is introduced into the experimental set up, the sensor sensitivity does not change, but the signal-to-noise ratio is improved, resulting into an enhanced resolution of 0.025 dB. We also obtain a remote sensing operation at a location of 50 km, showing the feasibility of this configuration to be used as a remote sensing application.