Bragg Grating-based Sensor Research Articles

Experimental Test Results for a Fiber Bragg Grating-based Flow Sensor

A fiber optic-based mass flow sensor has been developed that uses fiber Bragg gratings to deduce flow velocity. Flow velocity, local temperature, pressure measurements (which all can be extracted using fiber Bragg gratings), and geometric information can be combined to determine mass flow. A range of concepts have each been investigated and compared using the same “design of experiment” for each sensor. The most promising concept has been further developed into a prototype. The working prototype successfully demonstrated a thermally insensitive sensor design that has the capability to track flow velocities. The sensor design is incorporated directly with a structural beam element to magnify the strain effect while simultaneously compensating for thermally induced wavelength shifts in the sensor response. Further testing has been performed using three flexible beams at different angular positions, demonstrating that flow angles can be measured using a similar approach to that used for three-hole pneumatic probes. As a final test, the sensor has been tested in a shock tube, demonstrating superior performance to steady pneumatic measurements, which rely on tubing to reach the measurement location.

A Simple Fiber Bragg Grating-Based Sensor Network Architecture with Self-Protecting and Monitoring Functions

A novel fiber Bragg grating (FBG)-based passive sensor architecture, which can be used to protect the fiber cut and monitor the multiple sensors simultaneously, is proposed and experimentally demonstrated. Here, we employ a wavelength-tunable erbium-doped fiber (EDF) laser scheme with 25 km cavity length acting as the detecting light source in central office (CO). Each FBG sensor, serving as a feedback element, is used in proposed sensor architecture. By tuning the tunable bandpass filter (TBF) placing inside cavity to match the corresponding Bragg wavelength of FBG over the amplification bandwidth, we can retrieve the related wavelength lasing for the FBG sensing and monitoring simultaneously. Moreover, the survivability and capacity of the passive FBG sensor architecture can be also enhanced.

An in-fiber Bragg grating sensor for contact force and stress measurements in articular joints

We present an in-fiber Bragg grating-based sensor (240 µm diameter) for contact force/stress measurements in articular joints. The contact force sensor and another Bragg grating-based pressure sensor (400 µm diameter) are used to conduct the first simultaneous measurements of contact force/stress and fluid pressure in intact cadaveric human hips. The contact force/stress sensor addresses limitations associated with stress-sensitive films, the current standard tools for contact measurements in joints, including cartilage modulus-dependent sensitivity of films and the necessity to remove biomechanically relevant anatomy to implant the films. Because stress-sensitive films require removal of anatomy, it has been impossible to validate the mechanical rationale underlying preventive or corrective surgeries, which repair these anatomies, by conducting simultaneous stress and pressure measurements in intact hips. Methods are presented to insert the Bragg grating-based sensors into the joint, while relevant anatomy is left largely intact. Sensor performance is predicted using numerical models and the predicted sensitivity is verified through experimental calibrations. Contact force/stress and pressure measurements in cadaveric joints exhibited repeatability. With further validation, the Bragg grating-based sensors could be used to study the currently unknown relationships between contact forces and pressures in both healthy and degenerated joints.

Fiber Bragg grating-based fiber sensor for simultaneous measurement of refractive index and temperature

A simple sensor for simultaneous measurement of refractive index and temperature in solutions based on fiber Bragg grating (FBG) is proposed. The sensor contains only a normal FBG with a pigtail fiber. When the FBG is immersed in different liquids of refractive index, the side mode suppression ratio of the FBG varies due to the Fresnel reflection of the pigtail fiber. The sensor is also capable of simultaneous measurement refractive index and temperature combining the thermo-optic effect of the FBG. Its ease of fabrication and low-cost offer the attractive applications in chemical and biological sensing.

Comparison of Raman and Fiber Bragg Grating-Based Fiber Sensor Systems for Distributed Temperature Measurements

Two types of distributed fiber sensor systems for temperature measurements have been developed: the first one is multi-point Fiber Bragg Grating based system with interrogation by CW tunable laser and nonlinearity compensation by reference interferometer. The second device is Raman scattering system based on optical time domain reflectometry (OTDR) with a pulsed laser providing spatial resolution of several meters and efficient spectral filtering of the Stokes and anti-Sokes signals by means of WDM couplers. Physical effects important for the systems operation are analyzed and their parameters are compared and optimized for applications in oil-gas industry and turbogenerator temperature monitoring.

Strain Measurement Using Embedded Fiber Bragg Grating Sensors Inside an Anchored Carbon Fiber Polymer Reinforcement Prestressing Rod for Structural Monitoring

Results are reported from a study carried out using a series of Bragg grating-based optical fiber sensors written into a very short length (60 mm) optical fiber network and integrated into carbon fiber polymer reinforcement (CFPR) rod. Such rods are used as reinforcements in concrete structures and in tests were subjected to strain through a series of cycles of pulling tests, with applied forces of up to 30 kN. The results show that effective strain measurements can be obtained from the different sensors mounted along the rod. Additionally, the tests show that close agreement with the results obtained from the calibrated force applied by the pulling machine and from a conventional resistive strain gauge mounted on the rod itself is obtained. Calculations from strain to shear stress show a relatively uniform stress distribution along the bar anchor used. The results give confidence to results from various methods of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> monitoring of strains on such CFRP rods when used in different engineering structures.

Monitoring of Corrosion in Structural Reinforcing Bars: Performance Comparison Using In Situ Fiber-Optic and Electric Wire Strain Gauge Systems

This work addresses the problems of effective in situ measurement of the initiation or the rate of steel corrosion in reinforced concrete structures through the use of optical fiber sensor systems. By undertaking a series of tests over prolonged periods, coupled with acceleration of corrosion, the performance of fiber Bragg grating-based sensor systems attached to high-tensile steel reinforcement bars (ldquorebarsrdquo), and cast into concrete blocks was determined, and the results compared with those from conventional strain gauges where appropriate. The results show the benefits in the use of optical fiber networks under these circumstances and their ability to deliver data when conventional sensors failed.

Fiber Bragg Grating-Based Force-Torque Sensor with Six Degrees of Freedom

Robotic control and force-feedback applications require multi-axial force and torque sensing. One possible implementation of future sensors is seen in fiber optic force torque sensors, since the signal demodulation may be located in some distance to the actual sensor and they also do not have to include any magnetic material. This article presents a fiber Bragg grating-based force and torque sensor with six degrees of freedom. The general setup resembles a Stewart platform. Its connecting beams are formed by the fiber used to measure the deformation of the transducer. The element creating stiffness may be of arbitrary form. We demonstrate how the sensor is realized and show results of all six force and torque measurements. We present a theoretical model of the sensor. The results in this work demonstrate the feasibility of a fiber-optic force-torque sensor.

Fiber-optic Bragg gratings as magnetic field-insensitive strain sensors for the surveillance of cryogenic devices

While conventional electrical resistance strain gages show increasing cross-sensitivities to temperature and magnetic field with decreasing temperature down to liquid helium, it has been found that fiber-optic Bragg grating strain sensors show negligible thermo-optic and magneto-optic effects in cryogenic environments; therefore, they allow reliable strain measurements. These specific application advantages of optical fiber Bragg grating sensors at low temperatures, together with the electrical isolation and low electro-magnetic interference, low thermal conductivity and their multiplexing capability, make them attractive for structural health monitoring in cryogenic devices such as superconductive magnets. In this paper we present low temperature characteristics of fiber Bragg grating-based sensors and address application-based side effects such as induced birefringence.

Industrialization of advanced optical technologies for environmental monitoring

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 Avancadas para a Monitorizacao de Sistemas Estuarinos e Costeiros (PDCTM/P/MAR/15275/1999)], an 11 km optical fibre cable with Bragg sensors each 500 m 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.

Hybrid Fiber-Optic/Electrical Measurement System for Characterization of Railway Traffic and Its Effects on a Short Span Bridge

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The characterization of traffic effects on a short span railway bridge in Northern Portugal with a new hybrid platform that allows the simultaneous assessment of signals generated by a sensing network composed of both electrical and fiber Bragg grating-based sensors was demonstrated. A commercial fiber-optic-based train characterization system using a bridge weight-in-motion algorithm was also developed and tested. By using only three fiber Bragg grating sensors the system allows on-motion determination of train speed and weight distribution. </para>

Interrogation of Fiber Bragg Grating Dynamic Strain Sensors by Self-Mixing Interferometry

An innovative investigation of optical feedback or self-mixing interference within the cavity of a single-longitudinal-mode laser is described as an integral part of a novel interrogation scheme to be employed in a fiber Bragg grating-based sensor for strain measurement. The entire sensor device simply consists of a laser diode with an integrated photodiode which is coupled to a fiber Bragg grating under strain. A small percentage of the injected lightwave resonantly reflected off the grating structure reenters the laser cavity and modifies the emission properties of the laser, resulting in the formation of characteristic sawtooth fringes which contain the embedded strain information. The feasibility of demodulating the induced small wavelength shifts of fiber Bragg gratings under dynamic mechanical loading by self-mixing interferometry is thus presented analytically and experimentally verified. A relatively good corroboration has been achieved.

Bragg grating-based fibre optic sensors in structural health monitoring

This work first considers a review of the dominant current methods for fibre Bragg grating wavelength interrogation. These methods include WDM interferometry, tunable filter (both Fabry-Perot and acousto-optic) demultiplexing, CCD/prism technique and a newer hybrid method utilizing Fabry-Perot and interferometric techniques. Two applications using these techniques are described: hull loads monitoring on an all-composite fast patrol boat and bolt pre-load loss monitoring in a composite beam in conjunction with a state-space modelling data analysis technique.

Monitoring Ingress of Moisture in Structural Concrete Using a Novel Optical-Based Sensor Approach

The detection of moisture ingress in concrete is important for structural monitoring and in this work is realised by monitoring the shift in the characteristic wavelength of a fibre Bragg grating-based sensor. The sensor relies upon a moisture-sensitive polymer layer deposited on the fibre Bragg grating (FBG) and the strain induced on it as a result of polymer swelling is monitored. Moisture ingress experiments were carried out using two such optical fibre sensors, placed at varying distances from the edge of the face of standard concrete cubes to the inner part of the concrete sample and subjected to water at a constant temperature. Information on the properties of different types of concrete and thus potentially on the migration of dissolved salts and their effect on reinforcement bars within concrete can be obtained.

A novel temperature-insensitive fiber Bragg grating sensor for displacement measurement

This paper presents the design and development of an optical fiber Bragg gratingbased displacement sensor. A fiber Bragg grating is glued at a slant orientationonto the lateral side of a specially designed cantilever beam. It is found thatthe bandwidth of the FBG based sensor changes linearly with the variation ofthe displacement at the free end of the beam due to the displacement-inducedstrain gradient. Displacement sensing is realized by measuring the reflected opticalpower of the signal from the grating with a photodetector. A linear response of37.9 mV mm−1 was obtained within a displacement range of 9.0 mm. This sensor is also cost-effective dueto the use of a simple demodulation method and is inherently temperature insensitive,eliminating the need for temperature compensation.

Demonstration of etched cladding fiber Bragg grating-based sensors with hydrogel coating

A novel hydrogel-coated single-mode fiber Bragg grating (FBG) sensor is investigated. The sensor uses a swellable polymer material, hydrogel, as the active sensing component. The sensing mechanism is based on the stress that is induced in the chemically sensitive swellable hydrogel coating. The stress shifts the Bragg wavelength of the FBG. By reducing the cladding diameter of FBG through etching with hydrofluoric acid, the tension tuning force of FBG can be lowered. Therefore, the stress induced in hydrogel can shift the wavelength of FBG more and the sensitivity of the sensor is improved. When the grating fiber is etched 37.5 μm in diameter, the sensitivity of sensor is approximately 10-fold larger than that without fiber etching.

Strain evaluation of epoxy-cured fiber-optic connectors

Optical fiber connectors are passive components used to link two fiber links or a fiber link to a photonic device. One widely used type of fiber connector, a design that uses a thermally cured epoxy adhesive, has been evaluated via Bragg grating-based fiber strain sensors. Strain sensors were used to evaluate the strain incurred by the optical fiber as a result of installation and subsequent environmental testing. Preliminary mechanical modeling and a strain analysis using Bragg grating-based strain sensors are discussed. Since the strain sensors were not exposed to uniaxial loading, mechanical modeling was used to determine the optimum placement of the sensors and the expected response. Also discussed are ongoing studies to evaluate the viscoelastic behavior of the epoxy and its effect on the strain state of the connector assembly.

Distributed dynamic strain measurement using a correlation-based Brillouin sensing system

Conventional fiber Brillouin-based strain sensors are capable of distributed sensing, making them advantageous over fiber Bragg grating-based sensors for structural monitoring applications. However, Brillouin sensors have low spatial resolution and are inappropriate for dynamic strain measurements as they have large measurement times of several minutes. We present a correlation-based continuous-wave technique for high spatial resolution and distributed dynamic strain measurements using stimulated Brillouin scattering. Using our technique, we have successfully measured dynamic strain from a 5-cm vibrating section, at a sampling rate of 8.8 Hz with a strain accuracy of about /spl plusmn/38 /spl mu//spl epsiv/.

Novel hybrid interferometer stabilization scheme used in wavelength shift measurement for Bragg grating sensors

A novel hybrid interferometer stabilization scheme used for wavelength shift measurement, such as for in-fiber Bragg grating-based sensors, is described. This scheme combines the advantages of the long term optical imbalance stability offered by the locking system used and a comparative insensitivity to vibration-induced noise at frequencies up to 200 Hz, achieved with the use of a so-called “detection-and-compensation” scheme. By including an additional reference beam and the hybrid interferometer stabilization scheme, the vibration-induced fluctuation in the signal can be reduced by about 20 dB, and the value of the phase variation can be reduced by a factor of 20.

Bragg grating-based laser sensors systems with interferometric interrogation and wavelength division multiplexing

We describe the use of fiber Bragg grating based lasers as sensors. An interferometric detection technique is presented for interrogating laser wavelength shifts due to measurand induced laser cavity strain with high resolution from both single-mode and multimode lasers. The principle of integrating multiple sensors to form an array is demonstrated using a wavelength division multiplexing approach. >