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Bragg intragrating structural sensing

When a fiber-optic intracore Bragg grating is subject to an appreciable strain gradient, its reflective spectrum will not only be shifted but also be distorted because of the chirp of the grating. We employed the J-matrix formalism to calculate the influence of different strain gradients on the reflective spectra of Bragg gratings and have undertaken experiments to test these calculations. The results of these experiments have confirmed that intracore Bragg gratings can be used to evaluate strain gradients and can be thought of as quasi-distributed strain sensors. This adds a new dimension to structural sensing, permitting measurements in any situation where strain gradients exist. It also provides a warning of any sensor/host debonding.

Distributed strain measurement based on a fiber Bragg grating and its reflection spectrum analysis

A method of extracting the strain profile along a fiber Bragg grating from the intensity reflection spectrum is described. The procedure is based on a filter synthesis theory that relates the aperiodicity of a grating with its reflection spectrum. To illustrate the approach, we measured the strain profile near a hole in a plate and obtained a strain resolution of 80 micro. The spatial resolution depends on the strain gradient; i.e., the higher the gradient, the better the resolution. A resolution of 0.8 mm was achieved for a 5-mm grating with a gradient of 250 micro/mm.

Phase-based Bragg intragrating distributed strain sensor

A strain-distribution sensing technique based on the measurement of the phase spectrum of the reflected light from a fiber-optic Bragg grating is described. When a grating is subject to a strain gradient, the grating will experience a chirp and therefore the resonant wavelength will vary along the grating, causing wavelength-dependent penetration depth. Because the group delay for each wavelength component is related to its penetration depth and the resonant wavelength is determined by strain, a measured phase spectrum can then indicate the local strain as a function of location within the grating. This phase-based Bragg grating sensing technique offers a powerful new means for studying some important effects over a few millimeters or centimeters in smart structures.

A structurally integrated Bragg grating laser sensing system for a carbon fiber prestressed concrete highway bridge

Carbon fiber based composite material is of considerable interest for the replacement of steel in large concrete structures, such as bridges, where corrosion is a serious problem. A new two-span concrete highway bridge built in the city of Calgary in 1993 is the first in the world to use carbon fiber composite prestressing tendons in several of its precast concrete deck support girders. We have instrumented a number of these girders with an array of fiber optic intracore Bragg grating sensors in order to monitor the changes in the internal strain that take place over an extended period of time. A four-channel fiber laser demodulation system was developed for interrogating the set of Bragg grating sensors embedded within the bridge girders. This demodulation system was demonstrated to be rugged, compact and transportable to the bridge construction site where it allowed changes in the internal strain on all three types of prestressing tendon (steel and two types of carbon fiber composite) to be tracked over several months. The same set of structurally integrated Bragg grating sensors has also been used to measure the change in the internal strain within the deck girders arising from both static and dynamic loading of the bridge with a 21 ton truck. This first permanent testbed for structurally integrated Bragg grating sensors demonstrates the feasibility of building into new bridges fiber optic long-term structural monitoring sensing technology that will allow the use of these advanced composite materials to be monitored in a manner not previously practical. The strain information available from this type of monitoring system will assist engineers in their assessment of new materials and innovative design features, and has a potential role in maintenance and repair activities.

Bragg grating structural sensing system for bridge monitoring

Corrosion of steel within bridges and other large concrete structures has become a serious problem and consequently there is growing interest in replacing the steel within such structures with carbon fiber based composite materials. The first highway bridge in the world to use carbon fiber composite prestressing tendons was constructed and opened to the public in the fall of 1993. This two span bridge was also unique in another respect, it is the first highway bridge in the world to have been built with a set of fiber optic Bragg grating sensors structurally integrated into several of its precast concrete deck support girders. A four-channel fiber laser sensor demodulation system that was rugged, compact and transportable was developed for this project. This demodulation system monitored the changes in the internal strain on all three types (steel and two types of carbon fiber composite) of prestressing tendons over several months. The same structurally integrated fiber optic sensing system has also been used to measure the change in the internal strain within the deck girders arising from both static and dynamic loading of the bridge with a large truck.

Fiber optic Bragg grating sensor network installed in a concrete road bridge

The installation of a fiber optic Bragg grating strain sensor network in a new road bridge is described. These sensors are attached to prestressing tendons embedded in prefabricated concrete girders. Three types of prestressing tendons are being monitored: conventional steel strand and two types of carbon fibers reinforced plastic tendons. Sensor durability issues are reviewed and the installation is described. Initial measurements indicate that the sensors are operational and provide some early comparison of tendon performance.

Tunable fiber grating dispersion using a piezoelectric stack

In this paper, we propose a versatile and compact mechanism for tunable grating dispersion. A segmented piezoelectric stack with individually controlled segments was used to control the local strain at 21 positions along the grating. By step-wise monotonic increase or decrease of the strain along the grating a group-delay response close to that of a perfectly linearly chirped fiber Bragg grating (FBG) was obtained.

Tunable laser demodulation of various fiber Bragg grating sensing modalities

Fiber optic Bragg grating technology offers unrivaled sensing versatility combined with the promise of extremely compact, rugged and low-cost integrated optoelectronic microchip demodulation systems. We show that a tunable laser can be used to demodulate each of the different sensing modalities: short and long gage fiber optic strain sensors; serial and parallel multiplexed fiber grating sensor arrays and truly distributed sensing based on intragrating strain profile mapping. We also show that distributed strain sensing is not limited by the length of a single Bragg grating. We demonstrate that Fourier transform analysis combined with tunable laser interrogation of a string of gratings permits distributed sensing over distances of about a meter without the need for high-cost time division multiplexing or producing each grating with a different reference wavelength as required by wavelength division multiplexing.

Characteristics of fiber grating sensors and their relation to manufacturing techniques

This work examines the effects of different manufacturing techniques of fiber optic intracore Bragg gratings on their sensing and system parameters. Specifically, we have looked at tolerance with respect to center wavelength, strain, and temperature sensitivity. Our test results indicate a slight variation in the strain and temperature sensitivity of different gratings. In an attempt to quantify the effect of transverse loading on fiber gratings, three differently manufactured sensors were diametrically loaded and their spectrums studied. Our experimental results indicate a splitting of the grating spectrum with small loads for an optical fiber grating manufactured in either boron doped fiber or hydrogen loaded fibers. On the other hand, very small transverse sensitivity was observed for a fiber optic grating formed in a bend insensitive fiber suggesting some degree of control on the transverse sensitivity of these sensors.

Distributed Bragg grating sensing with a direct group-delay measurement technique

A new method of distributed in-fiber Bragg grating sensing is proposed. A method is outlined by which the strain field imposed upon a Bragg grating sensor is obtained by measurement of the sensors reflectivity and delay characteristics. The proposal is demonstrated by interrogation of two loaded samples. Strain distributions from these experiments are compared against a theoretical estimate. The data treatment is also discussed. Strain resolution is found to be +/-24 microepsilon, with a spatial resolution defined by a minimum spatial wavelength component of the coupling distribution of 1.65 mm.