Shang Yuan Huang

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.

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.

Bragg grating fiber optic sensing for bridges and other structures

We have demonstrated that fiber optic intracore Bragg grating sensors are able to measure the strain relief experienced over an extended period of time by both steel and carbon composite tendons within the concrete deck support girders of a recently constructed two span highway bridge. This is the first bridge in the world to test the prospects of using carbon fiber composite tendons to replace steel tendons. This unique set of measurements was accomplished with an array of 15 Bragg grating fiber optic sensors that were embedded within the precast concrete girders during their construction. We have also demonstrated that these same sensors can measure the change in the internal strain within the girders associated with both static and dynamic loading of the bridge with a truck. We are now studying the ability of Bragg grating fiber optic sensors to measure strong strain gradients and thereby provide a warning of debonding of any Bragg grating sensor from its host structure...one of the most important failure modes for any fiber optic strain sensor.

Fiber optic intra-grating distributed strain sensor

Fiber-optic intracore Bragg gratings have been widely used as strain sensors. In most applications, the strain along a fiber grating is uniform and the strain is measured by the shift of the reflective wavelength of the grating. If the grating is under a gradient strain field, however, its reflective spectrum will be not only shifted but also distorted due to the chirp of grating periods. The shape of this spectrum depends on the strain distribution. Therefore the location and the shape of the reflective spectrum together can provide information on the strain distribution over a small region (sub-centimeter). We have calculated the reflective spectra of gratings by using the T-matrix formalism which allows us to simulate gratings with any distribution of the refractive-index modulation depth and the chirp of the period length. The calculation results show a strong dependence of the reflective spectrum of a grating on the distributions of its refractive-index modulation depth and period length. Experimental results have verified the T-matrix calculations. These results indicate that fiber-optic gratings can be used as quasi-distributed strain sensors to detect strain gradients that is very useful for monitoring and studying the presence and the evolutionary process of fiber sensor malfunction or material fringe/damage.

Novel Bragg grating distributed-strain sensor based on phase measurements

We recently disclosed that the shape of the intensity spectrum reflected from a Bragg grating can serve as an indicator of strain gradient across this grating. However, in principle, to map the strain field distribution, both the intensity spectrum (Iota) ((lambda) ) and the phase spectrum (Psi) ((lambda) ) are needed. In this paper, we propose a method based on phase measurements. It relies on the correlation between the wavelength-dependent penetration depth of light in a nonuniform (chirped) grating and the phase delay of the reflected wave. If we know that the nonuniform strain field to be measured is a monotonic function of position along the grating (which is the most practical case), then the measured grating phase-spectrum (Psi) ((lambda) ) can be used to evaluate the penetration depth and consequently the distribution of the grating optical pitch-lengths, i.e. the strain distribution over the grating. Theory reveals that the phase slope spectrum d(Psi) ((lambda) )/d(lambda) induced by a grating can directly yield the strain field profile. For example, a linear phase slope spectrum indicates a linear chirp in the grating due to a constant strain gradient; and the strain gradient can be easily obtained from the dispersion constant d2(Psi) ((lambda) )/d(lambda) 2. The validity of this method is also discussed in this paper.

Multiplexed Bragg grating laser sensors for civil engineering

A portable, rugged, and compact 4-channel Bragg grating fiber laser sensor demodulation system was developed for interrogating an array of 20-intracore Bragg grating sensors that we embedded within several of the concrete girders used to support the deck of a new two span road in Calgary, Alberta. Another unique feature of this bridge is that two types of Carbon fiber based composite prestressing tendons are being tested in a number of the concrete girders. We have instrumented five of the approximately 60 ft long concrete girders with fiber optic Bragg grating strain and temperature sensors in order to monitor the loads on these girders and their response during the construction of the bridge and subsequently over an extended period of time.