Peter L. Fuhr

Statistical-mode sensor for fiber optic vibration sensing uses.

A method of sensing vibration using the detection of changes in the spatial distribution of energy in the output of a multimode optical fiber has been demonstrated. Two implementations of the sensor have been built and tested. The first implementation involved simple optical processing of the output fiber speckle pattern using spatial filtering. The second implementation involved projecting the pattern on a CCD array and digitally processing observed changes in the intensity distribution. A mathematical model has been developed which has shown good agreement with observed sensor behavior. The sensor technique has been used to detect induced structural vibration in laboratory test specimens. Simple field testing has also demonstrated the ability of the technique to detect personnel and vehicles passing over a buried and electrically undetectable sensing cable. The sensing technique is compatible with off-the-shelf components and fiber cable and even allows for simultaneous telecommunication and sensing using the same optical fiber cable. Near term application of this technology could provide significant benefits for vibration sensing, intrusion detection, and acoustic sensing.

Corrosion detection in reinforced concrete roadways and bridges via embedded fiber optic sensors

The problems associated with the application of chloride-based deicing agents to roadways and specifically bridges include chemical pollution and accelerated corrosion of strength members (especially the rebar) within the structure. In many instances, local ordnances are attempting to force state agencies to reduce, if not eliminate, the use of these chlorides (typically at the cost of increased driving hazards). With respect to the corrosion aspects of chloride application, cracks that occur in the roadway/bridge pavement allow water to seep into the pavement carrying the chloride to the rebar with the resultant increase in corrosion. In tandem with these efforts has been the continuing use of embedded fiber optic sensors for identification of faults or cracks within a highway structure - i.e., structural health monitoring. In this paper, we present multiple-parameter sensing fiber optic sensors which may be embedded into roadway and bridge structures to provide an internal measurement and assessment of its health. Such issues are paramount in determining if remedial or preventative maintenance should be performed on such structures. Laboratory results, comparisons with conventional sensing methods as well as a review of real-world issues in highway sensing are presented.

Performance and health monitoring of the Stafford Medical Building using embedded sensors

The University of Vermont is in the process of constructing a new major facility to house various biotechnology research laboratories and offices. This five-storey, 65000 square foot concrete structure, named the Stafford Building, is being equipped with fiber optic and conventional sensors embedded into the concrete superstructure. These sensors will allow monitoring of stresses incurred during the construction phase and monitoring of concrete curing as well as vibration sensing and internal crack sensing. Longitudinal studies of the buildings in-service performance and overall health will also be carried out. A description of the sensor choices, physical placements, use and review of relevant construction practices are presented in this paper.

Embedded fiber optic sensors for bridge deck chloride penetration measurement

The use of chloride-based deicing agents to help clear U.S. highways of roadway hazards leads to associated chemical related problems. Fouling of local rivers and streams due to runoff of the waterborne chlorides is significant and has contributed to local ordinances that are attempting to force state agencies to reduce, if not eliminate, the use of these chlorides (typically at the cost of increased driving hazards). With respect to the corrosion aspects of chloride application, cracks that occur in the roadway/bridge pavement allow water to seep into the pavement carrying the chloride to the rebar with the resultant increase in corrosion. The costs of this corrosion are considerable and have led to the widespread use of chloride/water impermeable membranes on roadways and especially within bridges. Fiber optic sensors have repeatedly been shown to provide measurement capabilities of parameters within such reinforced concrete structures. Development of fiber optic chloride sensors capable of being embedded within a roadway or bridge deck is reported.

Mode coupling and phase modulation in vibrating waveguides

When coherent light is coupled into a multimode waveguide (such as a fiber), a speckle pattern is formed at the fiber output. When the fiber is vibrated, the speckle pattern is modulated due to mode coupling and phase modulation of the waveguide modes. It is shown that when a highly coherent light source is used, the speckle pattern modulation is due primarily to phase modulation of the modes. When less coherent sources are used (such as low-cost laser diodes), mode coupling is the dominant effect. >

Fiber-optic rotary displacement sensor with wavelength encoding

A fiber-optic sensor is described in which an optical retardation plate is used to encode rotary displacement information as a notched minimum in a broadband optical signal. As the waveplate is rotated, the optical beam experiences a variable linear retardation. The signal wavelength at which the retardation is exactly one-half wave exhibits a minimum intensity transmission. The wavelength of the intensity minimum is then a function of the rotation of the retardation plate. The theoretical prediction of the sensors performance is developed and compared to experimental results.

Fiber Optic Sensing of a Bridge in Waterbury, Vermont

The use of fiber optic sensors for the internal state measurements of large civil structures has been increasing in recent years. Sensors are embedded into the reinforced concrete structure in order to obtain information about the internal structural health and overall structural performance. The installation process is of utmost importance for if incorrectly done, the sensors will not withstand the rigorous conditions present during the construction and concrete pouring process. A detailed description of the sensor installation and excavation process undertaken when a suite of fiber optic sensors were embedded into a 67 m long steel truss bridge spanning the Winooski River in Waterbury, Vermont (USA) arise is presented.

Neural network damage detection in a bridge element

Smart structures technology is being increasingly applied to civil structure applications. In particular, development of health monitoring for bridge structures is of considerable importance. In order to explore the possibility of developing such a system, an investigation was carried out on a scale model steel bridge element using an attached sensor system consisting of two point sensors (piezoelectric accelerometers) and one integrating sensor (fiber optic modal sensor). The model element was selectively configured to produce the equivalent of a number of damage conditions. For each condition, it was physically perturbed. The sensor outputs were then used as inputs to a neural net which then provided an estimate of structural damage. A reasonable correlation between net output and actual damage indicated that this type of health monitoring system offers potential for practical application on full scale bridge structures.

Twin-fiber interferometric method for measuring spatial coherence

A new method for measuring spatial coherence in which optical fibers replace the traditional slits in Youngs experiment is developed. The beam is sampled by the fiber ends, and the phase difference that causes the fringes is introduced by translating one fiber along its long axis by a few wavelengths. The technique has several advantages over the slit method: The fibers can be moved to arbitrarily chosen points, as opposed to removing and replacing sets of slits, giving greater flexibility in measurement as well as easing alignment problems. This method also does not rely on diffraction, and therefore enables measurement of much dimmer beams and dimmer points on beams. One can take as many data points as desired, thus tracing the shape of the coherence function accurately, whereas with slits one is restricted to the number of slit sets available. Finally, one does not have to assume beam symmetry. Although the magnitude of the spatial coherence function is not accurately reproduced, this can be corrected for, and the ability to reproduce the shape of the beam remains a useful measurement tool. We report, in particular, measurements of spatial coherence of a single-stripe diode laser, as well as a HeNe laser.

Impact detection and location system for smart skins applications

It is shown that both impact localization and impact magnitude determination can be achieved in rigid structures with embedded fiber-optic sensors. Impact-generated acoustic signals were detected by means of statistical mode fiber-optic vibration sensors that were embedded in a polymer matrix composite material panel. The 1-sigma error in inferred vs actual impact location, determined via relative-timing measurements, was found to be of about 4 cm.