Timothy A. Berkoff

Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry–Perot wavelength filter

The use of a fiber Fabry–Perot filter for detecting the wavelength shift of a fiber Bragg grating sensor or network of sensor elements along a commonfiber path is described. Results obtained by using a system with four sensor elements are presented.

Fiber-optic Bragg grating strain sensor with drift-compensated high-resolution interferometric wavelength-shift detection

The operation of a fiber Bragg grating strain sensor system that uses interferometric determination of strain-induced wavelength shifts and incorporates a reference channel to compensate for random thermal-induced drift in the output is described. This system is shown to be capable of resolving sub-microstrain changes in the quasi-static strain applied to a grating and has a resolution of ~6 x 10(-3) microstrain/ radicalHz at a strain perturbation frequency of 1 Hz.

In-line fiber étalon for strain measurement.

A description and demonstration of a fiber interferometer that uses a short segment of silica hollow-core fiber spliced between two sections of single-mode fiber to form a mechanically robust in-line cavity are presented. The hollow-core fiber is specifically manufactured to have an outer diameter that is equal to the outer diameter of the single-mode lead fibers, thereby combining the best qualities of existing intrinsic and extrinsic Fabry-Perot sensors. A dynamic strain resolution of ~22 nepsilon/ radicalHz at frequencies of >5 Hz with a sensor gauge length of 137 microm is demonstrated.

Determination of cantilever plate shapes using wavelength division multiplexed fiber Bragg grating sensors and a least-squares strain-fitting algorithm

An algorithm has been developed to determine the full deformation field of a cantilever honeycomb plate under arbitrary loading conditions. The algorithm utilizes strain information from a set of sixteen fiber Bragg grating sensors mounted on the plate so that all sensors measure strains along the clamped-free direction. The sensors were interrogated using a wavelength division multiplexing scheme. A two-dimensional polynomial function which represents the strain field was created using a least-squares surface-fitting algorithm. This function was integrated twice with the known boundary conditions applied to yield the deformation field for the plate. Finite-element comparisons were performed to test the accuracy of the strain-displacement algorithm. Single-point loading tests were also experimentally performed to further verify the accuracy of the algorithm.

Hybrid time- and wavelength-division multiplexed fiber Bragg grating sensor array

A nine element fiber Bragg grating sensor array is demonstrated utilizing a combination of wavelength and time division addressing techniques. This system utilized a pulsed broad-band source and a scanning Fabry-Perot element to recover sensor strain information from each of the FBG elements. By combining two multiplexing techniques, the potential number of FBG sensor elements that can be addressed is significantly increased.

Transient load monitoring on a composite hull ship using distributed fiber optic Bragg grating sensors

We describe strain data recorded using fiber optic Bragg grating sensors mounted on the hull of a GRP composite ship. Twelve gratings were attached to the structure, in three arrays of four elements. The electro-optic system used was able to monitor a single set of four elements at a time. The preliminary results indicate the usefulness of distributed fiber Bragg grating sensor systems for monitoring transient loading events on such structures.

Design and performance of a fiber Bragg grating distributed strain sensor system

We describe the design and performance of a prototype fiber Bragg grating demodulation system based on the use of a scanning fiber Fabry-Perot filter. The computer driven system is capable of demodulating several arrays of wavelength division multiplexed gratings at various scanning rates for real-time strain display and data logging. The instrument represents a new measurement tool which should be useful in a variety of structural health monitoring applications. Results obtained by the system in several applications are presented and system performance limitations are discussed.

Single-channel phase-tracker for the open-loop fiber optic gyroscope

The present, simplified analog signal-processing technique for an open-loop FOG is based on Kersey and Moellers (1990) closed-loop phase-shift-nulling concept. This approach emulates the operation of a true closed-loop gyroscope on the basis of low-cost phase tracking, and furnishes a linear output from an open-loop gyro configuration over a +/- 2-pi radian range in Sagnac phase shift. Good linearity and comparatively low noise and drift characteristics are obtained.

Eight element time-division multiplexed fiber grating sensor array with integrated-optic wavelength discriminator

An eight element time-division multiplexed fiber Bragg grating sensor array is demonstrated using an unbalanced integrated-optic discriminator to facilitate demodulation of Bragg wavelength shifts in the return signals. The system exhibits a wide sensing range with a detection capability of <1 (mu) - strain/(root)Hz rms at low frequencies.

Dynamic strain monitoring of an in-use interstate bridge using fiber Bragg grating sensors

We describe the monitoring of the dynamic strain response of an in-service I-10 interstate bridge due to traffic loading. FBG sensors were attached to the center support girder of one span of the structure. Using a fiber Bragg grating interrogation system based on a wavelength division multiplexer, the sensors were monitored for various vehicle loading conditions.