Peter K. C. Chan

A fibre-optic grating sensor for the study of flow-induced vibrations

Abstract A fibre-optic Bragg grating sensor for flow-induced vibration measurement is described. The sensor is based on monitoring shift in the Bragg wavelength of a fibre Bragg grating. The fibre Bragg grating, when bonded onto a structure, can measure local axial strain variation of the structure. The sensor was used to measure the flow-induced vibrations on a circular cylinder in a cross-flow. The measured strain e is consistent with the transverse structural bending displacement Y obtained from a laser vibrometer in terms of the natural frequency of the fluid–structure system and the vortex shedding frequency. The experimental data further indicated that e and Y are linearly correlated when the bending displacement is small. It is expected that the fibre Bragg grating sensor, because of its physical uniqueness, has an important role to play in the study of fluid–structure interactions.

FMCW multiplexing of fiber Bragg grating sensors

We report on the use of frequency-modulated continuous wave (FMCW) techniques for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity from a broadband source by a linear swept-frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in frequency domain and demodulated using a tunable optical filter. The potential and limitation of the technique are discussed. A three-sensor FMCW multiplexed FBG array of parallel topology and a six-sensor hybrid FMCW/WDM system were experimentally demonstrated with -30 dB crosstalk between sensors and 2 /spl mu//spl epsi/ resolution in terms of root mean square (RMS) strain value.

Multi-point strain measurement of composite-bonded concrete materials with a RF-band FMCW multiplexed FBG sensor array

We report the use of fiber Bragg grating (FBG) sensors for strain measurement in a rectangular composite-bonded concrete beam. Three FBGs are multiplexed by using a frequency modulated continuous wave (FMCW) technique in the RF-band and embedded at three different locations at the interface between the composite reinforcement and surfaces of the concrete beam specimen. Strain measurements were conducted with the concrete beam specimen subjected to a three-point bending test. The strains measured from the FBG sensors are compared with that from the external bonded strain gauges.

Multiplexing of fiber Bragg grating sensors using subcarrier intensity modulation

We report the theory and experimental demonstration of two multiplexing schemes for addressing fiber grating sensor arrays consisting of multiple branches. In the first scheme, light in each branch is intensity modulated at different subcarrier frequencies and signals from different branches are separated in frequency domain by using bandpass filters. In the second scheme, the modulation frequency in each branch can be the same and signal separation is performed in time domain by using an electronic switch after photodetection.

Strain monitoring of composite-boned concrete specimen measurements by use of FMCW multiplexed fiber Bragg grating sensor array

We report the use of fiber Bragg grating (FBG) sensors for strain measurement in a rectangular composite-bonded concrete beam. Three FBG sensors are multiplexed by using a frequency-modulated continuous wave technique and embedded at three different locations at the interface between the composite reinforcement and surfaces of the concrete beam specimen. Strain measurements were conducted with the concrete beam specimen subjected to a three-point bending test. The strains measured from the FBG sensors are compared with that from the external bonded electrical strain gauges.

Frequency domain technique for multiplexing of fiber Bragg grating sensors

We report on the use of a frequency-domain reflectometry technique for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity from a broadband source by a swept-frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in the frequency-domain and demodulated using a tunable optical filter. A three FBG sensor system is experimentally demonstrated. The potential of the technique for multiplexing a large number of FBG sensors is discussed.

FMCW addressing of a fiber Bragg grating sensor array

We report on the use of a frequency-modulated continuous wave (FMCW) technique for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity from a broadband source by a linear swept- frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in frequency-domain and demodulated using a tunable optical filter. The potential and limitation of the technique are discussed. A 3-sensor FMCW multiplexed FBG array of parallel topology and a 6-sensor hybrid FMCW/WDM system were experimentally demonstrated with -30 dB crosstalk between sensors and 2 (mu) (epsilon) resolution in terms of root-mean-square strain value.

Frequency division multiplexing of FBG sensors using an FMCW approach

We report on the use of a frequency-modulated continuous wave (FMCW) technique for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity for a broadband source by a linear swept-frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in frequency-domain and demodulated using a tunable optical filter. The potential and limitation of the technique are discussed. A 3-sensor FMCW multiplexed FBG array of parallel topology and a 6-sensor hybrid FMCW/WDM system were experimentally demonstrated with -30 dB crosstalk between sensors and a 2(mu) (epsilon) resolution in terms of root-mean- square strain value.

Utilization of fiber optic Bragg grating sensors in concrete columns confined with glass-fiber-reinforced plastic (GFRP) laminate under uniaxial compression test

In this paper we report of experimental studies on strain monitoring by using fiber Bragg grating sensors in concrete structures. The strain variation of the specimen under different loading conditions were monitored by the Fiber- optic Bragg grating (FBG) sensors. The FBG sensors have been pre-installed in the structure by embedding either inside the concrete specimen or at the interface between the concrete and the composites. The strain reading from the fiber grating sensor compares favorably with that obtained from the conventional strain gauge in uni-axial compression testing. The test result generally indicated that the concrete structures can be strengthened significantly by wrapping with glassfiber composites. The sensor embedded at the notch tip provides a very good indication of the health condition of the strengthened structure, especially in high stress concentration area. The strain sensitivity by using FBG sensor is 67 (mu) (epsilon) .

Measurement of flow-induced vibrations using a fiber Bragg grating sensor

A fiber-optic bragg grating sensor for flow-induced vibration measurement is described. The sensor is based on monitoring shift in the Bragg wavelength of a fiber Bragg grating. the fiber Bragg grating, when bonded onto a structure, can measure local axial strain variation of the structure. The strain can be related to the structural bending displacement. Experiments were conducted to measure the flow-induced vibration of a cylinder in a crossflow. The measurement results in terms of then natural frequency of the fluid-structure system and the vortex shedding frequency are consistent with the result obtained from a laser vibrometer. Fiber Bragg grating sensors have potential applications in the study of fluid-structure interactions of cylinder arrays in a crossflow.