Gordon M. H. Flockhart

Two-axis bend measurement with Bragg gratings in multicore optical fiber

We describe what is to our knowledge the first use of fiber Bragg gratings written into three separate cores of a multicore fiber for two-axis curvature measurement. The gratings act as independent, but isothermal, fiber strain gauges for which local curvature determines the difference in strain between cores, permitting temperature-independent bend measurement.

Distributed Feedback Fiber Laser Strain Sensors

The distributed feedback (DFB) fiber laser strain sensor has demonstrated strain resolution comparable to that obtained from high-performance fiber-optic interferometry. This manuscript describes the characteristics and performance of this fiber laser strain sensor and discusses the technological developments necessary to obtain comparable performance from a multiplexed array of laser sensors. The design of the Bragg grating and doped fiber are discussed, where possible providing simplified equations to quantify the relevant design parameters. Techniques based on fiber-optic interferometry to decode the wavelength shifts of the laser are presented and potential noise sources are described. Measurements conducted on a test laser demonstrate the capability of the DFB fiber laser to resolve effective length changes to less than 0.76 fm/Hz1/2 at 2 kHz. The accuracy of the strain measurement, calculated by subtracting the output of two lasers subjected to the same strain, is found to be less than 1%. Issues relating to multiplexing lasers, such as pump power depletion and optical feedback, are described along with methods to maximize the number of lasers serially multiplexed on a single fiber. Finally, the strain transduction mechanism and methods to mount the laser sensor are described. It is shown that for certain applications, the DFB fiber laser sensor provides significant performance benefits when compared with remotely interrogated fiber-optic interferometric sensing techniques.

Quadratic behavior of fiber Bragg grating temperature coefficients

We describe the characterization of the temperature and strain responses of fiber Bragg grating sensors by use of an interferometric interrogation technique to provide an absolute measurement of the grating wavelength. The fiber Bragg grating temperature response was found to be nonlinear over the temperature range -70 degrees C to 80 degrees C. The nonlinearity was observed to be a quadratic function of temperature, arising from the linear dependence on temperature of the thermo-optic coefficient of silica glass over this range, and is in good agreement with a theoretical model.

Implementation of vectorial bend sensors using long-period gratings UV-inscribed in special shape fibres

We report the implementation of vector bending sensors using long-period gratings (LPGs) UV-inscribed in flat-clad, four-core and D-shaped fibres. Our experiments reveal a strong fibre-orientation dependence of the spectral response when such LPGs are subjected to dynamic bending, which provided an opportunity to realize curvature measurement with direction recognition.

Efficient fiber Bragg grating and fiber fabry-Pe/spl acute/rot sensor multiplexing scheme using a broadband pulsed mode-locked laser

A pulsed broadband mode-locked laser (MLL) combined with interferometric interrogation is shown to yield an efficient means of multiplexing a large number of fiber Bragg grating (FBG) or fiber Fabry-Pe/spl acute/rot (FFP) strain sensors with high performance. System configurations utilizing time division multiplexing (TDM) permit high resolution, accuracy, and bandwidth strain measurements along with high sensor densities. Strain resolutions of 23-60 n/spl epsiv//Hz/sup 1/2/ at frequencies up to 800 Hz (expandable to 139 kHz) and a differential strain-measurement accuracy of /spl plusmn/1 /spl mu//spl epsiv/ are demonstrated. Interrogation of a low-finesse FFP sensor is also demonstrated, from which a strain resolution of 2 n/spl epsiv//Hz/sup 1/2/ and strain-measurement accuracy of /spl plusmn/31 n/spl epsiv/ are achieved. The system has the capability of interrogating well in excess of 50 sensors per fiber depending on crosstalk requirements. A discussion on sensor spacing, bandwidth, dynamic range, and measurement accuracy is also given.

High-resolution distributed-feedback fiber laser dc magnetometer based on the Lorentzian force

A low-frequency magnetic field sensor, based on a current-carrying beam driven by the Lorentzian force, is described. The amplitude of the oscillation is measured by a distributed-feedback fiber laser strain sensor attached to the beam. The transduction mechanism of the sensor is derived analytically using conventional beam theory, which is shown to accurately predict the responsivity of a prototype sensor. Excellent linearity and negligible hysteresis are demonstrated. Noise sources in the fiber laser strain sensor are described and thermo-mechanical noise in the transducer is estimated. The prototype sensor achieves a magnetic field resolution of 5 nT Hz for 25 mA of current, which is shown to be close to the predicted thermo-mechanical noise limit of the sensor. The current is supplied optically through a separate optical fiber yielding an electrically passive sensor head.

Pitch and roll sensing using fibre Bragg gratings in multicore fibre

We report the first use of a multicore fibre incorporating fibre Bragg grating strain sensors in each core as a fibre optic pitch and roll sensor. A length of four-core fibre supported at one end forms a cantilever. The differential strains between opposite grating pairs depend on the fibre’s orientation in pitch (in the vertical plane) and roll (azimuth) with respect to gravity. Resolutions of ±2◦ in roll and ±15◦ in pitch were measured.

Departure from linearity of fibre Bragg grating temperature coefficients

We have measured the Bragg wavelength shift of three gratings from -30/spl deg/C to +80/spl deg/C. The observed deviation from linearity is a quadratic function of temperature, arising from the non-linear thermo-optic coefficient of silica glass.

Differential phase tracking applied to Bragg gratings in multicore fiber for high-accuracy curvature measurement

We report interferometric interrogation of fiber Bragg gratings in separate cores of a multicore fiber for high resolution quasi-static and dynamic bend measurements. Two axis curvature measurements are made by measuring the differential strain between three FBG sensors formed in a singlemode four-core fiber using a common interrogating interferometer. Therefore a measurement of the differential phase from each FBG yields the differential strain and compensates for the common-mode random drift of the interrogating interferometer. A DC curvature accuracy of 3.4×10-3m-1, and an AC curvature resolution of 1.2×10-4m-1 / Hz1/2 are reported.

Polarization properties of interferometrically interrogated fiber Bragg grating and tandem-interferometer strain sensors

Lead sensitivity in low-coherence interferometric fiber-optic sensors is a well-known problem. It can lead to a severe degradation in the sensor resolution and accuracy through its effect on the fringe visibility and interferometric phase. These sensitivities have been attributed to birefringence in the various components. In the current work, an analysis of the polarization properties of fiber Bragg grating and tandem-interferometer strain sensors, using Stokes calculus and the Poincare/spl acute/ sphere, is presented. The responses of these sensors as a function of the birefringence properties of the various components under different illuminating conditions are derived. The predicted responses demonstrate very good agreement with experimentally measured responses. These models provide a clear insight into the evolution of the polarization states through the sensor networks. Methods to overcome the lead sensitivity are discussed and demonstrated, which yield a differential strain measurement accuracy of 18 n/spl epsiv//spl middot/rms for a fiber Bragg grating sensor.