Roy McBride

Remotely addressed optical fibre curvature sensor using multicore photonic crystal fibre

We demonstrate an all-fibre curvature sensor that uses two-core photonic crystal fibre (PCF) as the sensing element. The PCF acts as a two-beam interferometer in which phase difference is a function of curvature in the plane containing the cores. A broadband source illuminates both cores, and the spectrum at a single point in the far-field interferogram is recorded. Applying a three-wavelength phase recovery algorithm to the data provides an unambiguous measurement of the interferometer phase, and hence curvature.

Polarization mode dispersion minimization in fiber-wound piezoelectric cylinders.

We describe a technique for minimizing polarization mode dispersion (PMD) in broadband interferometry for use with phase modulators of the type formed by coiling nominally circular-core single-mode optical fiber onto piezoelectric cylinders. The technique involves using two piezoelectric elements, arranged to be optically orthogonal. We show that in the general case the two cylinders give rise to four distinct interferograms, but by careful alignment of the elements we reduce these to two coincident interferograms. Hence the large PMD of a single-element modulator is reduced to a negligible level.

Two-dimensional bend sensing with a single, multi-core optical fibre

Measurement of two-dimensional bending in a structural element using intrinsic optical fibre strain gauges would normally require three sensors to be attached to, or embedded within, the structure. The same measurement can now be made using a single multi-core optical fibre, reducing deployment cost and increasing practicality. Fabrication of a novel three-core photonic crystal fibre is described. The ability of the fibre sensor to measure bend in two dimensions is demonstrated in the laboratory using interferometric interrogation at a single wavelength. Deployment of the sensor to measure the deformation of a bridge undergoing loading trials is described.

Dispersion of birefringence and differential group delay in polarization-maintaining fiber

We present an interferometric technique for measurement of the dispersion of birefringence in polarization-maintaining fibers. The approach yields measurements over a broad spectral range from analysis of single interferograms obtained in a tandem inteferometer. The technique is demonstrated to measure first-, second-, and third-order dispersion of the differential propagation constant, corresponding to differential group delay (DGD) and its dispersion to second order; measurements are immune to asymmetry in the interferomgram that is being processed. The technique is further applied to measurement of the temperature dependence of DGD and its first-order dispersion.

Two-axis bend measurement using multicore optical fibre

Abstract We describe the first use of a four-core optical fibre to measure bending about two orthogonal axes simultaneously. Individual cores in the fibre act as independent strain gauges where local curvature determines the difference in strain between cores. The multicore sensing element, interrogated in reflection, generates a two-dimensional far-field interferogram. The component of fibre curvature in the plane of the two cores shifts the corresponding fringes in the interferogram. Bend angle is then calculated using phase values derived from Fourier analysis of the far-field interferogram. This technique achieved a bend angle resolution better than 120 μrad.

Measurement of the wavelength dependence of beam divergence for photonic crystal fiber

We report measurements of the wavelength dependence of beam divergence for single-mode photonic crystal fiber. These measurements confirm predictions of strongly wavelength-dependent beam divergence, consistent with the effective-index model for the photonic crystal cladding material.

Modelling and calibration of bending strains for iterative laser forming

We describe a simplified model for laser forming that identifies key features in bending behaviour and use approximations resulting from the model to characterize bending by the thermal gradient mechanism (TGM). The model predicts local curvature change in terms of area energy, interaction time and material thickness and from this derives the bending angle in terms of laser power, spot diameter and feed rate. It provides reasonable agreement between theory and measurement without the need for finite element (FE) analysis and identifies the areas where FE is needed to improve accuracy. In particular, it gives accurate predictions for threshold and saturation area energies, and so can be used in conjunction with the calibration tests to identify the optimum operating conditions for TGM laser forming.

Process control of laser conduction welding by thermal imaging measurement with a color camera

Conduction welding offers an alternative to keyhole welding. Compared with keyhole welding, it is an intrinsically stable process because vaporization phenomena are minimal. However, as with keyhole welding, an on-line process-monitoring system is advantageous for quality assurance to maintain the required penetration depth, which in conduction welding is more sensitive to changes in heat sinking. The maximum penetration is obtained when the surface temperature is just below the boiling point, and so we normally wish to maintain the temperature at this level. We describe a two-color optical system that we have developed for real-time temperature profile measurement of the conduction weld pool. The key feature of the system is the use of a complementary metal-oxide semiconductor standard color camera leading to a simplified low-cost optical setup. We present and discuss the real-time temperature measurement and control performance of the system when a defocused beam from a high power Nd:YAG laser is used on 5 mm thick stainless steel workpieces.

Interferometric fiber-optic sensing based on the modulation of group delay and first order dispersion: application to strain-temperature measurand

We describe an extension of low coherence interferometry which uses the complete interferogram rather than just the interferogram envelope. We measure the phase across the entire source spectrum using dispersive Fourier transform spectroscopy, and obtain group delay and dispersion from the phase curve. We apply the technique to a simultaneous quasi-static strain-temperature measurand in which strain and temperature are recovered from the modulation of group delay and first-order dispersion. We show these parameters have a negligible cross-term, yield a particularly robust transformation to strain and temperature, are immune to 2 /spl pi/ phase ambiguity and insensitive to fiber geometry. With an ultra broadband thermal source we achieved simultaneous resolution of strain acid temperature to accuracies of 5 /spl mu/strain.m and 0.35 K over strain and temperature ranges of 1200 /spl mu/strain.m and 22 K. We present detail of a diffraction grating-coupled multiple SELED source and its operation in optical fiber DFTS. With this source we achieved resolutions of >

COMBINED TEMPERATURE AND STRAIN MEASUREMENT WITH A DISPERSIVE OPTICAL FIBER FOURIER-TRANSFORM SPECTROMETER

We report simultaneous measurement of strain and temperature in single-mode optical fiber by broadband interferometry. A Mach-Zehnder interferometer, illuminated by a xenon-arc lamp, has a sensing element in one arm. Scanning an air path generates interferograms that are calibrated by a monochromatic reference interferogram. Values of group delay and dispersion, obtained from the phase of the fast Fourier transform of the sampled interferogram, give strain and temperature through a well-conditioned matrix transformation without phase ambiguity. We obtained measurement ranges and resolutions of 1500 +/- 12 microstrain and 25.0 +/- 0.4 K using a 0.8-m sensing element.