Two-axis bend measurement using multicore optical fibre

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 mu rad. (C) 2000 Published by Elsevier Science B.V.