Monte Carlo simulation of curvature gauges by ray tracing

We have proposed recently a fibre optic device sensitive to minute curvatures with radii in the kilometre range. This 'curvature gauge' is intended primarily for the monitoring of deflection of structures under mechanical loading in applications in which strain gauges have traditionally been used. The sensitive zone of the curvature gauge is precision machined into the plastic optical fibre on grinding or milling machines. The cut-out produced removes a part of the fibre core and introduces a loss of light propagating along. The extent of such loss is related to the bend-radius of the fibre. Such a relationship, embodying the essence of the sensor's modulation principle, is examined in this paper by ray-tracing based on Monte Carlo simulation. A calibration curve is shown that relates the relative light-loss to the fibre curvature for different parameters of the sensitive zone (its depth, length and pitch of cuts). Through its asymptotic decay character, this nonlinear sensor characteristic reveals for the first time the existence of the device's sensitivity barrier that has limited its past applications to the measurement of curvatures with radii below about 2 to 3 km. We have also included an overview of the curvature gauge and have compared its performance with strain measuring instruments as the established benchmark. © 2004 IOP Publishing Ltd.