A tunable laser source (TLS) is split at a 2x2 coupler into two arms of a measurement interferometer, one of which contains the strain sensing fiber. The interference pattern from this interferometer is measured using a polarization-diverse detection scheme, using a polarization controller (PC) to balance the signals on the S and P detectors. A second (trigger) interferometer with a known delay is used to compensate for nonlinearities in the laser tuning.
Precision Shape Sensing using OFDR
Shape and position sensing is performed through measurement of axial twist and curvature along the length of a monolithic, helical multi-core optical fiber (Figure 5). OFDR techniques are used to monitor distributed strain along the lengths of the center core and the three outer cores [3]. Members of this “triad” of outer cores are spaced at 120o azimuths, at a fixed radius from the center core. This geometry allows the shape sensing fiber to convert the multi-core distributed strain measurement to three-dimensional distributed position (shape).
Under curvature, each of the outer cores experiences alternating states of tension and compression on its helical path through the bend. The center core, which is precisely located in the center of the sensing fiber, experiences negligible first-order strain. The three outer cores exhibit sinusoidal