A material sensing apparatus comprises an excitation source configured to induce waves in a workpiece, and an optical waveguide interferometer configured to sense the induced waves in the workpiece. The optical waveguide interferometer comprises a probe segment having a probe segment end, and an adjustable coupler configured to permit setting a gap between the probe segment end and the workpiece. A controller is coupled to the adjustable coupler and configured to set the gap between the probe segment end and the workpiece.