Displacement sensing by spatial mode conversion between single mode to multi-mode fiber

Summary form only given. There have been many displacement sensing methods based on optical fibers such as using mode scrambling of multi-mode fibers or intensity change of single mode fibers due to coupling mismatch. In this work, we used as displacement sensing the mode excitation change caused by spatial variation of input beam. A single mode fiber is used as a point-like input beam source for a multi-mode fiber. Since the core size of the multi-mode fiber is about 10 times larger than that of the single mode fiber, if two fibers are closely located and the multi-mode fiber is sufficiently long, the input beam from the single mode fiber excites many modes of the multi-mode fiber without power loss.