Application of modal domain distributed fiber optic sensors to smart materials and structures

Modal domain distributed fiber optic sensors by using a birefringent single mode fiber can be applied for measurement of mode coupling distribution along a fiber attached to a structure. The mode couplings are caused by strain, stress, temperature, etc. In the present paper, some fundamental experiments were conducted to understand relations between the mode coupling and the external force. The relations between mode couplings in x and y polarized modes and the stress distribution at the cross section of a fiber subjected to a transverse load were derived theoretically by using a coupled mode equation. The stress distribution was calculated by two dimensional finite element method (FEM). Experiments were conducted to measure the mode couplings of a coated or uncoated fiber subjected to a transverse load at one point. Experimental results showed relation between a mode coupling and an external load was almost linear on a logarithmic graph. It was found that an uncoated fiber measured load more quantitatively than a coated fiber. A fiber optic load sensor module for a serial multiplexed sensor system was developed for future application of the present study. The above load identification technique can be combined with the OCDR (optical coherence domain reflectometry) technique developed by K. Hotate et al. to quantify the applied loads along a fiber on structures.