Measuring the optical path length of a plastic optical fibre using the sing-around method and its sensor applications

Abstract

An optical path length of a plastic multi-mode fibre was measured using a simple method for the purpose of sensor use. A single optical pulse is launched from one end of a fibre, and the next pulse is triggered electronically by the signal detected at the other end of the fibre after a delay time due to the propagation through the fibre. This cycle is repeated at the period determined by the length of the fibre. The optical path length can be estimated by measuring the period using a frequency counter instead of the conventional interferometric technique. In this work, the authors attempted to realize a low-cost optical fibre sensor using the present so-called `sing-around method' technique with a plastic optical fibre. The optical path length of plastic fibre is much more sensitive to stress and temperature than that of silica-glass fibre. A simple fibre optic sensor can be realized by the combination of a plastic optical fibre and the proposed method. Responses to pressure, lateral stress and temperature of a plastic optical fibre are examined experimentally using the sing-around method.