In this research, we developed an optical fiber sensor that can observe changes in the refractive index of light. This sensor is called an RR sensor because it uses the reflection and refraction phenomena of light. The advantage of the RR sensor is that it can obtain local information by using a method different from the conventional fiber optic sensor. In particular, the RR sensor is excellent in detecting presence of water. This paper presents an example of monitoring the arrival of infiltration water inside the concrete specimen and the freezing and thawing of water in the sand.

1. Introduction

Distributed fiber sensing methods that use optical fibers for wide area monitoring of various infrastructures are attracting considerable attention these days. In these methods1),2),3), a single optical fiber with a specially processed extension of several kilometers is placed on the structure in a single-stroke state, and laser light of a specific wavelength is sent to the optical fiber. In this way, it is possible to measure strain and temperature change that occur at an arbitrary location of the fiber. On the contrary, the authors have been developing a new method4),5),6) in which plastic optic fiber (POF, for short) is used, instead of glass fiber, to monitor change in the state of light at the tip of the POF. This method is called Light State Sensor System (abbreviation: LS3) because it is a method of recording changes in light state that is affected by various factors. This sensor system enables optic monitoring of movement of soil particles, turbidity of water between soil particles, movement of cement grout material in the ground, movement of air bubbles in water, movement of soil particles in liquefaction, etc. The authors have also developed a new type of POF sensor which is suited to identify presence of water in various situations. In this paper, this POF sensor is applied for monitoring phase change of water and seepage of water through concrete cracks so that its future potential for various applications in rock engineering projects could be evaluated.

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