## 2 Sensor Principle
### Response of Single Fiber Optic Sensor Probe  
We investigate, first, the tension effect of the metal wire because it may influence the bending loss before the response test of the sensor probes. Fortunately, the force of the WCS expansion is not enough to bend the metal wire because the wire is strongly tightened and very stiff. Thus, we neglect the wire tension effect on the bending loss due to the WCS expansion in this study. In addition, the  
Figure 5: Dynamic range of MOL-FOS.  
Figure 6: Fiber optic sensor probes using three types of vessels.  
single mode fiber (SMF) is tightly jacked with 1 mm-diameter plastic tube. If we assume that bare fibers are employed, the bending loss will be much larger than that of the 1 mm-loose tube SMF because 1 mm plastic jacketed SMF is stiffer than bare fibers. In order to study the response of a sensor probe, at first, we test a single sensor probe with the sensor system. In the result of this test, the reflected signals from the reference connector and the sensing connector are \(-25.8\) and \(-25.2\) dB, respectively, at the initial state of no-water absorption shown in Figure 7. We can know that these two reflections are located 2 m apart from each other.  
In Figure 8, the signal of the sensing connector is decreased about 3 dB because of the bending loss caused by the expansion of WCS absorbing water. The detection sensitivity of the sensor is governed by the relation between bending loss and water absorbance. Therefore, it is noteworthy that the maximum bending loss, sensor margin, of each sensor probe should be determined by considering the total power margin in order to operate the entire sensor probes through one optical fiber line. If it assumes that the bending loss is quite high even for absorbing a small amount of water, then the sensor probe is too sensitive. However, although a severe expansion of WCS material occurs, the bending loss is not fully developed because the water absorbing WCS becomes jellylike after absorbing water within several minutes in this experiment. Therefore, the relation of the bending loss with absorbing water should be carefully studied to design the whole multiplexing sensor system.  
Generally, the bending of SMF will be directly affected by the metal wire diameter when the optical fiber is bent on the surface of the wire by a sufficient force. However, the bending radius of the optical fiber in our sensor probe is more influenced by the expansion force of WCS. As shown in Figure 8, the optical fiber is bent with a relatively large curvature as compared with the metal wire diameter.  
Therefore, the bending of the fiber will not affected by the metal wires having the diameter of less than 0.6 mm of the present metal wire. Therefore, it is no need to be concerned about the effect of the diameter of the metal wire on the bending loss of SMF in our sensor probes. Also, our sensor probes are designed as simple water leakage detection tools. This means that they are to be exchanged after a water detection event. Thus, we will focus on studying the effect of several vessel types and water absorption in this study.  
Figure 7: Initial state and received signal of a sensor probe.