PAPER Special Issue on Optical Fiber Sensors

distributed sensings on temperature and strain have been extensively investigated based on Raman [1] and Brillouin [2] scattering methods, respectively. During the course of our developments on a new sensor which can work as one of sensing elements for multi-point detection using a Rayleighscattering based OTDR through an optical fiber network, a hetero-core fiber optic sensor was exploratively examined [3], [4] in which a hetero-core structure is implemented in a single mode, low-transmission-loss fiber line at the wave length of 1.3 μm by fusion splicing. In a series of experiments, two types of the sensor are typically found in depending on a small difference in the core diameters of 3 and 5 μm for the inserted hetero-core portion which are referred to as 9-3-9 and 9-5-9 types, respectively. It was interestingly found that their bending loss characteristics was distinctively different. In this paper, the explorative applications of the two types of hetero-core sensors are successfully demonstrated as a bending-to-linear displacement converter and a liquid adhesion sensor using a 9-5-9 structure with a low insertion loss and a cladding interactive 9-3-9 structure with a high sensitivity, respectively. Relatively large distortion in the order of several millimeters in length can be reproducibly monitored by means of an OTDR combined with 9-5-9 bending loss change on a curvature radius of about several tens of millimeters given in this experiment. On the other hand, experiments with the use of 9-3-9 type have shown an outstanding feature as a viable new technology in detecting liquid adhesion around the curved hetero-core portion. Because of possible building-up of cladding modes in the 93-9 type sensor, the loss mechanisms for such modes would be closely interactive to boundary conditions in the close vicinity of cladding surface. A conceivable model for the cladding mode interaction has been also discussed in terms of mode transition and possible recoupled power to a 9-μm transmission fiber which follows the hetero-core portion.