| 摘要 |
PROBLEM TO BE SOLVED: To improve durability, reliability of a detector and also simplify a structure by inserting an optical fiber into a coiled spring. SOLUTION: An optical fiber 1 for giving a bending loss is inserted into a coiled spring 2 or a hard rubber tube, a first roller 3 is mounted at an end of a movable member moving in response to opening or closing of a cover or a door, and a plurality of second rollers 9 are disposed at both sides of the first roller 3 so as to sandwich the spring 2 or the tube containing the fiber 1. The spring 2 or the tube is constituted to be guided to the roller 3 so that the member 4 is biased by a compression spring 6, and moved to give bending of a curvature of the roller 3 to the spring 2 or the tube. For example, in a state such that a handhole cover is closed, the detector does not give bending to the fiber. In an opened state of the cover, the bending is given to the fiber to generate its loss.
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