Changes in speckle patterns induced by load application onto an optical fiber and its detection through image processing

Speckle patterns observed in an output light spot from an optical fiber are likely to be changed due to external disturbances applied to the fiber, and such changes can be utilized for sensing applications. In this paper, certain load was applied onto an optical fiber through which laser beams emitted from a laser diode were propagating, and changes in speckle patterns observed in the output light spot from the optical fiber were investigated as image data via a CCD camera. For the purpose of realizing effective load application onto an optical fiber, a load application section was employed in which several ridges were provided onto opposite flat plates. A jacket-covered communication-grade multi-mode glass optical fiber was placed in the load application section so that corrugated bending of the fiber was intentionally induced via load application due to the ridges. The obtained results from appropriate image processing showed that the number of speckles in the observed patterns decreased upon load application (up to 15kg) onto the optical fiber with satisfactory repeatability. The load was then reduced from the total of 15kg, and the number of speckles was found to recover. With different arrangements of the optical fiber in the load application section in which the number of load application points was altered, slight differences in the observed characteristics were recognized. Thus, there are possibilities of utilizing changes in speckle patterns observed in an output light spot from an optical fiber for sensing of load application onto the optical fiber by employing appropriate load application arrangements.