Measurement of the bending loss of single-mode fibers by use of fiber loop cavity ring-down technique

The oscillations of the bending loss for single-mode fibers with the bending radius and the temperature are investigated experimentally by use of fiber loop cavity ring-down technique. When the fiber is bent, the light ejects from the fundamental mode to form whispering gallery (WG) modes produced by reflection at the cladding-coating interface, or at the coating-air interface. On the synchronous coupling condition, the light from the WG modes will couple back and interfere with the fundamental mode, which leads to bending loss oscillations. The experimental results demonstrate that the bending loss follows an oscillatory and exponential growth trend when the bending radius decreases from 27.63 mm to 9.33 mm. In addition, the secondary oscillations are observed because of the secondary coupling between the fundamental mode and the WG modes. Change of the bending loss with temperature is oscillated, and the oscillatory period decreases with the decrease of the bending radius and the increase of the temperature. The locations and the amplitudes of the loss peaks and the oscillatory period are consistent with the theoretical results.