Mode coupling and field distribution in sub-mm permanently bent single mode optical fibers

A simple mechanism useful to tailor the field profile in single mode optical fibers is proposed. It involves the local and permanent bend of the fiber with bending radius of few hundred micrometers. The permanent bend is obtained by local thermal treatment using the electric arc discharge method. The resultant sub-mm bend leads to a significant power coupling from the fundamental core mode to guided cladding modes. The order and number of the excited cladding modes depend on the geometrical features of the bent region, like angle and curvature rate. Also, since the light propagated by guided cladding modes interacts with the surrounding medium by evanescent wave, the field distribution can be additionally tailored by changing the refractive index of the medium surrounding the straight fiber region after the permanent bend. Here, the effects of permanent bend on the guided mode in single mode fiber is first theoretically introduced and then widely experimentally investigated. Finally, preliminary results of light re-coupling from excited cladding modes to the core mode using long period fiber gratings are reported opening new avenues for light manipulation within optical fibers.