the coupler 10 operates on evanescent field coupling principles in which guided modes of the strands 12 interact through their evanescent fields to cause light to be transferred between the strands 12 as previously indicated this transfer of light occurs at the interaction region 32 the amount of light transferred to dependent upon the proximity and orientation of the cores as well as the effective length of the interaction region 32 the length of the region 32 is in turn dependent upon the radius of curvature of the fibers 12 and to a limited extent the core spacing although it has been found that the effective length of the interaction region 32 is substantially independent of core spacing in one typical coupler 10 employing an edge to edge core spacing of abot 1 4 microns the radius of curvature is on the order of 25 centimeters and the effective interaction region is approximately one millimeter long at a signal wavelength of 633 nm with these dimensions the light makes only one transfer between the strands 12 as it travels through the interaction region 32 however if the length of the interaction region 32 is increased or core spacing decreased a phenomenon referred to herein as 34 overcoupling 34 will occur in which the light will transfer back to the strand from which it originated as the interaction length is further increased and or the core spacing further decreased the light transfers back to the other strand thus the light may make multiple transfers back and forth between the two strands 12 and it travels through the region 32 the number of such transfers being dependent on the length of the interaction region 32 and the core spacing the foregoing may be more.