Effect of Misalignments on Coupling Efficiency of Single-Mode Optical Fiber Butt Joints

The lowest-order mode in a clad optical fiber, the hybrid HEn mode, is the only propagating mode for core sizes less than a few wavelengths in diameter (v ^ 2.4).1 Hence, small-core, single-mode glass fibers are attractive for transmitting optical signals because of their potentially low dispersive effects. A possible disadvantage lies in the difficulty of joining small-core fibers end-to-end.2,3 It has been suggested4 that butt joining may be made less critical by reducing the size of the fiber in the vicinity of the joint. The computation and experimental measurements disclosed here evaluate the advantage to be gained by such a procedure. It is found that as the fiber gets smaller, the accuracy with which the ends of the fibers to be joined must meet on-centers is indeed reduced. At the same time, however, as might be expected, the required angular alignment of the fibers becomes more critical. Both calculations and experiments have been made under the assumption that the cladding is of sufficient extent that the role played by possible conversion of the zero-order mode to cladding modes need not be considered.4 The calculation was made by matching the fields of the zero-order modes at the joint. 1439