This invention relates to optical fiber-to-fiber couplers and more particularly to evanescent field coupling between single-mode fibers as well as multimode fibers.
The development of lasers that produce coherent light has brought about research in the use of laser light in communication systems using optical fibers. One of the major problems involving the use of optical fibers in a communication system concerns means for coupling optical signals from one fiber to another. Through the use of optical couplers, an optical signal can be shared by, and distributed among a multiplicity of optical fibers.
In multimode fibers, the core diameter is comparable to the core-cladding total diameter, and intermodal scattering prevails when a perturbation is introduced to the guiding system. Thus, multimode fiber couplers mix and distribute optical energy among a plurality of fibers by first converting guided modes to cladding modes or unguided modes, and then converting the cladding modes or unguided modes back to the guided modes. An example of this kind of mixer coupler can be found in U.S. Pat. No. 4,087,156. This kind of optical coupler or mixer is not proper for the applications where fine control of coupling efficiency is required. Furthermore, it would not work efficiently with single-mode fibers since the core diameter of single-mode fibers is much less than the total diameter and, more importantly, scattering loss would be much larger compared to the multimode fiber cases for the same amount of perturbation. In other words, in single-mode fibers, it is very difficult to couple an optical beam into the fiber core with high-efficiency after it has leaked out of the core.