The working principle of the optical fiber coupler is that the fusion cone type optical fiber coupler pulls the two optical fibers close together in the molten state to pull down the cone, and as a result of the operation, the two optical fiber cores are close to each other, so that the propagation field is expanded toward the optical fiber cladding so as to be equivalent. An effective power coupling occurs in the short cone neck region. From a strict mathematical point of view analysis. The transmission field needs to find the solution of the vector wave equation in the region formed by the core, the cladding, and the filling medium, and the mathematical derivation is complicated.
To simplify the analysis, the usual approach is to ignore the effects of the core. The condition for this simplification is that the mode within the coupler in the power coupling effective region (cone neck) is essentially a cladding mode. The propagation field exits the core, where the propagation field propagates in the new waveguide formed by the cladding and the external medium (air or other suitable filling medium). At the same time, the size of the core is reduced to a negligible extent due to the taper. The approximate model that ignores the influence of the fiber core can provide simple results for any fiber optic coupler with cross-section.