Modified Gaussian approach for the design of optical fiber couplers of arbitrary core shapes

Many fiber devices, e.g. fiber couplers, rely on the cladding evanescent field for their operation. We present a new method for determining the approximate fundamental mode fields, with particular reference to cladding field applications. In our method, by introducing a field which is accurate both near to and far from the core axis, coupling parameters in fiber couplers can be obtained in a simple form. We apply this to studies of couplers consisting of two identical cores of arbitrary shape. We have compared the results from the new approximation with those from the usual Gaussian in circular core cases where exact solutions can be found and have shown that the new approximation can give more accurate propagation constants and, especially, fundamental mode fields. The evanescent behavior of these fields is directly related to the coupling features of a fiber coupler. In particular, we apply this new method to twin elliptic core fibers to quantify their polarization transmitting and polarization coupling characteristics. Up to now, as far as we know, no method has seemed appropriate to handle this system. Design features of birefringent and polarization coupling of these fibers for device applications are also discussed.