Coupling of a laser diode to monomode elliptic core fiber via upside down tapered microlens on the fiber tip: Estimation of coupling efficiency with consideration for possible misalignments by ABCD matrix formalism

We investigate theoretically the coupling optics involving a laser diode and an elliptic core monomode step index fiber via an upside down tapered lens on the tip of the fiber in absence and presence of possible transverse and angular misalignments. By employing Gaussian field distributions for both the source and the fiber and also the derived ABCD matrix for upside down tapered lens, we formulate analytical expressions for the concerned coupling efficiencies with and without transverse and angular mismatches. Our formalism is very simple in comparison to the other existing methods involving cumbersome numerical integrations. Elliptic core single-mode fiber has already emerged as a potential candidate in polarization conserving fiber optic sensor, coherent fiber optical communication, etc. The investigations have been performed for two different emitting wavelengths of the laser diodes for the said elliptic core monomode fibers. Therefore, such analysis which as per our knowledge is the first theoretical investigation in the context, can also be treated as simple and novel prescription for choosing suitable parameters for the design of upside down tapered lens on the tip of elliptic core single-mode fiber for optimum launch optics. In addition, this analysis will be useful in case of inherent non-circularity that may arise due to fabrication problem in circular core fiber and thereby generates interests among the experimentalists for verification and future investigations on upside down tapered lens.