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In this work, we focus on designing heterogeneous step-index multi-core fibers (Hetero-SI-MCFs) with the standard cladding diameter of 125-μm, which enables the utilization of the existing optical cables, connector interfaces, and conventional optical components.To estimate the inter-core crosstalk (XT) in Hetero-SI-MCFs, an analytical expression with a particularly simple form is derived for the mode-coupling coefficients between non-identical step-index (SI) cores, which enables a quick estimation of the XT values. The analytical results are in good agreement with the results obtained by numerical simulations by finite-element method (FEM). Using the derived analytical expression, the feasibility of designing conventional Hetero-SI-MCFs with one-ring core layout in the standard 125-μm cladding diameter is discussed. We then propose a novel method of allocating the non-identical cores in heterogeneous MCFs (Hetero-MCFs), where the crosstalk and critical bending radius (Rpk) are improved as compared to that of Hetero-MCFs designed by the conventional method. We numerically reveal that the number of SI cores that incorporated in the standard cladding diameter can be further increased with the proposed method of core allocation. It is shown that the simple SI profile enables us to allocate 8 and 6 non-identical cores in the standard cladding diameter of 125-μm for the O- and C-bands, respectively, and the achieved sufficiently low XT values support hundreds of kilometers transmission of the QPSK format signal.It is also shown that the simple SI profile enables us to allocate 10 and 8 non-identical cores in the standard cladding diameter of 125-μm for the O- and C-bands, respectively, and the achieved XT values support several kilometers transmission of the QPSK format signal, which has the potential use in datacenter networks. In addition, we propose a relatively low refractive index trench layer which occupies the outermost part of the conventional fiber cladding enables us to suppress the loss of outer cores in MCFs. A new kind of novel Hetero-SI-MCF with outer trench (OT) layer is proposed for achieving high core multiplicity. The designed MCFs with the comparable optical transmission characteristics to the conventional single-mode single-core fibers have the potential application in the space-division multiplexing systems., (主査) 教授 齊藤 晋聖, 教授 大鐘 武雄, 教授 西村 寿彦, 情報科学院(情報科学専攻)