Quasi-single mode operation in few-mode fibers is a promising technique for extending the Kerr nonlinearity threshold of standard single mode fibers. Under this operation the design goals of few-mode fibers used for mode division multiplexing can be relaxed. Based on the MinΔneff, we calculated the maximum core radius of two-mode and four-mode fibers for quasi-single-mode operation, and is found to be 8 μm and 9.7 μm respectively. We also propose the design of a two-core fiber coupler, based on thermally diffused core technique, for effective excitation of the fundamental mode alone in the above two-mode fiber for the entire C-band. The robustness of the proposed design is studied by varying the design parameters and observing their sensitivity to coupling efficiency and extinction ratios and are found to be well above −0.5 dB and 25 dB respectively. © 2019 Elsevier Inc.