Single-mode-multimode-multimode (SMm) fiber structure provides an all-fiber solution for exciting a single radial mode in a multimode fiber (MMF). The proposed SMm design essentially exploits the interfering modes of the first MMF for exciting a single mode in the second. In this paper, the sensitivity of single-mode quality to small variations in the fiber parameters and geometrical misalignments is studied in detail. Different methods are suggested to mitigate the mode quality degradation caused by errors in the fiber parameters. An estimate of the acceptable geometrical misalignments in the structure is also given for achieving a high-quality single mode from the device. © 2016 Optical Society of America.