A gradient index (GRIN) lens (16) couples light from a single fiber (12) optic into a fiber bundle (14) or directly into the input port (104) of an illumination device, such as a medical illumination device. The single fiber (12) optic provides a high intensity beam of light having a high NA. An entry face (18) of the GRIN lens (16) is positioned adjacent to an exit aperture of the fiber (12) within the near field (28) of the fiber and thereby receives a beam having a substantially uniform cross section, rather than a Gaussian cross section as would occur with the GRIN lens (16) positioned in the far field of the fiber (12). An exit face of the GRIN lens (100) is positioned adjacent to the fiber bundle or input port (104) of the illumination device (106). As such, the GRIN lens effectively images the exit aperture of the fiber (102) of the input aperture of the fiber bundle as input port (104). The optical characteristics of the GRIN lens (100), including its length and radial variation in index of refraction, are selected to reduce the NA of light output from the fiber (102) to a value appropriate for use with the illumination device (106). An optical coupler employing the GRIN lens (100) is described as well as an entire optical system including a source of light (108), an off-axis coupler (110) for coupling light into the fiber (102), a gradient index lens (100), and an endoscope (106). A mechanical housing (105) for holding the single fiber (101) and GRIN lens (160) and for facilitating connection to a fiber bundle or input port (106) is also described.