Abstract: This invention relates to optical fiber systems where light is launched into and propagated in the fundamental mode of graded-index multimode fiber. Generally, it is not possible to launch light exclusively into the fundamental mode of graded-index multimode fiber by butt-coupling the multimode fiber to an arbitrary fiber carrying light from a source; several other modes may be launched. By inserting a length of a special singlemode fiber between an arbitrary fiber carrying light from a source and the graded-index multimode fiber, light can be launched exclusively into the fundamental mode of the graded-index multimode fiber. The special singlemode fiber has a modal pattern that matches the fundamental Gaussian modal pattern of the graded-index multimode fiber, which permits launching of the light into only the fundamental mode of the multi mode fiber. Several applications for propagation in the fundamental mode of graded-index multimode fiber arises.