An optical fiber with a core of non-circular cross-section is realized by a method including the steps of: creating a void having the desired cross-section in a preform housing and filling the void with an optical material. This structure is then collapsed to solidify it and drawn to desired dimensions. The optical material may be rods, soot or ground material. An active region which may be provided in the core does not necessarily have a different refractive index than that of immediately adjacent material. The active region of the drawn fiber may only support oscillation in the fundamental transverse mode. The fiber drawn by the method disclosed may be used to convert multi-mode pump light into single mode light, e.g., for pumping a single mode amplifier. |