Fabricating graded index plastic optical fiber by diffusing a high molecular weight dopant within a step index plastic optical fiber after the step index plastic optical fiber has been drawn from a preform using a conventional draw tower in a first embodiment. Also, the step index plastic optical fiber may be fabricated by extruding one material circumferentially around another material, e.g., by use of a concentric nozzle. The dopant is diffused after the drawing or extruding of the step index plastic optical fiber by heating the plastic optical fiber to a temperature that causes a high rate of diffusion state while measuring the transmission bandwidth of the plastic optical fiber. When the predetermined specified transmission bandwidth is measured, the plastic optical fiber is immediately returned to an ambient temperature. In addition, the plastic optical fiber may be gradually heated to an equilibrium temperature that is just below the temperature required to produce the high rate of diffusion state. Once the equilibrium temperature has been achieved, additional heating is carried out to raise the temperature to that required to produce the high rate of diffusion and the transmission bandwidth is tested.