SPIE Proceedings [SPIE 3rd Iberoamerican Optics Meeting and 6th Latin American Meeting on Optics, Lasers, and Their Applications - Cartagena de Indias, Colombia (Monday 28 September 1998)] 3rd Iberoamerican Optics Meeting and 6th Latin American Meeting on Optics, Lasers, and Their Applications - Method for determining threshold power of amplification in erbium-doped fiber by means of measurements of ASE on both directions on the fiber