example an optical fiber having a multi section core according to the present discussion may be fabricated by doping one core glass with erbium ions having a refractive index of 1 50 at 1 55 microns a second core glass may be doped with thulium ions having a refractive index of 1 50 u00b1 0 0005 at 1 55 microns thus the difference between the two core glasses is less than 0 001 the cladding class may be fabricated having a refractive index of 1 4935 resulting in a numerical aperture of 0 14 each core glass may be ground until they form semi cylindrical rods such that when the grounded surfaces are disposed in contact with one another the two core glasses form a cylinder the ground surface of each core glass may then be polished a cladding glass tube can be fabricated from the cladding glass such that the inner diameter of the tube equals the outer diameter of the core glass the outer diameter of the cladding being dependent upon the size of the fiber both the inner and outer diameters are polished once done the semi cylindrical rods of erbium doped glass and thulium doped class are inserted into the inner diameter of the cladding glass tube to form the fiber preform fig6.