OPTICAL FIBER FOR OPTICAL AMPLIFICATION

PROBLEM TO BE SOLVED: To keep the polarizing surface of a signal beam superior for amplifying the signal beam with good efficiency by providing a core of which the cross-section is formed in to an elliptical shape and concentrating the doping of the rare earth element only to the region within a circle corresponding to the mode field of the core. SOLUTION: A core (a) is formed to have the cross-section in an elliptical shape. A cladding (b) has the external circumference of the circular shape. A rare earth element (Er) is not doped uniformly to the core (a), but doping of Er is concentrated only to the region 1 in a circle (x), corresponding to the mode field of the core (a). Therefore, there is a region 2 where Er is not doped at all at both right and left end sections in the long axis side of the core (a). The mode of the doped region 1 of Er in the core (a) is not limited, but various adequate modes may be selected from those where doping is concentrated to the region with in the circle (x), corresponding to the mode field of the core (a).