The double clad optical fiber becomes, the core of the silica type glass, possessing inside clad and outer part clad. The core, radius under approximately 5 millimicrons, and has 1st refractive index n1, the active rare earth dopant does not include completely. The inside clad surrounds the core, at least radial thickness of approximately 25 millimicrons, at least approximately 0.25 numerical apertures, and n2