Patent ID: 8340487

Claim:
A rare earth-doped core optical fiber comprising: a core which comprises a silica glass containing at least aluminum and ytterbium; and a clad provided around the core and comprising a silica glass having a lower refraction index than that of the core, wherein aluminum and ytterbium are doped into the core such that a loss increase by photodarkening, T PD , satisfies the following inequality (A): T PD ≧10 {−0.655*(D Al )−4.304*exp{−0.00343*(A Yb )}+1.274} (A) wherein T PD represents an allowable loss increase by photodarkening at a wavelength of 810 nm (unit: dB), D Al represents the concentration of aluminum contained in the core (unit: % by mass), and A Yb represents the peak absorption coefficient of the absorption band which appears around a wavelength of 976 nm in the absorption band by ytterbium contained in the core (unit: dB/m), wherein T PD is 0.5 dB or less, and wherein the core includes no other dopant that increases the refractive index except for rare earth dopants.