| 摘要 |
An optical fiber for optical amplifying, capable of amplifying optical signa ls at least in the vicinity of wavelengths of 1.57 to 1.62 .mu.m with a high ga in in optical communication or the like. A clad (5) lower in refractive index than a core (1) is formed on the outer peripheral side of the erbium-added core (1), with the relative refractive index difference .DELTA. of the core (1) with respect to the clad (5) set to at least 0.3% and up to 1%. The core has a composition Er-Al2O3-GeO2-SiO2, and the clad has a composition SiO2, with erbium added to the entire region of the core at a concentration of 100 0 wtppm and with the cutoff wavelength of the optical fiber set to 1400 nm. A constant cutoff wavelength of the optical fiber and an optimized relative refractive index difference .DELTA. can optimize a core diameter, avoid a reduction in gain due to an optical fiber bending loss, and increase a gain per optical fiber unit length by increasing an erbium absorption amount per optical fiber unit length.
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