OPTICAL FIBER FOR AMPLIFICATION

PROBLEM TO BE SOLVED: To reduce the loss caused by bending without impairing preservation of the plane of polarization and miniaturize the device, by setting a specific optical parameter in case of using an optical fiber preserving the plane of polarization, as a light transmission path. SOLUTION: The optical fiber that amplifies signal light directly by an induced emission effect in the preserved state of the plane of polarization has a core X formed to have an elliptic cross section, and rare earth elements such as Er and Nd are doped in the core X. A clad Y with the circular periphery is formed outside of the core X of elliptic cross section. In this case, it is so set that the ratio (La/Lb) of the major axis length La of the core X to the minor axis length Lb is 4-5, that the specific refractive index difference Δbetween the core X and clad Y is 0.7-1.6% and that cutoff wavelength λ0 is in a range of 1.18-1.48μm. As a result, not only signal light is amplified in the well preserved state of the plane of polarization, but also bending loss is reduced, so that the optical fiber can be wound with a small bending diameter so as to miniaturize the device.