Double cladding fiber and optical fiber amplifier

Abstract

A double cladding fiber and an optical fiber amplifier, which have a function equivalent to that of a pumping light cut filter with a simple structure are provided. In the double cladding fiber, a core transmitting a signal light is covered with a first clad having a refractive index of n7b. A second clad having a refractive index of n7c, covers the surrounding surface of the first clad. At an output end of the fiber, a designated length of the second clad is removed and a substance having a refractive index of “n” (“n”>n7b) is coated on the first clad where the second clad has been removed. With this structure, a remaining pumping light leaks to this coated part. That is, and the substance coated part works as a filter for the remaining pumping light. As a result, a signal to noise ratio (SNR) of the amplified signal light is improved with a simple structure.

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