Abstract

Ultra-short length gain fibers are expected to be applied to range sensors and optical frequency combs because of their ability to fabricate cavities with high repetition rates around GHz. However, such an ultra-short length fiber with Yb ions requires very high concentration of Yb ions which, leading the optical loss due to the photo-darkening (PD) effect in Yb-doped silica fibers [1]. We previously reported that PD phenomenon can be suppressed by co-doping group 2 elements in Yb doped silica fibers [2]. Among them, Yb-Mg co-doped fiber (YbMgSGF, Yb:54,600 ppm) was the most effective, maintaining 94.3% transmittance even after 2 hours of irradiation. In addition, laser oscillation has been already demonstrated in the 37 mm YbMgSGF [3]. When pulse oscillation occurs in an ultra-short length fiber than this fiber, it is possible to generate a multi-GHz repetition rate. Therefore, confirming amplification in shorter fibers is the first step toward improving the repetition rate and miniaturizing the cavity.

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