Compared with traditional uniform fiber, the long tapered fiber has the advantage of suppressing nonlinear effects and maintaining excellent beam quality. Both the cladding size and core size of the long tapered fiber reported so far vary simultaneously in a constant core-to-cladding diameter ratio (CCDR) along the length. In this work, we propose a technique for preparing cladding uniform core tapered fiber (CUCTF) with a varying CCDR, and have successfully fabricated CUCTF. The core diameters of narrow end and wide end are 31.2 mu m and 52.5 mu m, respectively. The cladding diameter is always 400 mu m along the length. The laser performance of CUCTFs with different structures (wide to narrow, narrow to wide) are investigated based on a co-pumped fiber amplifier. Between them, the CUCTF with the wide to narrow structure has a better M2 factor of 1.455 due to mode filtering.
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