Liu C.-NLiu C.-MSHENG-LUNG HUANGCheng W.-H.2021-09-022021-09-02202107338724https://www.scopus.com/inward/record.uri?eid=2-s2.0-85102717495&doi=10.1109%2fJLT.2021.3065649&partnerID=40&md5=d97a151c2b8740a46008387fed7cd500https://scholars.lib.ntu.edu.tw/handle/123456789/581127We demonstrate a record net gain of 11-dB in a 34-cm fiber length for 300-nm broadband single-mode Cr-doped crystalline core fibers (CCFs) fabricated by precise laser-heated pedestal growth (LHPG) and tetrahedral chromium (Cr4+) optimization. The core composition of the CCF is a Cr4+-doped: yttrium aluminum garnet (YAG). The record gain achieved is mainly due to (1) precise LHPG of the CCFs to fabricate a longer fiber length of 34-cm employing a novel predictive control to constantly maintain conical molten-zone shape in the growth process and (2) the Cr4+ concentration optimization up to 150% utilizing thicker dielectric coating and thermal annealing and then the Cr4+ optimization by polarization pumping. The net gains of 6.4 and 11-dB are measured with and without Cr4+ optimization, respectively. The 72% of net gain improvement is directly related to Cr4+ optimization in the CCFs, as evidenced by SQUID and EPMA measurements, whereas the optimized thicker coating of the Cr4+ concentration provided the dominant net gain improvement of 33% in the CCFs. The development of the CCF is essential for practical use in the next-generation broadband fiber amplifier applications. ? 1983-2012 IEEE.Fiber amplifier; fiber annealing; fiber coating; fiber design and fabrication; fiber laser[SDGs]SDG7Broadband amplifiers; Chromium; Coatings; Fiber amplifiers; Fibers; Yttrium aluminum garnet; Broadband fiber amplifiers; Core composition; Crystalline core; Dielectric coatings; Growth process; Laser heated pedestal growth; Predictive control; Thermal-annealing; Chromium compoundsjournal article10.1109/JLT.2021.30656492-s2.0-85102717495