Broadband Single-Mode Cr-Doped Crystalline Core Fiber With Record 11-dB Net Gain By Precise Laser-Heated Pedestal Growth and Tetrahedral Chromium Optimization

We 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.