Enhancement of Tetrahedral Chromium (Cr4+) Concentration for High-Gain in Single-Mode Crystalline Core Fibers

Tetrahedral chromium (Cr
⁴⁺
)-ions is the main contributor to gain in the broadband single-mode Cr-doped crystalline core fibers (SCCFs). We have demonstrated a gross gain of 8-dB on a length of 27-cm for a 300-nm broadband SCCF using a thermal annealing technique. Gross gain and fiber length are the highest yet reported for the SCCF. The record gain is mainly due to both the longer fiber by aligned molten-zone growth and Cr
⁴⁺
-ions enhancement by thermal annealing. The enhancement mechanism of the Cr
⁴⁺
-ions in the SCCF is experimentally verified by the means of superconducting quantum interference device (SQUID) and electron probe X-ray micro-analysis (EPMA). As a practical application of the proposed 8-dB gain in the SCCFs in fiber transmissions, a 25-Gb/s error-floor free link is successfully demonstrated.