High Gain Bismuth-Doped Fiber Amplifier Operating in the E+S Band With Record Gain Per Unit Length

We experimentally demonstrate high-gain E+S band bismuth (Bi)-doped fiber amplifiers (BDFAs) using Bi-doped germanosilicate fiber (BGSF) with lengths shorter than those widely reported in the literature, ranging from 25.5 to 48 m. In a double-pass amplifier configuration, a 39.9 dB gain with 5.6 dB noise figure (NF) is achieved at 1440 nm using 35 m of BGSF, for a −23 dBm input signal. By reducing the fiber length to 25.5 m, we achieve the highest recorded gain per unit length of 1.33 dB/m, to the best of our knowledge, with 33.8 dB gain and 3.7 dB NF for an input signal of −23 dBm. The highest power-conversion-efficiency (PCE) is 18.3%, obtained by 48 m of BGSF using 375 mW pump power and −10 dBm signal power. From 1410–1490 nm, the in-band optical signal-to-noise ratio (OSNR) is >21 dB for a −23 dBm input signal and >33 dB for a −10 dBm input signal. The temperature-dependent gain is characterized from −60 to 80 °C, with the longer BGSF length exhibiting better thermal stability. Moreover, three BGSFs with an increasing GeO₂ concentration, measured to be in the range of 3.7-16 mol%, are studied in detail for their absorption and luminescence characteristics. An absorption band peaking at ∼1370 nm has appeared and is likely to be associated with the bismuth-active-center (BAC) connected to the Ge, BAC-Ge. By increasing the GeO₂ concentration to 16 mol%, the 1370 nm BAC-Ge absorption band starts to dominate over the 1405 nm BAC-Si absorption band, while simultaneously the other BAC-Ge absorption band at 1640 nm appears. The luminescence exhibits a wider bandwidth with an increase in GeO₂ content, favoring the E+S band amplification.