Realizing 48.5 dB Gain in the E+S Band Using 12 m Bismuth‐Doped Fiber

Abstract Bismuth‐doped fibers (BDFs) are renowned in wideband amplification due to their numerous gain bands, such as O, E, S, and U. However, the low amplification coefficient and excessive usage length hinder its further application, how to achieve a high amplification coefficient remains an unresolved challenge. In this study, a high‐efficiency bismuth‐doped germanosilicate fiber (BGDF) for E+S band amplification is developed using modified chemical vapor deposition (MCVD) combined with the solution doping technique. The BGDF achieves a gain of 48.5 dB using a length of only 12 m. The effects of bismuth active centers (BACs) and unsaturable loss (UL) determined by fiber drawing parameters and bismuth concentrations on amplification properties of the BGDFs are discussed. New observations and analysis are conducted on how variations in oxygen vacancies and the bismuth concentration impact BGDFs' luminescence intensity and lifetime. A successful optimization approach is devised to increase BACs levels and decrease UL by modifying drawing parameters and the bismuth concentration to facilitate the formation of low‐valence bismuth and oxygen vacancies. The results suggest that BGDFs are significant for the future expansion of the gain band to the E+S band.