Silica‐Based Er:YAG Nanocrystal Composite Fibers Enabling S+C+L Broadband Tunable Lasers

ABSTRACT The rapid advancement of optical communication and laser technologies demands materials that simultaneously exhibit broadband tunability and sufficient gain efficiency. However, achieving a harmonious balance between these two critical properties in conventional glass optical fibers remains a significant challenge. Here, we report the development and characterization of erbium‐doped yttrium‐aluminum‐garnet (Er:YAG) nanocrystal composite fibers. The fibers are fabricated using a post‐feeding melt‐in‐tube method by precisely controlling the interfacial reaction time (<10 min), which effectively resulted in Er:YAG nanocrystals embedded in the core region. Spectroscopic studies reveal that the nanocrystal size effect modifies the energy level splitting of erbium ions, leading to an extended emission bandwidth. When deployed in a ring‐cavity laser, a 30‐cm‐length fiber enables a low fusion splicing loss of 0.1 dB with standard single‐mode silica fibers and continuous tuning over 126 nm (1494–1620 nm) with an optical signal‐to‐noise ratio of ∼70 dB, making it suitable for S+C+L band tunable laser applications. These fibers demonstrate both broad gain bandwidth and high signal‐to‐noise ratio, positioning them as ideal candidates for wavelength‐division multiplexing systems and other optical communication devices.