Boosting the branching ratio at 900 nm in Nd3+ doped germanophosphate glasses by crystal field strength and structural engineering for efficient blue fiber lasers

Abstract

Fiber lasers operating at around 900 nm based on Nd³⁺ doped fibers have attracted intense attention recently for generating high performance pure blue laser sources by frequency doubling, which could be applied in the fields of laser color displays, laser driven solid lighting and high density storage. Although several approaches have been proposed to realize laser operation at this band, for example by nitrogen cooling or utilizing fibers with a special waveguide design, the intrinsic low fluorescence branching ratio of the three-level ⁴F_3/2 → ⁴I_9/2 transition (900 nm) is still a serious problem that limits the development of laser devices towards compactness and high efficiency. Here, we report a crystal field strength and structural engineering strategy; accordingly, not only can the branching ratio of Nd³⁺ be tuned towards enhancing the ⁴F_3/2 → ⁴I_9/2 transition, but also the near-infrared (NIR) emission intensity could be improved by six times. Furthermore, the lifetimes of the upper ⁴F_3/2 level were also prolonged from 124 to 246 μs, simultaneously. Systemically, optical and structural analysis implies that the fine-tuning of Stark splitting, the spectroscopic quality factor (χ) and the dispersed state of Nd³⁺ are the main reasons for this intriguing scenario. These results will help us design optical glass fibers by an inherent local structure based strategy to realize efficient and compact 0.9 μm fiber lasers for high performance pure blue fiber lasers.