Objective Compared with the traditional pump light source of 915 nm or 975 nm diode lasers, the 1018 nm Yb ³⁺-doped fiber laser (YDFL) with a tandem-pumping structure is a high-efficiency pump light source for high-power lasers and offers the advantages of excellent beam quality, high brightness, and low quantum loss. However, the output power of the 1018 nm YDFL is severely limited by the competition between the 1018 nm signal and the amplified spontaneous emission (ASE) mode. Consequently, the efficiency of the 1018 nm fiber laser is significantly lower than the theoretical efficiency of the system, which is mainly attributed to the selection of the gain fiber. The absorption and fluorescence intensity of the traditional Yb ³⁺-doped silica fiber at 1018 nm are weak, which reduces the overall conversion efficiency of the laser system and limits the further development of 1018 nm tandem-pumping technology. To improve the overall conversion efficiency of the laser system, two key materials—Yb ³⁺-doped silica fiber generating 1018 nm laser light using 976 nm laser diode (LD) pumping and another Yb ³⁺-doped silica fiber for absorbing the 1018 nm pump light—must be optimized urgently. For the former, the fluorescence at 1018 nm must be increased, whereas for the latter, the absorption at 1018 nm must be increased. Therefore, improving the absorption and fluorescence of Yb ³⁺ ions at 1018 nm by introducing codopants into the Yb ³⁺-doped silica glass to change the coordination environment of Yb ³⁺ ions is the key to improving the overall conversion efficiency of tandem-pumped silica-based YDFL systems.

Methods Herein, Yb-Al (YA) double-doped, Yb-Al-P (YAP) triple-doped, and Yb-P (YP) double-doped silica glasses were prepared using the sol-gel method combined with nanopowder sintering technology. The influence of Al and P codoping on the spectral characteristics of 1018 nm silica glass was systematically studied. By analyzing the difference in the Stark energy level splitting of Yb ³⁺ ions for different doping systems of silica glass, the influence of different codoping on the 1018 nm spectral performance was explained. Moreover, Raman spectroscopy and ultralow temperature (4 K) electron paramagnetic resonance (EPR) spectroscopy were combined to explore the influencing mechanism from the perspectives of glass structures and rare earth ion coordination environments.

Results and Discussions Currently, the spectral properties of the Yb ³⁺-doped multicomponent glass have been extensively studied; however, the spectral properties of the Yb ³⁺-doped silica glass at 1018 nm have rarely been reported. Here, the change rule of the spectral characteristics of silica glass at 1018 nm in three codoped systems of YA, YAP, and YP (Fig. 2, Fig. 3, and Fig. 4, respectively) were systematically studied. Furthermore, the reasons for the influence of different codoping on the 1018 nm fluorescence were explained based on the difference in the Stark energy level splitting of Yb ³⁺ ions (Table 2). Additionally, by comparing the normalized spectral properties of the best samples in different doping systems, the absorption and fluorescence intensities of the YP series at 1018 nm were found to be better than those of the YA and YAP series (Fig. 6). Moreover, Raman spectroscopy and ultralow temperature electron paramagnetic resonance spectroscopy were combined to prove the influence of Yb ³⁺ microenvironments on the spectral performance (Fig. 7 and Fig. 8).

Conclusions In the Yb ³⁺-doped silica glass system, increasing the doping content of Al ³⁺ induced the red shift of the absorption subpeak near 915 nm and enhanced the absorption at 1018 nm. Increasing the doping concentration of P ⁵⁺/Al ³⁺decreased the overall optical performance, which was significantly reduced when the molar ratio of P ⁵⁺ to Al ³⁺>1. Increasing the P ⁵⁺-ion doping content induced the blue shift of the fluorescence subpeak near 1030 nm. When the P ⁵⁺-ion doping concentration (molar fraction) was 8%, the fluorescence subpeak blue shifted to 1018 nm. Moreover, the absorption and fluorescence intensities of Yb ³⁺ ions at 1018 nm decreased as the P ⁵⁺-ion doping content further increased. By comparing the normalized spectra of the best samples from different series, the normalized fluorescence intensity of YP series at 1018 nm was significantly better than those of the other two series. In the YA series, rare earth ions were in an “Al-rich” environment and Yb—O—Si connections were gradually replaced by Yb—O—Al connections. In the YAP series, most Yb—O—Si connections were replaced by Yb—O—Al and Yb—O—P connections when the molar ratio of P ⁵⁺ to Al ³⁺<1, while most Yb—O—Si and Yb—O—Al connections were replaced by Yb—O—P connections when the molar ratio of P ⁵⁺ to Al ³⁺≥1. This indicates that the encapsulation of P ⁵⁺ by rare earth ions was better than that of Al ³⁺, i.e., the solubility of P ⁵⁺ in rare earth ions was greater than that of Al ³⁺. In the YP series, rare earth ions were in a “P-rich” environment and most Yb—O—Si connections were replaced by Yb—O—P connections. This is because the coordination environment of Yb ³⁺ ions in the codoping silica glass systems of YA, YAP, and YP was completely different, resulting in different spectral characteristics of Yb ³⁺ in these three systems at 1018 nm.