Stimulated emission and laser behaviors of Nd3+/Yb3+ Co-doped phosphate glass fiber

National Natural Science Foundation of China [61405215, 61505232]; Youth Innovation Promotion Association of the Chinese Academy of Sciences; National High Technology Research and Development Program of China [2016YFB0402201]The energy transfer phenomenon between Nd3+ and Yb3+ generates the research interest in Nd3+ / Yb3+ co-doping, because it provides a straight-forward way to combine the features of Nd3+ and Yb3+ to develop some potential applications, such as solar cells, high energy pulse and tunable lasers. Substantial research work has been conducted to study the spectroscopic properties of Nd3+ / Yb3+ in different glasses, crystal and ceramic host materials. However, it is still not very clear about the laser properties of the Nd3+ / Yb3+ co-doping system, especially the high rare-earth solubility phosphate glass. This work reports the stimulated emission and laser properties of an Nd3+ / Yb3+ co-doped phosphate glass fiber under singly 970 nm and 808 nm LD pumping. The molar doping ratio of Nd3+ : Yb3+ is 4 : 1. Using the free-space coupled method, the laser properties of the co-doped fiber under 970 nm pump are tested first in a laser cavity comprised of a butt-coupled dichroic mirror with high reflectivity (>= 99.5%) and a cleaved fiber ended with similar to 4.6% Fresnel reflectivity. It is found that with the increase of 970 nm pump power (P-970) two discrete laser peaks and one peak located at 1053 nm with a larger threshold can be observed for fiber length equal to and less than 0.7 m. The 1053 nm laser is produced by Yb3+ -> Nd3+ energy transfer, and its lasing threshold decreases with increasing fiber length in this length region. Then, the amplified spontaneous emission (ASE) spectra for fiber lengths of 0.35 m, 0.9 m and 5.0 m under 970 nm pumping are tested by cutting 6. at the output port. The test results indicate that the Yb3+ -> Nd3+ energy transfer has a modulation effect on fiber spectrum, and the modulation becomes more obvious for a longer fiber length. A two-fold promotion mechanism is suggested to explain the modulation effect: 1) the reabsorption effect of Yb3+ leading to relatively lifetime prolongation increases the Yb3+ -> Nd3+ energy transfer efficiency; 2) the red-shifted oscillator laser wavelength leads to a larger emission cross section difference between Nd3+ and Yb3+. Besides, the measurement results in 0.35-m-long fiber also suggest that the 1053 nm laser in fiber laser test may be due to a fiber temperature raising effect during the increase of P-970. The laser properties and ASE spectra of the fiber under 808 nm pumping have been studied in the same fiber test setup. However, the tested results are quite different from the 970 nm pumping case. Only one lasing peak at 1053 nm is detected, and it is found that the peak is not dependent on the 808 nm pump power (P-808)nor the fiber length. To explain this phenomenon, one energy transfer model with taking into consideration the stimulated emission of Nd3+ is derived. According to this theoretical model, Nd3+ -> Yb3+ energy transfer efficiency fast decreases with the increase of simulated emission intensity of Nd3+. This explanation is experimentally supported by a 0.05-m-long Nd3+ / Yb3+ co-doped phosphate glass fiber with varying P-808. Therefore, the adoption of Nd3+ to sensitize Yb3+ for developing some laser applications needs to consider the suppression effect of Nd3+ stimulated emission on Nd3+ -> Yb3+ energy transfer