Improving the Laser Performance of Yb3+-Doped Phosphate Fiber: Population Simulation of the Yb3+ Level.

Based on the Boltzmann distribution and multi-phonon relaxation probability criterion, an original Yb³⁺ population equation was proposed to describe the population distribution before and after laser generation, and the population distribution of Yb³⁺ under different pump ratios and temperatures was investigated by numerical simulation. The simulation results indicated that the laser wavelength of Yb³⁺-doped modified phosphate fibers have a high probability of being in the range of 1019 nm--1056 nm under the conventional pump ratio. Additionally, fibers lasing at a longer wavelength may have a lower laser threshold. For ultra-high pump ratio or high fiber temperature, the laser operation state changes from a quasi-four-level to a quasi-three-level scheme, and the laser wavelength may blue-shift. Experimental results verify the above simulation results, and in addition demonstrate an output power of 9.38 W with a slop efficiency of 27.4% in an Yb³⁺-doped phosphate modified fiber with a length of 35.4 cm and diameter of 280 μm from an optical path with a refrigeration patch and suppressing short-wave laser output. The results show that the laser performance of Yb³⁺-doped fibers can be improved by reducing the operating temperature and inhibiting short-wave laser output. [ABSTRACT FROM AUTHOR]