Numerical modeling of an ASE suppression and power scalable approach in 1908 nm LMA-TDFO

High-power Tm-doped fiber lasers operating at short wavelengths (< 1940 nm) are widely used in biomedical engineering, remote sensing, and pumping Ho-doped solid-state lasers. However, it is challenging to increase their output power. Amplified spontaneous emission (ASE) is a major bottleneck in scaling the power of Tm-doped fiber lasers operating at short wavelengths. This paper presents what we believe to be a novel approach for suppressing ASE and increasing output power in large-mode-area Tm-doped fiber oscillators (LMA-TDFOs) operating at short wavelengths. By designing a core with some particular doping profiles in LMA fibers, most ASE can be concentrated in the LP₁₁ mode and suppressed by bending the fiber to a suitable radius. At the same time, the signal laser can be wholly coupled to the LP₀₁ mode in the oscillator. A model describing the transverse mode competition and ASE is developed. A novel nested-ring doping scheme for LMA fibers is designed, and the output characteristics of a 1908 nm oscillator based on this scheme are numerically investigated. Theoretical analysis confirmed that this sophisticated doping design can effectively achieve the intended ASE suppression effect. Further simulations show that this method is expected to increase the output power of the 1908 nm LMA-TDFO to the kW level and has also demonstrated good practicability across a broader signal spectrum range (1848-1940 nm).