ASE-induced Quasi-Cross-Excitation Transfer Process (Quasi-CETP) mechanism in Erbium–Ytterbium co-doped fiber lasers: Theoretical modeling and experimental validation

The generation of high-quality seed signals at 1590 nm has been widely recognized as crucial for frequency doubling to 795 nm light required in spin-exchange relaxation-free (SERF) magnetometers. Conventional erbium-doped fiber systems face significant challenges at 1590 nm due to low gain efficiency and amplified spontaneous emission (ASE) noise, highlighting the need for advanced excitation mechanisms. However, the lack of a detailed understanding of internal ASE-mediated excitation pathways has hindered the development of efficient laser sources with high gain and signal-to-noise ratio (SNR) in this spectral region. In this work, we investigate a novel ASE-induced quasi-cross-excitation transfer process (quasi-CETP) in erbium–ytterbium co-doped fiber (EYDF) systems. A coupled rate equation model is developed to capture ASE spectral evolution and its impact on population dynamics. The model reveals characteristic nonlinear spectral features and identifies three operational regimes — gain, quasi-saturation, and degradation — based on fiber length. Experimental validation using a pump-recycling-assisted distributed feedback (DFB) fiber laser confirms the model and demonstrates a high-quality 1590 nm seed with enhanced signal-to-noise ratio and spectral purity, offering a stable source for frequency doubling and quantum sensing applications.