Modeling and optimization of intensity noise transfer in EYDF-based low-noise fiber amplifiers

The importance of intensity noise in laser systems has been widely recognized in advancing the performance of spin-exchange relaxation-free (SERF) magnetometers. Elevated noise levels in frequency-doubled light have been directly traced to fluctuations in the fundamental laser output, highlighting the need for effective suppression strategies in master oscillator power amplifier (MOPA) systems. However, the lack of a detailed understanding of the intensity noise transfer process has hindered the development of targeted noise reduction techniques. In this work, we present a theoretical and experimental investigation of intensity noise transfer in erbium–ytterbium co-doped fiber (EYDF) amplifiers. A steady-state model is developed to derive the light signal and intensity noise transfer functions from both pump and seed sources. The model reveals exponential relationship for pump influence and linear relationship for signal light influence in the MOPA system. Additionally, relative intensity noise (RIN) analysis reveals that pump-induced noise exhibits a low-pass response, while seed-induced noise spans the full frequency range. Experimental validation confirms the model and further identifies a trade-off introduced by pump saturation: although it suppresses pump noise, excessive saturation leads to additional low-frequency noise. With appropriate optimization, the output RIN closely follows that of the seed laser, demonstrating a high-efficiency, low-noise amplification scheme suitable for SERF magnetometry and other precision sensing applications.