PM-Fiber-Based Lyot Filter for Suppressing Gain Narrowing in High-Power Femtosecond Fiber Laser Amplification Systems

High-peak-power Yb-doped fiber femtosecond lasers are powerful tools widely used in scientific and industrial fields. To increase femtosecond pulse energy, the chirped-pulse amplification technique is commonly employed to suppress nonlinear effects and prevent fiber damage by temporally stretching pulses before high-gain amplifiers. However, the gain-narrowing effect severely limits the amplified spectral bandwidth and consequently constrains the achievable compressed pulse duration. In this work, we demonstrate a compact polarization-maintaining (PM)-fiber-based Lyot filter that enables flexible spectral pre-shaping to overcome this challenge. Utilizing the birefringence of PM fibers, precise control of the spectral modulation period is achieved through simple fiber-length selection, while continuous tuning of the modulation depth and spectral shift is realized via wave-plate rotations. Furthermore, the reflective configuration with PM-fiber pigtails minimizes insertion loss and device size, offering a cost-effective, environmentally stable, and high-damage-threshold design. Experimentally, the amplified spectral width increases from 8.5 nm to 13.6 nm at a 1-MHz repetition rate, enabling a reduction in compressed pulse duration from 264 fs (without the filter) to 198 fs for 27-μJ pulses. Even at 100 kHz, the system still maintains a bandwidth exceeding 13 nm for 229-μJ pulses, confirming effective suppression of gain narrowing at high pulse energies. This robust Lyot filter paves the way toward practical all-PM-fiber femtosecond laser systems delivering high-energy, sub-200-fs pulses for advanced manufacturing and strong-field physics.