Soliton rain is a typical multipulse dissipative structure in passively mode-locked fiber lasers, where the drift direction of drifting solitons (DSs) reflects the balance among gain, nonlinearity, dispersion, loss, and intracavity filtering. Here, we show that the drift symmetry of soliton rain can be tuned by changing the length of the polarization-maintaining fiber (PMF) in a passively mode-locked erbium-doped fiber laser. The PMF, together with the intracavity polarization elements, forms an equivalent Lyot filter whose wavelength-dependent transmission modifies the spectral-energy redistribution around the condensed soliton phase (CSP). Experiments and simulations indicate that gain accumulation enables soliton-rain formation, while the birefringence-induced filtering condition biases the DS drift direction and supports switching between unidirectional and bidirectional soliton rain. Correlation analysis, mutual information, cross-correlation, and three-dimensional phase-space reconstruction further reveal the internal interaction dynamics between the CSP and DSs. These results identify PMF-induced birefringence as an effective control knob for drift-symmetry selection, rather than the sole origin of soliton-rain formation.

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• Nonlinear optics
• Photonics
• Fiber lasers