Robust and simple thermally controlled all-PM Sagnac filter and its demonstration in a tunable mode-locked fiber laser

Sagnac filters have been widely used in tunable and multi-wavelength fiber lasers for applications such as sensing and biomedical imaging. However, previously reported tunable Sagnac filters usually rely on non-polarization-maintaining fibers and polarization-controller-based tuning, which limits long-term stability and repeatability. Here, we propose and demonstrate a simple, robust, all-polarization-maintaining (all-PM), thermally controlled Sagnac filter and apply it in an erbium-doped mode-locked fiber laser. The filter is realized by cross-splicing the two output ports of a 3-dB coupler, and its thermal tuning characteristics are verified through both calculated and measured transmission spectra. The filter enables stable single-wavelength thermal tuning >30 nm with a tuning efficiency of -0.83 nm/^∘C, while also supporting dual-wavelength mode locking. This design provides an important complement to the birefringence-filter family and offers a robust, low-insertion-loss filtering solution for stable wavelength-tunable and dual-wavelength ultrafast fiber lasers. In addition, the proposed all-PM Sagnac filter also shows potential for extension to other fiber systems, where improved immunity to polarization perturbations is desirable.