Polarization-insensitive stimulated Brillouin scattering filter with birefringence compensation.

Stimulated Brillouin scattering (SBS) has been extensively studied and repurposed for diverse practical applications, such as optical measurement and sensing, and microwave photonics filter. SBS based optical filters face inherent limitations due to polarization-dependent gain fluctuations caused by fiber birefringence and environmental perturbations. To address these challenges, we establish a comprehensive theoretical model of polarization evolution in the fiber and implement depolarization via the Faraday effect. Building on this insight, we propose and experimentally validate a polarization-independent SBS filter architecture enabled by dual orthogonally polarized pumps generated through a Faraday rotator mirror (FRM)-integrated Mach-Zehnder interferometer (MZI). This design achieves passive compensation for polarization fluctuations without requiring active or complex polarization state control. Experimental validation confirms that this design suppresses Brillouin gain variations to less than ±3% degree of polarization, enabling wavelength-independent operation across arbitrary input polarizations. In contrast, existing approaches based on complex polarization control schemes typically exhibit a degree of polarization exceeding ±5% and are susceptible to variations in both operating wavelength and filter parameters. Furthermore, we demonstrate the filter's versatility in optical spectrum analysis (OSA) using a two-stage SBS configuration, achieving a remarkable dynamic range of 80 dB, ± 0.3 pm wavelength precision, and improved power accuracy from ±2.4 dB to within ±0.2 dB, ensuring rapid and precise spectral acquisition. These advancements resolve long-standing limitations in SBS-based systems, enabling robust and polarization-independent operation for optical communications and sensing, integrated photonic devices and circuits, optical neural networks, microwave photonics, passive and active photonic devices.