A framework for spontaneous Brillouin noise: unveiling fundamental limits in Brillouin metrology.

Spontaneous Brillouin scattering (SpBS) enables non-contact probing of mechanical and thermodynamic material properties, underpinning transformative technologies such as distributed optical fiber sensing and high-resolution microscopy. Achieving ultimate precision in these systems demands a fundamental understanding of noise limits. Yet, an intrinsic SpBS noise phenomenon proposed over three decades ago has remained largely unexplored, particularly in metrological contexts. Here, we revisit the physical mechanism and stochastic nature of this long-overlooked noise source, developing a comprehensive analytical framework, validated through dedicated experiments. Crucially, we propose, for the first time, that SpBS noise constitutes a universal and fundamental limit capable of surpassing conventional constraints (e.g., the shot-noise limit) in spontaneous Brillouin metrological systems, such as imaging, microscopy and sensing. We experimentally demonstrate the SpBS-noise-limited regime in Brillouin imaging and sensing scenarios. This framework establishes a critical foundation for understanding and optimizing the performance of current and future Brillouin-based technologies across a broad range of applications.