Spontaneous Brillouin scattering (SpBS) is a fundamental light–matter interaction found in virtually all media, where light couples with thermally driven acoustic waves inside materials. Since its theorization in 1922, SpBS has become the cornerstone of many precision technologies, enabling long-distance fiber sensing, photon–phonon interactions in integrated photonics, and non-contact mechanical imaging in biological and medical samples. A seminal theoretical study in 1990 proposed and explained the intrinsic intensity fluctuations in Brillouin scattering arising from thermally excited phonons, and investigated their stochastic features via numerical simulations. Yet, the SpBS intrinsic fluctuations have remained experimentally unverified and have never been systematically explored in metrological contexts since it was first proposed, leaving a significant gap in spontaneous Brillouin metrology.
In a new paper published in Light: Science & Applications , a research team, led by Prof. Zhisheng Yang from State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Dr. Fan Yang from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences Shanghai, and their collaborators have revisited the physical formation mechanism and stochastic properties of this long-overlooked intrinsic noise, and developed and experimentally validated a comprehensive theoretical framework that describes intensity fluctuations arising from SpBS, bridging the gap between the theoretical study and measurable system performance.
Through both analytical modeling and carefully designed experiments, the researchers pictured the physical origin of SpBS intensity fluctuations (figure 1), and linked its stochastic behavior to key practical system parameters (phonon lifetime, measurement bandwidth, sampling interval, as well as total number of samples, figure 2). They further established theoretical models connecting the SpBS intrinsic noise directly to measurable signal-to-noise ratio (SNR), demonstrating that SpBS noise defines a universal and fundamental precision limit in Brillouin metrology—one that surpasses the traditional shot-noise boundary once a system enters the SpBS-noise-limited regime. Experimental validation in Brillouin imaging, microscopy (figure 2), and distributed fiber sensing (figure 3) confirmed the theoretical predictions. These researchers described the fundamental SNR limit:
“In the ideal full-bandwidth case, the DC and AC powers of scattered field spectrum are equal, yielding SNR=1. However, in practical systems with finite measurement bandwidth, only a portion of the AC term may be captured while the DC term remains unaffected, leading to a bandwidth-dependent SNR. As the system measurement bandwidth increases beyond the 3-dB bandwidth of Brillouin spectrum, the SNR approaches and eventually saturates at the value of 1. This relation implies that Brillouin systems operating in different bandwidth regimes will exhibit distinct, yet predictable, SNR behaviors when constrained by SpBS noise.”
This work enables the first quantitative understanding of how intrinsic SpBS noise fundamentally constrains the achievable performance of modern Brillouin metrological systems. The established unified framework is generalizable to a wide range of Brillouin sensing architectures, including standard schemes, polarization diversity detection schemes, pulse coding architectures, and even single-photon sensing schemes. In all of these systems, accounting for SpBS noise is essential for accurate system modeling and performance optimization. The insights gained from this study also open new avenues for optimizing the design and performance of next-generation Brillouin-based technologies, especially in high-power applications such as Brillouin spectroscopy and Brillouin Lidar systems.
“Due to the SpBS-noise-limited regime, there exists a well-defined pump power threshold beyond which increasing optical power no longer improves SNR. Instead, the performance can only be enhanced by extending the measurement time,” as the authors noted.
Light Science & Applications
A framework for spontaneous Brillouin noise: unveiling fundamental limits in Brillouin metrology