Infrared and Laser Engineering, Volume. 54, Issue 4, 20250114(2025)

Research progress on distributed fiber sensing using Brillouin phase-gain ratio (invited)

Zonglei LI1,2, Haijun HE1,2, Yin ZHOU1,2, Xihua ZOU1,2, Wei PAN1,2, and Lianshan YAN1,2、*
Author Affiliations
  • 1Center for Information Photonics and Communications, Southwest Jiaotong University, Chengdu 611756, China
  • 2Key Laboratory of Optoelectronic Integration and Communication Sensing, Ministry of Education, Southwest Jiaotong University, Chengdu 611756, China
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    Significance With intensive research for more than three decades, distributed Brillouin fiber sensing has found numerous applications in the past few decades. It relies on the linear relationship between Brillouin frequency shift (BFS) and physical quantities applied to sensing fibers. Since the BFS is an inherent frequency property of the sensing fiber that is only related to fiber material properties as well as temperature and strain applied to the sensing fiber, the Brillouin fiber sensors can provide ultra-high sensing accuracies and stabilities. The Brillouin phase-gain ratio method uses the linear relationship between BFS and Brillouin phase-gain ratio to map BFS directly, which is initially proposed to reduce the frequency scanning steps as well as raw data size. With a deep study of this method, it was found that it can solve many issues relevant for conventional frequency-scanning- and curve-fitting-based BFS estimation approaches, such as non-local effect, pump power and frequency fluctuations. More importantly, it truly combines Brillouin fiber sensing with image denoising and thus enhances the sensing speed considerably. These advances make the Brillouin phase-gain ratio method a suitable choice for the monitoring of both quasi-static and dynamic temperature variations and deformations of large infrastructures, such as bridges, railway tracks, freeways, and high voltage lines.Progress First, the operation principle of the Brillouin phase-gain ratio method is introduced, which shows that both Brillouin gain and phase-shift information are needed for its implementation. Since the direct detection throws away all the phase information, the coherent detection needed to be performed as it can provide both Brillouin gain and phase information simultaneously. The advances of coherent detection schemes are then introduced, including the elimination of the impacts of the fiber group velocity dispersion, fiber group delay fluctuations, the phase noises of RF driving sources, and relative intensity noises transferred from the Raman pump sources during distributed Raman amplification. These advances in Brillouin gain and phase extraction clearly reveal that ultra-precise Brillouin gains and phases can be obtained by differentiating two signals with different Brillouin gains and phases but the same noises. Such a differentiation process can be realized by using coherent detection schemes based on either digital or analog in-phase/quadrature (I/Q) demodulation. The advantages of the Brillouin phase-gain ratio method are further introduced. Benefiting from the division operation of this method, it avoids the impacts of pump depletion, pump power fluctuation, and pump frequency jitter on the estimation of the BFS, and thus considerably enhances the sensing reliability. Besides, the slope of the Brillouin phase-gain ratio spectrum has a much wider linear region compared to the Brillouin gain and phase spectra. A much wider temperature and strain sensing range for a single pump-probe frequency scanning is then enabled via the slope-assisted analysis of the Brillouin phase-gain ratio. A strain sensing range of more than 5000 microstrains can be reached by changing the pump-probe frequency difference rapidly via the frequency-agile technique. Furthermore, various advanced image denoising methods have been used recently for performance enhancements in Brillouin fiber sensors. However, with a deeper understanding of the image denoising methods, it was recently found that the newly-added image denoising action is redundant with the conventional signal processing that is composed of spatial-domain low-pass filtering and spectral-domain spectrum fitting. The slope-assisted analysis of Brillouin phase-gain ratio does not need spectral-domain spectrum fitting for BFS estimation and thus truly links Brillouin fiber sensing with image denoising. This enables a notable sensing speed acceleration of more than 20 times.Conclusions and Prospects Owning to the precise raw differential Brillouin gain and phase information provided by current advanced coherent detection schemes, the notable signal-to-noise enhancements gained from digital signal processing such as image denoising, as well as the advantages of the Brillouin phase-gain ratio method itself, ultra-precise, -reliable, and -fast Brillouin fiber sensing with ultra-wide temperature and strain range has been realized. This makes Brillouin fiber sensing a quite suitable choice for the structural health monitoring of large infrastructures. The successful measurements of both quasi-static and dynamic temperature and strain changes provide valuable information for in-time early-warning of their structural health conditions. The sensing performance of the Brillouin phase-gain ratio method is ultimately determined by residual noises existed in current sensing systems, further efforts may focus on developing new techniques to eliminate those noises.

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    Zonglei LI, Haijun HE, Yin ZHOU, Xihua ZOU, Wei PAN, Lianshan YAN. Research progress on distributed fiber sensing using Brillouin phase-gain ratio (invited)[J]. Infrared and Laser Engineering, 2025, 54(4): 20250114

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    Paper Information

    Category: Invited review

    Received: Dec. 24, 2024

    Accepted: --

    Published Online: May. 16, 2025

    The Author Email: Lianshan YAN (lsyan@home.swjtu.edu.cn)

    DOI:10.3788/IRLA20250114

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