Paper Express | 101 km · 60 s! Joint Research by AllianStream Photonics & SJTU Breaks Through Long-Distance Brillouin Sensing with FFT-Simplex Coded BOTDR

📢 Joint research conducted by Prof. Zuyuan He’s team from Shanghai Jiao Tong University (SJTU) and Ningbo AllianStream Photonics Technology Co., Ltd. has been published in Chinese Optics Letters. The proposed FFT-Simplex coded Brillouin Optical Time-Domain Reflectometry (BOTDR) innovates the traditional framework of Brillouin sensing, balancing signal gain and demodulation computing efficiency. It extends the practical sensing distance to over 100 km, delivering a brand-new technical solution for integrated communication & sensing monitoring of long-distance pipelines and submarine optical cables.

📖 Brief Introduction: What is BOTDR?

🔍Brillouin Optical Time Domain Reflectometry (BOTDR) is distributed fiber monitoring equipment for long-distance infrastructure. Based on fiber Brillouin scattering, it acquires full-line temperature and strain data by analyzing frequency shift of scattered light.

It supports single-end deployment, full coverage without blind spots, and reuses existing communication cables, widely applied in oil pipelines, submarine cables and power grid long-line safety monitoring.

Core Limitations of Traditional BOTDR

Though suitable for long-distance monitoring, traditional commercial devices have obvious drawbacks that restrict stable monitoring over 100-km trunk lines:

Conventional frequency sweep takes dozens of minutes per scan. Temperature drift and vibration cause data deviation, failing real-time early warning;
Traditional Short-Time Fourier Transform (STFT) adopts segmented demodulation. Computing load rises with fiber length, signals decay severely beyond 80 km with insufficient SNR, leading to unreadable far-end data.

✨ Technical Breakthrough: Simplex Coding + Full-Range FFT Demodulation

127-bit Simplex Orthogonal Coding Enabling Signal Amplifier

Adopting a 40 ns chip orthogonal pulse sequence, linear matrix decoding delivers a theoretical coding gain of ~7.5 dB, with a measured SNR improvement of 7.38 dB. Under identical transmit power, single-pulse signals saturate with noise at only 47.5 km, while our scheme retains a 2.6 dB effective signal margin at the far end of a 101.2 km fiber, laying a solid physical foundation for ultra-long-distance detection.

Full-Range Fast Fourier Transform (FFT) Demodulation Architecture Eliminating Redundant Segmented Computing

Abandoning repetitive window segmentation in STFT, the system performs one-time full-spectrum analysis on the complete backscattering signal via Fast Fourier Transform (FFT), followed by narrowband digital filtering and Inverse Fast Fourier Transform (IFFT) time-domain reconstruction. For datasets from ultra-long fiber links, the architecture drastically reduces hardware demodulation load and improves system scanning speed.

Coherent Detection Architecture Natively Compatible with Communication-Sensing Integration

A 10.80 GHz RF frequency-shifting coherent heterodyne detection scheme is adopted, ensuring beat signals fall within the linear working range of photodetectors. The system can directly reuse existing commercial communication optical cables without dedicated sensing fibers, enabling simultaneous data transmission and distributed temperature/strain monitoring on a single fiber.

Figure 1 Schematic diagram of FFT-Simplex coded BOTDR system

📊 Test Results at 101.2 km

The test link consists of two segments of G.652.D single-mode fiber with a total length of 101.22 km. A full scan takes only 60 seconds, with outstanding key performance indicators:

▫️ Maximum sensing distance: 101.2 km, supporting 100 km-level repeater-free distributed monitoring

▫️ Spatial resolution: 4.72 m, enabling precise localization of tiny abnormal points

▫️ Frequency shift error at 101.2 km fiber end: 1.83 MHz (equivalent to 1.83 °C/36.6 με)

▫️ High-precision coverage: Frequency shift accuracy better than 1 MHz within 90.7 km, satisfying acceptance standards for oil, gas and submarine cable industries

▫️ Temperature-frequency coefficient: 1.07 MHz/°C, linear fitting R² = 98.73%

Key Experimental Verification

Coding Gain Comparison Test

127-bit Simplex coding delivers prominent SNR improvement. Complete and distinguishable waveforms are obtained at 101.2 km, whereas single-pulse signals are fully submerged under noise under identical conditions.

Figure 2 SNR comparison between Simplex coded scheme and single-pulse scheme along full fiber length

Spectrum Reconstruction at Fiber Far End

Lorentzian fitting matches the measured far-end Brillouin Gain Spectrum (BGS) perfectly, and repeated 10 measurements demonstrate excellent stability of Brillouin Frequency Shift (BFS) data.

Figure 3 Full-length Brillouin Frequency Shift (BFS) distribution over 101.2 km & far-end Brillouin Gain Spectrum (BGS) fitting curve

Spatial Resolution Calibration via Temperature Step Test

A 30 °C ~ 70 °C temperature gradient is applied to a 50 m segment at the fiber far end. The measured temperature transition boundary reaches 4.72 m, with zero positioning offset.

Figure 4 Spatial resolution test curve based on step temperature excitation at fiber terminal

🌐 Application Scenarios

▪ Long-distance onshore oil & gas pipelines: Full-link real-time temperature monitoring for rapid leakage localization

▪ Repeaterless submarine communication cables: Real-time monitoring of stress anomalies induced by seabed settlement and ocean current scouring

▪ Ultra-high voltage Optical Fiber Composite Ground Wire (OPGW): Distributed early warning of ice coating and wire breakage hazards

▪ Intercity trunk & data center long-distance optical links: Reuse of communication fibers to realize integrated communication & sensing and reduce operation costs

📄 Paper Information

Yang Zhang, Jiageng Chen, Hanzhao Li, Xuhui Yu, Zuyuan He.

FFT-based signal processing for high-speed long-range Brillouin optical time-domain reflectometry

Chinese Optics Letters, Vol.24, No.5, May 2026

DOI: 10.3788/COL202624.051201

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