OFC2025|Long-Distance Fiber Monitoring Using a Repeaterless Brillouin Optical Time-Domain Reflectometry Sensing System

Introduction

Brillouin optical time-domain reflectometry (BOTDR) has emerged as a powerful technique for monitoring distributed temperature and strain variations along optical fibers. This technology is especially valuable for renewable energy infrastructure such as offshore wind farms and solar power installations, where high-voltage power cables require continuous monitoring over extended distances. Traditional BOTDR systems typically offer sensing ranges of around 100 kilometers, but recent advancements have significantly pushed this boundary [1].

Overview of Long-Distance Fiber Sensing

Renewable energy installations require robust monitoring solutions to ensure the integrity of their power transmission systems. Distributed fiber optic sensors (DFOS) based on Brillouin scattering have become the preferred option for long-range monitoring applications. These systems leverage naturally occurring Brillouin backscattering in optical fibers, which is highly sensitive to both temperature and strain changes.

The sensing range of conventional BOTDR systems is primarily limited by signal attenuation in the fiber. As the probe pulse travels further along the fiber, the backscattered Brillouin signal weakens and eventually falls below the detection threshold. To overcome this limitation, researchers have explored various amplification techniques, including distributed Raman amplification and remotely pumped erbium-doped fiber (EDF) amplifiers.

BOTDR Systems Assisted by Erbium-Doped Fiber

An innovative approach to extending BOTDR sensing range involves strategically placing EDF amplifiers along the sensing fiber. In the experimental setup presented in the research, two EDF segments were placed at 90 km and 140 km respectively.

Figure 1. Experimental setup for long-distance BOTDR with two EDF segments. OTS4: Brillouin OTDR, WDM: wavelength-division multiplexer, EDF: erbium-doped fiber.

The system uses a commercial BOTDR interrogator (OTS4 series by Luna Innovations), which generates a seven-bit coded pulse sequence at a wavelength of 1550 nm. This pulse coding technique extends the sensing range by approximately 35 km compared to uncoded methods. A 200 ns pulse width provides a spatial resolution of 20 meters.

The sensing fiber primarily consists of ultra-low-loss TeraWave SCUBA optical fiber with effective areas of 150 μm² (SCUBA150) and 80 μm² (SCUBA80). The SCUBA80 fiber is located at the end of the span to enhance Brillouin scattering due to its smaller effective area.

Continuous-wave (CW) pump lasers remotely power the two EDF segments (12 m and 11 m in length, OFS Rightwave LP980). Pumping is done via a separate ultra-low-loss SCUBA150 fiber to avoid distributed Raman amplification, which could distort the probe pulse. A 10 dB coupler at 90 km splits the pump power between the two EDF segments.

To achieve optimal performance, the peak probe power is set to 23.7 dBm, and 2.5 mW of pump power is delivered to each EDF segment. The total input pump power is 2.2 W. Under these conditions, the system successfully measured Brillouin peak frequencies over the entire 250 km range, with measurement uncertainties of 1.7 MHz at 90 km, 0.3 MHz at 140 km, and 3.5 MHz at the far end.

Mitigating Modulation Instability Using Normal Dispersion Fiber

One of the key challenges in long-distance BOTDR is the presence of nonlinear effects, particularly modulation instability (MI). MI arises from the interaction between Kerr-induced nonlinearity and anomalous dispersion in the fiber, producing spectral sidebands that reduce the central peak power and introduce noise into the measurement.

Figure 2. Measured Brillouin peak frequency: a) 250 km SCUBA fiber, b) spectral broadening due to MI at different peak probe powers (solid lines show simulation results), and c) Brillouin signals emitted to EDF under different pump powers.

Researchers examined the impact of MI on the detection spectrum using a setup with 260 km SCUBA fiber and a single EDF amplifier segment at 120 km. The results clearly show that increasing either the peak probe power or the pump power exacerbates MI, leading to significant spectral broadening. This broadening not only reduces Brillouin signal power but also increases Rayleigh scattering noise within the detector bandwidth.

To mitigate MI-induced noise, the researchers replaced a 50 km SCUBA150 segment after the second EDF with a 49 km section of TrueWave XL (TWXL) fiber, which exhibits normal dispersion of -3.0 ps/nm·km at 1550 nm. This strategic substitution offsets the anomalous dispersion present in the SCUBA fiber, thereby reducing the conditions favorable for MI.

Figure 3. a) Measured Brillouin peak frequency of a 250 km fiber with a 49 km TrueWave XL fiber connected after the second EDF at 140 km, b) peak frequency measured only over SCUBA fiber, and c) comparison of uncertainties in fitted peak frequencies.

The introduction of normal dispersion TWXL fiber at the far end of the fiber span significantly improved measurement quality. Specifically, the uncertainty in Brillouin peak frequency measurement at 250 km decreased from 3.5 MHz to 2.4 MHz. This improvement allows for more accurate temperature and strain measurements across the full sensing range.

However, researchers noted that the higher attenuation coefficient of TWXL fiber (0.20 dB/km vs. 0.15 dB/km for SCUBA fiber) presents a challenge in balancing pump power between the two EDF segments. They suggest that ideal future implementations should use specialty fiber that combines normal dispersion with the low-loss characteristics of modern telecom fibers.

Conclusion

The study demonstrates that the BOTDR sensing range can be extended to 250 km without distributed Raman amplification by strategically placing EDF amplifiers. Incorporating normal dispersion fiber into the setup effectively reduces nonlinear noise from modulation instability, enhancing measurement accuracy over ultra-long distances.

This breakthrough has significant implications for the monitoring of submarine power cables in offshore renewable energy installations, enabling more effective and reliable infrastructure management. As specialty fibers that combine normal dispersion and ultra-low loss continue to develop, the presented technology could see further improvement.

Reference

[1] N. M. Mathew, M. H. Vandborg, J. B. Christensen, Z. Wang, L. Grüner-Nielsen, L. S. Rishøj, R. Crickmore, T. North, T. Geisler, M. Lassen, and K. Rottwitt, "Repeaterless Brillouin OTDR Sensing," in OFC 2025, M1C.3.