The sensing range of Brillouin distributed fiber sensors (BDFS) is typically in the order of tens of kilometers due to the attenuation of the optical fiber and restricted input pump power. This limits the use of BDFS in certain long range applications such as oil and gas pipeline monitoring; where maintenance and safety monitoring requires sensing lengths up to hundreds of kilometers. This deterioration in the sensing performance cannot be counteracted by indefinitely increasing the pump power injected into the sensing fiber; as nonlinear effects such as modulation instability, self-phase modulation, and significant pump depletion occurs within the sensing fiber. In this paper, we demonstrate an extended sensing range system for pipeline monitoring using Brillouin optical time domain reflectometry (BOTDR) combined with Raman amplification and inline erbium-doped fiber amplifier (EDFA). Variations in pump light power, propagation direction, and injection location are explored to allow full control over the signal amplification in any particular section of the total sensing fiber length. Thus, the signal-to-noise ratio (SNR) for a certain location along the length of the fiber can be enhanced to provide more useful localized information. By using a continuous wave 1480nm Raman laser, and 980nm-pumped inline EDFA, the proposed system is theoretically validated over 150 km sensing fiber.
This paper proposes a new vector Brillouin optical time-domain analysis optical fiber sensor with large dynamic range and high signal-to-noise ratio that combines distributed Raman amplification with optical pulse coding. The optimized Raman pumping configurations are numerically simulated by solving the coupled differential equations of the hybrid Brillouin-Raman process, and experimentally investigated with respect to the Brillouin pump pulse. A vector network analyzer is adopted to extract both the amplitude and phase spectrograms of the Brillouin interaction in a distributed fashion which effectively lessens the impact of the Raman relative intensity noise transfer problem and achieve high accuracy measurement over a long sensing distance. Advanced pulse coding is further introduced to increase the sensing range under high spatial resolution. Initial experimental results of phase and amplitude from a custom built BOTDA system is presented. Compared to typically tens of kilometers measurement distance of conventional Brillouin optical time-domain analysis techniques, the proposed optical fiber Brillouin sensor has the potential to greatly enhances sensing range up to one hundred kilometers or greater, providing distributed temperature and strain monitoring of high spatial resolution and high sensing resolution in structures such as oil and natural gas pipelines.
In this paper, a novel technique was proposed to improve the sensing performance by employing wavelength diversity in Brillouin optical time domain reflectometry (BOTDR). This technique enables to maximize the launch pump power to achieve a higher measurement accuracy, without activating the nonlinear effects, which limit the conventional BOTDR performance. Experimentally, we have demonstrated the proposed technique, that provides measurement accuracy improvement of 3.6 times at far end of the sensing fibre compared to the conventional BOTDR system.