Overview: The distributed Brillouin optical fiber sensors have gain considerable interests, due to their capabilities of monitoring strain and temperature. The Brillouin optical time domain analysis (BOTDA) system, which is the typical representation of distributed Brillouin sensors, features high spatial resolution, long range and fast measurement. In recent years, the BOTDA sensors have been widely used in oil and gas pipelines to detect the leakage, civil structure health monitoring and cable temperature sensing. The distributed BOTDA sensors are promising tools to these and other applications that need for monitoring large distances.
However, the sensing range of BOTDA sensors is fundamentally determined by the signal to noise ratio (SNR). The continuous probe power is limited by the Brillouin threshold, while the modulation instability (MI) or the self-phase modulation (SPM) is the main obstacle to input high power pump pulse. We have systematically investigated the impact of MI and SPM on long-range BOTDA system theoretically and experimentally. Besides, the pump depletion and non-local effect is detrimental to the long-range BOTDA sensors performance.
Several impressive technologies have been proposed to extend the sensing range of BOTDA system. In 2010, the coding BOTDA is firstly proposed to increase the SNR effectively by Soto et al. Then they utilize the pre-amplification and Raman amplification to further extend the sensing range to 120 km with an accuracy of 2.2 ℃ or 44 με in 2012. In 2011, Dong et al proposed the time division BOTDA to decrease the SBS interaction range and realized a 100 km Brillouin fiber sensing. In the same vein, the frequency division combining the in-line EDFAs BOTDA achieved a 150km sensing range with a 2 m spatial resolution. In 2016, the Spanish group demonstrated a BOTDA sensor in loss configuration with the optical frequency of the probe wave modulating along the fiber. It had several advantages such as high Brillouin threshold, overcoming the non-local effect and no additional amplifier. In the same year, the Swiss team firstly utilized the image denoising and it was able to enhance the SNR 14 dB in 50 km fiber. Imaging denoising was an effective method to increase the SNR in BOTDA system with no hardware modification.
In summary, long range Brillouin sensing schemes based on time division multiplexing, frequency division multiplexing, pulse coding, wide-bandwidth frequency modulation and image denoising are introduced in this paper. With long range Brillouin sensors applied in practice, increasing demand for fast measurement emerges, which we believe will be dominant in the research of long range Brillouin fiber sensing in the future.