Distributed optical fiber sensing technologies provide the ability to detect and sense various environmental quantities like strain, vibration and temperature, making them suitable for various applications in various fields, including medical treatment, security detection and nano-/micro-sensing. Based on scattering mechanisms, distributed optical fiber sensing can be grouped into three main categories: Rayleigh, Raman and Brillouin. Among the three, Raman distributed optical fiber sensing technique is one of the most commonly used, especially in the linear detection of infrastructure temperature. Its advantages include simple structure, rapid measurement speed and remarkable sensitivity to temperature changes.
The sensing performance of Raman distributed fiber sensing can be improved through various advanced schemes available. However, the limitations associated with the pulse width and the optical time-domain reflectometry (OTDR) principle are a major problem to the efficient application of Raman fiber sensing. For instance, with a smaller detection area than the system’s spatial resolution, the detected temperature results are often relatively smaller than the actual temperature values, leading to inaccurate measurements. Consequently, the obtained temperature values can be highly underestimated for a small temperature variation spot located along the sensing fiber. Thus, accurate temperature detection in small-scale regions is of great significance in applications like gas production.
Numerous advanced demodulation techniques have been developed to address the above problems. These techniques focus either on improving the system’s spatial resolution or avoiding the spatial resolution limitations by using sensing fibers. Presently, the best spatial resolution that has so far been achieved experimentally is also limited to order of meters when operated over long sensing ranges, implying the difficulty of accurately detecting temperature in small-scale areas. Despite the significant research efforts devoted to achieving reliable small-scale area temperature detection, exiting methods can only realize positioning but fail to accurately achieve the desired temperature information. Nevertheless, previous findings revealed the importance of Raman distributed optical fiber sensing in achieving accurate temperature measurements at micros-scale areas.
To overcome these limitations, a group of researchers from the Taiyuan University of Technology: Dr. Jian Li, Dr. Xinxin Zhou, Dr. Yang Xu, Professor Lijun Qiao, Professor Jianzhong Zhang and Professor Mingjiang Zhang proposed a novel slope-assisted scheme based on Raman distributed optical fiber sensing to improve the ability to accurately monitor temperatures in micro-scale regions. The authors theoretically analyzed and simulated the pulse transmission characteristics of the temperature variation region of the sensing fiber and the superposition features of the resulting Raman OTDR signals. The work is currently published in the journal, Photonics Research.
The research team showed that the position information and true length of the fiber under test area could be demodulated effectively by the spatial length of the rising edge of the Raman OTDR trace. For the first time, the slope-assisted scheme enabled accurate temperature measurements in areas with a centimeter-level spatial scale even if the system was positioned at a meter-level scale. Unlike the existing advanced schemes, this scheme could eliminate special sensing fibers and expensive pulse-encoding sources to improve the accuracy of the final results, suggesting its feasibility for practical applications. The superimposed Raman anti-Stokes scattered signals equations were derived at various stages, providing a theoretical basis for demodulation of optical fiber systems as well as the design of various distributed optical fiber sensors like Brillouin optical time-domain reflectometry (BOTDR) and Rayleigh-OTDR systems based on OTDR principles.
In summary, the authors successfully demonstrated the feasibility of the newly proposed slope-assisted Raman distributed optical fiber sensing in improving micro-scale region temperature monitoring capabilities. The obtained results provided a thorough understanding of the scattering signal superposition and pulsed laser transmission. In a statement to Advances in Engineering, Professor Mingjiang Zhang, the corresponding author said the study findings will contribute significantly to improving the performance of various optical fiber sensing techniques.
Li, J., Zhou, X., Xu, Y., Qiao, L., Zhang, J., & Zhang, M. (2021). Slope-assisted Raman distributed optical fiber sensing. Photonics Research, 10(1), 205-213.