We propose and demonstrate a millimeter-level -spatial-resolution Brillouin optical correlation-domain analysis system based on a broadband chaotic laser, where the spatial resolution only depends on chaotic signal bandwidth. By adjusting the bandwidth of chaos, the single correlation peak has been compressed to be extremely narrow. The Brillouin gain spectrum was obtained by replacing a previous optical power meter with a lock-in amplifier. Thus, the signal-to-noise ratio improved and the data acquisition time reduced effectively. This experiment is the first in that the distributed tensile strain was observed via a chaotic Brillouin optical correlation-domain analysis system with a 3.5-mm spatial resolution over 165-m measurement range. The standard deviation in the reconstructed Brillouin frequency shift was ±2 MHz. To the best of our knowledge, the proposed system is a broadband-source optical sensing system with the highest reported spatial resolution.

We propose and experimental demonstrate a Dynamic Threshold Identification Method (DTIM) based on the Raman distributed fiber sensor (RDFS) system for detecting the pipeline leak. The DTIM can detect the position of pipeline leak according to distributed temperature measured by the RDFS. The DTIM includes three steps, which are the space segmentation detection, 3σ criterion and identification of historical data. The experimental results show that the proposed RDFS can detect and accurately locate the leak position by using the DTIM, and the positioning accuracy is 1 m. In addition, the temperature change rate of the leak position performs a quadratic function relationship with the leak rate, and the leak rate can be estimated by this method. It provides an effective distributed monitoring method for the leak monitoring of pipeline.

We propose and experimentally demonstrate a chaotic Brillouin optical correlation-domain analysis system for distributed fiber sensing. The utilization of a chaotic laser with a low coherence state ensures high spatial resolution. The experimental results demonstrate a 4 cm spatial resolution over a 906 m measurement range. The uncertainty in the measurement of the local Brillouin frequency shift is $\pm 1.2\mathrm{MHz}$. The analysis of the expected spatial resolution and signal-to-noise ratio is also given.

The field of tunnel fire detection requires a Raman distributed temperature sensor (RDTS) with high-accuracy and visual localization. A novel temperature demodulation method to improve the temperature measurement accuracy of the RDTS systems is presented. This method is based on the optical dynamic difference compensation algorithm, which can eliminate the optical power fluctuation. In addition, the visual localization technology is presented by using the longitudinal lining model (LLM) of a three-dimensional (3D) temperature display, which enhances the engineering application of RDTS in tunnel fire detection. Experimental results indicate that the temperature measurement accuracy is optimized from 7.0 °C to 1.9 °C at the sensing distance of 18.27 km by using the presented method. We provide a solution for temperature field monitoring as well as fire visual localization of the tunnel through RDTS systems.

Compared with conventional laser, random laser (RL) has no resonant cavity, reducing the requirement of cavity design. In recent years, the random fiber laser (RFL), a novel kind of RL, has made great progress in theories and experiments. The RFL has a simpler structure, a more flexible design, and higher reliability. It has valuable applications for earth sciences, biological life sciences, and national defense security, due to these unique properties. This paper reviews the development of RFLs in the last decade, including their configurations based on various optical fibers and their output properties, especially the method of control. Moreover, we also introduce their applications in the optical fiber sensing system, which is a very important and practical orientation to study. Finally, this paper presents the prospects of RFLs.

A new technology for Brillouin optical correlation domain analysis (BOCDA) based on chaotic laser is proposed, analyzed and demonstrated. The numerical simulation shows that the stimulated acoustic field has the secondary spurious peaks, which are the result of a weak amplitude autocorrelation of the chaotic signal occurring at the delay time of the external cavity, i.e., time delay signature (TDS). These secondary spurious peaks deteriorate the Brillouin gain spectrum and decrease the performance of the chaotic BOCDA system. The effect of the injection current and feedback strength on the TDS suppression is theoretically analyzed. By optimizing the two free parameters, chaotic laser sources operate in a TDS suppression region. Ultimately, a 3.2 km long single-mode fiber with a spatial resolution of 7.4 cm is experimentally demonstrated. The uncertainty of the local Brillouin frequency shift is ± 1.2 MHz.