# Advances in Random Fiber Lasers and Their Sensing Application
## 4 Sensing Application
### Distributed-Sensing  
RFLs based on long-distance fiber are perfect mediums for distributed sensing application, which can be used to improve the SNR in the distributed sensing system, because they are insensitive to the changed information of physical parameters, such as temperature and strain along the fiber. Currently, it is mainly used in the Brillouin optical time-domain analysis (BOTDA) system.  
An ultra-long distance distributed BOTDA sensing system was demonstrated in 2013 [126]. The experiment adopted a mixed composition of a second-order pump based on the RFL and a first-order pump based on the low-noise laser diode. The hybrid pump could optimize the gain distribution and extend the transmission distance. The results showed the repeaterless sensing distance is 154.4 km with 5 m spatial resolution and \(\pm\)1.4 degC measurement uncertainty. In this system, the pulses and probe light need to maintain certain SNR, and high SNR is beneficial to the extension of the sensing distance. There is no doubt that higher pump power is required for second-order emission, but it would result in the instability of modulation and obvious nonlinear effect. Relative intensity noise would cause the impairment of SNR. Thus, it is still a challenge for RFLs in the distributed sensing system. However, in 2019, due to optimizing the pump source with the second-order and third-order Raman laser, the sensing distance was further increased [127]. The experimental setup of 175 km BOTDA is shown in Figure 19. There are four configurations in this setup. The red part indicates the production of the pulses in BOTDA, and the green part presents the generation process of the probe light. The black part is the FUT that is also a part of RFL, and the blue part shows the detection module. In detail, the second-order Raman laser (1365 nm) is at the left of the FUT, and the third-order Raman laser (1280 nm) is at the right of the FUT. Both of them are pump sources. FBG1 and FBG2 are centered at the wavelength of 1461 nm and 1365 nm, and their bandwidths are 0.627 nm and 0.586 nm, respectively. The reflectivity of both is above 90%. This scheme realized a 175 km repeaterless BOTDA system with 8 m spatial resolution and \(\pm\)2.06 MHz uncertainty (Figure 20). This has been the longest distance of repeaterless BOTDA reported up to date.  
These schemes are all based on the hybrid pump, which have higher SNR compared with systems that adopted forward pump source. This is because the RFL possesses a lower effective noise figure in these configurations. Moreover, high-order RFL would perform stable gain distribution, avoiding power fluctuation along the transmission fiber caused by EDFA or distributed Raman amplification in the distributed sensing system. What's more, RFL is insensitive to the variation of temperature, making the BOTDA sensing system much more stable.  
As the distance of fiber sensing increases with the existence of the RFLs, RFLs could be applied to the safety monitoring of important facilities related to the national economy and people's livelihood: Bridges, tunnels, oil, and gas pipelines, as well as expressways. Moreover, the high SNR and the less  
Figure 18: Setup of RFL based on the strong RFG for ultrasound detection.  
gain fluctuation can be obtained, due to the RFLs, which is of vital importance in the sensing system. However, there are a few studies on point-sensing and distributed-sensing, which are based on RFLs. Challenges of huge loss caused by the ultra-long optical fiber and spectral broadening caused by the nonlinear effect would be required to overcome.