# Advances in Random Fiber Lasers and Their Sensing Application
## 4 Sensing Application
### Point-Sensing  
The sensing element requires over 100 km away from demodulation devices [21; 116] to apply ultra-long-distance safety detection (electric-lines, oil, and gas pipelines). FBG, a passive device, which is applied to the composition of RFL, could satisfy the practical requirements. As the external environment changes, the spectrum of FBG changes, and the spectrum of RFL will change accordingly. Because the fiber in the sensing system only plays the role of light transmission, the information to be measured is determined by the central wavelength of the FBG sensor. For example, a Raman RFL sensor for strain-temperature measurement was presented in 2009 [117]. FBGs remotely located at a distance of 10 km were characterized by the strain and temperature. The proposed system could measure strain and temperature simultaneously, using FBGs with low reflectively. The resolutions of temperature and strain are \(\pm 0.1\)\({}^{\circ}\)C and \(\pm 12.6\)\(\mu\varepsilon\), respectively. However, the sensing distance is only 10 km, which is far from satisfying the demand. Long-distance optical fiber remote sensing (beyond 100 km) was achieved in 2012, which is a mandatory requirement for sensing in harsh environments. This system was reported by [PERSON], which utilized first-order random lasing and second-order random lasing, as shown in Figure 17. For the first-order scheme, RFL is pumped by a 1365 nm pump source, while an FBG sensor whose central wavelength is 1455 nm corresponding to the first-order Stokes light is placed at the near end of the fiber span. For the second-order scheme, generated first-order Stokes light is the source of pumping. An FBG sensor with the central wavelength of 1560 nm, which is corresponding to the second-order Stokes light, is placed at the far end of the fiber span. The wavelength of output would shift according to the change of temperature of the FBG--namely, the retrieval of the sensing parameter only depends on the central wavelength of the FBG sensor, which makes the system robustness against degradation of the pump source or fiber span [43]. Moreover, for the first-order random lasing and second-order random lasing, the SNR is as high as 20 dB and 35 dB, respectively, which is sufficient for sensing at a distance of 100 km. Thus, this scheme is attractive for practical applications. However, the pump power of RFL is relatively high (\(>\)1.7 W), due to the Raman gain. After that, they optimized the scheme of long-distance  
Figure 16: Experimental setup of RFL. OC, optical coupler; CIR, circle. remote sensing [118], having placed the AOF in the middle of two SMF segments and taken the reverse RFL pumping scheme. The sensing distance is longer than that of the setup without AOF, and the SNR is over 30 dB. With a similar method, a point sensing RFL system of an ultra-long-distance of 200 km was obtained in 2013 [119]. This scheme can multiplex 11 FBGs, and the SNR is high enough to observe output emission. Thus, it has the capability of the multiplex. In 2016, a high-resolution point sensing RFL system with better stability was attained [45], in which the temperature sensitivity is less than 0.01 \({}^{\circ}\)C, and the strain sensitivity is less than 0.2 \(\mu\varepsilon\), and the resolution could be further increased by combining this system and improving-FBGs. To further decrease the threshold of the pump, [PERSON] et al. introduced a remote temperature sensor by adopting a single-wavelength erbium-doped RFL [120]. In 2019, a high-resolution static strain RFL sensor was proposed with RFGs, and a high precision \(\pi\)-phased shifted FBG. The static strain resolution is 196 \(p\varepsilon\) in a period of 160 s [121]. However, the scheme is incredibly complex, and the operation is difficult. Moreover, ultrasound sensing is a valuable technique, which can be widely used in the areas of ultrasound-related, such as biomedicine, geological disaster surveillance, material testing, dimensional measurement, as well as structural health monitoring.