# Advances in Random Fiber Lasers and Their Sensing Application
## 2 RFLs Based on Various Optical Fibers
### SMF-Based RFLs  
SMF is an alternative approach to form the RFL, avoiding the complex operation of filling the fiber with the material. RFLs based on SMF could be applied in long-distance and high-capacity fiber communication systems, fiber local area networks, and fiber sensors. The gain of SMF-based RFLs has two obvious principles: SRS and SBS.  
A random distributed feedback laser based on common SMF with a length of 83 km was proposed in 2010 [31], of which SRS provides the gain. The operation principle scheme of random distributed feedback fiber laser is illustrated in Figure 2. Two lasers with a wavelength of 1455 nm act as bi-directional pumping sources at the center of SMF. Random Rayleigh scattering provides random feedback, due to irregularity of the SMF, and backward Rayleigh feedback light is stimulated by Raman amplification continuously. Eventually, lasing emits from the two sides of the SMF. One of the biggest advantages of this setup is that integration of feedback and gain would limit light in one direction, compared with the previous RLs. Moreover, the threshold of this structure is lower than that of structures at other wavelengths because the minimum loss window of SMF is at the wavelength of 1550 nm (0.22 dB/km). Moreover, the proposal of this innovation has inspired the application of the RFLs in communication, as the traditional communication window is about at the wavelength of 1550 nm.  
Even though the threshold of the RFLs [31] is at a low-level compared with the same setup of which output is not at the wavelength of 1550 nm, the minimum pump power is still at the order of watts. If the gain mechanism of the RFLs is SBS, the threshold of the pump will decrease to the order of milliwatts. Because the Brillouin gain coefficient is much higher, it is possible to provide enough amplification in the case of extremely week random feedback. As shown in Figure 3, two beams at two ends are pumps. Scattering stokes light travels along the opposite direction and agglomerates on both ends of the fiber, because the length of fiber is relatively short and instant pump power is higher. Moreover, because of random distributed Rayleigh scattering caused by inhomogeneity of  
Figure 1: (**a**) Cross-section of the photonic crystal fiber (PCF); (**b**) optical setup for probing laser emission.  
SMF, the cavity mirror is formed along the optical fiber. However, valid Brillouin stokes feedback only occurs in the pump's deleted regions, i.e., \"mirror cloud\" at the ends of SMF [77]. This, therefore, suggests that SMF can make up not only Raman RFLs but also Brillouin RFLs.  
SMF-based RFLs that take advantage of Brillouin gain can obtain much narrower line-width emitted radiation than that of the Raman gain, which is due to the narrower bandwidth of the Brillouin gain. The research group of [PERSON] has made significant investigations on SMF-based Brillouin RFLs. In 2013, they firstly observed a narrow-line-width peak of coherent Brillouin RFL, in which the line-width is \(\sim\)10 Hz [78]. Theoretical and experimental research of coherent Brillouin RFL for frequency stability was reported in Reference [79], and the line-width of RFL is 50 Hz, but still relatively narrow. To understand the lasing mechanism of this kind of laser, they studied the frequency and intensity of noise characteristics of the Brillouin RFL based on bidirectional pumping [80]. In 2016, they presented a tunable ultranarrow-line-width microwave Brillouin RFL (under 3 dB bandwidth, the line-width is less than 10 Hz), which could provide a simple and economical design for microwave source [81]. This expands the application of the Brillouin RFLs. In Reference [82], six order stokes random lasing was realized with a high optical SNR of \(\sim\)47 dB.