# Advances in Random Fiber Lasers and Their Sensing Application
## 1 Introduction
However, the pump power is high up to watt-level for the operation of the all-fiber RFLs [31], due to the low Rayleigh backscattering coefficient of the SMF, leading to the narrow application. Experimental attempts on the threshold-decreasing approaches [54] have been performed continually. Using the fiber Bragg gratings (FBGs) with high reflection is an effective method. For example, a half-open RFL structure [55] was proposed, which FBG is on the one side of the RFL scheme. In this way, the central wavelength of the output beam is determined by the FBG, and the FBG provides much stronger feedback than that of optical fiber. Thus, the threshold of a half-open RFL scheme is nearly two times lower than that observed in a completely open cavity [39]. Another approach that improved gain by active fiber was considered, such as erbium [56] or ytterbium-doped [57; 58] fiber, which could serve to decrease the threshold of the RFLs and shorten the length of optical fiber. As a result of the existence of rare-earth ion, the power of the pump source drops to the order of milliwatt magnitude. Moreover, RFLs of higher efficiency and the lower threshold would be obtained with strong enhanced random feedback of random fiber grating (RFG) from Rayleigh scattering, by increasing the index changes along with the fiber to the order of \(10^{-4}\)-\(10^{-5}\). The intensity of random scattering is three orders of magnitude more than that of the Rayleigh scattering. Nevertheless, the fabrication of RFG requires complex technical conditions and processes. It may take a lot of time, but cannot work out. Moreover, the femtosecond laser with ultra-short pulse width and very strong instantaneous power can induce nonlinear multi-photon absorption of materials to modulate the refractive index of the fiber [59; 60]. An alternative approach of introducing strong backscattering, a segment of random reflectors artificially controlled, was inscribed in the fiber by using a femtosecond laser micromachining system. This process is less complicated than that of the fabrication of RFG, which can be realized by two methods. In detail, the first is that the pulse energies are from 0.32 to 0.36 mJ and the number of adjacent reflectors on the same position is less than 10. The second is that the pulse energies range from 0.32 to 0.4 mJ and the number of adjacent reflectors on the same position is less than 2 [61; 62]. As the number of random reflectors increases, the random feedback is enhanced, but together with a larger loss in the system. Therefore, an appropriate number of reflectors that should be optimized to obtain a better output beam of the RFLs. Even though the high power emission of the RFLs is restricted by the low pump source power [63; 64], many studies have been carried out to improve its experimentally [65]. Take the way of gain as an example. If only considered SRS and stimulated Brillouin scattering (SBS) in the fiber, it requires a long distance to compensate for the loss. Thus, advances in the change of gain medium have been carried out to improve the efficiency of the pump, such as the active gain. Moreover, the application of shorter fibers is preferable for high-efficiency generation, in which the power of the output beam is in the level of hundreds of watts. Optical conversion efficiency can be high up to 94%, which is close to the quantum limit (95%) [49]. There is no doubt that the power of the output emission cannot ever exceed the power of the pump in all approaches. The most direct and effective way is that optimizing the configuration of the power pump source. Later on, various studies on the pump scheme, including both symmetric and asymmetric configuration, are used to find the optimal structure. Increasing the power of one-side pumped RFLs or using a two-side pump source is considered, making the output power reach hundreds of watts [39]. However, temporal instability of the output lasing would happen, due to the self-pulsing property of the coherent pump source.