# Advances in Random Fiber Lasers and Their Sensing Application
## 1 Introduction  
'Laser' is an abbreviation of 'light amplification by stimulation emission of radiation' [1]. This originates from an atom or molecule that can produce stimulated emission of photons [2; 3]. In the 1950s, [PERSON] et al. realized microwave amplification by stimulation emission of radiation (MASER), based on the theory by [PERSON] [4; 5; 6]. In 1960, stimulated optical radiation in ruby was achieved by [PERSON] et al. [7]--this indicated that the era of the laser had arrived. A conventional laser scheme [8] comprises three key elements: A pump, an active medium, and a resonator. The pump is used to provide power for the population inversion of the active medium, and the active medium is utilized to offer optical amplification to compensate for losses. There are two functions for the resonator: The first is the mode selection, which means only a single or a few longitudinal modes can oscillate to improve the coherence of lasers; and the second is that the resonator offers the feedback of the longitudinal mode. Lasing can be generated when total gain overcomes total loss in the cavity. Therefore, the spectral and temporal performances of conventional lasers are determined by the active medium and the resonator. On the contrary, random lasers (RLs) [9; 10; 11] have no well-defined mirror configuration like the Fabry-Perot (F-P) cavity [12], where feedback depends on multiple scattering. The concept of self-generation of light in a random scattering medium without cavity was introduced by [PERSON] in 1968 [13; 14]. It ignited the interest in the fascinating field of random lasing [15; 16]. In 1994, [PERSON] et al. introduced laser action in strongly scattering media for the first time [17]. In 1995, the discussion of [PERSON]'s experiment [17] showed that random lasing has developed intoan exciting area of research [18; 19]. Different types of materials, such as powders, liquid crystals, quantum dots, biological tissues, and polymers, have been used to realize random lasing [20; 21; 22; 23; 24]. The spectral and temporal characteristics of RLs are defined by the randomly embedded localized spatial modes that can coexist with non-localized extended modes [25]. Usually, RLs have simple structures and complex emission properties. Therefore, RLs have gotten considerable attention in the last two decades. They have potential applications in speckle-free imaging, holography, and biology. Especially in the sensing area, we can tune the interplay of random media and cavity to optimize sensing applications [26; 27]. Distributed feedback (DFB) lasers were first represented in 1971, which were referred to as mirrorless by authors [28]. Random DFB lasers utilize random Bragg gratings as feedback medium [29], while RLs utilize various disorder medium, including random Bragg gratings for feedback. However, the directionality and efficiency of emission are challenges in the development of RLs' applications. Since optical fiber can transversely confine light in the fiber core, it is extensively used to improve RLs' performance. [PERSON] et al. [30] filled rhodamine 6G solution doped with TiO\({}_{2}\) nano-particles in the hollow photonic crystal fiber (PCF) core and realized the first RFL in 2007, in which the realization of random lasing combined optical fiber waveguide and random distributed nano-particles. This opens the door of a novel kind of RL. In 2010, an alternative approach that utilized the intrinsic disorder to obtain random feedback was proposed by [PERSON] et al. [31], in which Rayleigh scattering and stimulated Raman scattering (SRS) provide feedback and gain, respectively. After that, RFLs have gotten much more development in optical science and technology [32; 33; 34; 35; 36; 37; 38; 39], such as the physical mechanism of the RFLs [40; 41], optical communication and optical sensing [42; 43; 44; 45; 46], speckle-free imaging [47], and high power [48; 49; 50; 51; 52; 53].