Random fiber lasers (RFLs) utilize the distributed feedback in optical fibers to form resonant cavities of random lengths. They combine the low coherence of random lasers with the high brightness of fiber lasers. Therefore, RFLs have been widely used in fields such as environmental sensing and optical communications.

Because all-fiber-integrated RFLs were proposed using Rayleigh backscattering as a feedback mechanism, RFLs have attracted significant attention in the power scaling and wavelength extension of fiber lasers. Rayleigh backscattering occurs in silicon fibers owing to disordered fluctuations in the refractive index, which provides randomly distributed feedback for RFLs. Because the output of an RFL is the sum of the resonant cavities with random lengths, the RFL is free of the self-pulsing effect, unlike traditional fiber laser generated in a fixed cavity. Amplifying the RFL through the master oscillator power amplification (MOPA) configuration can suppress spectral broadening because no peak power from the RFL seed is enhanced during this process. Control experiments were conducted in a 10 kW-level MOPA system by adopting an RFL and a fiber laser with a fixed cavity as the seed. The results demonstrate a suppression effect.

Rayleigh backscattering provides broadband reflection in optical fibers, which can replace the reflection component in wavelength-tunable fiber-laser systems and supercontinuum (SC) sources. Therefore, RFLs can easily achieve wavelength tunability using a single-frequency selection component. By taking advantage of cascaded Raman scattering, the operating wavelength of the RFL can cover the transmission band of the silicon fiber. Research on 1.1‒2.0 μm RFLs proves the flexibility of their output wavelength. In addition, the modulation instability provides a wide band gain for light waves near the zero-dispersion wavelength of the optical fiber. This type of light wave can be generated by cascaded Raman scattering in the RFLs. In addition, the nonlinear effect is enhanced by Rayleigh backscattering because it increases the effective length. Thus, RFLs are an excellent choice for SC sources. The feedback provided by the optical fiber itself, without extra optical components, promises great capability for high-power handling; therefore, RFLs can be a good platform for high-power SC generation.

In the second section, generation methods for high-power RFLs are introduced and hundred-watt- to kilowatt-level RFL oscillators are summarized (Fig. 2). Furthermore, MOPA configuration seeding by RFLs is introduced to further scale their power (Fig. 4, Table 1). The suppression of stimulated Raman scattering (SRS) contributes to the 10 kW-level amplification of the RFL. In the third section, the flexibility of the operating wavelength enabled by the RFL configuration is reviewed and the wavelength-tunable RFLs gained by rare-earth ions are summarized (Fig. 6). The broadband feedback of Rayleigh backscattering simplifies the structure for wavelength tuning. Taking advantage of cascaded Raman scattering, random Raman fiber lasers (RRFLs) are introduced (Fig. 7). An amplification configuration adopting a hybrid gain of ytterbium ions and Raman scattering is used to achieve a high-power RRFL (Fig. 8). In the fourth section, SC generation in optical fibers using an RFL is reviewed. Both half-open-cavity (Fig. 9) and full-open-cavity RFLs (Fig. 10) can be utilized to realize an SC output whose spectral range can cover the transmission band of the silicon fiber. By combining Rayleigh backscattering and continuous-wave pumping, the average power of the SC generation in the RFL is scaled from the hundred-watt to 3 kW level (Fig. 11, Table 2). In the fifth section, practical applications of RFLs are discussed. Owing to their high brightness and low coherence, RFLs enable speckle-free imaging and are compatible with fiber-integrated imaging systems (Figs. 12 and 13).

RFLs pave the way for power scaling and wavelength extension of high-performance fiber lasers. Their temporal stability contributes to the suppression of spectral broadening during power amplification. The broadband feedback of Rayleigh backscattering and the gain of the cascaded Raman effect make them suitable for the wavelength extension of fiber lasers. Rayleigh backscattering not only simplifies the structure to achieve fiber lasers operating in a broad spectral range but also improves the power-handling ability of the reflection component. Because of their high brightness and low coherence, RFLs have been widely used in fields such as imaging through fibers and inertia-confinement fusion.