# Advances in Random Fiber Lasers and Their Sensing Application
## 3 Control of Output Characteristics
### Output Power  
High power RFLs are an important orientation in optical fields. The power of the RFL is only a few watts at first [47], but it becomes in kilowatts level after experiencing rapid progress in nearly a decade [99]. At first, the pump source that has high power and high-efficiency of light conversion is adopted to improve the output power of RFLs. Ytterbium-doped fiber laser (YDFL) is a decent choice for the pump source [100]. In 2011, 7.5 W YDFL was used to pump 10.7 km TWF for the first time, and only 3.8 W emission was obtained, the scheme of RFL pumped by YDFL is shown in Figure 9[47]. By 2016, a single-terminal RFL of 112 W output with 84.8% light conversion efficiency was observed, which used YDFL to pump a half-open-cavity RFL comprised of 320 m long passive fiber and an FBG [64]. Furthermore, using an incoherent pump to obtain high power RFL was proposed. In 2017, a hundred-watt linear polarization RFL was generated by using a broadband ASE source as the pump, and the second-order Stokes light that centered at the wavelength of 1178 nm reached the maximum power of 100.7 W. Moreover, the theoretical result suggested that the RFL with an incoherent pump could get 300 W linear polarization single-mode first-order Stokes light [67]. In 2019, the maximum power of the first-order Stokes light was up to 117 W, by optimizing the bandwidth of incoherent ASE pump source (the scheme of RFL is pumped by ASE source is depicted in Figure 10) [68]. However, the problems are the power of the incoherent pump is usually low, and amplifiers that result in the complexity of the RFL structure are required. An alternative approach to improve the output power is by optimizing the gain. Although Raman gain [101] is a considerable principle to realize high power RFL rather than the Brillouin gain, the main oscillator power amplifier (MOPA) [102], which has a much higher gain, is a typical way. The traditional MOPA system has a nonlinear spectral broadening phenomenon, and the line-width of output remains stable throughout the high power amplification process. However, it is of profound significance for further power scaling in the spectral domain to suppress the broadening of line-width, as the combination of spectral beams requires narrow line-width and high power laser. In 2015, a high power RFL with kilowatt-level was observed, which utilized the MOPA structure (Figure 11 shows the setup of the RFL seed and the amplifier.) [103]. In 2017, a high power linearly polarized RFL based on the structure of the MOPA was reported. Near-diffraction limited beam quality and high polarization of 93.9% were obtained under the maximum output power [104]. In 2019, random lasing emission with the highest power of 3.03 kW was achieved by using the structure of tandem-pumped MOPA [105]. Using the same medium of gain, [PERSON], [PERSON] et al. presented a monolithic high power RFL with a tandem-pump scheme and obtained 4.4 kW output power [99]. This is the ever report of the highest power RFL. It is worth noting that although the power of the RFL reaches up to kilowatts level in this approach, the structure is much more complicated. Table 1 summarizes some of the evolution of high power RFL utilizing these approaches. Figure 8: The scheme of all polarization-maintaining fiber (PMF) Raman RFL. Figure 11: The setup of the RFL seed and the amplifier. BPF, bandpass filter; P4, an extra port of WDM; RDFB, random distributed feedback; HR-FBG, high-reflectivity fiber Bragg grating; LD, 976 nm laser diode; PD, photo-detector; OSC, oscilloscope. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Year** & **Approach** & **Pump Power** & **Output Power** \\ \hline
2011 & YDFL & 7.5 W & 3.8 W \\
2014 & YDFL & 10 W & 7 W \\
2014 & YDFL & 98.6 W & 73.7 W \\
2015 & YDFL & 221.4 W & 193.5 W \\
2015 & MOPA & 1381.4 W & 1.03 kW \\
2017 & ASE source & 127 W & 104.8 W (second-order) \\
2019 & MOPA & 3.61 kW & 3.03 kW \\
2019 & YDFL & 1248 W & 985 W \\
2019 & MOPA & 4.343 kW & 4.02 kW \\ \hline \hline \end{tabular}
\end{table}