Conventional fiber lasers generate lasing output through feedback between two mirrors at either end of the laser cavity. In contrast, random fiber lasers (RDFLs) utilize random distributed feedback enabled by Rayleigh backscattering, which allows for a simplified structure, no longitudinal modes, and emission at any wavelength through cascaded Raman processes. One of the main research topics of RDFLs is high-power operation. On the one hand, RDFL can serve as a robust seed for high-power fiber amplifiers, on the other hand, it can directly achieve high-power output. Since 2013, the output power of single-stage RDFLs has rapidly increased by implementing several techniques, which include replacing full-opened cavities with half-opened structures, shortening the length of passive fiber, increasing the mode field area, reducing the number of fiber modes (appropriately reducing the numerical aperture of the fiber core), and adopting a more temporally stable pump source. Thanks to these techniques, high power output of up to 1.57 kW have been achieved in 2021. However, the generation of high-power backward light has become a new obstacle limiting the power scaling of single-stage RDFLs.

Recently, Prof. Pu Zhou and his colleagues from National University of Defense Technology reported a 2-kW-level RDFL in Chinese Optics Letters, Volume 21, Issue 9, 2023 (Jun Ye, Yang Zhang, Junrui Liang, Xiaoya Ma, Jiangming Xu, Tianfu Yao, Jinyong Leng, Pu Zhou. 2 kW random fiber laser based on hybrid Yb-Raman gain [Invited][J]. Chinese Optics Letters, 2023, 21(9): 090004). By the aid of generalized nonlinear Schrödinger equations and steady-state rate equations, they clarified the physical mechanism of high-power backward light generation in traditional high-power RDFLs [see Fig. 1(a) for a typical experimental setup]. At kilowatt-level output power, the optical spectrum of the random lasing exhibits substantial broadening such that a portion of light could leak from the fiber Bragg grating (FBG) [see Fig. 1(b) for the simulated leaked optical spectrum]. Although the leaked power may seem very small when compared to the kilowatt-level output, it can be amplified to dozens of watts when injecting into the pump source's main amplifier [see Fig. 1(c)]. They further numerically verified that this issue is unavoidable in traditional high-power RDFLs, thus, optimizing the system structure is of utmost importance.

Fig. 1. (a) A typical structure of traditional high-power RDFLs. (b) Power amplification of backward leaked random lasing in pump source's main amplifier. (c) Optical spectrum of the leaked random lasing.

Given that the amplification of FBG leakage by the pump source is inevitable, Prof. Pu Zhou and his colleagues proposed incorporating the main amplifier of the pump source into the RDFL, as illustrated in Fig. 2(a). By moving the highly reflective FBG to before the main amplifier of the pump source, this structure combines Raman gain and doping ion gain and is referred to as a "RDFL based on hybrid gain". To investigate the power characteristics of hybrid-gain RDFLs, they established a theoretical model by considering stimulated Raman scattering and Rayleigh backscattering in the steady-state rate equations. Figure 2(b) shows the longitudinal power distribution at kilowatt-level output power, the power of the random lasing at the highly reflective FBG is approximately 16 W [see Fig. 2(c)], and the power of the FBG leakage is only 0.16 W, indicating that the hybrid-gain RDFL can effectively suppress the generation of high-power backward lasing.

Fig. 2. (a) Schematic of a RDFL based on hybrid Yb-Raman gain. (b) Longitudinal power distribution (F.w.: forword, B.w.: backward). (c) Zoom-in of the longitudinal power distribution near the 1130 nm FBG.

Based on the theoretical design, a hybrid-gain half-opened RDFL is constructed, as depicted in Fig. 3(a). Previous reports have shown that a temporally stable pump source is of great importance for high-performance RDFLs, therefore, a superfluorescent fiber source (SFS) with customizable optical spectrum is used as the Raman pump seed. The hybrid gain and random distributed feedback are provided by a 25-m-long 20/400 μm ytterbium-doped fiber and a 35-m-long 20/400 μm passive fiber. A backward pump structure is employed in the active-gain stage, which consists of 21 LDs with a maximum pump power of more than 3 kW. Figure 3(b) shows the output spectrum with the highest spectral purity. At the LD pump power of 2789 W, the spectral purity of the signal light reaches as high as 98.1%, which is the highest spectral purity among kilowatt-level RDFLs to the best of our knowledge. The power evolution of each spectral component is shown in Fig. 3(c). At the LD pump power of 2881 W, the maximum output power of the signal light reaches 1972 W with a corresponding conversion efficiency of 68.4%. Since the conversion efficiency reported here is relative to the LD pump power, it is higher than the conversion efficiency reported for previously developed kilowatt-level RDFLs. Moreover, at the highest output power, the backward leaked power is only 0.12 W.

Fig. 3. (a) Experimental setup of the RDFL based on hybrid Yb-Raman gain. (b) Output spectrum with the highest spectral purity. (c) Power evolutions of residual 1075 nm pump wave, forward 1st-order Stokes wave (random lasing) and 2nd-order Stokes wave.

Associate Prof. Jiangming Xu from this group believes that hybrid gain and SFS pumping is an effective way to realize high-power RDFLs with high efficiency and high spectral purity. Furthermore, he posits that this work could also serve as a valuable reference for high-power Raman fiber lasers and long-wavelength Yb-doped fiber lasers. In the follow-up researches, this group intends to further optimize the system parameters with the aim of achieving higher output power and higher spectral purity.