In this paper, the ytterbium-doped tapered fiber with core/inner cladding diameter varying from 31/250 μm to 62/500 μm was designed and prepared by the improved chemical vapor deposition and solution doping technology. An all-fiber nanosecond pulse amplifier was built based on the ytterbium-doped tapered fiber, and the influence of the longitudinal structure on the output characteristics of nanosecond pulsed laser was investigated. A nanosecond pulsed laser output with a central wavelength of 1064 nm, an average power of 832 W, a single pulse energy of 8.32 mJ and a peak power of 24.8 kW at a repetition rape of 100 kHz was achieved based on the ytterbium-doped tapered fiber with a large diameter uniform region length ratio of 62.5%. Compared with 50/400 μm uniform fiber, the ytterbium-doped tapered fiber showed obvious suppression effect on stimulated Raman scattering and beam degradation at a similar output power.
Yb-doped fiber (YDF) lasers and amplifiers have been widely employed to industrial processing, medical treatment, 3D printing, and military defense owing to the high conversion efficiency, efficient heat dissipation, compactness, and good beam quality [1-3] . However, their outstanding evolution is interrupted by stimulated Raman scattering (SRS) [4] and transverse mode instability (TMI) [5] effects. It is effective to suppress both of them by concentrating on the large mode area (LMA) YDF design. Specially designed fibers, such as constant-cladding tapered-core fiber [6] , spindle-shaped fiber [7] , low-NA fiber [8] , and gain-tailored fiber [9] , have been fabricated and demonstrated to suppress those nonlinear effects (NLEs) with effect. Hereon, we proposed a novel low-numerical-aperture (NA) confined-doped long-tapered fiber, which was fabricated by a modified chemical vapor deposition (MCVD) process combined with solution doping technique (SDT). As described in Fig. 1 (a), the core and inner cladding diameter were 25μm and 400 μm at both ends and 37.5 µm and 600 μm in the middle, respectively. The numerical aperture (NA) of fiber core was ~0.05, as shown in Fig. 1(b). Additionally, the gain dopant was restrictedly distributed in smaller area in fiber core, whose diameter ratio was 70%.
A high power short-cavity random fiber laser employing the gain mechanism of the Yb-doped fiber and the half-open cavity structure and the temporal optical rogue waves (RWs) behavior are observed and investigated in the paper. The record output power without the stimulated Raman scattering (SRS) is promoted to 26.6 W in the YDRFL with the GDF length of 120 m. The stochastic pulses and temporal optical RWs are observed and demonstrated in the short cavity YDRFL for the first time. It is found that the proportion of RWs depends on the GDF length which can also affect the stability of output lasing. The research results reveal that achieving the relative stable output power requires the greater pump power for the shorter GDF length, although decreasing the GDF length will promote the maximum output power of the YDRFL without the SRS.
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