# High Power All-Fiber Supercontinuum System Based on Graded-Index Multimode Fibers
## 3 Results and Discussion  
The spectral evolution of the signal light from the oscillator to passing through the picosecond amplifier is shown in Figure 2a. The spectral width of the seed was reduced from the initial 25 nm to 18 nm after the three amplifiers, which was mainly due to the gain narrowing effect. The output power of the last amplifier stage and the pump power is shown in Figure 2b. We obtained a picosecond pulse with the maximum average output power of about 30 W. The amplification slope efficiency was 75% at the beginning. At the last amplification, there was nonlinear amplification with slight stimulated Raman scattering effect when the output power was higher than 27.5 W, which led to a decrease in the amplification efficiency. The amplified pulse width increased to 34 ps at 27.5 W average output power, corresponding to a pulse peak power of about 90 kW. The spectrum of the supercontinuum was detected by three optical spectrum analyzers (OSAs) covering the spectral range from 200 to 950 nm (Maya 2000 Pro, Dunedin, FL, USA), 950 to 1500 nm (YOKOGAWA AQ6370D, Tokyo, Japan), and 1500 to 2400 nm (YOKOGAWA AQ6375, Tokyo, Japan), respectively. Figure 3 shows the spectrum of the supercontinuum after the 10 m GRIN fiber. Compared to the signal light, the spectrum in the GRIN fiber shows a significant broadening, both in the long and short wavebands. The supercontinuum extended from 480 nm to 2440 nm. The spectrum showed good flatness in the 1000 nm range centered on the signal light wavelength. Due to the broad spectrum of the signal light, a flatter evolution of the supercontinuum was promoted. The generated supercontinuum in the 10 m GRIN fiber with different input powers is presented in Figure 4. When injected by the low peak power pump pulses, the spectral broadening was mainly caused by stimulated Raman scattering effect. Several intense Stokes peaks can be observed in the spectrum. Broad spectral emerges when the SRS extended to the zero-dispersion wavelength (~1310 nm) of the fiber. As the input power increased, the generation of shorter wavelengths begun to appear. The generation of visible wavelengths was mainly caused by the generation of dispersion waves [17]. At the same time, four-wave mixing effect further contributed to the intensity and flatness of the visible spectrum. At the peak pump power of 90 kW, long and short waveband spectra were significantly enhanced. The increase of the peak power promoted spectral broadening and flatness enhancement. \begin{table}
\begin{tabular}{c c} \hline
**Components** & **Parameter** \\ \hline Seed & 1030 nm, 8.27 MHz, 23 ps, 25 nm \\ Amplifier & 0.5 m PM-HI-YSF; 1 W, 976 nm LD \\ Amplifier 1 & 1 m PM-10/125-YDF; 5 W, 976 nm LD \\ Amplifier 2 & 1.3 m PM-30/250-YDF; 2 \(\times\) 25 W, 976 nm LD \\ GRIN fiber & 5 m/10 m/80 m 50/125 \(\upmu\)m GRIN fiber (Corning, ClearCurve OM4) \\ \hline \end{tabular}
\end{table} 
Table 1: The parameters of components in the system. Figure 3: The spectrum of the supercontinuum after 10 m GRIN fiber of output power 22.3 W.  
Figure 2: **(a)** Spectral evolution in each amplifier stage. (**b**) Output power versus pump power of the amplifier. Figure 5 shows the evolution of the visible part of the supercontinuum (obtained by transmission grating spectroscopy) when the peak power of the pulses was increased. At a peak power of roughly 40 kW, the visible light was observed in the observation screen after the spectroscopy, and initially appeared as distinct peaks near green 530 nm, yellow 650 nm, and red 730 nm. Subsequently, with these peaks as centers, the brightness increased, the spectrum continuity rose, and finally, at the peak power of 90 kW, a continuously visible spectrum was formed. Figure 6 shows supercontinuum obtained at different lengths of 5 m, 10 m, and 80 m of the GRIN fiber. The 80 m GRIN fiber generated the flattest spectrum, and the 5 m GRIN fiber generated the worst flatness of the spectrum. As the fiber length increased, the spectral intensity rose in the visible waveband and long waveband beyond 1500 nm.