# High Power All-Fiber Supercontinuum System Based on Graded-Index Multimode Fibers
## 3 Results and Discussion
Although the supercontinuum generated by the 5 m GRIN fiber showed an increase in the spectral intensity above 2000 nm in the spectrogram, this was due to the high-order diffraction of  
Figure 4: Spectral evolution at different input peak powers after 10 m GRIN fiber. Figure 5: Evolution of visible light with increasing input peak power after 10 m GRIN fiber. laser near 1 \(\upmu\)m in the spectrometer. This part of the spectrum should be ignored so that the range of spectrum was the shortest in the long waveband with a fiber length of 5 m. For the fiber length of 10 m and 80 m, the high laser intensity around 2000 nm can mask the effect of the high-order diffraction. Due to the characteristics of silicon-based fibers, in which the optical loss at wavelengths above 2200 nm was significant, the decreasing trend of the last segment of the spectrum with 80 m fiber length was more noticeable than the 10 m fiber length. The longer fiber also promoted the conversion in the visible waveband, and the intensity of the visible waveband is higher. The experimental results showed that the longer fiber length improved the flatness and spectral extension of the supercontinuum. The output power of the supercontinuum decreased with the increasing length of GRIN fiber. The maximum output supercontinuum average power was about 24.9 W, 22.3 W, and 15.2 W at the fiber length of 5 m, 10 m, and 80 m, respectively. This was mainly due to the high absorption loss of graded-index fiber for some wavelengths. While ensuring a high-quality supercontinuum output, the GRIN fiber length should be controlled to avoid power degradation. Figure 7 shows the efficiency curves of the pump power and supercontinuum power at the GRIN fiber length of 5 m. The overall conversion efficiency of the supercontinuum was about 82%, which was higher than that with photonic crystal fiber (about 50%), demonstrating the effectiveness of direct splicing of a GRIN fiber with the gain fiber. Continuous operation at the highest power for 4 h is realized, and the power instability is less than 1.8% with the spectrum and beam quality remaining basically unchanged. Figure 8 shows the evolution of the output beam profile (1030 nm) from the GRIN fiber during the increase of pumping power. At low peak powers, the spot showed a poor beam quality. During the increase of the peak power of the pump pulse from 10 kW to 90 kW, it was clearly observed that the beam profile tends to be Gaussian. The M\({}^{2}\) at the wavelength of 1030 nm was measured from 5.1 to 2.5 at the output power of 6 kW to 90 kW. Near-field beam profiles with wavelengths of 580, 760, 850, and 1030 nm were measured and shown in Figure 9. It can be seen that all beam distributions tend to be Gaussian-like at different wavelengths, although GRIN fiber supported multiple modes. The similar clean-up process was also observed for far-fields beam profiles (not shown here). This clean-up process was similar to the phenomenon observed by [PERSON] et al. [12]. Kerr self-cleaning (KSC) is caused by the coupling among the fundamental mode and high-order modes and could wash out the linear mode mixing and attain a cleaned self-sustained beam. Figure 6: Spectral evolution after different GRIN fiber length at 30 W input power. A simple spectroscopic device was devised to help obtain different wavelengths of light. As shown in Figure 10, the light from the supercontinuum source was emitted to the transmission grating through the collimating mirror. The diffracted light from the transmission grating was focused onto the focal plane by a focusing lens. An optical fiber with a core diameter of 500 \(\upmu\)m was placed on the focal plane to receive light at different wavelengths. This fiber can be moved on the focal plane to change the wavelength of light received, and the core diameter of the fiber can be changed to change the spectral width. With this simple spectroscopic device, the light of any wavelength in the supercontinuum can be obtained.