## 3 Results and Analysis
### Experimental Analysis of Hybrid EYDF Amplifier  
Figure 1 presents a schematic diagram of the hybrid EYDF amplifier. The power performance and spectra properties were experimentally analyzed. The power scaling of hybrid and uniform EYDF amplifiers was recorded by an optical power meter (Thorlabs, S425C-L and PM100D) and is plotted in Figure 4a. The output power increases linearly with the pump power. The hybrid EYDF amplifier with 30 mW seed can launch 4.33 W of output power under a 20 W pump, and the corresponding slope efficiency is 22.3%. However, the slope efficiency of the typical uniform SM-EYDF-6/125 fiber amplifier with the same seed and fiber length is 19.6%, which is less than that of the hybrid EYDF amplifier by 2.7%. The experimental output power and slope efficiency were less than the theoretical results. We consider that this difference was caused by the fiber fusion loss in the signal laser, and the leakage of pump light due to no coating at the fusion point. In addition, the mismatch between the signal wavelength and the transmission spectrum peak of the SMS structure will also lead to the decrement in efficiency. Unfortunately, we cannot further improve the output power because of the limitations of the experimental conditions. Nevertheless, compared with the uniform fiber amplifier, the hybrid gain fiber amplifier has advantages in regard to both output power and slope efficiency. One of the reasons is that the insertion of multimode fiber adds extra gain. Another is that the MMI effect of the SMS structure can suppress Yb-ASE and promote pump energy transfer, which was demonstrated by the simulation results and following spectral measurement. The output spectra of the hybrid and uniform fiber amplifiers at the maximum output power were recorded by an optical spectrum analyzer (Anritsu, MS9701C) and are plotted in Figure 4b. The Yb-ASE intensity of the hybrid EYDF amplifier was decreased by more than 12 dB compared with that of the uniform fiber amplifier. The spectral results illustrate that the SMS structure has a good effect on Yb-ASE suppression and has the potential to alleviate the bottleneck effect in EYDF.  
Figure 4: (**a**) The power scaling of the hybrid and uniform EYDF amplifier. (**b**) The output spectra of the hybrid and uniform fiber amplifier at the maximum output power in the 900 to 1700 nm region.  
The zoomed spectra in the range of 1000 to 1100 nm are plotted in Figure 5a to analyze the detail of the Yb-ASE. The output spectrum of the hybrid fiber was obviously modulated. The maximum Yb-ASE suppression occurs at 1060.2 nm where the suppression intensity was 15.2 dB. However, the zoomed spectra in the 1530 to 1560 nm region, which is plotted in Figure 5b, demonstrate that this hybrid fiber amplifier cannot suppress the Er-ASE. The Er-ASE of the hybrid fiber amplifier was higher than that of the uniform fiber amplifier by 2-3 dB. These experimental results can be seen clearly in the insertion in Figure 5b, which is the zoomed spectra around 1535 nm with a 7 nm range. The Yb-ASE suppression promotes the pump power transfer and the pump leakage from the non-coated bare fiber can improve the gain in the 1.5-\(\upmu\)m band. At the same time, the multimode fiber length variation caused by the instability in the fiber temperature may increase the signal light loss. A signal with insufficient power cannot effectively extract the upper-level population of Er\({}^{3+}\), which will also increase the Er-ASE intensity. High quality coating treatment at the fusion point and accurate fiber temperature control may be a possible way to avoid this phenomenon.