## 1 Introduction
The multimode interference (MMI) effects in the multimode fiber segment causes output spectral modulation of the SMS structure. The transverse modes excited in the multimode fiber segment are coherently superimposed at the fusion point of the output end single-mode fiber. Due to this MMI effect, the number of excited transverse modes, which is related to the fiber core diameter, determines the coherence intensity, and then affects the modulation depth of the output spectrum [20]. In step-index fiber, the normalized frequency \(V\) is inversely proportional to the wavelength \(\lambda\) and directly proportional to the mode number \(N\)[21]. The laser with a shorter wavelength can excite more transverse modes in the same core diameter fiber. It can be inferred that the laser with a shorter wavelength can obtain greater modulation depth in a specific SMS structure. Specifically, in EYDF amplifiers, the Yb-ASE with a wavelength near 1 \(\upmu\)m can result in more transmission loss than the Er-ASE with a wavelength range of 1500-1600 nm, when propagating through an SMS structure. This provides a possible way to suppress Yb-ASE in EYDF amplifiers by using an SMS structure. Although the SMS fiber structure made with passive fiber can be used as a bandpass filter [21; 22], the increment in fiber length caused by insertion in the structure is disadvantageous to the fiber amplifier as it suppresses the nonlinear effect, especially the SBS. In our previous work, two fibers with different core diameters were spliced together to suppress reverse propagating light [23]. If the SMS structure can be made from commercial gain fiber with different core diameters, it could filter the ASE while providing gain without increasing the total length of the amplifier. However, there are two limitations to the usage of SMS structures based on active fiber. One reason is that the core size of commercial double cladding gain fiber is too small to obtain significant MMI effects at their working wavelength. The other is that the fusion point on the gain fiber will reduce the thermal stability of the amplifier. Broadening the working wavelength range or considering other light with a shorter wavelength could obtain sufficient modes to achieve an obvious MMI effect, while the second problem can be reduced by fiber thermal management. In this work, we attempted to make an SMS structure by using commercial EYDF with different core diameters, and analyzed its properties in an amplifier, not only at the signal but also the Yb-ASE band. A single-frequency hybrid erbium-ytterbium co-doped fiber amplifier with an SMS structure is proposed. Two kinds of EYDFs with core diameters of 6 and 12 \(\upmu\)m were spliced in sequence to form a hybrid gain fiber with an SMS structure. The output performance of the hybrid EYDF amplifier was numerically and experimentally analyzed. The length of multimode fiber in the SMS structure was preliminarily optimized by simulation. The experimental results demonstrated that compared with a typical uniform EYDF amplifier, the hybrid EYDF amplifier with the same fiber length can suppress the Yb-ASE by over 12 dB without any reduction in output power. The efficiency of the amplifier can be improved by 2.7% because of Yb-ASE suppression and the additional gain provided by the SMS active fiber structure. This provides a monolithic structure for the EYDF amplifier to overcome the pump bottleneck effect and achieves high efficiency and high-power output.