## 1 Introduction  
Mid-infrared sources around 3-5 \(\upmu\)m have been widely used in many fields, such as environment monitoring, medical diagnosis and Terahertz (THz) generation [1, 2, 3, 4]. Recently, frequency conversion of single frequency (SF) lasers has attracted great attention [5, 6, 7]. Two main methods have been used to achieve frequency down-conversion of SF lasers. The first is to utilize SF laser as the pump source of optical parametric oscillators (OPOs). OPOs are well-established mid-infrared sources, offering high power, tunable wavelength and broad bandwidth. However, OPOs require high power pump sources, and it is difficult for SF lasers to achieve high power output directly. The master oscillator power amplifier (MOPA) structure is an effective way to enhance power, but the system is relatively complicated, which includes a multi-amplifier stage. The fiber nonlinear effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and four-wave mixing are also easily induced during the amplification process. Besides, unexpected seed instability and back reflection from the output end may damage the MOPA system. In 2011, [PERSON] et al. reported a mid-infrared singly-resonant OPO pumped by an SF pump source which was a MOPA system [8]. When the pump power was 52.8 W, a 7.2 W idler power at 3.4 \(\upmu\)m was achieved, indicating a conversion efficiency of 13.6%. In 2018, [PERSON] et al. demonstrated a singly-resonant OPO which was pumped by an SF Yb-fiber laser [5]. When the pump power was 10.6 W, a 1.6 W idler power and the highest efficiency of 15% were obtained. However, a further increase in pump power led to a decrease in idler power, which limited the further improvement of conversion efficiency. Another method is to use the difference frequency generation (DFG) process [6], specifically, to mix the SF pump source around 1 \(\upmu\)m and the signal source around 1.5 \(\upmu\)m. DFG is a single-pass process with a relatively low efficiency. This method not only requires a high power SF pump source, but also a signal source with high enough power to realize the frequency down-conversion of the SF pump source. In 2018, [PERSON] et al. realized mid-infrared output using DFG between the SF pump source and signal source [6]. Both sources were multi-stage amplified, which increased the damage risk and the complexity of the system. Meanwhile, the conversion efficiency was relatively low, where a 62.4 mW idler power was obtained under 23.2 W pump power and 4.31 W signal power. Narrow linewidth mid-infrared sources have great potential in environment monitoring, gas trace sensing and medical diagnosis [9; 10; 11]. The common method to achieve narrow linewidth mid-infrared radiation is to add etalons in the OPO cavity. In 2003, [PERSON] et al. demonstrated a continuous-wave (CW) singly resonant SF mid-infrared OPO which was pumped by a CW SF Neodymium-doped Yttrium Aluminium Garnet (Nd:YAG) source [12]. A solid-state etalon was inserted in the resonant cavity to achieve a narrow linewidth output. However, it was found that the inserted etalon caused a reduction in idler power by approximately 50%. In 2018, [PERSON] et al. demonstrated a narrow linewidth mid-infrared output based on OPO [7]. When adding an etalon to the cavity, the idler power was reduced from 1.3 W to 1.1 W. Undoubtedly, adding etalons in the cavity will cause additional losses and reduce the efficiency. Our research group has made some achievements in utilizing a high-power intracavity signal beam to achieve mid-infrared output. In 2018, our group realized a tunable dual-wavelength mid-infrared output by using the 1060 and 1070-1090 nm tunable fiber laser as the pump source [13]. By utilizing intracavity DFG, the 1070-1090 nm tunable fiber laser, whose power was insufficient to build the OPO process, was successfully converted to the mid-infrared region. However, the phase-matching conditions were not satisfied in the experiment, resulting in a relatively low conversion efficiency.