. The paper has interesting results that the community will appreciate because the single-frequency laser with the wavelength of 397.5 nm and perfect beam quality as well as the long lifetime is important resource for the available quantum internet. The grammar however needs to be improved before publication. For example:
1) “The optical coupler composed of two lens (f1 and f2) and a HWP (HWP2) not only ensures the mode-matching between the injected laser beam with the external enhancement ring cavity but also align the polarization of the injected fundamental-wave laser to satisfy the requirement of the frequency doubler” is clumsy.
2) In the paper, the author should distinguish “the frequency doubler” and “external enhancement-cavity”; for example, (EOM), which can be utilized to lock the frequency doubler to the injected fundamental-wave laser via the PDH method.
3) “In the experiment, the nonlinear conversion coefficient of the MgO:PPSLT crystal was firstly measured by;
4) “because the external enhancement cavity is difficult to locked on the top of its transmission peak”
2. The stability of the fundamental-wave laser directly influences that of the obtained 397.5 nm laser. The author should give the stability of the fundamental-wave laser.
3. The obtained is useful for the quantum internet. The intensity noise characteristic should be given in the paper.