Femtosecond Green Light Generation Using a MgO-Doped Periodically Poled Lithium Niobate Crystal Pumped by a Yb-Doped Fiber Laser

Round 1

Reviewer 1 Report

The manuscript is rather solid. However, it does not bring any scientific or technological novelty. At least the novelty is not clearly stated. Mover, it suffers from some incompleteness and inaccuracy in presentation and interpretation of results. Therefore, I suggest addressing the issues listed below prior to final acceptance of the manuscript.

1. It is necessary to emphasize clearly what is the novelty in the results presented.
2. It is necessary to enrich the introduction by mentioning alternative methods for wavelength conversion (from IR to the green) of radiation from femtosecond all-fiber laser systems. For example, an approach based on harmonics generation in fiber-coupled nematic liquid crystals studied in https://doi.org/10.1070/QE2013v043n02ABEH015048
3. It is necessary to clarify the origin of the MgO:PPLN crystal. Was it a commercial sample? If yes, the trademark has to be indicated.
4. It is necessary to avoid confusing designations in Figs. 1 and 2. Actually the more appropriate common designation for laser source used to pump the MgO:PPLN crystal would be “MOPA” (Master Oscillator – Power Amplifier). In the current manuscript one can see in Fig. 1(b) that inside the structure called “Yb-doped fiber laser” there is another embedded laser which is called both as “oscillator” and as “seed laser”. That is very confusing. The correct and universal designation would be “MOPA” which consists of a “master oscillator” (instead of the “seed laser”) and a power amplifier (which has two stages). The “seed laser” is inappropriate term in the given context because normally it relates to injection seeding techniques.
5. Description and parameters of the master oscillator (which is indicated as “oscillator” in Fig. 1(b)) are almost totally missing in the manuscript. What was the pulse duration and average power of this oscillator?
6. What is the meaning of the designation “stretched fiber” in Fig. 1(b)? How it is stretched? In the text it is said to be a conventional “segment of 15-m-length PM single-mode fiber for spectral broadening”
7. The authors should validate those measurements of output power at each amplification stages were not affected by contribution from unabsorbed (residual) pump radiation.
8. It is not clear how the authors have calculated “the Fourier transform-limited duration of 515-nm pulse as 220 fs, shown in Fig. 4(d)”. What were the spectral shape and width assumed for this calculation? How those assumptions were justified? If the authors rely upon their assertion that “The spectrum roughly matches sinc² fitting curves” as shown in Fig. 4(a), then it is not clear how they derived the bell-like shape shown in Fig. 4(b). The Sinc² spectral shape has to correspond to a triangle temporal profile according to the Fourier transform.
9. There is a misprint in the reference to Fig. 4(b) in the following statement: “Furthermore, we calculate the Fourier transform-limited duration of 515-nm pulse as 220 fs, shown in Fig. 4(d).”

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper demonstrates  a green laser based on SHG in a MgO:PPLN, which was pumped by an all-PM Yb-fiber femtosecond laser. The results are technically interesting; however, the following points need to be clarified.

1. I noticed that Hong et al. recently demonstrated a picosecond green laser based on SHG in a MgO:PPLN, pumped by an Yb fiber picosecond laser [Ikjun Hong, et al., "532-nm second harmonic generation with enhanced efficiency using subharmonic cavity modulation-based quasi-Q-switched-mode-locked pulses," Opt. Express 28, 25431-25443 (2020)]. According to the work by Hong et al, it is straightforward to predict a SHG conversion efficiency more than 40 % in MgO:PPLN. I am wondering the reason why the authors claim that a 40% SHG efficiency in their laser is a big technical achievement, as mentioned in the introduction. The authors need to compare their work with the work by Hong et al.
2. The authors are advised to provide the measurements results on the output beam quality (M square value) at 515 nm.
3. The authors are advised to provide the autocorrelation trace of the output pulses at 515 nm.

Author Response

Please see the attatchment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors report on the realization of all PM Yb-doped fibre laser system which includes a MgO:PPLN SHG module.  A green laser is produced with 820-mW average power by a fundamental laser with 280-fs duration, 20-MHz repetition rate, and 2-W average power.

The realization of compact and low-cost all-fibre based femtosecond laser source is a field of growing interest with several application in material science, ultrafast spectroscopy, and multi-photon imaging.

However, the paper seems do not introduce significant novelty with respect the existing literature. Yb-based fs laser oscillator are also commercially available along with their SHG module. It is not clear if the main point of the work is the Yb-baser fibre laser of the efficient SHG process which is obtained with a commercial PPLN crystal.

Before any publication can be considered the author must address a few points:

1. Better introduce and motivate the novelty of their work with respect literature.
2. Describe with more experimental details the scheme of the Yb fibre oscillator.
3. List all the component of their laser system with part number and manufacturer to allow reproducibility of their work
4. Add details and characterization of the pulse-to-pulse laser intensity fluctuation and the long-term stability of the optical output.
5. Jitter is usually an issue in such systems. Can the authors comment on that?

Author Response

Please see the attatchment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The revised manuscript merits publication in Applied Sciences

Reviewer 2 Report

The revisions are acceptable.

Reviewer 3 Report

The authors have addressed all the point raised in my review. The paper can be accepted in the present form