Influence of Gain Saturation Effect on Transverse Mode Instability Considering Four-Wave Mixing
Round 1
Reviewer 1 Report
1. Present the manuscript in the format of the j. of photonics, such as i) Introduction ii) Materials and Methods iii)Results and Discussions iv) Conclusions, References etc.
2. As such it is not in the format except introduction, conclusions and references
3. Check the equation (1), it seems not correct. Also give reason, why did you choose first order form instead of second order of Helmholtz equation.
4. You have lot of results but weak in providing scientific/physical concept.
5. In Ref. 30 provide details/publisher and bold the years in references: 19, 20, 30, 39 etc.
Author Response
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Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper describes the theoretical investigation on the transverse mode instability (TMI) effect based on stimulated thermal Rayleigh scattering (STRS), together with the consideration of four wave mixing (FWM). The connection between the FWM threshold and TMI threshold was studied by analyzing the mode gain characteristics of a fiber amplifier. In addition, the impact of gain saturation on the maximum TMI threshold power of fiber amplifiers were investigated. The authors also considered other fiber laser system parameters such as seed power, pumping direction, and core numerical aperture (NA), which were previously not considered in previous perturbation theory models. I believe this work would be beneficial as it provides further understanding of the TMI effect when scaling up the power of high brightness lasers. However, there are some comments that the authors should address;
1. Page 2 Line 54 to 56: The authors considered only two modes (LP01 and LPmn) for their model. This probably could be accepted, except for fibers with very large mode areas (LMAs). Could the authors provide the maximum dimensions of LMA fibers that this assumption is valid?
2. The authors investigate the influence of seed power on TMI threshold in forward- and backward-pumped fiber amplifiers. It would be a great addition if the authors could investigate the same influence for a bi-directionally pumped fiber amplifier.
3. Page 4 Line 121: What if a longer fiber length is considered? Wouldn’t a longer fiber length induce a stronger FWM effect?
4. Figure 1 (a): Does Z refer to fiber length? This wasn’t clearly defined in the manuscript.
5. Page 1 Line 30 to 31: “Compared with the above models, the perturbation theory model proposed by Michalis can not only obtain similar results as the TMI static model based on STRS [19], …”. I believe it would be more accurate to use the last name of the author, i.e. Zervas et al.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
In this paper, authors investigate the transverse mode instability in fiber amplifier by considering the four-wave mixing and gain saturation. However, I cannot consider it to be published before the authors well address my concerns.
1. Can the Eq (21) be applied through the total length? How much is the input signal at most that can be seen as the small input?
2. By considering gain saturation, TMI predicted by this model is extremely higher than other results reported when the core diameter is small. Is there any reported experimental data supporting that?
3. The figure 4 and figure 5 are confusing. Does the black curve represent TMI threshold instead of output power?
4. Can authors compare the result solved by numerical simulation to that of the analytical expression when the pump is backward and input signal is small? I want to see the difference.
Author Response
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Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
Authors well addressed all my concerns. I would like to recommend it to be published.
