First Demonstration and Comparison of 5 kW Monolithic Fiber Laser Oscillator Pumped by 915 nm and 981 nm LDs

Round 1

Reviewer 1 Report

The article was well motivated i.e. to mitigate TMI without compromising beam quality.  The method used was sound i.e. 981 nm wavelength to reduce both the quantum defect and the thermal load on the fiber since longer fiber would be needed due to reduced cross-section.  Looks like 915 nm pumping worked as well but the efficiency was lower due to higher quantum defect.  The results were convincing and the power level achieved was also very admirable.  It would have been good to see the limits of this type of pumping scheme – perhaps it was pump limited.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

I consider that the MS under review presents novel, interesting, and potentially useful for practice results in the field of high-power Ytterbium fiber lasers (YDFL); so I strongly recommend its publication in PHOTONICS.

The MS clearly demonstrates an advantage of using laser diodes at wavelength 981 nm as pump sources for YDFL over those at 915 nm (as directly compared experimentally in this MS) as well as 'standard' ones at 976 nm (as shown from an analysis of the correspondent literature on the matter). It is a real novelty. After careful reading of the MS, I can reveal that the set of experimental data provided is comprehensive (all the basic characteristics of the new YDFL assembled in its two versions where the different pump wavelengths are used) and reliable. The MS is organized logically, from the Introduction to the Conclusions.

There are only two (but minor) problems of this work: (1) poor English grammar (the reader encounters many mistakes throughout the text: see the examples below) and (2) some technical negligences in the part dedicated to modelling of the YDFL (see my questions below, too). Meanwhile, I believe that authors could easily address my criticism in their revised MS. 

1) Examples of poor grammar to be corrected by authors:

(a) "...915 nm pumped fiber laser oscillator LD with the output power..." (line 41); 

(b) "...to be studied in detailed." (lines 49-50);

(c) "...and obtain a good the beam quality..." (lines 85-86);

(d) "...The laser excited in the cavity is output from a quartz beam head." (lines 87-88);

(e) "...little decrease in of efficiency..." (line 135);

(f) "... the Raman light intensity variety." (line 153);

(g) "...doped icon..." (line 180);

(h) "...absorption of pump power is more sufficiently..." (line 188);

(i) "...the laser intensity is not only decided by the output power." (line 215);

(j) "And there is an effective way to reflect both output power and beam quality of a fiber laser, that is the brightness. " (lines 216-217);

(k) "...and revealed excessive stability during the test that power fluctuations were less than 1%." (lines 253-254).

...And there are many other lapses throughout the text.  

2) Questions regarding the modelling:

(a) Eq. (1): It is written for signal waves but (after the equation) there appear sigma for absorption and emission cross-section for pump waves (indices "p"); a similar lapse is for loss factor (in Eq. (1) it applies for signal waves but below it it is said "pump loss". Furthermore: what does the index "m" mean in Eq. (1)?

(b) What does the index "n" mean in Eq. (2)?

(c) The majority of the quantities entering Eqs. (1 - (3) are not defined in the text. Given that Ref. [18] to which authors refer to when presenting these equations is almost inaccessible, the errors / lapses exemplified by me above look bad. 

(d) It is not clear which are the values of the quantities / parameters entering Eqs. (1)-(3) that were used by authors in their modelling. I recommend them to add a Table where each quantity / parameter ought to be specified; otherwise, their results are merely not reproducible.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript, the authors studied several parameters of the output beam of a fiber laser pumped by either a 915nm or a 981nm laser diode (LD), aiming to find the one that could optimize both the transverse mode instability (TMI) and stimulated Raman scattering (SRS) suppression. This study shows the laser oscillator has better performance while pumped by a 981 nm LD. The methodology, the experiments set up and the simulation seem to be solid and well elaborated. However, the novelties are not sufficient or not well elaborated,

1.       The title “Comparative study of 5 kW monolithic fiber laser oscillator pumped by 915 nm and 981 nm LDs”, does not reflect any novelty and it’s difficult to catch attention.

2.       The abstract summarized the results from the comparative study of 915nm-pump and 981nm-pum, however lacks the statements of the novelty in this manuscript.

3.       Regarding the novelty, the authors claim that “this is the first report of a more than 5 kW all-fiber laser with a beam quality M2 less than 1.4 and a detail description of all the design of the laser, including fiber parameter, pump source, and so on” (lines 72-74).  However, Ref. 6 reports a 5-kw single stage all fiber laser with M2 of 1.3 in 2018. Hence this statement conflicts with the fact.

4.       Regarding the advantages of the 981nm-pump, the comparisons of pumping methods and optimization of the pump wavelength to 976nm have been published in Ref. 17 by the same group.  Later, the same group published another paper that established the idea to further optimize the pump wavelength to 981 nm (Ref. 15), and these advantages for TMI and SRS suppression are already discussed in Ref. 15.

5.       Line 84, the authors gave the cladding pump absorption at 915 nm, what about at 981nm?

6.       Line 85 should be “In order to control the high order mode excitation and obtain a good the beam quality

7.       Line 93, what are the sources/manufacturers and part number details for the commercial parts?

Author Response

We deeply appreciate the time and effort you spent in reviewing our manuscript. Your comments and suggestions are very valuable and helpful for improving our research paper. According to your valuable comments, we have revised all the relevant parts in the manuscript carefully. The revisions are marked out in the revised manuscript. The responses to the comments are listed one by one. Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The reviewer thanks the authors for detailed explanations and clarifications. The manuscript has been improved significantly. Only a typo needs to correct: the table in line 241 should be “Table 2 “.