High-Efficiency Hemispherical Short-Cavity Continuous-Wave Yb:YAG Laser by High-Intensity Pumping

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.There are too many Keywords. The author should sort them by importance and cut out some unimportant keywords.

 

2.In the experimental section, the efficiency starts to saturate when the pump power approaches its maximum. Since saturation effects are not included in the theory, so would it be possible to incorporate these effects to better match experimental data and enhance the accuracy of the model?

 

3.You performed experiments with two different pump wavelengths (940 nm and 968 nm). Could future studies investigate the long-term performance differences between these two pump wavelengths? For example, could efficiency decay or other performance changes be observed over time at one of the wavelengths?

 

4.This work demonstrates the potential of high-intensity pumping for improving Yb: YAG laser efficiency. Do you have plans for future research to study the long-term stability and durability of lasers under high-intensity pumping? For example, could optical components experience damage due to high pump intensity, or should new optical materials be considered to enhance stability?

 

5. There are too many repetitions between this manuscript and the References [17], so it is suggested to make some deletions to this manuscript.

Comments on the Quality of English Language

It is suggested to improve the readability of English language.

Author Response

Dear Sir,

Please see the attachment.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

In the manuscript, It is first demonstrated theoretically that, for a Yb:YAG laser with a simple structure, the optical-to-optical conversion efficiency's dependence on the minimum laser spot radii decreases significantly as pump intensity and efficiency increase. The manuscript provides a well-written introduction and effectively summarizes the experimental and theoretical results, making it a valuable contribution to the field. I recommend its publication with minor revisions.

1.      The direct pump does not follow a linear trend beyond a certain pump power. This is particularly evident in Figure 4(c-e). I recommend adding a detailed explanation to clarify the underlying reasons for this behavior, as it is critical to understanding the observed phenomena.

2.      Visualization Improvements: For better visualization and understanding, I suggest changing the color schemes in the diagrams, particularly in Figure 2 and Figure 3. Improved contrast and clarity would enhance the reader’s experience and comprehension.

Author Response

Dear Sir, 

Please see the attachment. 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The authors present a cost effective and simple method of increasing the efficiency of a Yb:YAG laser with hemispherical cavity through a systematic experimental study of effects of high-intensity pumping on the laser efficiency. The experimental procedure was carried out according to and supported by a sound theoretical analysis of the pupmping schemes and the various parameters related to the efficiency coefficients necessary to reveal the level of pumping intensitites and pumping configurations that would yield the highest input-output laser efficiency. The paper is well structured and the theoretical discussions are clearly discussed and the experimental methods and results are also systematically presented. The article is recommeded for publication to the Photonics Journal after some minor issues as follows:

1. From Figure 4, it shows that beyond ~600mW of incident pump power for various transmitances, the output laser power keeps increasing with pump level pumping while the output laser power starts to taper off when using direct pumping. It is apparent that pump level pumping gives a better performance, however there is a larger output wavelegnth shift.  What other trade offs are there to using the pump level pumping?

2. In Figures 5 and 6, there should be a legend placed as an inset in the graphs to label the theoretical (solid line) and the experimental (open shapes). These distictions have been mentioned in the figure captions but must be clearly indicated in the graphs also.

3. The manuscript needs a minor spelling and grammar and language usage check. 

After these minor issues have been addressed, this article is ready for publication in Photonics Journal.

 

Author Response

Dear Sir,

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

Dear author,

Laser modeling has advanced to a point where parameters such as cavity, absorption length, doping level, cooling, temperature, and others can be optimized in advance. However, this approach has not been implemented in the present case. The theoretical modeling presented in the article is inadequate to explain the measured results, as the author has acknowledged. The plots in Figures 2 and 3 are largely superfluous due to the absence of discernible differences in both the horizontal and vertical axes. In conclusion, the text is replete with extraneous content, failing to adequately address the salient aspects of the pump wavelengths. Consequently, this article is, regrettably, of negligible relevance to its intended audience. That is why I voted for reject it for publication in the journal Photonics.   

Comments on the Quality of English Language

The article is challenging to read due to excessive repetition and paraphrasing. 

Author Response

Dear Sir,

Please see the attachment.

 

Author Response File: Author Response.docx

Reviewer 5 Report

Comments and Suggestions for Authors

In this manuscript, the performance of a longitudinally pumped Yb:YAG laser medium placed in a short, hemispherical resonator (the mirror with high reflectivity is flat) is studied. The laser efficiency is discussed and it is shown that a high value of it is obtained without a precise optimization of the resonator characteristics (of the laser extraction mirror). A theoretical model and many experimental results are presented, an aspect that is to be appreciated.

 

Some comments follow.

 

1. The optical pumping is done at 940 nm (called in the manuscript as “the pump-level pumping”) and at 968 nm (expressed in the manuscript as “direct pumping of the upper-laser-level”). These expressions are then used in the manuscript, every time explanation is made to how the optical pumping was done! From my point of view, this expression becomes tiring for the reader! In the case of Nd lasers, these expressions are simpler, because the energy levels in which the pumping is done can be mentioned.

I suggest that in the introduction an explanation be made of how the optical pumping is done and what each one represents. Then, I think that in the manuscript it is sufficient to say that the optical pumping was done at 940 nm or at 968 nm.

This is a suggestion. The author will decide whether to take it into account or not.

 

2. Introduction, lines 61-63 it says: "... it was found that the dependence of the optical-to-optical conversion efficiency on the minimum laser spot radii decreases extremely with increasing pump intensity and efficiency."!

What does that "efficiency" at the end of the sentence mean? What does "the dependence of the optical-to-optical conversion efficiency on the minimum laser spot radii decreases extremely with ... efficiency" mean?

 

3. Some comparisons are made between the laser geometry analyzed here and thin-disk lasers or fiber lasers! These comments are not entirely correct, the last two types of laser mentioned being developed for much higher power levels and/or other operating regimes.

 

4. Figure 1. The characteristics of the output mirror could also be given on the figure, as in the case of the other components of the resonator.

 

5. Lines 143-145, it is stated: "At this pump absorption efficiency, the length of the gain medium is about 1 mm when the concentration of Yb ions is 20 at.%. Therefore, the concentration of Yb ions was set to 20 at.% and the length of the gain medium was set to 1 mm.".

"The pump absorption efficiency" is not mentioned!

Some absorption efficiencies are indicated on the previous page! Shouldn't the characteristics of the Yb:YAG medium be given there?

 

6. The theory used in this manuscript is well known, discussed in detail in various publications (which were also cited). It is not very clear what is meant by (line 175) “The theory in this paper is optimized for the discussion of this research …”! Rather the theory is applied for the laser configuration investigated here. I would suggest that before each equation the reference be used!

 

7. Equation (4) is denoted by (1); it needs to be corrected. Then, the statement (lines 197-199) “this Eq. (4) is obtained by approximation when the total loss in the cavity 𝐿 is small (at most about 5%, the maximum transmittance of the output coupler), as in this paper” needs to be explained! Alternatively, references can be added!

 

8. I suggest that instead of “minimum laser spot radii” or “minimum pump spot radii” the authors use “laser beam waist radius / radius of the laser beam waist” and “pump-beam waist radius / radius of the pump beam waist”, respectively. That is, “minimum” could be replaced with “waist”, a well-established expression of this parameter!

 

10. The discussion presented on page 2, lines (436-443) is not argued! Readers will have to take what is stated there for granted!

 

11. The experimental results are convincing. However, are three figures needed, i.e. Fig. 4(c-e) for exemplification? Wouldn't a single figure, one considered representative, suffice?

 

12. Again, I think that in Fig. 5 and Fig. 6 the simulations could be presented only for the 750 mW pump power (or at most for two pump powers)! Maybe the manuscript would be easier to follow this way! In addition, the pump power and the output mirror transmission could be indicated / written on the figures!

It remains for the author to decide how many figures to use, this is just a suggestion!

 

13. The discussion of the results is acceptable, but hard to follow! I think this has an influence on the quality and impact of this manuscript.

 

 

This is a typical paper, which may be of interest to those starting research in this field. The writing style can be improved, but the comments in the report can be considered minor. Consequently, the manuscript can be published in the journal Photonics, after minor revision.

Author Response

Dear Sir,

I would like to thank you for your very helpful comments.
Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

Dear author,

It is still the same paper and if the dependence on the higher pump intensity is so obvious, then there is no need for this paper. That's why my conclusion is the same as before.

 

Comments on the Quality of English Language

Dear author,

There are still a lot of repetitions in the text that impair the reading flow.