Study on Side-Pumping and Electro-Optical Q-Switched Laser Performance of a Novel Near-Infrared Laser Crystal Nd:GYSAG

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

The manuscript should be improved.

I recommend careful addressing of following comments.

1. The references should be modified appropriately. More specific references relevant to the subject should be added.
2. The chemical formula appearing on line 45 should be verified.
3. Figure captions are incomplete.
4. Sketch of laser set-up employed for free running and/or for Q-switching operation could be added.
5. How is the mode matching condition satisfied with this laser set-up?
6. Beam caustics along the resonator could be shown.
7. What is the pump beam dimension?
8. What is the thermal lens focal length at highest pump energy level for both Nd:GYSAG crystals (of diameters 3 and 4 mm)?
9. What is the resolution of the spectrometer? What is the FWHM bandwidth of laser emission spectrum?
10. The introduction begins with significance of 940 nm emission or dual wavelength laser operation, however, in this paper neither 940 nm emission is explored, neither dual wavelength operation is described. Perhaps the introduction could be improved to emphasize more on the significance of the results reported here in this paper.
11. All graphs seem to be saved with low resolution.
12. Arrows and labels on Figures 2 and 3 could be added denoting the main elements of the experimental set-up.
13. Comparison with other results from literature could be done and appropriate references should be added in that case.
14. Usually side pumping has the disadvantage of obtaining lasers with high M². In this paper improvement of M² is discussed however the overall M² value remains about 10. How the authors intend to improve M² in the future?
15. The authors claim that the maximum pump pulse energy was 604 mJ however, in their graphs the maximum pump pulse energy seems to exceed 650 mJ level. This is ambiguous.
16. On line 141 electro-optical efficiency should be replaced by optical to optical efficiency, most probably.
17. By comparing the free running results of Nd:GYSAG crystals with different diameters, we observe that for the 3 mm diameter Nd:GYSAG the tendency of the laser is to have a decrease in the optical to optical efficiency, at high pump energy levels. On the contrary, for the 4 mm diameter Nd:GYSAG the laser optical to optical efficiency seems to increase. Could the authors comment on this?
18. The time scale in Figure 10(e) should be added.
19. The temporal pulse shape for the Q-switch laser pulse obtained at maximum pump energy level could be shown for voltage applied vs voltage removed cases.

Best regards,

Author Response

Dear Reviewer, We sincerely appreciate your careful review of our manuscript and the constructive, insightful comments and valuable suggestions you have provided. Your thoughtful feedback is of great importance for us to further improve the quality of our paper and refine the research content. We have carefully addressed all your comments one by one and made corresponding revisions and supplements to the manuscript accordingly. The detailed responses to your comments are presented below, and the revised parts in the manuscript are marked for your easy check. Comments 1: The references should be modified appropriately. More specific references relevant to the subject should be added. Response: We appreciate the reviewer’s valuable comment. We have appropriately revised and supplemented the references as suggested, and added more representative references closely relevant to the topic of this study, which further improves the background integrity and academic rigor of the introduction. Comments 2: The chemical formula appearing on line 45 should be verified. Response: We sincerely appreciate the reviewer’s careful check. We have corrected the inaccurate chemical formula in line 45 of the revised manuscript. The original formula (NdxGdyY1-x-y)3SczAl1-zO12 has been revised to the correct form (NdxGdyY1-x-y)3SczAl5-zO12. Comments 3: Figure captions are incomplete. Response: We appreciate the reviewer’s valuable comment. We have carefully checked and revised all figure captions throughout the manuscript, supplementing the necessary descriptive information to ensure they are complete and clear, so that the content and experimental conditions can be fully explained for better readability and understanding. Comments 4: Sketch of laser set-up employed for free running and/or for Q-switching operation could be added. Response: We appreciate the reviewer’s valuable suggestion. We have added a schematic diagram of the laser setup under Q-switching operation in the manuscript, to help readers better understand the experimental configuration and operation mode. Comments 5: How is the mode matching condition satisfied with this laser set-up? Response: We sincerely appreciate the reviewer’s valuable question, which helps to elaborate on the key structural optimizations of our laser system. For our side-pumped Nd:GYSAG rod laser, the mode matching condition (maximizing the spatial overlap between the pump light and the fundamental cavity mode (TEM00)) is achieved through three targeted design measures: 1.Roughened side surface of the Nd:GYSAG rod The side surface of the Nd:GYSAG crystal rod was treated with a roughening process to enhance the diffuse reflection of the incident pump light inside the crystal. This modification expands the effective absorption region of the pump light, making its spatial distribution more consistent with the Gaussian profile of the fundamental cavity mode. 2.Resonator configuration with plano-convex mirrors We adopted a resonator constructed with plano-convex mirrors, which allows flexible adjustment of the waist position and size of the fundamental mode. By optimizing the curvature radius of the convex mirror and the cavity length, we precisely tuned the fundamental mode waist diameter inside the Nd:GYSAG rod to match the effective absorption diameter of the pump light, thus maximizing the mode overlap factor. 3.Water cooling system for thermal lensing mitigation Side-pumping inevitably induces radial temperature gradients inside the crystal, leading to thermal lensing effects that can distort the cavity mode and degrade mode matching. A dedicated water cooling sleeve was mounted on the Nd:GYSAG rod to achieve uniform temperature distribution, which significantly reduces the focal length fluctuation of the thermal lens and maintains the stability of mode matching under different pump power conditions. Comments 6: Beam caustics along the resonator could be shown. Response: We sincerely apologize for not being able to present the beam caustics along the resonator in this manuscript. The main reason is that, given our current technical capabilities, it is challenging to directly and intuitively characterize the beam caustics for the side-pumped laser configuration. The asymmetric pump light distribution inside the Nd:GYSAG rod, together with the dynamic thermal lensing effect induced by side pumping, makes the beam propagation path inside the resonator highly complex. We greatly appreciate the reviewer’s constructive suggestion, and we will consider adopting advanced optical simulation tools (e.g., ray-tracing software) or experimental characterization methods to investigate and visualize the beam caustics in our future research work. Comments 7: What is the pump beam dimension? Response: We appreciate the reviewer’s question regarding the pump beam dimension, which helps to clarify the key parameters of our laser system. The pump source adopted in this work is a side-pumped semiconductor laser (LD) array. The length of the Nd:GYSAG crystal rod is 73 mm; The pump light is focused on the middle segment of the crystal rod, with an effective irradiation length of 50 mm along the rod axis. This design ensures that the pump light acts on the core gain region of the crystal uniformly, avoiding excessive pump intensity at the ends and thus mitigating the end-face thermal damage of the crystal. Comments 8: What is the thermal lens focal length at highest pump energy level for both Nd:GYSAG crystals (of diameters 3 and 4 mm)? Response: We sincerely appreciate the reviewer’s insightful question regarding the thermal lens focal length, which is a critical parameter affecting the resonator stability of side-pumped lasers. Due to the limitations of our current experimental conditions, we were unable to measure or accurately calculate the thermal lens focal length of the Nd:GYSAG crystals (with diameters of 3 mm and 4 mm) at the highest pump energy level. The main constraints lie in two aspects: (1) The lack of dedicated characterization equipment (e.g., laser beam profiler or shearing interferometer) for dynamic thermal lens focal length measurement; (2) The high complexity of theoretical calculation, which is affected by multiple factors including the non-uniform pump light absorption distribution, the radial temperature gradient inside the crystal, and the temperature-dependent refractive index coefficient of Nd:GYSAG. Nevertheless, based on our previous research experience on side-pumped rare-earth-doped laser crystals and relevant theoretical formulas describing the thermal lens effect, we infer that the thermal lens focal length of the two Nd:GYSAG crystals (3 mm and 4 mm in diameter) at the highest pump energy level falls within the range of several hundred millimeters to over 1000 millimeters. We will prioritize the configuration of specialized thermal lens measurement equipment in our subsequent research, and carry out precise quantitative measurement of the thermal lens focal length of Nd:GYSAG crystals with different diameters under various pump energy levels. Comments 9: What is the resolution of the spectrometer? What is the FWHM bandwidth of laser emission spectrum? Response: We appreciate the reviewer’s valuable comment. The resolution of the spectrometer used in this work is 0.1 nm, and the full width at half maximum (FWHM) of the laser emission spectrum is 0.48 nm. These details have been added to the revised manuscript. Comments 10: The introduction begins with significance of 940 nm emission or dual wavelength laser operation, however, in this paper neither 940 nm emission is explored, neither dual wavelength operation is described. Perhaps the introduction could be improved to emphasize more on the significance of the results reported here in this paper. Response: We appreciate the reviewer’s valuable comment. We have revised the introduction accordingly and added relevant content to make it more consistent with the research content and significance of this paper. Comments 11: All graphs seem to be saved with low resolution. Response: We thank the reviewer for this helpful reminder. The relatively low resolution of the figures is caused by automatic image compression during Word document saving. We will provide all high-resolution figure files separately to the journal editor at the later proofreading stage to ensure good quality for publication. Comments 12: Arrows and labels on Figures 2 and 3 could be added denoting the main elements of the experimental set-up. Response: We appreciate the reviewer’s valuable suggestion. We have added a schematic diagram of the experimental setup in Figure 3, with clear labels and annotations for the main optical components to help readers better understand the experimental layout. Comments 13: Comparison with other results from literature could be done and appropriate references should be added in that case. Response: We appreciate the reviewer’s valuable suggestion. We have added a comparison between the experimental results of this work (EO Q-switching with side-pumping) and those reported in existing literature (passive Q-switching with end-pumping) at the corresponding position in the manuscript, analyzed the main reasons for the differences in laser performance between the two, and supplemented relevant references. This makes the discussion of the experimental results more targeted and sufficient, and further highlights the technical characteristics and value of this study. Comments 14: Usually side pumping has the disadvantage of obtaining lasers with high M2. In this paper improvement of M2 is discussed however the overall M2 value remains about 10. How the authors intend to improve M2 in the future? Response: We appreciate the reviewer’s valuable comment. As pointed out, side-pumping configuration usually leads to relatively high M² values. In this work, the optimized M² is about 10. In future studies, we plan to further improve the laser beam quality by optimizing the resonator structure, adopting a hybrid pumping scheme combining end-pumping and side-pumping, and further optimizing the thermal management and cooling scheme of the laser medium. Comments 15: The authors claim that the maximum pump pulse energy was 604 mJ however, in their graphs the maximum pump pulse energy seems to exceed 650 mJ level. This is ambiguous. Response: We thank the reviewer for the careful observation. We have checked the relevant data and revised the maximum pump pulse energy in the manuscript to 646 mJ to resolve the ambiguity and ensure consistency. Comments 16: On line 141 electro-optical efficiency should be replaced by optical to optical efficiency, most probably. Response: We thank the reviewer for the careful correction. We have revised “electro-optical efficiency” to “optical-to-optical efficiency” as suggested to ensure accurate and standard expression. Comments 17: By comparing the free running results of Nd:GYSAG crystals with different diameters, we observe that for the 3 mm diameter Nd:GYSAG the tendency of the laser is to have a decrease in the optical to optical efficiency, at high pump energy levels. On the contrary, for the 4 mm diameter Nd:GYSAG the laser optical to optical efficiency seems to increase. Could the authors comment on this? Response: We have added a detailed analysis of the performance difference between the 3-mm and 4-mm Nd:GYSAG crystals. The inferior performance of the 3-mm crystal is mainly attributed to its more pronounced thermal lensing effect: as pump power increases, the thermal focal length of the Φ3×73 mm rod decreases continuously and significantly, which distorts the beam wavefront and exacerbates the mismatch between the cavity mode size and the crystal cross-section. Although the pump wavelength redshift (beneficial for reducing quantum defect) is observed, its positive effect is outweighed by thermal lensing-induced diffraction loss and mode distortion. This comprehensive analysis has been supplemented in the results and discussion section. Comments 18: The time scale in Figure 10(e) should be added. Response: We sincerely appreciate the reviewer’s valuable comment. The sampling frequency of 100 Hz for Figure 10(e) has been clearly stated in the figure caption, from which the time interval can be derived as 0.01 s. We have added the corresponding time scale in the figure and further clarified this point in the caption for better readability and understanding. Comments 19: The temporal pulse shape for the Q-switch laser pulse obtained at maximum pump energy level could be shown for voltage applied vs voltage removed cases. Response: We appreciate the reviewer’s suggestion. We have systematically compared and analyzed the temporal pulse shapes of the Q-switched laser under both voltage-applied and voltage-removed conditions in the manuscript. The discussion covers the pulse characteristics under various pump conditions with sufficient detail, so we did not provide an additional separate comparison at the maximum pump energy level. We thank you again for your time and efforts devoted to the review of our work. We hope the revised manuscript can meet your requirements and look forward to your further comments. Sincerely yours, The Authors

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is devoted to the development of a high-power Q-switched laser on the new laser crystal Nd:GYSAG. The previous results obtained with this crystal are well presented in the Introduction. The relevance of the work is well substantiated. Pretty good output parameters were achieved. The experimental results are presented in detail, and their correctness is beyond doubt.

The main disadvantage of the work is the lack of analysis. The following remarks should be considered.

1. The authors claim that the 3-mm crystal demonstrated worse results than the 4-mm crystal. The possible reasons should be analyzed.
2. The authors compared voltage-applied and voltage-removed switching modes. It should be explained, what these regimes are and why one of them gave higher efficiency.
3. The authors compared two resonator designs, discussing the influence of thermal lensing. However, analysis is also poor here. What was the thermal lens focal distance? What was the size of the fundamental cavity mode? The relation between the mode size and the crystal size should influence the efficiency. What is the reason for the efficiency reduction? These calculations should be carried out, and conclusions should be drawn. The authors claim: “Proper cavity design can significantly enhance the spatial characteristics of laser output.” So proper cavity design should be calculated and analyzed.
4. The elements in fig. 3 should be labeled, and beam path should be indicated. The word “diagram” is better replaced with “scheme”.

Author Response

Dear Reviewer,   We sincerely appreciate your careful review, positive comments, and constructive suggestions on our manuscript. We are grateful for your recognition of the research background, the novelty of the Nd:GYSAG crystal, the detailed experimental results, and the good output performance achieved in this work. According to your comments, we have carefully revised the manuscript, supplemented in-depth analysis where feasible, and made necessary revisions to the figures. The detailed responses are as follows:  

1. Analysis of the performance difference between 3-mm and 4-mm crystals   We have added a detailed analysis of the performance difference between the 3-mm and 4-mm Nd:GYSAG crystals. The inferior performance of the 3-mm crystal is mainly attributed to its more pronounced thermal lensing effect: as pump power increases, the thermal focal length of the Φ3×73 mm rod decreases continuously and significantly, which distorts the beam wavefront and exacerbates the mismatch between the cavity mode size and the crystal cross-section. Although the pump wavelength redshift (beneficial for reducing quantum defect) is observed, its positive effect is outweighed by thermal lensing-induced diffraction loss and mode distortion. This comprehensive analysis has been supplemented in the results and discussion section.  
2. Explanation of voltage-applied and voltage-removed Q-switching modes   We have supplemented clear definitions of the voltage-applied and voltage-removed switching modes, and elaborated on the physical mechanisms underlying the efficiency and pulse width differences. As analyzed in the revised manuscript: (1) The voltage-removed mode requires continuous high-voltage application, inducing thermal effects and leakage current in the electro-optic crystal that degrade switching-off efficiency and cause weak oscillation (leading to upper-level population loss); (2) it is highly sensitive to 1/4λ voltage accuracy and resonator polarization matching, resulting in suboptimal performance under non-ideal practical conditions. In contrast, the voltage-applied mode operates at static zero voltage in the off-state, minimizing the crystal’s thermal load, ensuring stable operation, and achieving cleaner switching-off. These advantages effectively suppress population loss and weak oscillation, ultimately enabling narrower pulses and higher single-pulse energy—thus explaining why the voltage-applied mode outperforms the voltage-removed mode. The relevant detailed description and analysis are now included in Section 3.2 of the manuscript.  
3. Analysis of resonator design and thermal lensing   We sincerely acknowledge your valuable suggestion regarding the calculation of thermal lens focal length, fundamental cavity mode size, and optimal cavity design. However, as Nd:GYSAG is a newly developed laser crystal, the key intrinsic parameters (e.g., thermal conductivity, thermal expansion coefficient, refractive index temperature coefficient) required for quantitative calculation of thermal focal length and cavity mode size are not yet fully characterized and publicly available. Due to the lack of these essential parameters, we are unable to perform the corresponding quantitative calculations at present. We have supplemented qualitative analysis in the manuscript to clarify the physical mechanism: the efficiency reduction is mainly caused by the mismatch between the cavity mode size and the crystal cross-section, as well as the enhanced diffraction loss induced by thermal lensing. We will conduct systematic measurements of the crystal’s intrinsic thermal and optical parameters in future work, and further optimize the resonator design based on quantitative calculations.  
4. Revision of Figure 3   As suggested, we have comprehensively revised Figure 3:  
   1. We have added a clear schematic scheme alongside the original experimental setup photograph, which explicitly shows the optical layout.
   2. All optical components in the schematic (e.g., Total Reflector, Pump Source, crystal, Polarizer, Electro-Optic Switch, quarter-wave plate, Output Coupler) have been clearly labeled.
   3. The term “diagram” in the figure caption has been officially replaced with “Scheme” to conform to academic expression norms.
    

  We sincerely hope that the revised version meets the publication requirements. Thank you again for your insightful suggestions, which have helped improve the quality and completeness of our work. We will continue to deepen the relevant research based on your comments in future studies.   Sincerely,   The Authors

Reviewer 3 Report

Comments and Suggestions for Authors

Investigating the laser properties of new materials and optimizing them is a pressing issue. The peer-reviewed article compares the laser generation parameters of two rods made of the new Nd:GYSAG material, first produced last year. The paper demonstrated the generation of nanosecond laser pulses with a maximum energy of 179.4 mJ. The References appears to be up-to-date and requires no additions. The tables and figures are well-written.

I have a few comments:

- the superscripts in lines 73, 181, and 265 need to be corrected;

- the caption to Figure 4 needs to be corrected;

- section 2.1 needs to be expanded to include the reagents, purity, and atmosphere in which the crystal was grown.

Author Response

Dear Reviewers,   Thank you very much for your careful review and valuable comments on our manuscript. We highly appreciate your recognition of our work. We have carefully addressed all your comments and revised the manuscript accordingly. The detailed responses are as follows:  

1. Regarding the comment on superscripts in lines 73, 181, and 265:   The superscripts in the aforementioned lines have been carefully checked and corrected to ensure consistency with the manuscript’s citation format and academic standards.  
2. Regarding the comment on the caption to Figure 4:   The caption of Figure 4 has been revised to improve clarity. The corrected version accurately describes the content and key information of the figure.  
3. Regarding the comment on Section 2.1:   Section 2.1 has been expanded as suggested. We have added detailed information about the starting reagents, their purity, and the crystal growth atmosphere: the Nd:GYSAG crystal was grown by the Czochralski method using high-purity raw materials (Gd₂O₃, Y₂O₃, Sc₂O₃, Al₂O₃, and Nd₂O₃) with a purity of 99.999% (5N), and the growth process was conducted under a nitrogen (N₂) atmosphere. These details have been integrated into the revised Section 2.1 to enhance the completeness and reproducibility of the work.  

  We sincerely hope that the revised version meets the publication requirements. Thank you again for your constructive suggestions, which have greatly improved the quality of our manuscript.   Sincerely,   Lei Huang

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

Not all revisions were done as suggested. The manuscript was revised however could still be improved, in my opinion. The graph data points could be higher, the pulse shape achieved under voltage applied vs voltage removed Q-switch could be shown as well.

The schematic diagram with the laser set-up should be corrected.

Best regards,

Author Response

Dear Reviewer,   Thank you for your constructive comments.   The schematic diagram of the laser setup has been revised accordingly.   Regarding the other two suggestions, we are currently unable to process them temporarily. We are in a different place and cannot access the raw data stored on our desktop computer, so we are not able to supplement the graph data points and the comparison of pulse shapes at this stage.   We apologize for any inconvenience caused.   Best regards,   Authors