Sub-Nanosecond, Room-Temperature, Mid-IR Fe:ZnSe Gain-Switched Laser: Experimental Characterization and Modeling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

The authors present few and sub nsec pulse durations applying gain switch,  close to mJ energy per pulse level  , which is a distinguished achievement at this spectral range.

It seems that this research group showed this results at the last Photonics West  SPIE conference, Paper 13341-55. They can cite this at the current manuscript, similar to ref 14 that is given from the same conference during 2024. 

In order to accept  this article for publication and to improve its quality, I recommend that the author address the following comments:

1.It is recommended to include supporting references for the applications you mentioned in the introduction line 26-31.

2. The authors used 2.79um  with 52 ns pulse duration and 2.98um  with 9 ns pulse durations, beside the difference of pulse duration, dose  the difference between  the cross section values of those different wavelengths has any influence on the results?

3. The given model doesn't include any interference effect between the beams that goes back and forth in the cavity. Is it a correct assumption in your case?

 

4. There is a big deference of the beam quality between the two GS results as reflected from figure 3 and 4 , please elaborate about this issue (is it  the case also between the two pump  sources) ?

5. Dose thermal lensing effects was seen or investigated in this research? And how it can be affected?

 

6. Pulse train measurement for the stability of pulses souled be add to the paper and discuss.

 

7. In order to give more general view about gain switch using Fe:ZnSe crystal it is recommended to add the following references as well:

Liang, X., Zhou, S., Liu, Z. et al. Modeling and analysis of actively Q-switched Fe: ZnSe laser pumped by a 2.8 µm fiber laser. Optoelectron. Lett. 19, 513–518 (2023). https://doi.org/10.1007/s11801-023-3016-y

 

Stanislav Leonov, Mikhail Frolov, Yury Korostelin, Yan Skasyrsky, and Vladimir Kozlovsky, "Gain-switched Fe:ZnSe laser pumped by a coupled-cavity Q-switched Er:ZrF₄ fiber laser," Opt. Lett. 48, 2957-2960 (2023)

 

Author Response

The authors present few and sub nsec pulse durations applying gain switch,  close to mJ energy per pulse level  , which is a distinguished achievement at this spectral range.

It seems that this research group showed this results at the last Photonics West  SPIE conference, Paper 13341-55. They can cite this at the current manuscript, similar to ref 14 that is given from the same conference during 2024.

Response: We thank the Reviewer for the valuable comment. The results presented at the 2025 SPIE Photonics West conference have now been cited in the manuscript.

[39] Ghimire, S.; Danilin, D.; Martyshkin, D.; Fedorov, V.; Mirov, S. Room Temperature Sub-Nanosecond Fe:ZnSe Gain-Switched Laser Characterization and Modeling. Proceedings of the Solid State Lasers XXXIV: Technology and Devices; Clarkson, W.A., Shori, R.K., Eds.; SPIE: San Francisco, United States, February 26 2025, 55, doi:10.1117/12.3043285.

In order to accept  this article for publication and to improve its quality, I recommend that the author address the following comments:

Comment 1: It is recommended to include supporting references for the applications you mentioned in the introduction line 26-31

Response 1: We thank the Reviewer for the valuable comment. We agree and have included supporting references for the applications mentioned in the introduction.

 

Comment 2: The authors used 2.79um  with 52 ns pulse duration and 2.98um  with 9 ns pulse durations, beside the difference of pulse duration, dose  the difference between  the cross section values of those different wavelengths has any influence on the results?

Response 2: We thank the Reviewer for the valuable comment. The absorption cross-sections at 2.79 µm and 2.98 µm are close to each other: 0.82 x 10^-18 cm² and 0.96 x 10^-18 cm², respectively, while the emission cross-sections at these wavelengths are negligible. This small variation in cross-section values does not significantly influence laser efficiency or oscillation dynamics. Additionally, these cross-section differences are fully incorporated into our rate-equation model, with explicit values used for each case.

 

Comment 3: The given model doesn't include any interference effect between the beams that goes back and forth in the cavity. Is it a correct assumption in your case?

Response 3: We thank the Reviewer for the valuable comment. We agree that rate equations are a basic theoretical model of laser operation, and interference effects could play a significant role in the CW regime with a narrow oscillation spectrum. However, there are no interference effects in our laser system due to its broadband oscillation with a spectrum exceeding 50 nm. Moreover, this model has been successfully applied to many laser systems, including Fe:ZnSe lasers, as demonstrated by several research groups [1-3].

[1]        Wan, Y.; Shen, Y.; Wang, K.; Chai, T.; Wang, Y.; Chen, Z.; Zhu, F. Study on Electrically Modulated Quasi-Continuous Wave Fe:ZnSe Solid-State Laser with Hundred-Hertz. Micromachines 2023, 14(12), 2194, doi:10.3390/mi14122194.

[2]        Xu, F.; Pan, Q.; Zhang, Y.; Zhang, R.; Chen, Y.; Yu, D.; Chen, F. Pulse High Energy Fe:ZnSe Laser Pumped by Q-Switched Er:YAG Laser. Optics Express 2023, 31, 26807–26814, doi:10.1364/OE.497566.

[3]        Dormidonov, A.E.; Firsov, K.N.; Gavrishchuk, E.M.; Ikonnikov, V.B.; Kazantsev, S.Yu.; Kononov, I.G.; Kotereva, T.V.; Savin, D.V.; Timofeeva, N.A. High-Efficiency Room-Temperature ZnSe: Fe²⁺ Laser with a High Pulsed Radiation Energy. Appl. Phys. B 2016, 122, 211, doi:10.1007/s00340-016-6489-6. (Reference 30 in the article)

 

Comment 4: There is a big deference of the beam quality between the two GS results as reflected from figure 3 and 4 , please elaborate about this issue (is it  the case also between the two pump  sources)?

Response 4: We thank the Reviewer for the valuable comment. The difference in the beam quality between the GS lasers (in Figures 3 and 4) was influenced by the pump beam profile. The Cr:Er:YSGG pump beam exhibits a ‘near-Gaussian’ profile. However, the idler beam from the KTA-OPO exhibits poor beam quality due to several factors: phase-matching conditions and a short OPO cavity length. We added the following text to the manuscript (lines 213-216):

“As shown in the figure, the beam quality of the KTA-OPO idler was poor due to several factors, including phase-matching conditions and a short OPO cavity length. However, the efficiency of the Fe:ZnSe laser under OPO excitation was comparable to its efficiency under Cr:Er:YSGG laser excitation.” 

 

Comment 5: Dose thermal lensing effects was seen or investigated in this research? And how it can be affected?

Response 5: We thank the Reviewer for the valuable comment. The Fe:ZnSe laser operates at a low repetition rate, near the laser threshold, and with a relatively large beam diameter. The crystal was installed in a copper crystal holder. Therefore, thermal effects could be neglected in our experiments and were not studied.

 

Comment 6: Pulse train measurement for the stability of pulses souled be add to the paper and discuss.

Response 6: We thank the Reviewer for the valuable comment. The following text was added to the manuscript (lines 221-224):

“Although we did not measure the temporal stability of the oscillation pulses, the close values of the Fe:ZnSe laser pulse durations under KTA-OPO excitation suggest good temporal stability. The measured pulse durations in three trials were 622 ps, 649 ps, and 625 ps, prior to ×√2 corrections.”

Comment 7: In order to give more general view about gain switch using Fe:ZnSe crystal it is recommended to add the following references as well:

Liang, X., Zhou, S., Liu, Z. et al. Modeling and analysis of actively Q-switched Fe: ZnSe laser pumped by a 2.8 µm fiber laser. Optoelectron. Lett. 19, 513–518 (2023). https://doi.org/10.1007/s11801-023-3016-y

Stanislav Leonov, Mikhail Frolov, Yury Korostelin, Yan Skasyrsky, and Vladimir Kozlovsky, "Gain-switched Fe:ZnSe laser pumped by a coupled-cavity Q-switched Er:ZrF₄ fiber laser," Opt. Lett. 48, 2957-2960 (2023)

Response 7: We thank the Reviewer for the valuable comment. We agree with your point; the recommended articles have been added as cited references [22] (line 47) and [29] (line 55), respectively.

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript Title: Sub-Nanosecond, Room-Temperature, Mid-IR Fe:ZnSe Gain-Switched Laser: Experimental Characterization and Modeling

Authors: Saugat Ghimire, Daniil Danilin, Dmitry Martyshkin, Vladimir Fedorov, and Sergey Mirov

Journal: Photonics

This manuscript presents a well-structured and comprehensive study of a room-temperature, sub-nanosecond, gain-switched Fe:ZnSe laser operating in the mid-IR region (4.4-4.8 µm). The authors combine experimental characterization with numerical modeling to investigate the laser's performance under different pumping conditions (2.79 µm Q-switched Cr:Er:YSGG laser, 2.98 µm KTA-OPO). The study demonstrates impressive pulse durations and provides valuable insights into the optimization of such lasers, particularly regarding the potential for microchip configurations. The work is technically sound, and the results are clearly presented and discussed. I recommend publication after addressing a few minor points.

• The introduction could be strengthened by a more compelling discussion of the broader impact and applications of sub-nanosecond mid-IR lasers. While applications are mentioned, emphasizing their significance in specific areas (e.g., remote sensing, medical diagnostics) would enhance the manuscript.
• The introduction is good, but it could be more compelling by discussing the broader impact of the research in more detail.
• In the introduction section, the phrase "crucial for a variety of scientific and industrial applications" is a bit generic. Provide a few specific examples of why these pulse durations are crucial for those applications. For instance, mention specific molecules that can be targeted with these wavelengths in environmental monitoring or the advantages of this pulse duration in materials processing.
• Can you provide a citation for the damage threshold issue with the LGS crystal?
• It would be beneficial to briefly explain the rationale for using the √2 factor to estimate the fundamental pulse duration from the second harmonic measurement. This isn't necessarily common knowledge.
• It would be good to explain why a configuration with a Eth = 3 mJ with an Lcry = 5.4mm thickness had this behavior. Is there any comparison to published results by other authors?
• While generally well-written, a few sentences could be rephrased for improved clarity and conciseness (see comments below).
  • "It offers a practical balance between accuracy and computational simplicity while still allowing distinct absorption and emission parameters." - reword it.
  • Is "small disease of the pulse duration" the correct intention?
  • Is the final "disclaimer" paragraph required?

I recommend this manuscript for publication in Photonics after the authors address the minor comments and suggestions outlined above. The research is valuable and contributes to the advancement of mid-IR laser technology. The improvements suggested will enhance the manuscript's impact and clarity.

Comments on the Quality of English Language

The English could be improved to more clearly express the research. While generally well-written, some sentences could be rephrased for greater clarity and conciseness.

Author Response

This manuscript presents a well-structured and comprehensive study of a room-temperature, sub-nanosecond, gain-switched Fe:ZnSe laser operating in the mid-IR region (4.4-4.8 µm). The authors combine experimental characterization with numerical modeling to investigate the laser's performance under different pumping conditions (2.79 µm Q-switched Cr:Er:YSGG laser, 2.98 µm KTA-OPO). The study demonstrates impressive pulse durations and provides valuable insights into the optimization of such lasers, particularly regarding the potential for microchip configurations. The work is technically sound, and the results are clearly presented and discussed. I recommend publication after addressing a few minor points.

Comment 1: The introduction could be strengthened by a more compelling discussion of the broader impact and applications of sub-nanosecond mid-IR lasers. While applications are mentioned, emphasizing their significance in specific areas (e.g., remote sensing, medical diagnostics) would enhance the manuscript.

Comment 2: The introduction is good, but it could be more compelling by discussing the broader impact of the research in more detail.

 

Response 1 and 2: We thank the Reviewer for the valuable comment. We have significantly updated the introduction and more clearly described the background and history of the work, notable accomplishments in the field, and broader impact.

 

Comment 3: In the introduction section, the phrase "crucial for a variety of scientific and industrial applications" is a bit generic. Provide a few specific examples of why these pulse durations are crucial for those applications. For instance, mention specific molecules that can be targeted with these wavelengths in environmental monitoring or the advantages of this pulse duration in materials processing.

Response 3: We thank the Reviewer for the valuable comment. In the updated introduction, we referenced articles describing specific molecules detected with the use of Fe:ZnSe mid-IR laser [4] and articles [6,7] explaining the advantages of sub-ns pulses for laser scalpel, enabling the most negligible damage of the tissue during surgical procedures.

We also provided references to the sentence (lines 69-71): “Achieving this optimization allows for enhanced pulse quality and temporal control, enabling key applications in micromachining [33], materials processing [34], and pumping of CPA systems [10].”

 

Comment 4: Can you provide a citation for the damage threshold issue with the LGS crystal?

Response 4: We thank the Reviewer for the valuable comment. We added the following citation on optical damage in LGS crystal in the text (line 136).

[38] Lan, H.; Liang, F.; Lin, Z.; Yu, H.; Zhang, H.; Wang, J. Langasite Family Midinfrared Nonlinear Optical Oxide Materials: Structure, Property, And Applications. Int. J. Opt. 2017, 2980274, doi:10.1155/2017/2980274.

 

Comment 5: It would be beneficial to briefly explain the rationale for using the √2 factor to estimate the fundamental pulse duration from the second harmonic measurement. This isn't necessarily common knowledge.

Response 5: We thank the Reviewer for the valuable comment. We added the following text to justify our procedure instead of the last sentence in section 3.

“Considering only the temporal dependence of second-harmonic generation (SHG) in the fixed-field approximation, the intensity of the second harmonic is proportional to the squared intensity of the laser oscillation:  [40]. In our experiments, the detected temporal profiles could be well-fitted by a Gaussian shape. Therefore, the measured pulse duration at the second harmonic was multiplied by √2 to estimate the pulse duration at the fundamental wavelength.”

 

Comment 6: It would be good to explain why a configuration with a Eth = 3 mJ with an Lcry = 5.4mm thickness had this behavior. Is there any comparison to published results by other authors?

Response 6: We thank the Reviewer for the valuable comment. There is nothing special about the laser cavity under consideration. As mentioned in the introduction, several research groups have studied RT GS Fe:ZnSe lasers in similar cavities. However, most researchers have focused on the development of high-energy, high-efficiency laser systems. In these cases, the pulse duration of the oscillation typically includes several relaxation spikes, with the overall pulse duration being close to that of the excitation pulses. In our study, we focused solely on optimizing the first oscillation spike. To clarify this, we have revised our introduction as previously described. The studied cavity was chosen for its simplicity to validate the laser model and demonstrate sub-nanosecond oscillation pulses. As demonstrated in the last paragraph of Section 4, even shorter pulses could be achieved in a 3 mm long cavity.

 

Comment 7: While generally well-written, a few sentences could be rephrased for improved clarity and conciseness (see comments below).

• "It offers a practical balance between accuracy and computational simplicity while still allowing distinct absorption and emission parameters." - reword it.
• Is "small disease of the pulse duration" the correct intention?
• Is the final "disclaimer" paragraph required

Response 7: We thank the Reviewer for the valuable comment. Here are the updated versions:

• The sentence was reworded to “This provides a practical balance between accuracy and computational simplicity while preserving the ability to define distinct absorption and emission parameters.”
• That was a typo; it has been corrected to “… small decrease in the pulse duration …”
• The “disclaimer” paragraph is required.

 

I recommend this manuscript for publication in Photonics after the authors address the minor comments and suggestions outlined above. The research is valuable and contributes to the advancement of mid-IR laser technology. The improvements suggested will enhance the manuscript's impact and clarity.

Response: We thank the reviewer for the comments and for recommending the manuscript for publication. We trust that our revisions address the reviewers' comments and make the manuscript suitable for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

The article under review studies the characteristics of a laser system on a Fe:ZnSe crystal. The results obtained are of interest for laser physics.

However, in my opinion, the article needs significant revision:

- a large number of works on a laser on a ZnSe:Fe crystal can be found through the scholar.google search engine, which are not cited in the article under review. The authors should significantly expand the literature review and indicate the novelty of the results obtained.

- in Fig. 1, the arrow corresponding to the pumping does not fall into the maximum of the absorption cross-section; a similar situation is visible for the radiative transition. What is the reason for this?

- in Table 1, the refractive index is indicated for which wavelength?

- is temperature not involved in the system of equations (1)?

- from the appearance of Fig. 2, it seems that radiation with a wavelength of 2.79 μm does not participate in the experiments in any way. This figure needs to be redone.

- is the ZnSe:Fe crystal commercial (it is necessary to indicate the manufacturer)?

-what is the reason for the deterioration of the KTA-OPO beam profile at the input of the Fe:ZnSe crystal?

-in references 2, 3, 10, 12, and 16, the title of the article needs to be corrected;

Author Response

The article under review studies the characteristics of a laser system on a Fe:ZnSe crystal. The results obtained are of interest for laser physics.

However, in my opinion, the article needs significant revision:

Comment 1: a large number of works on a laser on a ZnSe:Fe crystal can be found through the scholar.google search engine, which are not cited in the article under review. The authors should significantly expand the literature review and indicate the novelty of the results obtained.

Response 1: We thank the Reviewer for the valuable comment. We have significantly updated the introduction and more clearly described the background and history of the work, notable accomplishments in the field, a broader impact, and the novelty of the obtained results.

Comment 2: in Fig. 1, the arrow corresponding to the pumping does not fall into the maximum of the absorption cross-section; a similar situation is visible for the radiative transition. What is the reason for this?

Response 2: We thank the Reviewer for the valuable comment. The arrow in Figure 1 indicates the oscillation and pump wavelength used in the experiments. Er:YAG/YSGG lasers operating at ~2.8–2.9 μm are among the most promising for practical applications; this is why we used pump sources in this spectral range. Although pump radiation of 2.8-2.9 µm is slightly shifted from the maximum of the Fe:ZnSe absorption band, the decrease in the absorption cross-section is minimal. The Fe:ZnSe laser demonstrated approximately the same efficiency even when using a pump source based on a Cr:ZnSe laser with an oscillation wavelength of 2.65 μm. The oscillation wavelength shifts due to a strong overlap between the absorption and emission bands.

We modified the caption to Figure 1 (lines 89-95) to make the figure clearer:

“Figure 1. (A) Absorption (solid red curve) and emission (dashed red curve) cross-sections of Fe:ZnSe gain media. The red arrow indicates excitation by radiation with a wavelength of approximately 2.8–2.9 µm, as used in the experiments. The long blue arrow represents the stimulated emission process. The oscillation wavelengths in the experiments shift from the maximum of the emission cross-section due to a strong overlap between the absorption and emission bands. (B) Cavity geometry used in the numerical model along with the energy level diagram of Fe²⁺ ions in the tetrahedral field of ZnSe.”

 

Comment 3: in Table 1, the refractive index is indicated for which wavelength?

Response 3: We thank the Reviewer for the valuable comment. The refractive index, n is 2.44 for wavelengths 2.79 µm and 2.98 µm. At the oscillation wavelengths, it is slightly smaller ~2.43, which results in negligible effects on the model. The Table was corrected appropriately. 

 

Comment 4: is temperature not involved in the system of equations (1)?

Response 4: We thank the Reviewer for the valuable comment. The Fe:ZnSe laser operates at a low repetition rate, near the laser threshold, and with a relatively large beam diameter. The crystal was installed in a water-cooled copper crystal holder. Therefore, thermal effects could be neglected in our experiments and were not studied.

 

Comment 5: from the appearance of Fig. 2, it seems that radiation with a wavelength of 2.79 μm does not participate in the experiments in any way. This figure needs to be redone.

Response 5: We thank the Reviewer for the valuable comment. The figure was redone to show the switch between the two pump sources. To make it clear, we added the following text (lines 153-154).

“The transition between the pump lasers is illustrated by repositioning Beam Blocker 2, as shown in Figure 2.”

 

Comment 6: is the ZnSe:Fe crystal commercial (it is necessary to indicate the manufacturer)?

Response 6: We thank the Reviewer for the valuable comment. Yes, the Fe:ZnSe crystals used during the experiment were commercial and obtained from IPG Photonics. It has been added to the article in line 162 as “… Fe:ZnSe gain element (IPG Photonics) …”

 

Comment 7: what is the reason for the deterioration of the KTA-OPO beam profile at the input of the Fe:ZnSe crystal?

Response 7: We thank the Reviewer for the valuable comment. The difference in the beam quality between the GS lasers (in  Figures 3 and 4) was influenced by the pump beam profile. The Cr:Er:YSGG pump beam exhibits a ‘near-Gaussian’ profile. However, the idler beam from the KTA-OPO exhibits poor beam quality due to several factors: phase-matching conditions and a short OPO cavity length. We added the following text to the manuscript (lines 213-216):

“As shown in the figure, the beam quality of the KTA-OPO idler was poor due to several factors, including phase-matching conditions and a short OPO cavity length. However, the efficiency of the Fe:ZnSe laser under OPO excitation was comparable to its efficiency under Cr:Er:YSGG laser excitation.”

 

Comment 8: in references 2, 3, 10, 12, and 16, the title of the article needs to be corrected;

Response 8: We thank the Reviewer for the valuable comment. The errors have been addressed and corrected in the submitted version of the manuscript.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed all the comments raised and in the current version the article can be accepted for publication.

Author Response

Thank you for your thoughtful review and feedback. We greatly appreciate your time and effort in evaluating our manuscript. We are pleased to hear that the revisions addressed all the comments raised, and we are glad the article is now suitable for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

I believe that the article in its current form can be accepted for publication.

Author Response

Thank you for your thoughtful review and feedback. We greatly appreciate your time and effort in evaluating our manuscript. We are pleased to hear that the revisions addressed all the comments raised, and we are glad the article is now suitable for publication.