High-Repetition-Rate 2.3–2.7 µm Acousto-Optically Tuned Narrow-Line Laser System Comprising Two Master Oscillators and Power Amplifiers Based on Polycrystalline Cr²⁺:ZnSe with the 2.1 µm Ho³⁺:YAG Pulsed Pumping

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

A narrow-linewidth widely tunable laser system, composed double master-oscillators and power amplifiers, based on the polycrystalline Cr²⁺:ZnSe elements pumped by repetively-pulsed 2.1 mm Ho³⁺:YAG laser operating at the tens of kilohertz pulse ratio is reported. The procedure of ZnSe-element doping and surface improvement are applied to increase the laser-induced damage threshold and to increase the output power of laser system. The output wavelength is widely tunable within 2.3-2.7 mm by an acoustio-optical tunable filter inside the master oscillator cavity.

The experimental procedures are well described in this article. The descriptions of the experimental results are also quite detialed. In addtion, the thermal lensing effect in the  Cr²⁺:ZnSe active element has been presented. The artice provides important reference for the design of high power, narrow-linewidth, tunable 2.3-2.7 mm laser. The article can be published after the format of some formulas being carefully modified.

The letters in some physical quantities, such as σabs, σem, Nup,5T2, Nda in lines 108-112, and in Eq.(1) should be marked with supscripts.

3+ in Ho³⁺:YAG and -2 in e^-2 should be marked with superscripts.

Comments on the Quality of English Language

 Minor editing of English language required

Author Response

COMMENT 1. The letters in some physical quantities, such as σabs, σem, Nup,5T2, Nda in lines 108-112, and in Eq.(1) should be marked with supscripts.

3+ in Ho3+:YAG and -2 in e-2 should be marked with superscripts.

Answer: The physical quantities were corrected.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

In the following paper, entitled, “High repetition-rate 2.3-2.7 μm acoustic-optically tuned narrow line laser system comprising two master oscillators and power amplifiers based on polycrystalline Cr2+:ZnSe with the 2.1 μm  Ho3+:YAG pulsed pumping” Antipovet al, report a narrow-linewidth widely-tunable laser system based on the polycrystalline Cr2+:ZnSe elements pumped by the repetitively-pulsed 2.1 μm Ho3+:YAG laser operating at the tens of kilo-20 hertz pulse rate. The results are interesting. The paper is well-written and all sections have been clearly described. A good comparison of the experimental and theoretical study is presented. I would recommend the manuscript be accepted after minor revision.

1.      What is the advantage of using a double master oscillator?

2.      How the distance of the optical components was optimized in the Fig. 3.

3.      What’s the 3 dB bandwidth of the optical spectra presented in Fig. 4(a)?

4.      The output power varies as a function of wavelength depicted in Fig.13; however, the beam quality of the laser remained the same.

5.      Please comment about the stability of the device.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

The authors thank the respected reviewers for the assessment of the work, useful remarks and recommendations.

REVIEWER 2.

QUESTION 1. What is the advantage of using a double master oscillator?

Answer: A double master oscillator provided automatic time synchronization in the Cr2+:ZnSe amplifying element of pumping pulses of 2091 nm radiation amplified in the Ho:YAG amplifier and the delayed Cr2+:ZnSe laser pulses which came a little later.

This sentence has been added to the text [Lines 384-386].

 

QUESTION 2. How the distance of the optical components was optimized in the Fig. 3.

Answer: The resonator lengths of the Ho:YAG and Cr:ZnSe lasers were varied to maximize the average emission power at 2.1 μm and 2.3-2.5 μm.

 

QUESTION 3. What’s the 3 dB bandwidth of the optical spectra presented in Fig. 4(a)?

Answer: The estimated 3-dB linewidth of the Ho:YAG laser was less than 0.3 nm (the measurement accuracy was limited by the spectrometer used).

This sentence has been added to the text [Lines 152-153].

 

QUESTION 4. The output power varies as a function of wavelength depicted in Fig.13; however, the beam quality of the laser remained the same.

Answer: The beam quality at 2.3 µm was slightly higher than at 2.5 µm, which can be explained by the decrease of heat generation with the decrease of the pump quantum defect.

 This sentence has been added to the text [Lines 359-361].

 

QUESTION 5. Please comment about the stability of the device.

Answer: At stable temperature and environmental conditions in the laboratory, the instability of output pulse peak power and average power was less than 10-15% at 20-30 kHz PRR (instability of the Ho3+:YAG laser oscillator pulses was less than 5%, the Cr2+:ZnSe laser oscillator instability was less than 7-10%).

 This sentence has been added to the text [Lines 404-407].

 

All additions in the text are marked in yellow.

 

GENERAL COMMENT. Minor editing of English language required

Answer: Additional editing of the English language was performed.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Solid-state lasers have been a subject of extensive research for over half a century. The technologies used to construct these lasers and the methods for analyzing them are well-developed and mature, including Cr2+: ZnSe lasers (P. A. Berry and K. L. Schepler, "High-power, widely tunable Cr2+: ZnSe laser," CLEO/QELS: 2010 Laser Science to Photonic Applications, San Jose, CA, USA, 2010, pp. 1-2; S. B. Mirov et al., "Progress in Mid-IR Lasers Based on Cr and Fe-Doped II–VI Chalcogenides," IEEE Journal of Selected Topics in Quantum Electronics, vol. 21, no. 1, pp. 292-310). This manuscript reports on a high-power tunable Cr2+: ZnSe laser system with oscillator and amplifier integrated, with comprehensive experimental data and analysis. However, the major concerns are the novelty, which needs to be more clearly articulated in introduction (how this manuscript differs from previous studies and what unique contributions it makes), and the figures, which require significant improvement. Here are my specific suggestions:

Figure Presentation:

In Figure 2, I recommend using a shaded area to indicate the operational wavelength span of the Cr2+: ZnSe laser, rather than two red dotted pointers.

Legends should be used to denote results obtained under different conditions in Figures 5, 6, 8, 9, 10, and 13.

The purpose of presenting Figure 2 is unclear, and further discussion is needed.

Figure 4's caption mentions input powers of 20 W (green) and 21.6 W (red), but the figure shows these lines starting below 17.5 W and 20 W, respectively.

In Figure 6, the y-axis requires units. The caption "50 mm long rod (red) or the 65 mm long rod" should include "(black)" at the end.

The figure markings in Figures 6(a) and 6(b) are inconsistent; (a) uses red and black to denote different rod lengths, whereas (b) uses dashed and solid lines. A unified marking system is recommended.

Scientific Rigor:

Certain statements should be supported with references or calculations. For example: "The gain increase in a large signal at higher PRR can be explained by the influence of the thermal lens and the electronic self-focusing effect, which reduced the diameters of the pumping and amplified beams in the laser amplifier." "The decrease in gain of the 65-mm long amplifier with increasing input pulsed power can be explained by the saturation effect (Fig. 6)." "Assuming the absorption cross section of Cr2+: ZnSe at 2091 nm of σabs = 1.4×10^-19 cm2, the final concentration of the Cr2+ ions in the laser elements was estimated to be Nd = 3-6×10^18 cm-3."

Clarify if "electronic self-focusing" refers to self-focusing caused by electron movement, as this phrasing is uncommon.

Other Issues:

The manuscript suggests thermal effects may degrade beam quality. It would be beneficial to discuss how heat was managed throughout the system.

Figure 13 appears to be undiscussed in the manuscript.

The manuscript states "The Cr2+: ZnSe laser operated in the gain-switch mode: the laser pulse was delayed with respect to the pumping pulse, and the delay time decreased with an increase in the pump pulse energy (Figure 8)." This information, however, is not shown in Figure 8.

To enhance the novelty of this paper, it would be prudent to elaborate on key innovative aspects. Some common experiments, such as those shown in Figure 6, might be abbreviated or omitted to focus on more unique contributions.

Author Response

The authors thank the respected reviewers for the assessment of the work, useful remarks and recommendations.

 

REVIEWER 3.

COMMENT 1. The technologies used to construct these lasers and the methods for analyzing them are well-developed and mature, including Cr2+: ZnSe lasers (P. A. Berry and K. L. Schepler, "High-power, widely tunable Cr2+: ZnSe laser," CLEO/QELS: 2010 Laser Science to Photonic Applications, San Jose, CA, USA, 2010, pp. 1-2; S. B. Mirov et al., "Progress in Mid-IR Lasers Based on Cr and Fe-Doped II–VI Chalcogenides," IEEE Journal of Selected Topics in Quantum Electronics, vol. 21, no. 1, pp. 292-310).

Answer: We agree with the comment. Several references to the initial papers on Cr:ZnSe lasers were added to the text:

17. Mirov S. B., Fedorov V. V., Graham K., Moskalev I.S., Badikov V.V,, Panyutin V. Erbium fiber laser-pumped continuous-wave microchip Cr(2+):ZnS and Cr(2+):ZnSe lasers. Optics Letters, 2002, 27: 909-11.
18. Berry P. A., Schepler K. L. High-power, widely tunable Cr2+: ZnSe laser, Proceedings of Conference on Lasers and Electro-Optics 2010, CLEO’2010 (Optica Publishing Group, 2010), paper CMDD5.

 

Additional papers concerning the Cr:ZnSe crystal manufacturing were added:

15. Gavrishchuk E. M. Polycrystalline Zinc Selenide for IR Optical Applications, Inorg. Mater., 2003, 39, 883–899.
16. Levchenko V. I., Yakimovich V. N., Postnova L. I., Konstantinov V. I., Mikhailov V. P., Kuleshov N. V. Preparation and properties of bulk ZnSe :Cr single crystals, Journal of Crystal Growth, 1999, 198/199, 980—983.

 

41. Gavrishchuk E.M., Kurashkin S. V., Savin D. V., Timofeev O. V. Effect of Magnetorheological Polishing on Laser-Induced Damage in ZnSe and ZnSe:Cr Polycrystals, Appl. Phys. B, 2023, 129, 1.
42. Rodin S.A., Gavrishchuk E.M., Ikonnikov V.B., Savin D.V. Effect of Annealing Atmosphere on Chromium Diffusion in CVD ZnSe, Inorg. Mater., 2018, 54, 21–25.

 

Several references for the 2-3 µm laser applications were also added to the text:

3. Hodgkinson J., Tatam R. Optical gas sensing: a review, Meas. Sci. Technol., 2013, 24, 012004.
4. Hossain M., Nakamura Y., Yamada Y., Kimura K., Matsumoto N., Matsumoto K. Effects of Er,Cr:YSGG laser irradiation in human enamel and dentin: ablation and morphological studies, J. Clin. Laser Med. Surg., 1999, 17(4):155-9.
5. Vogel A., Venugopalan V. Mechanisms of Pulsed Laser Ablation of Biological Tissues, Chem. Rev., 2003, 103, 577- 644.

 

11. Fedorov V., Mirov S., Gallian A., Badikov D., Frolov M., Korostelin Y., Kozlovsky V., Landman A., Podmar'kov Y., Akimov V., Voronov A. 3.77-5.05-μm tunable solid-state lasers based on Fe²⁺-doped ZnSe crystals operating at low and room temperatures, IEEE Journal of Quantum Electronics., 2006, 42, 907-917.

 

 

COMMENT 2. In Figure 2, I recommend using a shaded area to indicate the operational wavelength span of the Cr2+: ZnSe laser, rather than two red dotted pointers.

Answer: Figure 2 was modified to use a gray-filled area to indicate the operational wavelength span of the Cr²⁺:ZnSe laser. The figure caption was also updated accordingly.

 

COMMENT 3. Legends should be used to denote results obtained under different conditions in Figures 5, 6, 8, 9, 10, and 13.

Answer: Additional information about the system parameters was added in the captions of Figures 5, 6, 8, 9, 10, and 13.

 

COMMENT 4. The purpose of presenting Figure 2 is unclear, and further discussion is needed.

Answer: The following discussion of Figure 2 was added to the text:

“The calculation of the effective gain cross section by Exp. 1 showed an increase of the operation wavelength span of the Cr2+:ZnSe laser (where the total laser gain G(λ) ×N_da×L can exceed the cavity losses) with an increase of the inversion fraction (Fig. 2). However, these calculations also showed that at high pump power (when the inversion fraction exceeds 0.2), pump absorption at 2091 nm ceased (where G(λ = 2091) = 0) due to the mutual compensation of up and down transitions ⁵E ↔ ⁵T₂. The Cr²⁺:ZnSe laser crystal can bleach at the high pump power. Therefore, the low-quantum defect pump of the Cr2+:ZnSe laser at 2091 nm demonstrated the self-limiting behavior.”

 

COMMENT 5. Figure 4's caption mentions input powers of 20 W (green) and 21.6 W (red), but the figure shows these lines starting below 17.5 W and 20 W, respectively.

Answer: The reviewer apparently meant Figure 5, not Figure 4. The input power values in the Figure 5 caption were corrected to match the figure lines.

 

COMMENT 6. In Figure 6, the y-axis requires units. The caption "50 mm long rod (red) or the 65 mm long rod" should include "(black)" at the end.

Answer: The y-axes in Figures 6a and 6b are the dimensionless gain axes, so they don’t require units.

The captions were corrected to indicate the line colors, adding "(black)" for the 65 mm long rod.

 

COMMENT 7. The figure markings in Figures 6(a) and 6(b) are inconsistent; (a) uses red and black to denote different rod lengths, whereas (b) uses dashed and solid lines. A unified marking system is recommended.

Answer: The figure markings in Figures 6(a) and 6(b) were changed to use a uniform system with consistent line colors and different line types.

 

COMMENT 8. Certain statements should be supported with references or calculations. For example: "The gain increase in a large signal at higher PRR can be explained by the influence of the thermal lens and the electronic self-focusing effect, which reduced the diameters of the pumping and amplified beams in the laser amplifier." "The decrease in gain of the 65-mm long amplifier with increasing input pulsed power can be explained by the saturation effect (Fig. 6)." "Assuming the absorption cross section of Cr2+: ZnSe at 2091 nm of σabs = 1.4×10^-19 cm2, the final concentration of the Cr2+ ions in the laser elements was estimated to be Nd = 3-6×10^18 cm-3."

Answer: The appropriate references were added at the end of the statements. One more cited paper was added to the reference list:

“47.    Elliott C. J. Gain saturation and self‐focusing considerations in the design of optical amplifiers, Appl. Phys. Lett., 1974, 24, 91–93.”

 

COMMENT 9. Clarify if "electronic self-focusing" refers to self-focusing caused by electron movement, as this phrasing is uncommon.

Answer: The word "electronic" has been removed from this term throughout the text.

 

COMMENT 10. The manuscript suggests thermal effects may degrade beam quality. It would be beneficial to discuss how heat was managed throughout the system.

Answer: Description of the thermal management of the laser elements was added to the text:

“The Cr²⁺:ZnSe active elements were wrapped in indium foil and inserted into the copper radiators with the temperature stabilized at 5±0.1 ºC”

and

“The Ho³⁺:YAG rods were wrapped in indium foil and inserted into the copper radiators with the temperature stabilized at 5±0.1 ºC”.

 

COMMENT 11. Figure 13 appears to be undiscussed in the manuscript.

Answer: Figure 13 was discussed in the sentences under the figure:

“The average output power of the electronically-tuned Cr²⁺:ZnSe laser at 23 W pumping power and 30 kHz PRR had a maximum of 3.1-3.3 W at 2410 nm, and decreased to 1.1-1.2 W at 2300 nm and to 0.6-0.8 W at 2700 nm (Fig. 13).”

 

COMMENT 12. The manuscript states "The Cr2+: ZnSe laser operated in the gain-switch mode: the laser pulse was delayed with respect to the pumping pulse, and the delay time decreased with an increase in the pump pulse energy (Figure 8)." This information, however, is not shown in Figure 8.

Answer: The delay of the laser pulse with respect to the pump pulse was shown by blue lettering on Figure 8a (“250 ns”) and Figure 8b (“50 ns”).

 

COMMENT 13. To enhance the novelty of this paper, it would be prudent to elaborate on key innovative aspects. Some common experiments, such as those shown in Figure 6, might be abbreviated or omitted to focus on more unique contributions.

Answer: To enhance novelty of the paper, the additional information about application of the created high-repetition rate Cr:ZnSe laser system was added in Conclusion:

“Several pollutant gases, such as CH₄, NH₃, CO, and HF, have specific absorption bands in the wavelength range of 2.3-2.7 µm and can be detected using the developed laser source [58-61]. The small mass-dimensional parameters of the created Cr²⁺:ZnSe laser system, as well as the electronic control of all its components including wavelength tuning, allow its use for remote monitoring of the pollutant gases near the surface or in the upper atmosphere, both from the Earth's surface and from an aircraft or low-orbit spacecraft. In addition, the high-average-power repetitively-pulsed tunable Cr²⁺:ZnSe laser system can find applications in other fields: dental hard tissue ablation [62], precision polymer processing [63], pumping of mid-infrared optical parametric oscillators, and Fe²⁺-doped solid-state lasers [64,65].”

 

“59.    Richter D., Fried A., Wert B. P., Waleg, J. G., Tittel F. K. Development of a tunable mid‐IR difference frequency laser source for highly sensitive airborne trace gas detection, Appl. Phys. B, 2002, 75: 281.

60. Tittel F.K., Richter D., Fried A. Mid‐infrared laser applications in spectroscopy, In: Solid‐State Mid‐Infrared Laser Sources, Top-ics in Applied Physics, 2003, vol. 89 (eds. I.T. Sorokina and K.L. Vodopyanov), Berlin: Springer.
61. Bobrovnikov S.M., Matvienko G.G., Romanovsky O.A., Serikov I.B., Sukhanov A.Y. Lidar Spectroscopic Gas Analysis of the Atmosphere; IOA SB RAS: Tomsk, Russia, 2014; p. 510.
62. Santos C. R., Tonetto M. R., Presoto C. D., Bandéca M. C., Oliveira O. B., Calabrez-Filho S., Andrade M. F. Application of Er:YAG and Er,Cr:YSGG Lasers in Cavity Preparation for Dental Tissues: A Literature Review, World Journal of Dentistry, 2012, 3(4):340-343.
63. Duval S. “Laser processing of polymer in the mid-infrared – Why?” www.Femtum.com, 2021 (available on May 19, 2024)
64. Yumoto M., Saito N., Wada S., Energy ZnGeP2 Optical Parametric Oscillator Pumped with Rapidly Tunable Cr: ZnSe Laser, Nonlinear Optics Technical Digest, 2013, paper NW4A.19.
65. Kozlovsky V.I., Korostelin Y.V., Podmar'kov Y.P., Skasyrsky Y.K., Frolov M.P. Middle infrared Fe2+:ZnS, Fe2+:ZnSe and Cr2+:CdSe lasers: new results. J. Phys.: Conf. Ser., 2016, 740, 012006.”

 

GENERAL COMMENT. Minor editing of English language required

Answer: Additional editing of the English language was performed.

 

All additions in the text are marked in yellow.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

This version looks fine