Review Reports - Optical Frequency Comb Generator Employing Two Cascaded Frequency Modulators and Mach–Zehnder Modulator

Round 1

Reviewer 1 Report

The paper presents a novel approach to achieve a flat optical frequency comb (OFC) by sequentially connecting two frequency modulators (FMs) followed by a single-drive Mach-Zehnder modulator (MZM). The study includes a comprehensive investigation of the theoretical and initial analysis of the cascaded FMs and MZM. The proposed technique is implemented and tested for a frequency of 8 GHz, resulting in an optical spectrum consisting of 71 subcarriers spaced at 4 GHz with a power fluctuation of approximately 2 dB. It is worth noting that the proposed method is based on theoretical and simulation outcomes, and further experimental validation is necessary to confirm its practical feasibility. The sole revision I would propose is to place greater emphasis on the fact that the tests conducted were simulated, and while the paper acknowledges their practical implications, their validity ultimately hinges on future experimental tests.

Author Response

Response: Thank you for your appreciation. The authors sincerely appreciate that you review this manuscript and recognize this work.

We have simulated the setup in Optisystem 19.0.0 and confirmed it by theoretical analysis. With the minimum facility available in our laboratory, we tried to experiment with the results through simulation, almost matching the experimental setup, and compared them with theoretical analysis results.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear authors, in the following my comments:

1. What is the main question addressed by the research?

The authors propose a novel technique to achieve a flat Optical frequency comb (OFC) by serially cascading two frequency modulators (FMs) followed by a single-drive Mach-Zehnder modulator.

2. Do you consider the topic original or relevant in the field? Does it address a specific gap in the field?

From my point of view, I consider that the authors are boarding a topic of interest in the field of research. Always is good to have technological alternatives to generate signals using optoelectronics techniques. This work is well compared with reported works allowing us to remark on the importance of this work.

3. What does it add to the subject area compared with other published material?

Considering the comparison with the state of the art presented by the authors, this work adds an alternative in the implementation of Optical frequency combs whose impact is compared and well supported by the references used. Therefore, this allows remarking on the main contribution with respect to others' works.

4. What specific improvements should the authors consider regarding the methodology? What further controls should be considered?

I consider that the methodology used by the authors is well established. Therefore, this allows the readers an easy understanding of this work.

5. Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed?

Yes, the conclusions are consistent with the developed work. The theory presented allows supporting the experimental results.

6. Are the references appropriate?

Yes, the references are appropriate and are of recent publications.

7. Please include any additional comments on the tables and figures.

All equations must be referenced.

There are some typo mistakes, this must be corrected

Author Response

The authors study a resource allocation algorithm for AR requests. The use case is interesting, but the paper has many issues:

Response: Thank you for your appreciation. The authors sincerely appreciate that you review this manuscript and recognize this work.

Corrections to be done:

Question 1: What is the main question addressed by the research?

Response: The authors propose a novel technique to achieve a flat Optical frequency comb (OFC) by serially cascading two frequency modulators (FMs) followed by a single-drive Mach-Zehnder modulator.

Question 2: Do you consider the topic original or relevant in the field? Does it address a specific gap in the field?

Response: From my point of view, I consider that the authors are boarding a topic of interest in the field of research. Always is good to have technological alternatives to generate signals using optoelectronics techniques. This work is well compared with reported works allowing us to remark on the importance of this work.

Question 3: What does it add to the subject area compared with other published material?

Response: Considering the comparison with the state of the art presented by the authors, this work adds an alternative in the implementation of Optical frequency combs whose impact is compared and well supported by the references used. Therefore, this allows remarking on the main contribution with respect to others' works.

Question 4: What specific improvements should the authors consider regarding the methodology? What further controls should be considered?

Response: I consider that the methodology used by the authors is well established. Therefore, this allows the readers an easy understanding of this work.

Question 5: Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed?

Response: Yes, the conclusions are consistent with the developed work. The theory presented allows supporting the experimental results.

Question 6: Are the references appropriate?

Response: Yes, the references are appropriate and are of recent publications.

Question 7: Please include any additional comments on the tables and figures.

Response: Thank you very much for your suggestion. Following your suggestion, we have included additional comments on the figures.

Question 8: All equations must be referenced.

Response: Thank you very much for your valuable suggestion. Following your suggestion, we have referenced all the equations in the revised manuscript and highlighted them in yellow.

Question 9: There are some typo mistakes, this must be corrected.

Response: Thank you for your careful reading and valuable suggestion. Following your suggestion, we have tried our best to correct all the typo errors; however, if any particular point is still available, kindly let us know.

Author Response File: Author Response.pdf

Reviewer 3 Report

In the reviewer’s point of view, this paper’s main contribution is that the authors have proposed a novel technique to achieve a flat optical frequency comb (OFC) by serially cascading two frequency modulators (FMs) followed by a single-drive Mech-Zehnder modulator (MZM). Particularly, they have also studied the impact of the phase of the RF signal on the power variation of the OFC spectrum. The results are quite interesting to the community, and the manuscript is well written. Nevertheless, there are still a few issues that need to be resolved before possible final publication, which are listed in the following:

1. In the "Abstract" Section, it is mentioned "The simulation results show good performance; therefore, our proposed approach can be used in next-generation optical networks." For better understanding of the potential readers, it is highly recommended the authors might at least add one or two sentences in the conclusion section, elucidating how the proposed technique using optical frequency comb can be widely applied to the real next-generation optical network applications.

2. Also, it is strongly recommended that the authors might at least add one or two sentences in the conclusion section explaining if they would like to experimentally demonstrate this technique using optical frequency combs, what might be the major challenges?

3. There are up to 11 equations throughout the manuscript, whose format seem somehow not uniform. For example, it is suggested that each equation ends with a period.

4. Besides, it is also recommended that the sentence before each equation be ended with a colon.

5. Last but not least, please carefully edit your manuscript by correcting typos, and please have a person fluent in English proofread your paper to address all the language issues.

If English is your second language and editing services are required for your manuscript, you may find the following editing websites useful for a cost:

1. IEEE Professional Editing Services at http://www.prof-editing.com/ieee/

2. American Journal Experts at http://www.journalexperts.com/

Comments for author File: Comments.pdf

N/A

Author Response

Response: Thank you for your appreciation. The authors sincerely appreciate that you review this manuscript and recognize this work.

Corrections to be done:

Question 1: In the "Abstract" Section, it is mentioned "The simulation results show good performance; therefore, our proposed approach can be used in next-generation optical networks." For better understanding of the potential readers, it is highly recommended the authors might at least add one or two sentences in the conclusion section, elucidating how the proposed technique using optical frequency comb can be widely applied to the real next-generation optical network applications.

Response: Thank you very much for your suggestion. Following your suggestion, we have added a sentence in the conclusion section, elucidating how the proposed technique using optical frequency comb can be widely applied to real next-generation optical network applications, in the revised manuscript and highlighted with yellow color.

Change and location:

Using our proposed approach, 71 subcarriers spaced at 4 GHz are generated within a power deviation of around 2 dB. The proposed method is also tested for f_m = 16 GHz, and it is shown that this approach can work in all scenarios with lower power fluctuations. These many sub-carriers with good spectral flatness can be employed as carriers to facilitate super-channel transmission, which serves as the building block for the next-generation intelligent optical transport network. Section 4, Page No. 12.

Question 2: Also, it is strongly recommended that the authors might at least add one or two sentences in the conclusion section explaining if they would like to experimentally demonstrate this technique using optical frequency combs, what might be the major challenges?

Response: Thank you very much for your constructive and kind suggestion; following your suggestion, we have added a few phrases to the conclusion section outlining the main problems that would arise during the experimental demonstration of this technique.

Change and location:

However, some challenges are there during the experimental implementation of this approach, such as noise accumulation caused by a cascaded laser and modulators in the OFC. Particularly for optical communications applications, the accumulated noise affects the overall comb flatness. In order to lessen the influence of noise, phase noise reduction techniques might be used. Furthermore, carefully adjusting the driving characteristics of the modulators is necessary to produce a comb source with a flat spectrum. The system is inherently nonlinear, even in the most basic electro-optic modulator configuration, and when the RF signal consists only of a single frequency, the power distribution along each comb line is not even. Section 4, Page 12.

Question 3: There are up to 11 equations throughout the manuscript, whose format seem somehow not uniform. For example, it is suggested that each equation ends with a period.

Response: Thank you very much for your constructive and kind suggestion. Following your suggestion, we have ended each equation with a period in the revised manuscript.

Question 4: Besides, it is also recommended that the sentence before each equation be ended with a colon.

Response Thank you for your careful reading and valuable suggestion. Following your suggestion, we have included a colon at the end of the sentence before each equation in the revised manuscript and highlighted it with yellow color.

Question 5: Last but not least, please carefully edit your manuscript by correcting typos, and please have a person fluent in English proofread your paper to address all the language issues.

Response: Thank you for your careful reading and valuable suggestion. Following your suggestion, we have tried our best to correct all the typo errors. However, if any particular point is still available, kindly let us know.

Author Response File: Author Response.pdf

Reviewer 4 Report

In this article, the author proposed a novel technique for obtaining a flat optical frequency comb by serial cascading of two frequency modulators (FMs) followed by a single driving Mech Zehnder Modulator (MZM).

The manuscript is somewhat innovative and largely satisfies the requirements. However, the abstract and conclusions need to be rewritten. These two sections are described less like a scientific paper than a lab report.

A number of errors of expression in the manuscript were checked.

Author Response

Corrections to be done:

Question1: In this article, the author proposed a novel technique for obtaining a flat optical frequency comb by serial cascading of two frequency modulators (FMs) followed by a single driving Mech Zehnder Modulator (MZM). The manuscript is somewhat innovative and largely satisfies the requirements. However, the abstract and conclusions need to be rewritten. These two sections are described less like a scientific paper than a lab report.

Response: Thank you for your appreciation. The authors sincerely appreciate that you review this manuscript and recognize this work. Following your suggestion, we have rewritten the abstract and conclusion section, in the revised manuscript and highlighted them with yellow color.

Question 2: Comments on the Quality of English Language

Response: Thank you for your careful reading and valuable suggestion. Following your suggestion, we have tried our best to correct all the typo errors. However, if any particular point is still available, kindly let us know.

Question 3: A number of errors of expression in the manuscript were checked.

Response: Thank you very much for your constructive and kind suggestion. Following your suggestion, we have checked the manuscript thoroughly and tried to remove all the errors.

Author Response File: Author Response.pdf

Reviewer 5 Report

Please see enclosed, thanks.

Comments for author File: Comments.pdf

Please see enclosed, thanks.

Author Response

Corrections to be done:

Question 1: The abstract does not mention the results of the experiments and whether they meet the expectations of the experiments, which need to be added by the authors accordingly.

Response: Thank you for your valuable suggestion. We have simulated the setup in Optisystem 19.0.0 and confirmed it by the theoretical analysis. With the minimum facility available in our laboratory, we tried to experiment with the results through simulation, almost matching the experimental setup, and compared them with theoretical analysis results. Following your suggestion, we have included it in the revised manuscript and highlighted them with yellow color.

Change and location:

In addition, we have also studied the impact of the phase of the RF signal on the power variation of the OFC spectrum. A theoretical investigation of the ultra-flat spectrum generated by cascaded FMs and MZM is conducted, and the results of simulations support the findings. Page No. 1.

Question 2: Several citations in the article are not standardized and do not meet the requirements of MDPI journals; please check and correct them by the authors.

Response: Thank you for your valuable suggestion. Following your suggestion, we have re-checked and corrected the references.

Question 3: In the first section of the article, "Introduction", there are no two or more subheadings. Could the author please check whether "1.1, Justice" in line 62 is necessary?

Response: Thank you for your suggestion. The phrase "1.1, Justice" is not present in our manuscript.

Question 4: The second section, "Materials and Methods", is too long. Please revise it and use tables, diagrams and schemes to make it more attractive to the readers.

Response: Thank you for your suggestion. The section titled “Materials and Methods" is not present in our manuscript.

Question 5: For the image labeling in line 251, a short caption that summarizes the entire complete image (including all the smaller images in it) should be used, with the left and right images separated by the (a), (b) signs. Ditto for lines 260, 270 in the article.

Response: Thank you for your suggestion. Lines 251, 260, and 270 of our manuscript do not contain any images. All the images in our manuscript are labeled according to the recommended format.

Question 6: Figure 6 already contains Figures 3 and 4, which the authors were asked to recheck and correct.

Response: Thank you for your suggestion. We re-checked it, and Figure 6 shows the optical spectrum at the output of MZM, whereas Figures 3 and 4 show the optical spectrum at the output of the second FM.

Question 7: In line 299, the title should be changed to "Results and Discussion"

Response: Thank you for your suggestion. We already have a section titled "Results and Discussion" in our manuscript, and line 299 appears in the Reference section.

Question 8: Check the clarity of Figure 5 and Tables 2 and 3; these should be submitted at a higher resolution.

Response: Thank you for your suggestion. We have checked the clarity of Figure 5, and our manuscript does not contain Tables 2 and 3.

Question 9: Tables 1, 2, and 3 in the article are more like inserts and are not in editable format. In addition, the table numbers and descriptive headings are at the top and the decimal points of the numbers should be aligned.

Response: Thank you for your suggestion. Our manuscript does not contain Tables 1, 2, and 3.

Question 10: There is an error in the reference at the end of the article, please check and correct it by the author.

Response: Thank you for your suggestion. Following your suggestion, we have re-checked and corrected the references.

Question 11: The data in the results section should be depicted with more graphs, which can highlight the experimental results more visually.

Response: Thank you for your suggestion. We have provided enough graphs to demonstrate our proposed approach. We looked it out more and found no scope to add more graphs.

Author Response File: Author Response.pdf

Reviewer 6 Report

This work demonstrates a comb frequency generator based on Mach-Zehnder modulators by a numerical simulation. The cascade configuration is interesting, and the number of lines achieved can be attractive for some research groups. However, the manuscript in some parts needs to be clarified. In addition, the analysis is limited, and the practical implementation challenges limit the manuscript's impact. As a result, it is hard to recommend the acceptance of the manuscript in Electronics.

1. The simulation analysis needs detailed information:

*What is the wavelength operation used for the analysis?

*What is the propagation medium? Is the system based on optical fiber?

*The authors mention an optical spectrum analyzer as a monitoring device; however, the system response is analyzed in the frequency domain. The authors must clarify if an RF analyzer is used for the analysis or if they transform the wavelength spectrum to the domain time.

*What is the input power used for the analysis? Moreover, the authors need to validate the “good power efficiency.”

2. The authors need to include a discussion about practical implementation challenges.

*The authors do not mention the propagation medium; then, it is essential to mention this medium to discuss dispersion, losses, and the crosstalk effect.

*What are the limits of the cavity length? The total length of the system needs a discussion

3. The result needs a comparison with prior works and experimental results.

*A comparative table regarding frequency lines, method, and frequency range should be included considering theoretical and experimental results.

*Several techniques have been reported based on ring resonators, phase modulators, bandpass filters, and delay lines. The authors need to highlight their contribution to the literature.

Author Response

Corrections to be done:

The simulation analysis needs detailed information:

Question 1(a): What is the wavelength operation used for the analysis?

Response: The central wavelength of the laser used in optical frequency comb generation is set at 1552.52 nm (equivalent frequency of 193.1 THz ).

Question 1(b): What is the propagation medium? Is the system based on optical fiber?

Response: Optical frequency combs are a broad optical spectrum with discrete and equidistant frequency lines produced using a single laser source. The optical frequency comb generator (OFCG) is an efficient optoelectronic device that is included in many important applications in various fields, such as optical communications, optical arbitrary waveform generation, optical sensing, high-accuracy optical metrology, spectroscopy, etc.

In the context of this work, we are not using any propagation medium. We proposed an optical frequency comb generator.

Question 1(c): The authors mention an optical spectrum analyzer as a monitoring device; however, the system response is analyzed in the frequency domain. The authors must clarify if an RF analyzer is used for the analysis or if they transform the wavelength spectrum to the domain time.

Response:

An optical spectrum analyzer allows the user to calculate and display optical signals in the frequency domain.

RF spectrum analyzer allows the user to calculate and display electrical signals in the frequency domain.

Optical frequency combs are a broad optical spectrum with discrete and equidistant frequency lines, so it is measured using an Optical spectrum analyzer.

Since we are generating an optical spectrum, an RF spectrum analyzer is not being used in the context of this work.

1(d): What is the input power used for the analysis? Moreover, the authors need to validate the “good power efficiency.”

Response: The output power of the CW laser is 1 mW.

Our proposed OFC generator produces 71 comb lines within a power deviation of 2 dB. Here, the modulators are driven directly by RF signals without a phase shifter or amplifier, making our approach simple, stable, and power efficient.

Question 2(a): The authors need to include a discussion about practical implementation challenges.

Response: Following your suggestion, we have included a few sentences about practical implementation challenges in the conclusion section of the revised manuscript and highlighted them in yellow.

Question 2(b): The authors do not mention the propagation medium; then, it is essential to mention this medium to discuss dispersion, losses, and the crosstalk effect.

Response: We didn't use any propagation medium. We generated an optical frequency comb that may be applied to a variety of tasks, including optical communications, optical arbitrary waveform generation, optical meteorology, spectroscopy, etc.

Thus, dispersion, losses, and the crosstalk effect are out of the scope of this manuscript.

Question 2(c): What are the limits of the cavity length? The total length of the system needs a discussion.

Response: In this work, we proposed a novel technique to achieve a flat Optical frequency comb by serially cascading two frequency modulators (FMs) followed by a single-drive Mach-Zehnder modulator.

The optical comb sources based on optical modulators have no cavity configuration. Thus, the limits of the cavity length and the total length of the system are not discussed here.

Question 3(a): The result needs a comparison with prior works and experimental results. A comparative table regarding frequency lines, method, and frequency range should be included considering theoretical and experimental results.

Response: Thank you for your valuable suggestion. We have simulated the setup in Optisystem 19.0.0 and confirmed it by the theoretical analysis. With the minimum facility available in our laboratory, we tried to experiment with the results through simulation, almost matching the experimental setup, and compared them with theoretical analysis results.

Question 3(b): Several techniques have been reported based on ring resonators, phase modulators, bandpass filters, and delay lines. The authors need to highlight their contribution to the literature.

Response: Thank you very much for your suggestions. To have a better survey on this topic, we have revised the Introduction section. We have added seven new references (from Ref. 32 to Ref. 38) and highlighted them with yellow color in the revised manuscript. The new references are listed in the following:

Change and location：

The seven new references are as:

Demirtzioglou, I.; Lacava, C.; Bottrill, K.R.; Thomson, D.J.; Reed, T.; Richardson, D.J.; Petropoulos, P. Frequency comb generation in a silicon ring resonator modulator. Optics express 2018, 26, 790–796.

Zhang, M.; Wang, C.; Buscaino, B.; Shams-Ansari, A.; Kahn, J.M.; Loncar, M. Electro-optic frequency comb generation in ultrahigh-Q integrated lithium niobate micro-resonators. In Proceedings of the CLEO: QELS_Fundamental Science. Optica Publishing Group, 2018, pp. FW3E–4. 372 34.

Kulagin, V.; Valuev, V.; Kontorov, S.; Kornienko, V.; Prokhorov, D.; Cherepenin, V. Optical frequency comb in optoelectronic oscillator with delay line and microresonator. In Proceedings of the 2022 International Conference Laser Optics (ICLO). IEEE, 2022, pp. 1–1.

Wu, X.; Tsang, H. Flat-top frequency comb generation with silicon microring modulator and In Proceedings of the 2017 Conference on Lasers and Electro-Optics (CLEO). IEEE, 2017, 377 pp. 1–2. 378.

Liu, Y.; Wu, S.; Shen, X. Ultra-flat optical frequency comb generation based on electro-optic intensity modulator with digital driving signal 2022. 380.

Cui, Y.; Wang, Z.; Xu, Y.; Jiang, Y.; Yu, J.; Huang, Z. Generation of flat optical frequency comb using cascaded PMs with combined harmonics. IEEE photonics technology letters 2022, 34, 490–493.

Shen, J.; Wu, S.; Li, D. Ultra-flat optical frequency comb generation based on phase modulation with simple digital driving signal. Optik 2019, 198, 163254.

Author Response File: Author Response.pdf

Round 2

Reviewer 5 Report

Please see enclosed.

Comments for author File: Comments.pdf

Please see enclosed.

Author Response

Corrections to be done:

Question 1: Could the author please check if it is necessary to keep the "1" in line 15 of the article?

Response: Thank you for your careful reading and valuable suggestion. Following your suggestion, we have removed "1" in line 15 in the revised manuscript.

Question 2: The authors should have added a paragraph in Section 2 to explain why the simulations were performed only for f_m = 8 GHz and f_m= 16 GHz.

Response: Thank you very much for your suggestion. Following your suggestion, we have added a paragraph in section 2 that explains why the simulations were performed only for f_m = 8 GHz and f_m = 16 GHz, in the revised manuscript and highlighted with the yellow color.

Change and Location:

Our proposed approach is tested for two sets of RF1, RF2, and RF3 signals: 16 GHz, 8 GHz, 32 GHz, and 8 GHz, 4 GHz, and 16 GHz, respectively.

However, this proposed approach can also be used with other sets of RF signal frequencies, but the power deviation is minimal at f_m = 8 GHz and 16 GHz. Section 2, Page No. 4

Question 3: The third section of the article, "Results and Discussion", contains mostly simulation results and equations, and the discussion part is less mentioned, which should be supplemented by the authors.

Response: Thank you very much for your suggestion. Following your suggestion, we have included a few more points in the result and discussion section for a better understanding of the readers in the revised manuscript and highlighted with the yellow color.

Question 4: The experiments in the article are all simulations, and I would like to ask the authors if they can conduct practical experiments and verify the results against each other?

Response: Thank you for your valuable suggestion. With the minimum facility available in our laboratory, we tried to experiment with the results through simulation, almost matching the experimental setup, and compared them with theoretical analysis results.

Question 5: Typically, the standard paragraph indentation is to add a space at the beginning of each new paragraph, usually a tab (Tab key) or some spaces. Ask the author to recheck and revise to ensure consistent indentation formatting throughout the article.

Response: Thank you very much for your constructive and kind suggestion. Following your suggestion, we have indented every paragraph throughout the manuscript.

Question 6: Each equation starts with E_mzm(t) = ..., which is not well distinguished. Please ask the authors to recheck and explain the symbols inside the equations accordingly.

Response: Thank you very much for your suggestion. Following your suggestion, we tried to adequately explain each symbol used in every equation for a better understanding of the readers in the revised manuscript and highlighted with the yellow color.

Question 7: At the end of the Results and Discussion section, it is suggested that the authors summarize and compare the two different cases of f_m in the form of a table, which can make the reader feel more intuitively the advantages and feasibility of the authors' proposed method.

Response: Thank you very much for your suggestion. Following your suggestion, we have included a table at the end of the Results and Discussion section to compare the two different cases of f_m, in the revised manuscript and highlighted with the yellow color.

Change and location:

Dear Sir, please find the graph in an attached pdf file.

Section 3, Page No. 12,

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Author Response File: Author Response.pdf

Reviewer 6 Report

The authors attend to most of the comments and include suitable information; It is suggested to attend to the following concern:

*As the authors mention, "In the context of this work, we are not using any propagation medium"; however, it is not clear from what medium propagates the initial CW laser source described by the authors. Please clarify this point,

Author Response

Question 1: As the authors mention, "In the context of this work, we are not using any propagation medium"; however, it is not clear from what medium propagates the initial CW laser source described by the authors. Please clarify this point,

Response: Thank you very much for your question. The initial CW laser is coupled into the first frequency modulator using optical fiber and connectors.

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Author Response File: Author Response.pdf