The Optimal Operating Point for Linearizing an Integrated Optical Lithium Niobate Directional Coupler Modulator^†

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Fig. 2 - give more description of a) and b) in caption and text

Fig. 4. - discuss or note Vbias 1 and Vbias 2 labels (specifically) in caption and text

Discuss importance of second harmonic not being suppressed

Highlight the most "promising" aspect in the last paragraph and let the reader know if you expect the device to find commercial use and in what way it is better than standard MZI devices.

A comparison of experimental and simulation results would be nice, to see how closely the measurements follow simulations. 

Comments on the Quality of English Language

Some minor modifications needed, but readable as-is

Author Response

First of all, I would like to express my sincere gratitude to the reviewer for his attention to our manuscript and valuable criticism, which will undoubtedly contribute to a better presentation of our research. Our detailed responses to each reviewer comment are here.

Reviewer #1 

Fig. 2 - give more description of a) and b) in caption and textEfficient simultaneous transmission of light and an optical carrier for transmitting a high-frequency analog signal was showed.   I

This is Fig. 5 in the revised extended version of the Manuscript . The figure has been changed, in particular an experimental setup has been added. A more detailed description of the methods for measuring the spectral dependences of DCM, as well as a discussion of the results obtained, has been added to the Manuscript above this figure.

Fig. 4. - discuss or note Vbias 1 and Vbias 2 labels (specifically) in caption and text 

This is Fig. 7. in the revised extended version of the Manuscript. A description of the figure and a discussion of the criteria for optimal operating point have been added to the manuscript above this figure.  

Discuss importance of second harmonic not being suppressed 

A discussion of the influence of the second harmonic, which leads to modulation bandwidth limitation and so-called sub-octave modulation, has been added to the introduction. 

Highlight the most "promising" aspect in the last paragraph and let the reader know if you expect the device to find commercial use and in what way it is better than standard MZI devices. 

The discussion of the advantages and disadvantages of DCM was extanded in the conclusion. 

A comparison of experimental and simulation results would be nice, to see how closely the measurements follow simulations.  

Figure 1d demonstrates good agreement between the experimentally measured transmittance of the fabricated DCM sample with the theoretical prediction given by Equation 1. Quantitative theoretical analysis of the remaining characteristics was difficult, since many technical characteristics of various components influence the results. We limited ourselves to only experimental studies that clearly demonstrate the applicability of the proposed method and provide estimates of achievable parameters. 

Reviewer 2 Report

Comments and Suggestions for Authors

Considering the fact that optical modulators are of great interest in the field of optical communications and other important applications, the work presented here on the lithium niobate optical directional coupler modulator and its linearization may be of importance in this field and represents an original approach. I recommend that this work be published if some clarifications and improvements are made:

1) The work in this paper is similar to a previously published journal article and the authors have not cited this published article.

- Parfenov, M.V., Tronev, A.V., Il’ichev, I.V. et al. Photorefractive Correction of the Coupling Ratio of an Integrated Optical Directional X-Coupler on a Lithium Niobate Substrate. Tech. Phys. Lett. 45, 187–189 (2019). https://doi.org/10.1134/S1063785019030131

- Parfenov, M.; Agruzov, P.; Tronev, A.; Ilichev, I.; Usikova, A.; Zadiranov, Y.; Shamrai, A. Metal Electrodes for Filtering the Localized Fundamental Mode of a Ridge Optical Waveguide on a Thin Lithium Niobate Nanofilm. Nanomaterials 2023, 13, 2755. https://doi.org/10.3390/nano13202755

Authors should cite the above papers and explain the difference between this article and previously published papers. Previously published papers must be cited between the references of this article.

2) The authors mention wireless broadband communication networks (5G) in the introduction, but they do not provide a reference for it. It would be useful for readers of MDPI Photonics if the authors cited review articles on 5G and other important applications in the introduction, such as:

1. M. A. Ilgaz et al, "A flexible approach to combating chromatic dispersion in a centralized 5G network", Opto-electronics review, vol. 28, no. 1, pp. 35-42, 2020.

2. L. Maleki, "The opto-electronic oscillator (OEO): Review and recent progress," 2012 European Frequency and Time Forum, Gothenburg, Sweden, 2012, pp. 497-500, doi: 10.1109/EFTF.2012.6502432.

3. M. A. Ilgaz, et al, "Phase-Noise Degradation of an Optically Distributed Local Oscillator in a Radio Access Network", Radioengineering, vol. 30, no. 1, pp. 10-15, 2021.

3) I assume that the equations presented are not new. The authors should use references when the equations do not show their own results from their own derivation. For example, for equation (1).

 4) All figures must be referenced in the text. Figure 1 (a), (b) and (c), for example, is not referenced in the text at all.

5) The resistor and capacitor are shown in Figure 1 (b). Authors should provide the values for these electronic components.

6) The authors should provide the test setup for the measurement of S21 and S11.

7) Some photos of a test setup are more than welcome. The authors should provide a photo of the experimental setup.

8) The authors mention attenuators in the system after the generators, but these attenuators are not shown in Figure 5. What were the values of these attenuators?

9) The authors should indicate the wavelength of the laser used and the type of laser.

10) In Figure 7, it is very difficult to distinguish green and blue lines. The authors  should use two different figure legends.

11) In the experimental verification section, the authors have made measurements with frequency dependence. I think some measurements depending on the wavelength of the laser should also be presented.

Author Response

First of all, I would like to express my sincere gratitude to the reviewer for his attention to our manuscript and valuable criticism, which will undoubtedly contribute to a better presentation of our research. Our detailed responses to each reviewer comment are here.

1) The work in this paper is similar to a previously published journal article and the authors have not cited this published article.- Parfenov, M.V., Tronev, A.V., Il’ichev, I.V. et al. Photorefractive Correction of the Coupling Ratio of an Integrated Optical Directional X-Coupler on a Lithium Niobate Substrate. Tech. Phys. Lett. 45, 187–189 (2019). https://doi.org/10.1134/S1063785019030131- Parfenov, M.; Agruzov, P.; Tronev, A.; Ilichev, I.; Usikova, A.; Zadiranov, Y.; Shamrai, A. Metal Electrodes for Filtering the Localized Fundamental Mode of a Ridge Optical Waveguide on a Thin Lithium Niobate Nanofilm. Nanomaterials 2023, 13, 2755. https://doi.org/10.3390/nano13202755Authors should cite the above papers and explain the difference between this article and previously published papers. Previously published papers must be cited between the references of this article. 

These papers related to the topic of lithium niobate integrated optics, but not directly to the modulator based on the directional coupler. Nevertheless, we have added references to these works ([38] and [37]) in the part of the Manuscript dedicated to the production of experimental samples. 

2) The authors mention wireless broadband communication networks (5G) in the introduction, but they do not provide a reference for it. It would be useful for readers of MDPI Photonics if the authors cited review articles on 5G and other important applications in the introduction, such as:1. M. A. Ilgaz et al, "A flexible approach to combating chromatic dispersion in a centralized 5G network", Opto-electronics review, vol. 28, no. 1, pp. 35-42, 2020. 2. L. Maleki, "The opto-electronic oscillator (OEO): Review and recent progress," 2012 European Frequency and Time Forum, Gothenburg, Sweden, 2012, pp. 497-500, doi: 10.1109/EFTF.2012.6502432. 3. M. A. Ilgaz, et al, "Phase-Noise Degradation of an Optically Distributed Local Oscillator in a Radio Access Network", Radioengineering, vol. 30, no. 1, pp. 10-15, 2021. 

We have extended the list of cited papers covering the most important applications ([12 – 19]), including references suggested by the reviewer. 

3) I assume that the equations presented are not new. The authors should use references when the equations do not show their own results from their own derivation. For example, for equation (1). 

References have been added. 

4) All figures must be referenced in the text. Figure 1 (a), (b) and (c), for example, is not referenced in the text at all. 

We checked that all figures were mentioned and discussed in the text. 

5) The resistor and capacitor are shown in Figure 1 (b). Authors should provide the values for these electronic components. 

The values of 30 Ohms and 100 pF have been added in Figure 1 (b). 

6) The authors should provide the test setup for the measurement of S21 and S11. 

A block diagram and photograph of the setup  have been added to Figure 5. 

7) Some photos of a test setup are more than welcome. The authors should provide a photo of the experimental setup. 

We have added photographs of the experimental setups. 

8) The authors mention attenuators in the system after the generators, but these attenuators are not shown in Figure 5. What were the values of these attenuators? 

This is Figure 8 in the revised manuscript. Corrections have been made. 

9) The authors should indicate the wavelength of the laser used and the type of laser.

The experiments were carried out using a TeraXion PureSpectrum™-PS-TNL tunable continuous-wave semiconductor laser. In most experiments, the laser was tuned to the center of the telecommunications C-band at a wavelength of 1555 nm, excluding measurements of the spectral dependence of the splitting ratio of the directional coupler. Appropriate explanations have been added to the text (in the description of the experimental setups). 

10) In Figure 7, it is very difficult to distinguish green and blue lines. The authors  should use two different figure legends. 

This is Figure 10 in the revised manuscript. During the measurements, the RF oscillators had the same power, which led to equal power of the two third-order intermodulation terms, so the curves in Fig. 10 for the first harmonics (775 and 776 MHz), as well as the curves for the third order intermodulation terms (774 and 777 MHz) are superimposed on each other. Appropriate clarifications have been added to the text. 

11) In the experimental verification section, the authors have made measurements with frequency dependence. I think some measurements depending on the wavelength of the laser should also be presented. 

We have added the dependence of the splitting ratio of the experimental DCM sample on the wavelength of light (Fig. 3.) and discussed the effect of wavelength in the text. A more detailed study of the influence of wavelength is beyond the scope of this work.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

My principal objections to the manuscript are answered, corrected, and added as further explanations. Since I don’t have other concerns about the manuscript, the work may be accepted as it stands.