Performance Comparison of Different Modulation Formats for a 40 Gbps Hybrid Optical CDMA/DWDM System against ISI and FWM
Round 1
Reviewer 1 Report
In the paper, the authors compared three different optical modulation formats. After carefully reading this paper, I see the following serious flaws with this paper:
1) It is difficult for the reader to tell whether the paper is based on simulations or experiments. This should be made clear in the abstract by saying that this is a theoretical investigation and that the results were obtained by simulations.
2) It would be useful for the reader of Photonics if the authors cited review articles on optical networks in the introduction, e.g., M. Vidmar, "Optical-Fiber Communications: Components and Systems", Informacije MIDEM, vol. 31 (2001), no. 4, pp. 246-251 http://www.midem-drustvo.si/Journal%20papers/MIDEM_31(2001)4p246.pdf and B. Batagelj, V. Janyani, and S. Tomazic, ''Research challenges in optical communications towards 2020 and beyond,'' Informacije MIDEM, vol. 44 (2014),no. 3, pp. 177-184 http://www.midem-drustvo.si/Journal%20papers/MIDEM_44(2014)3p177.pdf
3) Also missing are some very important references on optical modulation formats, e.g., P. J. Winzer and R. -. Essiambre, ''Advanced Optical Modulation Formats,'' in Proceedings of the IEEE, vol. 94, no. 5, pp. 952-985, May 2006, doi: 10.1109/JPROC.2006.873438.
4) Sometimes no space is inserted between the numeric value and the units, but space should always be the case. For example, in line 56, the 40 Gbps should be written with space. The same problem with spaces is repeated in lines 153, 154, 155, 194.
5) It is not clear from Figure 1 where the data inputs and outputs are located. The authors should add the arrows for the data input and output in Figure 1.
6) The DCF should be shown in Figure 1.
7) 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.
8) It is a mistake that equation 8 is repeated twice.
9) The authors should take care of their English. Some sentences are not clear. For example, "The EOPM is used to module simultaneously modulate the signals'phase which are carried by DWDM channels to eradicate the intra-channel FWM effect."
10) Reference to the statement "it has been reported in the literature that the performance of MDRZ in purely WDM systems outperforms the RZ modulation format" is missing. The authors should add the reference.
11) The discussion of experimental results in the figures is not detailed enough. It is not enough to just discuss the general trends and say that it has better performance. The authors should expand the discussion section. I prefer to have the results and discussion in two separate sections.
12) In the conclusion, it should be written that the results were obtained by simulations.
13) A very relevant research area, but it would be more useful to the community if the work was extended to include an experimental implementation of the modulation formats in a real optical network.
Author Response
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Author Response File: 
Author Response.docx
Reviewer 2 Report
1. It is not clear why the authors choose 40 Gbit/s per channel. The total Bit rate or Baud rate of the system is not been evaluated.
2. Please clarify why the expression 8 is mentioned with different indexes and with different variables – some italic some not?
3. In the scheme of operation of the system are splitters in the transmitter and receiver. What is the split ratio in these splitters?
4. Fig. 2- Fig. 4 the average BER and transmitted power for random channels are shown, so it is not clear why this principle was chosen when it is known that the system throughput is affected by the worst channel performance?
5. The reference [11] mentioned by the authors in line 65 and in the conclusions does not contain a detailed description of the EOPM parameters and the operating principles of the component. Please, authors, provide this information.
6. How do the authors propose to separate FWM from other nonlinear optical effects that arise from the same physical reason?
7. Chapter 4 mentions the number of channels 15. Why was this number of channels chosen and in which WDM band are these channels? Is the asymmetry of the channel spacing taken into account to reduce the effect of FWM on the signal quality?
8. The paper doesn’t contain any spectrum and eye diagrams to prove the results and conclusion, just BER values. The BER threshold is also not mentioned.
9. The paper shows simulation results, however, it would be nice to see some results validated by experimental data.
Comments for author File: 
Comments.pdf
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
The authors have investigated three different modulation formats including carrier suppressed return to zero, modified duobinary return to zero and return to zero to choose the optimum modulation format for the hybrid optical CDMA/DWDM system with the implementation of electro-optic phase modulator at a data rate of 40Gbps per channel. The manuscript is interesting and useful for the design and application of hybrid optical CDMA/DWDM system. The paper is acceptable to be published in Photonics, provided the following issue can be addressed
- Some abbreviations should be clarified when they appear for the first time.
- Add more details in the table to describe the physical parameters of the considered transmission setup.
- Improve the quality of Figure 1.
- Add some discussion and explanation why the measured BER can go down to 10^-14. This is such a low value.
- Add some discussions regarding the transmission impairments in the considered optical communications, such as dispersion and phase noise
See e.g.
C Jin et al., Nonlinear coherent optical systems in the presence of equalization enhanced phase noise, IEEE Journal of Lightwave Technology, 2021.
A. Ghazi et al, A Systematic review of multi-mode fiber based on dimensional code in optical-CDMA, Journal of Physics: Conference Series, 2021.
Author Response
Please check the attached report
Author Response File: Author Response.docx
Reviewer 4 Report
Using numerical simulations, the authors have investigated three different modulation formats (CSRZ, MDRZ and RZ) for the hybrid optical CDMA/DWDM system. Such system used electro-optic phase modulator at a data rate of 40Gbps per channel. The main objective of this paper was to find the optimum modulation format for the proposed system. They have analyzed the average BERs versus the transmitted power. They have also found that the performance of the proposed hybrid system based RZ format is better than the performance of that based on MDRZ format.
Overall, the manuscript needs a lot of improvement in writing and English usage. More importantly, the proposed modulation formats should be compared with other modulations (RZ-DPSK, NRZ-DPSK and CRZ-DPSK).
In conclusion, I believe that the manuscript needs a lot of improvement in the presentation and discussion of the results. The authors are suggested to consider the comments below.
1. Other than numerous instances of poor grammar and a number of sentences without a verb, the paragraph/sentence describing the obtained results in the presented work is repeated several times at different places in the manuscript.
2. It is thought that the novelty of the proposed modulation formats used to tolerant to optical fiber nonlinearity has not been discussed (deeply and in detail) enough by using references in the introduction section. In addition to the existing references, some up-to-date references must be given in this section.
3. The construction and principles differences between the different modulation formats should be more explained by the authors. Moreover, the optical generation of such modulations formats should be discussed in the second section.
4. In the literature, various modulations formats have already been well proposed with high-tolerance to chromatic dispersion and polarization mode dispersion (Baghdouche, L. K., & Simohamed, L. M. (2013, May). In 2013 8th International Workshop on Systems, Signal Processing and their Applications (WoSSPA) (pp. 476-480). IEEE, Kassa-Baghdouche, L., & Simohamed, L. M. (2011, May). In International Workshop on Systems, Signal Processing and their Applications, WOSSPA (pp. 159-162). IEEE. ). These modulation formats should be discussed in the manuscript. Moreover, the difference between these modulation formats and the proposed modulations should be included in the manuscript.
5. In the fourth section (Results and Discussion), the authors used only the average BER for analyzing the performances of the hybrid optical CDMA/DWDM system based on different modulation formats. However, there are many criteria for analysis the performances of the optical systems such as : the quality factor and the eye diagram. Such criteria’s (quality factor and eye diagram) should be also used for the analyzing the performances of the proposed hybrid system.
6. In this manuscript, only numerical simulations were performed by the authors for analyzing the performance of different modulation formats for 40 Gbps hybrid optical CDMA/DWDM system, and any experimental results were not given. If the numerical results of this study had been extended with additional experimental results, it would be better.
Author Response
Please check the attached report
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
My main objections to the manuscript are answered, corrected, and added as further explanations. The authors have addressed all comments and updated the manuscript.
I have only one more correction request to the manuscript, then the paper can be accepted.
1) The space between the numerical value and the units should always be written. The authors should insert the space in lines 16 and 17 at 40 Gbps, 22 dBm and 105.075 km.
Reviewer 4 Report
The authors have implemented my suggestions.
