Design and Investigation of Linearly Polarized Modal Next-Generation Passive Optical Network–Free Space Optics System Considering Fiber-Wireless Link Losses

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Here are a few Comments:

1. Some sentences are too long and complex. Consider breaking them into shorter, more direct statements. Example: "The enhancement has been observed by moving from single to other multi-wavelength, polarization, and modulation techniques." Suggested Improvement: "Enhancements have been achieved through multi-wavelength, polarization, and modulation techniques." 

The phrase “faithful transmission” is used often. Consider “reliable” or “accurate” transmission instead.

The motivation section could be made more precise.

Equations should have more context before and after they are introduced.

Some references lack full details (e.g., missing page numbers, journal volume/issues).

The future scope mentions higher-order modes and amplifiers but lacks specific recommendations.

Consider discussing:

Possible hardware implementation challenges.

Integration with 6G and smart city networks.

Comments on the Quality of English Language

Needs a thorough improvement.

Author Response

Responses to the comments from Reviewer #1

 

Comment 1): Some sentences are too long and complex. Consider breaking them into shorter, more direct statements. Example: "The enhancement has been observed by moving from single to other multi-wavelength, polarization, and modulation techniques." Suggested Improvement: "Enhancements have been achieved through multi-wavelength, polarization, and modulation techniques."

 

Response: Thank you. As suggested, the given statement and other long statements are modified into simple statements.

Please see page no. 2.

 

Comment 2): The phrase “faithful transmission” is used often. Consider “reliable” or “accurate” transmission instead.

 

Response: As per suggestion, the “faithful” word is replaced by “reliable” in the text.

 

Comment 3): The motivation section could be made more precise.

 

Response: As suggested, the motivation section is précised.

Please see page no.3.

 

Comment 4): Equations should have more context before and after they are introduced.

 

Response: All equations are context before their introduction.

Please see page no.7, 8, 11, 14.

 

Comment 5): Some references lack full details (e.g., missing page numbers, journal volume/issues).

 

Response: All references are updated with full details.

Please see page no. 16-17.

 

Comment 6): The future scope mentions higher-order modes and amplifiers but lacks specific recommendations.

 

Response: As suggested, the future scope is mentioned with higher-order modes and amplifiers recommendations.

Please see page no. 16.

 

Comment 7): Consider discussing:

Possible hardware implementation challenges.

Integration with 6G and smart city networks.

 

Response: As suggested, the possible hardware implementation challenges and integration with 6G & smart city networks are added. 

Please see page no. 15.

 

Comment 8): The manuscript "Design and Investigation of LP Modal Next Generation PON-FSO System Considering Fiber-Wireless Link Losses” presents the ability of MDM multiplexing to enhance system capacity of bidirectional NG-PON-FSO under the impact of MMF fiber non-linearities and link losses for both downlink and uplink transmission.

 

Response: Thank You.

 

Comment 9): The manuscript is well-organized and comprehensively described. This work significantly contributes to the field and is highly interesting to readers. Therefore, I recommend accepting this manuscript.

 

Response: Thank You.

 

 

 

 

 

 

 

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript "Design and Investigation of LP Modal Next Generation PON-FSO System Considering Fiber-Wireless Link Losses” presents the ability of MDM multiplexing to enhance system capacity of bidirectional NG-PON-FSO under the impact of MMF fiber non-linearities and link losses for both downlink and uplink transmission.

The manuscript is well-organized and comprehensively described. This work significantly contributes to the field and is highly interesting to readers. Therefore, I recommend accepting this manuscript.

Author Response

We sincerely appreciate your positive feedback and recommendation for the acceptance of our manuscript, "Design and Investigation of LP Modal Next Generation PON-FSO System Considering Fiber-Wireless Link Losses."

We are grateful for your recognition of our work's significance and its contribution to the field. Your encouraging comments motivate us to continue our research in advancing high-capacity and energy-efficient optical communication systems.

Thank you for your time and effort in reviewing our manuscript. We truly appreciate your valuable insights and support.

Reviewer 3 Report

Comments and Suggestions for Authors

This work presents simulation results of bidirectional 4-channel fiber optic communication system with bit rate 10Gbps per channel, operating at “L”-band” wavelengths (1596.0, 1596.8, 1597.6 and 1598. 4nm) with link, combined multimode graded-index optical fiber (GI-MMF) 50/125 with length about 5.5 km and 1 m free-space. Authors performed simulations in commercially available OptiSystem software. They researched BER to distance (GI-MMF link length) dependance, as well as how lens reflectance and focal length impact on BER, and estimated excited mode power fraction and their phase changing. Simulation results confirmed, that designed 4-channel 10Gbps optical communication system provides BER=10E-9 under GI-MMF length up to 5.5 km and free space distance 1 m.

The paper corresponds to the journal scope. It is suitable for publication in Photonics after major revision / improvements / corrections / suggestions and answering on following questions / comments:

1. First of all, proposed and designed optical communication system does not provide mode division multiplexing (MDM) technique. There are no any selected mode excitation and following detection, as well as there are no any mode multiplexers and demultiplexers (or mode filters) in the presented 4x10Gbps optical system scheme. Finally, authors simulated and researched 4-channel dense wavelength division multiplexing (DWDM – while selected channel division is 0.8 nm grid, it corresponds to DWDM technique) optical communication system with data transmission over 5.5 km GI-MMF and 1 m free space combined link. Therefore, it is proposed to correct “NG-PON/MDM” and change “MDM” to “DWDM”.
2. It should be explained, why so strange combined link (5.5 km of MMF with 1 m free space) was selected and researched? Are there any cases from practice?
3. More details should be added to system description. For example, TIA/ANSI Category of MMF (it will also help to confirm utilized in simulation selected attenuation 2.6 dB/km), and parameters of “spatial connector”. Finally, what is the “spatial connector”? Is it free space alignment system?
4. While authors considered 4-channel DWDM group signal propagation over GI-MMF, which differs by large core diameter (and respectively large effective area) in comparison with single mode optical fiber, occurrence of non-linear effects (e.g. simulated four-wave-mixing) should be proven.
5. While did the authors consider the chosen wavelength? Why these channels were from “L”-band, while GI-MMF was utilized, which is optimized for “O”-band or 850 nm wavelength range (SWDM technique – Short Wavelength Division Multiplexing – 850…950 nm band)?
6. What does it mean “negative” azimuthal order of LP-mode”? According to Fig.3, it corresponds to mode polarization statement so vectorized spatial mode typical designation (HE12 etc.) is more correct. While authors finally considered vectorized spatial modes, how did they take into account polarization correction / propagation constant difference between modes with the same LP-order and different state of polarization?
7. How to explain local deviations of BER(distance), BER(lens focal length), BER(lens reflectance) from linear trend for 1524 nm channel?
8. It is recommended change “parabolic index” to “graded index” – well known and utilized typical abbreviation “GI-MMF” instead “PI-MMF”.
9. It is recommended to make Fig. 3, 8 and 9 more compact. While Fig. 3 represents well known “ideal” mode field distributions, they may be strongly decreased without any losses of information for readers. Also, it is proposed to set 4 eye-diagrams per 1 row on Fig. 5.
10. In the Table #1, optical fiber core radius is 25 um, while fiber cladding radius is 10 um. It is obviously misprint: assuming, that authors considered typical telecommunication graded index MMF 50/125 (core/cladding diameter 50/125), cladding radius should be 62.5 um
11. There are lot of misprints: #78 “ad”; Table No1  “Aperture diamter”, “Fiber attaunation”; #284 “Fi Bers”; #254 “non-linerities”; #259 “mini mum” et al.

Comments on the Quality of English Language

There are lot of misprints: #78 “ad”; Table No1  “Aperture diamter”, “Fiber attaunation”; #284 “Fi Bers”; #254 “non-linerities”; #259 “mini mum” et al.

Author Response

 Responses to the comments from Reviewer #3

 

This work presents simulation results of bidirectional 4-channel fiber optic communication system with bit rate 10Gbps per channel, operating at “L”-band” wavelengths (1596.0, 1596.8, 1597.6 and 1598. 4nm) with link, combined multimode graded-index optical fiber (GI-MMF) 50/125 with length about 5.5 km and 1 m free-space. Authors performed simulations in commercially available OptiSystem software. They researched BER to distance (GI-MMF link length) dependance, as well as how lens reflectance and focal length impact on BER, and estimated excited mode power fraction and their phase changing. Simulation results confirmed, that designed 4-channel 10Gbps optical communication system provides BER=10E-9 under GI-MMF length up to 5.5 km and free space distance 1 m. The paper corresponds to the journal scope. It is suitable for publication in Photonics after major revision / improvements / corrections / suggestions and answering on following questions / comments:

 

• Thank you for the suggestions.

 

Comment 1): First of all, proposed and designed optical communication system does not provide mode division multiplexing (MDM) technique. There are no any selected mode excitation and following detection, as well as there are no any mode multiplexers and demultiplexers (or mode filters) in the presented 4x10Gbps optical system scheme. Finally, authors simulated and researched 4-channel dense wavelength division multiplexing (DWDM – while selected channel division is 0.8 nm grid, it corresponds to DWDM technique) optical communication system with data transmission over 5.5 km GI-MMF and 1 m free space combined link. Therefore, it is proposed to correct “NG-PON/MDM” and change “MDM” to “DWDM”.

 

Response: Thank you. In this work, a proposed design of LP modal bidirectional NG-PON system using fiber-FSO link is realized and it is corrected in the manuscript. Based on next generation PON stage 2, ITU-T standardised NG-PON is realized in the proposed work, which incorporates a set of four downlink and uplink wavelengths at aggregate data rate of 40Gbps. For these specified wavelengths, 0.8 channel spacing is used as per ITU-T standard (G.989 series).

Please see page no. 4.

 

Comment 2): It should be explained, why so strange combined link (5.5 km of MMF with 1 m free space) was selected and researched? Are there any cases from practice?

 

Response: This mistake is improved and FSO range is modified. FSO range is considered under the impact of clear air weak turbulence condition.

Please see page no. 6-7.

 

Comment 3): More details should be added to system description. For example, TIA/ANSI Category of MMF (it will also help to confirm utilized in simulation selected attenuation 2.6 dB/km), and parameters of “spatial connector”. Finally, what is the “spatial connector”? Is it free space alignment system?

 

Response: In the proposed system, TIA-492AAAB standard OM2 MMF is used at 2.6dB/km attenuation. Also, the use of spatial connector is explained.

Please see page no. 4.

 

Comment 4): While authors considered 4-channel DWDM group signal propagation over GI-MMF, which differs by large core diameter (and respectively large effective area) in comparison with single mode optical fiber, occurrence of non-linear effects (e.g. simulated four-wave-mixing) should be proven.

 

Response: The occurrences of nonlinear effects are added. 

Please see page no. 7-8.

 

Comment 5): While did the authors consider the chosen wavelength? Why these channels were from “L”-band, while GI-MMF was utilized, which is optimized for “O”-band or 850 nm wavelength range (SWDM technique – Short Wavelength Division Multiplexing – 850…950 nm band)?

Response: The proposed system is based on ITU-T standardised next generation PON stage 2. This system incorporates a set of four downlink (L-band) and uplink (C-band) wavelengths at aggregate data rate of 40Gbps. For these specified wavelengths, 0.8 channel spacing is used as per ITU-T standard (G.989 series). In this work, the impact of using LP modes on PON-FSO networks is investigated. Besides, various existing works on LP modes using L or C band wavelengths are compared with the proposed work.

Please see page no. 14.

 

Comment 6): What does it mean “negative” azimuthal order of LP-mode”? According to Fig.3, it corresponds to mode polarization statement so vectorized spatial mode typical designation (HE12 etc.) is more correct. While authors finally considered vectorized spatial modes, how did they take into account polarization correction / propagation constant difference between modes with the same LP-order and different state of polarization?

 

Response: In Figure 3, LP modes are displayed in field amplitude representation means in spatial distribution of the electric field. In this representation, the spatial values indicate the mode's transverse profile instead of actual intensity in power units. Both space width X (horizontal spatial window=50µm) and space width Y (vertical spatial window=50µm) are measured in µm. Besides, the phase is shown in radian for all operating modes.

In the full vector model, LP_lm (l= azimuthal order, m= radial order) modes are analogous to the HE_lm modes. In this work, LP-modes are assumed to be scalar approximation, where ±l are equivalent. The same LP mode index corresponds to considerable vector modes such as LP11-⟩ HE11, TM01, TE01. Also, each of these modes has a somewhat different propagation constant owing to waveguide effects.

Please see page no. 4-6.

 

Comment 7): How to explain local deviations of BER(distance), BER(lens focal length), BER(lens reflectance) from linear trend for 1524 nm channel?

 

Response: As suggested, the reason of given BER deviations are added.

Please see page no. 9-11.

 

Comment 8): It is recommended change “parabolic index” to “graded index” – well known and utilized typical abbreviation “GI-MMF” instead “PI-MMF”.

 

Response:

Please see page no. 2.

 

Comment 9): It is recommended to make Fig. 3, 8 and 9 more compact. While Fig. 3 represents well known “ideal” mode field distributions, they may be strongly decreased without any losses of information for readers. Also, it is proposed to set 4 eye-diagrams per 1 row on Fig. 5.

 

Response: As suggested, the given figures are modified.

Please see page no. 4-6, 12, 13 and 9.

 

Comment 10): In the Table #1, optical fiber core radius is 25 um, while fiber cladding radius is 10 um. It is obviously misprint: assuming, that authors considered typical telecommunication graded index MMF 50/125 (core/cladding diameter 50/125), cladding radius should be 62.5 um

 

Response: The fiber specifications are corrected.

Please see page no. 2.

 

Comment 11): There are lot of misprints: #78 “ad”; Table No1  “Aperture diamter”, “Fiber attaunation”; #284 “Fi Bers”; #254 “non-linerities”; #259 “mini mum” et al.

 

Response: page 2, page 7, page 7, page 15, page 14, page 14.

Please see page no. 6-7.

 

 

 

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

Review comments:
Innovation and Contribution
The paper proposes a bidirectional NG-PON FSO system based on the LP [0,1] mode, which combines PIMMF and FSO links and is innovative in improving transmission capacity and reducing crosstalk. The feasibility of a symmetrical speed of 40Gbps within 5.5 kilometers was verified through simulation, and the influence of lens parameters on performance was analyzed, providing new ideas for future optical access network design. However, the paper did not fully explain why the LP [0,1] mode was chosen over other higher-order modes. It is suggested to supplement theoretical basis or compare data with other modes to highlight its unique advantages.
Methodology and Experimental Design
The experiment used OptiSystem simulation tool, with reasonable parameter settings (such as PIMMF length, lens reflectivity, etc.), but lacked detailed theoretical derivation or reference support for key parameters (such as fiber attenuation coefficient, nonlinear effect model). In addition, it was not mentioned whether actual environmental factors such as temperature fluctuations and atmospheric turbulence were considered in the simulation, which may affect the universality of the results. Suggest supplementing the theoretical background of parameter setting and adding discussion on actual environmental interference.
Results and Analysis
The data shows that the BER of the system is lower than 1e-9 within 5.5 kilometers, and the lens focal length and reflectivity have a significant impact on performance. The conclusion is reliable. However, the performance differences of different wavelengths in Figure 4 (relationship between distance and BER) (such as the sharp increase in BER at 1597.6nm at 5.4km) have not been reasonably explained, which may be due to insufficient exploration of fiber nonlinear effects or wavelength dependence. Suggest increasing the analysis of wavelength sensitivity and verifying the reproducibility of the results.
Writing and Structure
The paper has a clear structure and rich charts (such as pattern distribution diagram and eye diagram), but some charts are not clearly labeled (for example, the coordinate axes of the 3D pattern diagram in Figure 3 are not labeled with units). In addition, the citation and contextual connection of formulas (1) and (4) are stiff, and the logical coherence of formula derivation needs to be optimized. There are a few grammar errors in the language (such as spelling errors in "fiber nonlinearity"), it is recommended to unify the terminology throughout the text and polish it.
Practical application and comparative analysis
The paper highlights the advantages of high channel capacity (4 × 10Gbps) and low BER (1e-9) compared with existing research through Table 4, but does not discuss system costs (such as the deployment cost difference between PIMMF and SMF) or scalability (such as the potential to support more modalities). In addition, the adjustment of lens parameters in actual deployment may face engineering challenges (such as dynamic alignment issues). It is recommended to supplement the feasibility analysis of practical application scenarios and explore optimization directions for future hardware implementation.

Author Response

 Responses to the comments from Reviewer #4

 

Comment 1): Innovation and Contribution

The paper proposes a bidirectional NG-PON FSO system based on the LP [0,1] mode, which combines PIMMF and FSO links and is innovative in improving transmission capacity and reducing crosstalk. The feasibility of a symmetrical speed of 40Gbps within 5.5 kilometers was verified through simulation, and the influence of lens parameters on performance was analyzed, providing new ideas for future optical access network design. However, the paper did not fully explain why the LP [0,1] mode was chosen over other higher-order modes. It is suggested to supplement theoretical basis or compare data with other modes to highlight its unique advantages.

 

Response: Thank you. As suggested, the reason to choose the LP[0,1] mode over other higher-order modes is added.

Please see page no. 4.

 

Comment 2): Methodology and Experimental Design

The experiment used OptiSystem simulation tool, with reasonable parameter settings (such as PIMMF length, lens reflectivity, etc.), but lacked detailed theoretical derivation or reference support for key parameters (such as fiber attenuation coefficient, nonlinear effect model). In addition, it was not mentioned whether actual environmental factors such as temperature fluctuations and atmospheric turbulence were considered in the simulation, which may affect the universality of the results. Suggest supplementing the theoretical background of parameter setting and adding discussion on actual environmental interference.

 

Response: As suggested, the text is modified with detailed with reference support. Table 1 shows various performance parameters used in the paper.

Please see page no. 4-7.

In this work various environmental factors are given in manuscript. Please see page no. 7.

 

Comment 3): Results and Analysis

The data shows that the BER of the system is lower than 1e-9 within 5.5 kilometers, and the lens focal length and reflectivity have a significant impact on performance. The conclusion is reliable. However, the performance differences of different wavelengths in Figure 4 (relationship between distance and BER) (such as the sharp increase in BER at 1597.6nm at 5.4km) have not been reasonably explained, which may be due to insufficient exploration of fiber nonlinear effects or wavelength dependence. Suggest increasing the analysis of wavelength sensitivity and verifying the reproducibility of the results.

 

Response: The relationship between distance and BER with sharp increase in BER at 1597.6nm at 5.4km is explained. 

Please see page no. 9.

 

Comment 4): Writing and Structure

The paper has a clear structure and rich charts (such as pattern distribution diagram and eye diagram), but some charts are not clearly labeled (for example, the coordinate axes of the 3D pattern diagram in Figure 3 are not labeled with units). In addition, the citation and contextual connection of formulas (1) and (4) are stiff, and the logical coherence of formula derivation needs to be optimized. There are a few grammar errors in the language (such as spelling errors in "fiber nonlinearity"), it is recommended to unify the terminology throughout the text and polish it.

 

Response: In OptiSystem tool, the generated LP modes are displayed in field amplitude representation means in spatial distribution of the electric field. In this representation, the spatial values indicate the mode's transverse profile instead of actual intensity in power units. Both space width X (horizontal spatial window) and space width Y (vertical spatial window) are measured in µm. Besides, the phase is shown in radian for all operating modes.

As suggested, Figure 3 is modified with explanation.

Please see page no. 4-6.

Also, the spelling errors of given text is corrected.

All formulae are more derived.

 

Comment 5): Practical application and comparative analysis

The paper highlights the advantages of high channel capacity (4 × 10Gbps) and low BER (1e-9) compared with existing research through Table 4, but does not discuss system costs (such as the deployment cost difference between PIMMF and SMF) or scalability (such as the potential to support more modalities). In addition, the adjustment of lens parameters in actual deployment may face engineering challenges (such as dynamic alignment issues). It is recommended to supplement the feasibility analysis of practical application scenarios and explore optimization directions for future hardware implementation.

 

Response: Thank You. As suggested, Table 4 is modified.

Please see page no. 14-15.

The practical engineering challenges for the deployment of the proposed system are explored.

Please see page no. 15.

Responses to the comments from Journal

 

Comment 1): To facilitate transparent and open science, we encourage authors to publish their results and experimental methodology in as much detail as possible so that results can be reproduced. We noticed that the main text of your manuscript is quite brief which may mean that the materials and methods, research background, future research directions, or possible applications of the research are not described in enough detail.

 

Response: Thank you. The result section is improved. Also the proposed work, research background, future research directions and applications are described in detail.

 

Comment 2): Please consider the following points in your revisions: adding full experimental details, presenting completely all the results, and describing a comprehensive background to the research in the introduction section.

 

Response: As suggested, the proposed work, obtained results with discussions, background of the research are added in detail.

 

Comment 3): Furthermore, please add a section of discussion. A well prepared discussion section will greatly improve the value of the paper. Requirements of Discussion Section: It is essential to discuss one's results in detail. One should take into consideration other published work on the subject. One should tell the readers how one's results compared with other studies and/or how one's work is a groundbreaking study. In discussions, it is essential to appropriately refer to other published work, relative to the results obtained in one's paper.

 

Response: As per suggestion, a separate discussion section is added with comparison analysis of the proposed with existing ones.

Please see page no. 14-17.

 

Thank you again for your valuable comments and suggestions.

 

 

 

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

This work presents simulation results of bidirectional 4-channel fiber optic communication system with bit rate 10Gbps per channel, operating at “L”-band” wavelengths (1596.0, 1596.8, 1597.6 and 1598. 4nm) with link, combined multimode graded-index optical fiber (GI-MMF) 50/125 (Cat. OM5) with length about 5.5 km and 100 m free-space. Authors performed simulations in commercially available OptiSystem software. They researched BER to distance (GI-MMF link length) dependance, as well as how lens reflectance and focal length impact on BER. Also excited mode power fraction and their phase changing were estimated. Simulation results confirmed, that designed 4-channel WDM 10Gbps optical communication system provides BER=10E-9 under GI-MMF (Cat. OM5) length up to 5.5 km and free space distance 100 m.

Authors strongly improved the paper, made necessary corrections, according to previous review round comments and suggestions. The paper corresponds to the journal scope. It may be published in the Photonics in the present version.

Reviewer 4 Report

Comments and Suggestions for Authors

Accept in present form