Modeling S-Band Communication Window Using Random Distributed Raman Laser Amplifier

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the authors present numerical and some experiments results demonstrating how a random Distributed Feedback DFB Raman amplifier with a half-open cavity design enhances the S-band (196.2–201.1 THz) telecommunication window, making it suitable for high-density WDM systems. I recommend that the paper be accepted for publication upon the authors addressing the revisions suggested.

1. The abstract is not physically relevant. What are the novelty and concepts compared to the other studies (ref 6, 7, and 15)? I recommend that the authors rewrite the abstract.
2. In the introduction, lines 64–65 contain a sentence with unclear punctuation. I suggest the authors revise it for clarity.
3. In the introduction the authors provide a summary of research of C-/S-band systems Raman Laser amplifier. I suggest that the authors include additional details about the limitations and comparisons between their study and references 6, 7, 15.
4. For the experimental setup, I suggest authors to give more information of the measurement of Fig 2, 3, and 4 in the main text.

Author Response

Thank you for your time and useful comments to help to improve the manuscript. Below I try to answer your comments. I'm also attaching w pdf file with tracked changes:

Comments 1 [The abstract is not physically relevant. What are the novelty and concepts compared to the other studies (ref 6, 7, and 15)? I recommend that the authors rewrite the abstract.]

Response 1 [

Abstract was rewritten and reads “This study simulates an open-cavity random distributed Raman amplifier for optimal performance across a 5 THz S-band spectrum (196.2–201.1 THz; 1490.76–1527.99 nm), evaluating its capacity via a 50-channel WDM grid with 100 GHz spacing. The primary Raman pump wavelength was tuned from 1318–1344 nm to identify the optimal point. A Fiber Bragg Grating (FBG), placed at the end of a 60 km single-mode fiber and upshifted 88 nm from the pump, enhances efficiency by transferring energy to amplified signal, minimising power variation. Results yield <2 dB gain ripple across channels using raw Raman amplification without flattening filters with minor degradation from residual channels, confirming the DRA design's viability for high-density S-band optical communication expansion."

Comments 2 [In the introduction, lines 64–65 contain a sentence with unclear punctuation. I suggest the authors revise it for clarity.]

Response 2 [This explanation was not needed in introduction, hence was fully removed.]

Comments 3 [In the introduction the authors provide a summary of research of C-/S-band systems Raman Laser amplifier. I suggest that the authors include additional details about the limitations and comparisons between their study and references 6, 7, 15.]

Response 3 [I believe, that the reasoning of the particular choice of an amplifier is already stated “The choice of a random DFB Raman amplifier is motivated by its superior performance over conventional Raman amplification schemes, as demonstrated in prior work on long-haul transmission systems [16-19].”

Ref. 15 published in 2015  was our first paper with full numerical description of the amplifier that was found to actually work with a real transmission data (this is based after our results published in march 2015 at OFC “Extended Reach of 116 Gb/s DP-QPSK Transmission using Random DFB Fiber Laser Based Raman Amplification and Bidirectional Second-order Pumping”). Based on the superior results (I mean experimental transmission data) we decided to continue to use this configuration in all further publications -although in few cases URFL or first order Raman configuration would be more efficient we avoided it because we know it’s never gonna work in a transmission systems due to RIN noise.

In the same year (2015) in NLO conference, we published our first paper on asymmetry optimization where we compared 3 Raman amplifiers, and it happened that random DFB laser amplifier gave us best results in terms of asymmetry optimization. References 6 and 7 were published in 2022 and 2023 and refers to asymmetry optimization for an OPC system, hence its not so straight forward to compare the results from these particular references to this work as the optimisation procedures are different (asymmetry vs gain optimisation).]

Comments 4 [For the experimental setup, I suggest authors to give more information of the measurement of Fig 2, 3, and 4 in the main text.]

Response 4 [This is now explained and referenced “Using modified optical time domain reflectometer (OTDR) technique together with the tunable laser [28] it was possible to experimentally measure attenuation profile of standard SMF depicted in Figure 2. These in turn were used as parameters in simulations for the Raman pump (, FBG ) and each spectral component. Such approach ensures realistic modeling of signal and pump losses across the whole spectrum.”]

      

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

General comments

The article presents the simulation of an open cavity random distributed Raman amplifier to be optimized in the 196.2-201.1 THz (1490.76-1527.99 nm) range with a pump wavelength varying from 1318 to 1344 nm. To enhance amplification efficiency and mitigate signal loss, a fiber Bragg grating was incorporated at the end of a 60 km single-mode fiber span upshifted by 88 nm with reference to the Raman pump wavelength. The aim of the study was to investigate the nonlinear effects and pump efficiency limitations related to distributed Raman amplification in the S-band (1460–1530 nm) with the further aim to facilitate the use of the S-band in WDM links.     

The content of the article is within the scope of the Journal.

The main contribution of the article is the simulation of this particular WDM link transmitting 50 channels with a 100 GHz spacing and using an open cavity random distributed Raman amplifier optimized for operation in the 196.2-201.1 THz sub-region of the S-band.

The article provides a sufficient literature review, mainly included in the “Introduction” section. The simulated configuration is shown in fig. 1 while the mail results of the study are shown in figs 5-8 (net gain and OSNR variation with respect to pump wavelength and pumping power) as well as in fig. 11 (showing the theoretical prediction of four-wave mixing power). The conclusions presented in section 4 are in accordance with the results of the study and include suggestions for further work.

Specific comments

Line 213: Please check whether the “1318/1408” should read “1318/1406”.

Figs 5-8: The “P₁” notation is used for both the pump wavelength and the pumping power. The author could use the “λ_P1” and “P₁” notations respectively.

Fig 8: The “box” inside the graph refers to values of wavelength, instead values of the pumping power, so it should be replaced.

Though the study regards attenuation and amplification that are hardly affected by the transmission rate, a reference to the bit-rate carried by each wavelength should, nevertheless, be made for the sake of completeness. If this bit-rate is 10 Gb/s or higher (and though dispersion is outside the scope of this study) a short comment could be made (by just adding a few lines in section 4) with regard to also taking into account the effects of chromatic and/or polarization-mode dispersion when it comes to the design of actual links.

Use of English

The article is well written so regarding the use of English, a minor editing would be sufficient.

Review decision

Given that the article provides novel results that are presented in a complete and scientifically sound way, I consider it publishable subject to the revisions mentioned above.

Comments on the Quality of English Language

The article is well written so regarding the use of English, a minor editing would be sufficient.

Author Response

I would like to thank you for your useful comments and spotted mistakes within the figures that are crucial. I have changed all the graphs with your recommendations. Im attaching pdf with tracked changes. 

Comments 1 [Line 213: Please check whether the “1318/1408” should read “1318/1406”.]

Response 1 [This is correct, the first result was actually with a 90 nm shift. All the other results are with 88 nm shift (pump -> FBG).]

Comments 2 [Figs 5-8: The “P₁” notation is used for both the pump wavelength and the pumping power. The author could use the “λ_P1” and “P₁” notations respectively.]

Response 2 [This is all changed now with your recommendations]

Comments 3 [ Fig 8: The “box” inside the graph refers to values of wavelength, instead values of the pumping power, so it should be replaced.]

Response 3 [Thank you for spotting this mistake, it is now changed accordingly. ]

Comments 4 [Though the study regards attenuation and amplification that are hardly affected by the transmission rate, a reference to the bit-rate carried by each wavelength should, nevertheless, be made for the sake of completeness. If this bit-rate is 10 Gb/s or higher (and though dispersion is outside the scope of this study) a short comment could be made (by just adding a few lines in section 4) with regard to also taking into account the effects of chromatic and/or polarization-mode dispersion when it comes to the design of actual links.]

Response 4 [There is no data transmission involved in this study, hence no reference to bit rate; however, it is worth noting that pumps are fully depolarized. It is now added in simulation description:

“The simulation model accounts for residual Raman gain from fully depolarized primary pump”]

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This review work is an interesting work to the readers. However, some other comments are needed to address as follows:

 

1. Abstract:

The author claimed that “To to solve boundary value problems for differential equations, system was numerically simulated with a 4th order Runge Kutta method with a 1 m step”. This sentence contains “To to”, and it seems that there is an extra word “to”. Please confirm.

2. In the Introduction of this manuscript, the author cited 26 relevant references (including self-published papers). Please include and explain the novelty of this research in the Introduction.
3. The authors claimed that “The attenuation parameters for the Raman pump (𝑃₁^+/-), FBG (𝑃₂^+/-) and each spectral component were derived from the experimentally determined attenuation profile of standard SMF, as depicted in Figure 2, ensuring realistic modeling of signal and pump losses across the whole spectrum”. Please explain how the attenuation spectrum information of the standard (Figure 2) was determined experimentally so that readers can understand and evaluate it.
4. The authors claimed that “The attenuation profile of the WDM grid, illustrated in Figure 3, offers essential insights into the spectral loss characteristics across the 50-channel array spanning the S-band”. Please explain how the attenuation spectrum of the WDM gride (Figure 3) was experimentally determined.
5. Figure 4. Normalised Raman gain spectrum in standard SMF silica, the unit of the physical quantity (Gain shift) on the horizontal axis can be expressed in nm since the study discusses S-band, it may make it clear to readers.
6. Page 6 of 14, Line 216-218: the author claimed that “The on-off net gain, measured in decibels (dB), was plotted for each channel to assess the amplification performance across the random DFB Raman amplifier system over a 60 km SMF”, as shown in Fig. 5. Please explain how “the on-off net gain” is defined and calculated in this work. In addition, what does the negative net gain (dB) in Figure 5 mean? Please explain it clearly.
7. Figure 8. OSNR dependance due to primary Raman pump power, please explain why the OSNR (dB) data under the condition of P1=1344 nm/FBG=1432 nm does not appear in this graph?
8. The reference format is inconsistent. Please edit it according to the reference format of “Electronics” journal.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Thank you for your in depth review and comments -few points were also spotted by other reviewers so I try to address it all. I'm also attaching a pdf with tracked review.

Comments 1 [The author claimed that “To to solve boundary value problems for differential equations, system was numerically simulated with a 4th order Runge Kutta method with a 1 m step”. This sentence contains “To to”, and it seems that there is an extra word “to”. Please confirm.]

Response 1 [The abstract is now completely rewritten]

Comments 2 [In the Introduction of this manuscript, the author cited 26 relevant references (including self-published papers). Please include and explain the novelty of this research in the Introduction.]

Response 2 [The novelty of the proposed manuscript is the study of random DFB laser with the open cavity in the S band communication window, which was never done before. It is actually highlighted in last chapter of the introduction. For the clarity it is now said “This study, for the first time[…]”

This study, for the first time, employs a random Distributed Feedback (DFB) [11-13] Raman amplifier with a half-open cavity [14,15] design to optimize S-band amplification across a 5 THz spectrum (196.2–201.1 THz), addressing challenges such as nonlinear impairments and pump efficiency limitations. The choice of a random DFB Raman amplifier is motivated by its superior performance over conventional Raman amplification schemes, as demonstrated in prior work on long-haul transmission systems [16-19]. Unlike traditional cavity-based Raman amplifiers that rely on two FBGs to form a Fabry-Perot cavity[20-22], the random DFB configuration uses Rayleigh scattering as distributed feedback alongside a single FBG, enabling bidirectional second-order pumping without increasing relative intensity noise (RIN). This is critical for any WDM transmission systems, where forward pumping in conventional setups can transfer RIN to the signal, degrading performance [23-26].]

Comments 3 [The authors claimed that “The attenuation parameters for the Raman pump (?1+/-), FBG (?2+/-) and each spectral component were derived from the experimentally determined attenuation profile of standard SMF, as depicted in Figure 2, ensuring realistic modeling of signal and pump losses across the whole spectrum”. Please explain how the attenuation spectrum information of the standard (Figure 2) was determined experimentally so that readers can understand and evaluate it.]

Response 3 [   Thank you for this comment, it was also pointed out by another reviewer. This is now explained and referenced “Using modified optical time domain reflectometer (OTDR) technique together with the tunable laser [28] it was possible to experimentally measure attenuation profile of standard SMF depicted in Figure 2. These in turn were used as parameters in simulations for the Raman pump (P_1^{+/-}), FBG (P_2^{+/-}) and each spectral component. Such approach ensures realistic modeling of signal and pump losses across the whole spectrum.”]

Comments 4 [The authors claimed that “The attenuation profile of the WDM grid, illustrated in Figure 3, offers essential insights into the spectral loss characteristics across the 50-channel array spanning the S-band”. Please explain how the attenuation spectrum of the WDM gride (Figure 3) was experimentally determined.]

Response 4 [I believe this is now explained in previous comment 3.]

Comments 5 [Figure 4. Normalised Raman gain spectrum in standard SMF silica, the unit of the physical quantity (Gain shift) on the horizontal axis can be expressed in nm since the study discusses S-band, it may make it clear to readers.]

Response 5 [Great majority of publications describe it as a frequency shift, hence the X axis is now changed to “Frequency shift [THz]”. We work both, in wavelength and frequency domain, so I guess in this particular matter it is choice how one wishes to view the graph. I personally prefer to refer as frequency shift.]

Comments 6 [Page 6 of 14, Line 216-218: the author claimed that “The on-off net gain, measured in decibels (dB), was plotted for each channel to assess the amplification performance across the random DFB Raman amplifier system over a 60 km SMF”, as shown in Fig. 5. Please explain how “the on-off net gain” is defined and calculated in this work. In addition, what does the negative net gain (dB) in Figure 5 mean? Please explain it clearly.] 

Response 6 [This is described in section 2 . Methodology and Simulation:

“The backward pump power was dynamically adjusted to achieve a net gain of 0 dB for a single optimized channel, ensuring transparent transmission for the reference channel. The remaining 49 WDM channels were simulated using the fixed pump power settings determined for the optimized channel, enabling assessment of gain uniformity across the entire simulated spectrum. The transmitted power per channel was set to -10 dBm to replicate realistic WDM system conditions, consistent with high-density optical communication requirements. ASE noise and signal propagation were modeled in a 125 GHz bandwidth.”
 
Negative/positive net gain means that particular channel is either underpowered or overpowered. In this research, a central channel was selected as a baseline for optimization, hence it always gives 0 dB net gain, and the rest is simulated with the exact values as the central channel.   

“Detailed analysis of this profile indicates that the midpoint of the grid serves as an optimal reference for aligning simulation parameters to ensure accurate modeling of signal propagation and amplification dynamics. Accordingly, the central channel, designated as channel #25, was selected as the baseline for calibration due to its position at the median of the WDM spectral loss distribution.”   ]

Comments 7 [Figure 8. OSNR dependance due to primary Raman pump power, please explain why the OSNR (dB) data under the condition of P1=1344 nm/FBG=1432 nm does not appear in this graph?]

Response 7 [Thank you for pointing out this mistake, as the other reviewer pointed out, whole legend was wrongly described. The figure shows only the best configuration, which was 1342/1430, under different pump powers. This is now corrected and clear. ]

Comments 8 [The reference format is inconsistent. Please edit it according to the reference format of “Electronics” journal.]

Response 8 [All references were re-edited accordingly]

 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

 

1. The title of this manuscript has been revised to “Modeling S-Band communication window using Random Distributed Raman Laser Amplifier”, which is considered appropriate for this study.
2.  After reviewing the revised manuscript, the author has addressed the reviewer comments and made revisions. Overall, the quality of the manuscript has improved.

Comments on the Quality of English Language

The English could be improved to more clearly express the research.