Binary-Tree Structure for Extended Range-Distributed Acoustic Sensing

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comments are included in the pdf

Comments for author File: Comments.pdf

Author Response

Comments 1:The work describes acoustic sensing using a binary-tree structure, which has broad applicability in both research and industry. The adopted method is interesting and shows novelty. However, the authors do not present any comparison with current standards or even mention them. I would recommend publication only after the following queries are addressed:

The introduction is well written; however, it lacks a discussion or comparison with current state-of-the art techniques using similar approaches.

Response 1: We agree that a context regarding current long-range DAS techniques is necessary. We added a paragraph (Page 2, lines 57-72) with 4 new references (References [16-19]) in the Introduction to systematically review existing long-range DAS methods.

Comments 2: The schematic is somewhat confusing. It is unclear how the heterodyned signal is acquired. The roles of OF3 and the up fiber as the two components in heterodyning should be explicitly clarified. Additionally, no explanation of the heterodyning process is provided.

Response 2: Thank you for pointing this out. Our system utilizes the dual-pulse heterodyne demodulation technique (HD-DAS) that is different from the C-OTDR based DAS. In the C-OTDR based DAS system, a local light is required to interfere with the Rayleigh back-scattered light, while our system utilizes the interference of the Rayleigh back-scattered light from each of the dual pulses. The heterodyne pulse pair will cause Rayleigh scattering during transmission in OF₂, and the Rayleigh back-scattered light from OF₂ is directed to the photo-detector (PD₁) via CIR₁, OF₃ and a fiber connector (FC₂). As the insert figure in Fig. 1 shows, the Rayleigh back-scattered light (RBS) from pulse 1 (orange curve) and pulse 2 (green curve) will superimpose and interfere at PD₁, forming a heterodyne interference signal. We have added several sentences in the manuscript to clarify this point (Page 4, lines 120-125). We also modified Fig. 1 to show the heterodyne pulse pair and their Rayleigh back-scattered light clearly (Page 3, Fig. 1).

Comments 3: A comparison between results with and without heterodyning would help illustrate the advantages of the proposed scheme.

Response 3: The primary focus of this paper is extending the range of HD-DAS, not the performance comparison of HD-DAS vs. non-HD-DAS systems. As stated in Response 2, our system has a unique structure that uses dual pulses to achieve heterodyne detection. Without heterodyne detection, the system will not work.

Comments 4: Figure 3 is unclear. A color bar should be added, and the phase noise in the plots should be properly explained. The caption is too brief and should be expanded.

Response 4: We agree with this suggestion. We have added a color bar to Fig. 3 to clearly indicate the magnitude range of the phase noise. We also expanded the caption. (Page 6, Fig. 3.)

Comments 5: In Figure 4, harmonic features (a–c) are observed for certain pump currents, but these are neither addressed nor explained.

Response 5: Thank you for this careful observation. The low-frequency peaks (e.g., 26.6 Hz and its harmonics) in Fig. 4 typically originate from ambient low-frequency vibrations (e.g., vibrations generated by air conditioning operation), which are independent of the optical nonlinear effects. We have added one sentence in the manuscript to clarify this point (Page 6, line 191-194).

Comments 6: In Figure 4, the Y-axis units are confusing. The units should be clearly explained. Note that “Hz” under the square root should not be italicized; it should appear as $\sqrt{\text{Hz}}$.

Response 6: The unit means 1 rad/sqrt(Hz) corresponds to 0 dB. We have added one sentence in the manuscript to make this clear (Page 7, the caption of Fig. 4). We changed the Y-axis units in all parts of Fig. 4 to the standard format.

Comments 7: In Figure 4(c), the legends extend outside the plot area. Consider placing them within the figure, possibly in two columns.

Response 7: We agree with this suggestion. We have adjusted the position and format of the legend in Fig. 4 to ensure it is fully contained within the plot area and presented in a readable manner (Page 7, Fig. 4).

Comments 8: Error bars are missing in all measurements. Including them is essential to assess the robustness and reproducibility of the method.

Response 8: We accept the reviewer's suggestion. We have calculated and added error bars (standard deviation) to plots in Fig. 4(d) to reflect the robustness and reproducibility of the data (Page 7, Fig. 4.).

Comments 9: The statement “In the process of calculating the SNR, a bandwidth of 1.2 nm around the center wavelength (1550.12 nm) is used as the signal, while the rest is as the noise” is not convincing. The approach to quantifying SNR should be clarified and justified more rigorously.

Response 9: Thank you for pointing out the potential confusion. This SNR definition is used to quantitatively analyze the spectral degradation caused by MI in the transmission fiber. We provided a more rigorous explanation in Section 3.1.2 (Page 9, lines 231-239): Since the input optical pulse has an extremely narrow linewidth, its power should be concentrated around the center wavelength. MI is characterized by the transfer of power from the carrier (signal) to symmetrical sidebands (noise). Therefore, in the process of calculating the SNR, a bandwidth of 1.2 nm around the center wavelength (1550.12nm) is used as the signal while the rest is as the noise, to quantitatively measure the degree of optical power transfer from the primary carrier to the incoherent noise sidebands due to MI.

Comments 10: The authors use 2, 4, and 6 km fibers for pump power optimization but employ a 10 km fiber for actual deployment while keeping the same optimized parameters. A 10 km fiber can introduce additional nonlinear effects or modulation instability. A convincing argument or justification for this assumption should be provided.

Response 10: This is a crucial and valuable question. We agree that nonlinear effects increase with fiber length. We have added the following argumentation to Section 3.2 (Page 9, lines 252-259):

Fig. 4(d) shows that as the length of the transmission fiber increases, the optimal pump current approaches 70 mA. For example, for a transmission fiber of 6 km, the minimum noise floor is -64.81 dB when the pump current is 70 mA. Therefore, the choice of 70 mA was made to provide a robust margin for the 9.960 km length. Although the MI threshold for the 9.960 km transmission fiber will be slightly lower than that for 6 km, the laboratory findings indicate that setting the EDFA pump current to 70 mA is a conservative and robust setting that maintains sufficient SNR while successfully avoiding the severe performance degradation caused by MI. The field test results (achieving a noise floor of -59.79 dB) strongly validate that the 70 mA optimized parameter is successful and effective for the 9.960 km transmission length.

Comments 11: A direct comparison between HD-DAS and BNS-DAS systems would strengthen the claim of robustness and highlight the advantages of the proposed method.

Response 11: As stated in the manuscript, a direct comparison at the 10 km distance is physically impossible due to the inherent range limitation of traditional HD-DAS (limited to 2500 m with our parameters). In the Conclusion, we explicitly state that the main advantage of the BTS-DAS is the enabling of long-distance capability, which is unattainable for conventional HD-DAS.

Comments 12: Only one field trace using a truck test is presented, which does not adequately demonstrate reliability. The authors should discuss the challenges encountered in field deployment compared to the laboratory setup.

Response 12: We agree to enhance the discussion on field deployment. We have added a discussion paragraph to Section 3.2 to address reliability and depth (Page 10, lines 273-278):

While only one vibrator test analysis is shown (Fig. 7), this test is sufficient to demonstrate the system's basic functionality and high fidelity at the 9.960 km far-end (clear waterfall plot, coherent time-domain waveform, and accurate frequency identification). During on-site deployment, careful management of connectors and splices is required to reduce the impact of long-distance cable connection losses on the optical power budget.

Reviewer 2 Report

Comments and Suggestions for Authors I have some critical concerns with respect to some points.

• The Pulse Repetition Rate (PRR) and the range ambiguity. Your system has a 20 Km round-trip distance, but you use a 40 kHz PRR. ¿How was the data processed to eliminate or account for the severe range ambiguity (aliasing)?
• Heterodyne frequency justification. Why was an extremely low heterodyne frequency of 10 kHz chosen? What evidence shows that this choice did not compromise the high-performance by increasing low-frequency system noise?
• Clarify in the abstract and conclusion that this work demonstrates a remote sensing capability over a 10 Km transmission link, rather than a 10 km continuous distributed acoustic sensing range. Considering the sensing occurs only in the final 250 m segment.

 

Author Response

Comments 1: The Pulse Repetition Rate (PRR) and the range ambiguity. Your system has a 20 Km round-trip distance, but you use a 40 kHz PRR. ¿How was the data processed to eliminate or account for the severe range ambiguity (aliasing)?

Response 1: This comment addresses a crucial misunderstanding of the fundamental purpose and benefit of our binary-tree structure DAS (BTS-DAS). The system does not suffer from range ambiguity or aliasing because the 40 kHz PRR is applied only to the short sensing segment (OF₂), not to the full 10 km transmission link.

Comments 2: Heterodyne frequency justification. Why was an extremely low heterodyne frequency of 10 kHz chosen? What evidence shows that this choice did not compromise the high-performance by increasing low-frequency system noise?

Response 2: The 10 kHz heterodyne frequency is intentionally chosen to achieve a balance between sensing distance and noise, which is a common practice in dual pulse HD-DAS system. As shown in Figs. 4 and 6, a 10kHz heterodyne frequency can achieve good noise performance.

Comments 3: Clarify in the abstract and conclusion that this work demonstrates a remote sensing capability over a 10 Km transmission link, rather than a 10 km continuous distributed acoustic sensing range. Considering the sensing occurs only in the final 250 m segment.

Response 3: We agree with the reviewer that precise terminology is critical to avoid misrepresentation of the system's capabilities. Our system is a "remote" DAS, not a "continuous" 10 km DAS. We have modified the Abstract and Conclusions to make this clear.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents new distributed acoustic sensing (DAS) method: binary-tree structure DAS, which is based on dual-pulse heterodyne demodulation DAS method.

The manuscript presents a description of the experimental setup, the results of test experiments, the main sources of noise is analyzed, the optimal pump current is chosen reasonably, the on-site test result is demonstated. 

The conclusions of the manuscript are substantiated and confirmed by the presented experimental data. It is shown that the presented method allows sensing for fibers ~10 km long at a pulse repetition rate of 40 kHz.

The manuscript is well-structured, the figures and tables are clear and appropriate. There is one comment on the figures:

On figures 3, 6b, 7a thre is no color scale. Without this scale, it is impossible to correlate the data on different figures (is the range the same in all figures?).

In my opinion the article should be accepted after minor revision.

Author Response

Comments 1: This manuscript presents new distributed acoustic sensing (DAS) method: binary-tree structure DAS, which is based on dual-pulse heterodyne demodulation DAS method.

The manuscript presents a description of the experimental setup, the results of test experiments, the main sources of noise is analyzed, the optimal pump current is chosen reasonably, the on-site test result is demonstated.

The conclusions of the manuscript are substantiated and confirmed by the presented experimental data. It is shown that the presented method allows sensing for fibers ~10 km long at a pulse repetition rate of 40 kHz.

The manuscript is well-structured, the figures and tables are clear and appropriate. There is one comment on the figures:

On figures 3, 6b, 7a there is no color scale. Without this scale, it is impossible to correlate the data on different figures (is the range the same in all figures?).

In my opinion the article should be accepted after minor revision.

Response 1: We agree with this suggestion. We added color bars to Fig. 3, 6, 7 to clearly indicate the magnitude range. (Page 6, Fig. 3; Page 10, Fig. 6; Page 11, Fig. 11.)

Reviewer 4 Report

Comments and Suggestions for Authors

Overall, I think this paper is interesting and well presented with good research and results. However, the literature review is almost non-existent, which for Applied Sciences, a Q1 journal, is unacceptable. In this journal alone there are 20 articles in DAS, and in Sensors there are 137. The authors have not discussed how others have tried to resolve the issue with different modes and methods, and the general literature is not well presented. If this is to engage the readership of Applied Sciences, it should engage with the previous work in Applied Sciences!

This comment about literature feeds into the "can be improved" for the introduction, more details about the technology and competing approaches must be discussed.

In terms of the research design, I don't think additional work is needed, but discussion of the limitations, and potentially proposed future work that address them, needs to be included. This is in particular concerns the 250m sensing length relative to what is achievable. More important, is the fact that the advantage shown in Figure 1 is the ability to daisy chain sensors with circulators, and then this is not demonstrated. Why not? Talk about this, even if it is just to say to be done in future work!

Author Response

Comments 1: Overall, I think this paper is interesting and well presented with good research and results. However, the literature review is almost non-existent, which for Applied Sciences, a Q1 journal, is unacceptable. In this journal alone there are 20 articles in DAS, and in Sensors there are 137. The authors have not discussed how others have tried to resolve the issue with different modes and methods, and the general literature is not well presented. If this is to engage the readership of Applied Sciences, it should engage with the previous work in Applied Sciences!

This comment about literature feeds into the "can be improved" for the introduction, more details about the technology and competing approaches must be discussed.

Response 1: We sincerely apologize for the lack of depth in the original literature review. We have added a paragraph (Page 2, lines 57-72) with 4 new references (References [16-19]) in the Introduction to systematically review existing long-range DAS methods.

Comments 2: In terms of the research design, I don't think additional work is needed, but discussion of the limitations, and potentially proposed future work that address them, needs to be included. This is in particular concerns the 250m sensing length relative to what is achievable. More important, is the fact that the advantage shown in Figure 1 is the ability to daisy chain sensors with circulators, and then this is not demonstrated. Why not? Talk about this, even if it is just to say to be done in future work!

Response 2: The 250 m sensing fiber used in the field test was selected to represent a local monitoring section, and the longest length of the sensing section can be 2500 m. As shown in Fig. 1, the advantage of the BTS structure is its potential to use optical circulators to achieve multi node sensing. The focus of this experiment is to verify the range extension capability and nonlinear noise management under a single node. In future research, we will investigate multi node integration. We have added one paragraph in the Conclusions to address this point (Page 12, lines 300-305).

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors have addressed the comments/queries. Even though there are different methods which show longer distances of sensing, this method can be utilized to further the sensing capabilities. I recommend the manuscript for publication in its current form.