Wide-Range All-Fiber Optical Current Transformer Based on Spatial Non-Reciprocal Phase Modulation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present an AFOCT using a spatial non-reciprocal phase modulation technique with a low-voltage LiNbO₃ waveguide. The method shows clear improvement over previous approaches and should be accepted with revisions. However, the manuscript would benefit from improved clarity.

1. Since Ref. [25] has already demonstrated a spatial non-reciprocal modulation scheme, the authors should briefly explain how the proposed dual-polarized LiNbO₃ waveguide differs from the earlier bulk-crystal implementation and why it achieves a 150-fold reduction in half-wave voltage.
2. The manuscript mentions applicability to power systems and relay protection, yet all experiments focus solely on DC currents. Because AC currents dominate practical scenarios, adding AC measurement results or a short discussion on its AC performance would strengthen the work.

Comments on the Quality of English Language

The overall English is understandable, but several typographical and grammatical issues should be corrected to improve clarity and professionalism. For example, on Page 1, the author affiliation contains a typo (“Chin”), which should be “China.” In addition, on Page 15, the phrase “we have proposed a AFOCT” uses an incorrect indefinite article; since “AFOCT” begins with a vowel sound, it should be written as “an AFOCT.” A careful proofreading is recommended to resolve such issues throughout the manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear colleagues!
You've been developing the methods and structures implementing them, presented in this article, for quite some time. This raises a number of questions.
1. Could you provide a retrospective of your previous results in the introduction to understand the novelty of the results presented in this article?
2. This is also related to the fact that the reviewer found only one previously published work by you listed in the reference list.
3. The 150-fold reduction in half-wave voltage is a truly important result, to which you devote an entire paragraph. In my opinion, this is the foundation of electro-optical crystallography. Moreover, the premise for the process you call optimization is your five-year-old article. Optimization is a complex process that involves criteria and the study of many parameters, not just one.
4. The use of the term "modulation" twice in the title and its length make it very complex. Could you simplify it? Moreover, the phrase "flexible modulation" is used only in the title and keywords.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript reports an all-fiber optical current transformer (AFOCT) that uses a spatial non-reciprocal phase modulation scheme based on a LiNbO₃ dual-polarized waveguide placed between two Faraday rotators, aiming to remove the intrinsic Sagnac frequency limitation and to reduce the half-wave voltage by about a factor of 150. The authors present a detailed Jones-matrix model and an experiment covering 30–3600 A DC with reported 0.2S-class accuracy according to the Chinese national standard.  The topic fits the scope of Photonics and AFOCT technology is of practical interest for digital substations. However, the current version still has unclear points about the true level of novelty versus earlier spatial non-reciprocal modulation schemes, the physical interpretation of the dual-polarized waveguide, and several aspects of the experimental characterization and error analysis. I would recommend major revision before further consideration. Below are ten technical questions that the authors should address in detail.

1. Novelty relative to Ref. [25] and other prior spatial non-reciprocal phase modulation work: The paper claims that the proposed scheme“fundamentally overcomes the limitations imposed by intrinsic frequency and does not require delay coils”and that it provides“a novel phase modulation scheme”. Please explain clearly what is structurally and functionally new compared with the phase modulator in Ref. [25] and other AFOCT schemes using LiNbO₃ based spatial non-reciprocal modulation. A comparison table of structure, required voltage, operating principle and performance would help.
2. Quantification and validation of the“×150”reduction in half-wave voltage: In Section 2.2 you derive Eqs. (10) and (14) and then state that the dual-polarized waveguide reduces the half-wave voltage from about 600 V to 4 V.  Please show a numerical example using concrete n_e, n_o, γ₃₃, γ₁₃, γ₂₂, l and d to reproduce this factor in the text, and provide experimental calibration data (measured transfer curve of phase versus applied voltage) that confirm Uπ≈4 V for the actual device.
3. Impact of waveguide loss, bandwidth and drive electronics on system performance: The dual-polarized waveguide is introduced mainly from the viewpoint of half-wave voltage. However, insertion loss, residual amplitude modulation, and electro-optic bandwidth also affect AFOCT performance. Please provide measured or specified values of loss, Vπ·L, modulation bandwidth, and explain how these parameters influence noise, linearity, and maximum measurable current in your prototype.
4. Clarification of “flexible modulation frequency” and absence of intrinsic frequency limit: The introduction and Section 2.1 state that the scheme is no longer constrained by the Sagnac intrinsic frequency and that even a 200 Hz sawtooth can give “satisfactory and stable demodulation”.  Please clarify quantitatively what sets the remaining limits on modulation frequency (for example, by the electro-optic device, digital sampling, demodulation algorithm, or polarization drift) and provide data or simulations that demonstrate performance for several different modulation frequencies (for example 200 Hz, 1 kHz, 5 kHz).
5. Jones-matrix model and assumptions: The derivation leading to Eq. (21) assumes ideal polarizers, couplers, Faraday rotators and λ/4 plates, and it appears to ignore differential loss between the two counter-propagating paths.  Please discuss how non-ideal extinction ratios, coupling ratio errors, and differential loss would modify Eq. (21), and estimate their impact on the measured ratio error and peak error.
6. Noise sources and scaling with current: In Section 3.1 and Fig. 8 you state that the noise in the detected waveform is inversely proportional to the primary current and that circuit noise is independent of current.  Please provide a more rigorous noise analysis: identify shot noise, thermal noise of the amplifier, and optical interference noise, and show experimentally (for example by plotting noise spectral density versus current) that the dominant term scales as described. It would also help to convert the noise into an equivalent current noise floor in A/√Hz.
7. Dynamic range and low-current performance: The system shows quite large peak error (≈16.6 %) at 1 % of rated current, while meeting 0.2srequirements above 20 % of rated current.  Please discuss the practical impact of this limitation for grid applications, and whether there are straightforward ways (for example increased optical power, different amplifier gain, alternative demodulation) to improve the low-current performance without sacrificing high-current accuracy.
8. Temperature dependence and long-term stability: The introduction reviews temperature and reliability issues in AFOCTs, but the prototype experiments appear to be performed at a single laboratory temperature and over a short time.  Please provide at least preliminary data or analysis on how temperature changes in the range, for example, −20°C to 60°C would influence the LiNbO₃ waveguide phase shift, the Faraday coil, and the overall ratio error. If experimental data are not yet available, a quantitative estimate based on known thermo-optic and Verdet-constant coefficients is needed.
9. DC versus AC and bandwidth specification: The experiments use DC currents up to 3600 A. For many protection and metering tasks, AC currents and transient events are critical. Please clarify the effective bandwidth of the prototype (for example, up to what frequency can it track sinusoidal or step inputs within the quoted error limits), and explain how the spatial non-reciprocal modulation scheme and 200 Hz drive interact with higher-frequency current components.
10. Practical implementation and comparison with conventional CTs and other AFOCTs: The conclusion states that the proposed device can meet the 0.2S special-purpose transformer standard and reduce cost and size. Please quantify the current prototype in terms of footprint, optical power, required electronics, and compare it directly with a representative electromagnetic current transformer and at least one commercial or literature AFOCT solution. A table summarizing accuracy, dynamic range, size, required high voltage (or lack of it), and complexity of the demodulation hardware would make the practical advantages and trade-offs clearer.

Addressing these points with additional analysis and, where possible, new measurements would significantly strengthen the manuscript and clarify its true contribution beyond earlier AFOCT work.

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors!
Thank you for your careful consideration of my recommendations. After changing the article title, a comment still remains. You use the word "current" twice. Perhaps in the second instance, the term "measurement range" should be used more anonymously. Then we'd get "All-fiber optical current transformer with a wide measurement range based on spatial non-reciprocal phase modulation" or simply "Wide-range all-fiber optical current transformer based on spatial non-reciprocal phase modulation."

Author Response

Thank you for pointing this out. We agree with this comment. Therefore, we have chosen the article title based on your suggestion. We have changed the article title ---- “Wide-range all-fiber optical current transformer based on spatial non-reciprocal phase modulation."

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Thanks for author's revision.

Author Response

Thank you very much again for your suggestion, it has been very helpful to me.

Author Response File: Author Response.docx