Functional Coatings for Fiber Bragg Gratings: A Critical Review of Deposition Techniques for Embedded and Harsh-Environment Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Cristian et al. reviewed functional coatings for fiber Bragg gratings in terms of deposition techniques for embedded and harsh-environment applications. The work is very important for the development of the coating based FBGs. But there remain some issues. I can’t recommend the acceptance till all my concerns below are well responded:

1. Though the authors summarized the environmental drivers and coating strategies in table 1, the features are hardly for comparison. It is better to sort the specific and similar parameters for the clear demonstration. In addition, the authors should prepare a graph to demonstrate the importance of coating related FBG in view of materials, methods, applications, etc.
2. There have many reviews about the FBGs, where many of them related coating fields. But the authors seem seldom to mention them. In fact, it is very important to demonstrate the difference with these reviews instead of focusing on the coating importance as this is a revie work.
3. The paper structure is pretty bad with very long paragraphs, making it messy. It is better to further sort the materials in order and logically, especially for section 2. In addition, section 2 related with the fundamentals but none of the equation and theory are cited. Many subtitles have a name which is not matched with the contents described.
4. All the abbreviations must be clearly defined before the first citations.
5. What is the definition of the normalized temperature sensitivity?
6. 3 gives out the sensitivity multipliers for different metals. But the authors do not give out any discussion and explanation about it. Similarly for figure 2, Table 2, etc. The authors should add more discussion about these figures and tables.
7. Where is the table 2 mentioned in line 405 and Table 7 in 590?
8. The English is casual at some places. The authors should further check and revise the manuscript thoroughly and carefully.

 

Author Response

Dear reviewer,
We would like to thank you for your time, spent reading our review paper, and for your  thoughtful and constructive comments.
We have thoroughly reviewed the manuscript, giving careful consideration to all of the suggestions that were provided.
We believe these revisions address all concerns and substantially strengthen the manuscript. We appreciate your guidance.

Below we respond point by point to your comments. At each point We also indicate where changes have been made in the revised manuscript. The updated version of the manuscript is attached.

Sincerely,
The Authors

Reviewer n. 1

Comments and Suggestions for Authors

Cristian et al. reviewed functional coatings for fiber Bragg gratings in terms of deposition techniques for embedded and harsh-environment applications. The work is very important for the development of the coating based FBGs. But there remain some issues. I can’t recommend the acceptance till all my concerns below are well responded:

1. Though the authors summarized the environmental drivers and coating strategies in table 1, the features are hardly for comparison. It is better to sort the specific and similar parameters for the clear demonstration. In addition, the authors should prepare a graph to demonstrate the importance of coating related FBG in view of materials, methods, applications, etc.

Response: We reworked Table 1 to use consistent, comparable fields (environmental driver → recommended stack → route → key parameters/post-treatments → normalized sensitivity ηᵀ → trade-offs → refs). We also added Figure 1, a landscape schematic mapping application domains → coating materials/stacks → deposition routes, so readers can see at a glance how environment drives both materials and process choices.

2. There have many reviews about the FBGs, where many of them related coating fields. But the authors seem seldom to mention them. In fact, it is very important to demonstrate the difference with these reviews instead of focusing on the coating importance as this is a revie work.

Response: We inserted Section 1.2 “Position of this review within existing literature,” which explicitly contrasts our focus on coatings, standardized definitions (ηᵀ, plateau thickness, transient penalty), and integrated surface-prep → deposition → post-treat protocols with prior surveys.

3. The paper structure is pretty bad with very long paragraphs, making it messy. It is better to further sort the materials in order and logically, especially for section 2. In addition, section 2 related with the fundamentals but none of the equation and theory are cited. Many subtitles have a name which is not matched with the contents described.

Response: We reorganized early sections, shortened paragraphs, and inserted the core equations in Section 2 (Bragg condition; bare-fiber temperature slope; coated-fiber effective slope). Subsection 2.1 now ties equation (3) to layer properties (E, α, t) and the strain-transfer function κ. Headings were aligned to content throughout.

4. All the abbreviations must be clearly defined before the first citations.

Response: Abbreviations are now defined at first use in the text and consolidated in the list of abbreviations (e.g., EL/ED, PVD, CVD/ALD). We also normalized casing (e.g., “electroless (EL) Ni-P”). See Abstract/Intro and Section 1.1 lead-in.

5. What is the definition of the normalized temperature sensitivity?

Response: We define and use normalized temperature sensitivity explicitly in Section 2 and Table 2 (“Normalized sensitivity… sensitivity amplification due to coating”). We also aligned table footnotes to reflect ηᵀ = S_T,coated / S_T,bare over the same T-range. See Table 2 and the metrics paragraph preceding it.

6. 3 gives out the sensitivity multipliers for different metals. But the authors do not give out any discussion and explanation about it. Similarly for figure 2, Table 2, etc. The authors should add more discussion about these figures and tables.

Response: We expanded the explanations around the archetypes (Figure 2), the plateau behavior and trade-offs, and the performance metrics (Table 2). The text now explains why softer high-CTE metals (Zn/Sn) reach higher ηᵀ at modest thickness while Ni offers adhesion and high-T stability, and why duplex/multilayers balance these. See Section 2.1 (mechanistic link) and the paragraphs introducing Table 2.

7. Where is the table 2 mentioned in line 405 and Table 7 in 590?

Response: Both items are now present and cross-referenced: Table 2 (key metrics) and Table 7 (per-specimen reporting template) in the comparative/qualification sections.

8. The English is casual at some places. The authors should further check and revise the manuscript thoroughly and carefully.

Response: We performed a language edit to tighten tone, split long sentences, and remove repetition (especially in the Introduction and Sections 2–4).

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This review article discusses and distills valuable information focused on deposition techniques for functional coatings on Fiber Bragg Gratings used as sensors under embedded and harsh environment applications. The authors have provided a comprehensive yet focused treatment of coating FBGs for harsh environmental sensor applications specifically on materials and deposition techniques they manage to classify into five focused types as well as surface prepatrations and protocols used in current trends. All of these review results have been arrived at in the point of view of well-defined measurement metrics and parameters that determine the perfomance of the FBG sensors with these specialized coatings. The article is well-organized and the methodology at which the lierature review was carried out is highly methodical. This review article is well-suited for this Coatings Special Issue and is therefore recommended for publication after one minor revision, which is as follows:

For the purpose of giving clarity to the general reader and for completeness of the review article, the authors should create a table in Section 2 (Fundamentals and performance metrics for coated FBGs) that contain the perfomance metrics on one column, their corresosnding  brief definition in the next column, and the parameters of the FBG spectrum and physical/material parameters these metrics were derived from. After this minor addition, this review article is ready for publication in Coatings Journal. 

Author Response

Dear reviewer,
We would like to thank you for your time, spent reading our review paper, and for your  thoughtful and constructive comments.
We have thoroughly reviewed the manuscript, giving careful consideration to all of the suggestions that were provided.
We believe these revisions address all concerns and substantially strengthen the manuscript. We appreciate your guidance.

Below we respond point by point to your comments. At each point We also indicate where changes have been made in the revised manuscript. The updated version of the manuscript is attached.

Sincerely,
The Authors

Reviewer's comment:

1. For the purpose of giving clarity to the general reader and for completeness of the review article, the authors should create a table in Section 2 (Fundamentals and performance metrics for coated FBGs) that contain the perfomance metrics on one column, their corresosnding  brief definition in the next column, and the parameters of the FBG spectrum and physical/material parameters these metrics were derived from. After this minor addition, this review article is ready for publication in Coatings Journal. 

Response: We added Table 2 (Key Performance Parameters) with columns for the metric, definition/meaning, and typical ranges; the surrounding text ties each metric to spectral features (FWHM, SNR) and physical parameters. See Section 2 and Table 2.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Optical fibers are the sensitive element of high-precision sensors. The fibers are coated with various coatings to protect them from the external environment, moisture and aggressive environments, as well as to ensure the required functionality. Polymer coatings are limited in operating temperature conditions. The fibers are coated with metallic coatings for ultra-high and cryogenic temperatures. The problem of forming coatings and their coupling with glass fiber elements exists. The coating technology also influences the production process and the mechanical properties of the fiber. Analysis of coating technologies and materials is relevant. Developing an understanding of these effects and technologies is a crucial step in the development of the industry. The authors present an interesting overview.

 

Comments and recommendations:

1. The issue of fiber Bragg grating coatings is worth highlighting in the introduction. The challenges of fiber Bragg gratings in general are also worth considering. The industry's development trends and their features are also worth describing.
2. Recording of gratings using UV radiation through a polymer coating is not considered, especially the disadvantages.
3. Cryogenic temperature sensors are being developed, including using specialized fibers (such as Panda). However, sensors and modern developments in this area are not discussed. This would be interesting.
4. The architecture diagrams presented in Figure 1 require a more detailed description: features, areas of application, problems, etc.
5. It's also worth adding a classification of coatings and recommendations for their use. The accuracy of sensors for different coatings should be reflected.
6. The separator between the fractional and integer parts must be a period (Figure 2).
7. The data series labels in Figure 3 should be enlarged; the font size is significantly smaller than the font size of the figure.
8. The data series labels in Figure 4 are very close to the data series. This should be corrected.
9. Recommendations for selecting coating application technologies and coating materials should be included in the conclusion. Directions for the development of coating application technologies and materials for the industry's development should be emphasized.
10. References to literary sources are not listed in the order they were cited. References are not formatted according to journal standards. This should be corrected.

Author Response

Dear reviewer,
We would like to thank you for your time, spent reading our review paper, and for your  thoughtful and constructive comments.
We have thoroughly reviewed the manuscript, giving careful consideration to all of the suggestions that were provided.
We believe these revisions address all concerns and substantially strengthen the manuscript. We appreciate your guidance.

Below we respond point by point to your comments. At each point We also indicate where changes have been made in the revised manuscript. The updated version of the manuscript is attached.

Sincerely,
The Authors

Reviewer's comments

Comments and Suggestions for Authors

Optical fibers are the sensitive element of high-precision sensors. The fibers are coated with various coatings to protect them from the external environment, moisture and aggressive environments, as well as to ensure the required functionality. Polymer coatings are limited in operating temperature conditions. The fibers are coated with metallic coatings for ultra-high and cryogenic temperatures. The problem of forming coatings and their coupling with glass fiber elements exists. The coating technology also influences the production process and the mechanical properties of the fiber. Analysis of coating technologies and materials is relevant. Developing an understanding of these effects and technologies is a crucial step in the development of the industry. The authors present an interesting overview.

 Comments and recommendations:

The issue of fiber Bragg grating coatings is worth highlighting in the introduction. The challenges of fiber Bragg gratings in general are also worth considering. The industry's development trends and their features are also worth describing.

Response: The Introduction now foregrounds coating-driven needs across aerospace, energy, and civil settings (vibration, radiation, large ΔT), and clarifies limitations of bare fibers and polymer jackets at high T and deep cryo.

Recording of gratings using UV radiation through a polymer coating is not considered, especially the disadvantages.

Response: We added a paragraph in the Introduction noting UV-through-polymer inscription and the drawbacks for harsh environments (uniformity, mechanical stability, thermal endurance).

Cryogenic temperature sensors are being developed, including using specialized fibers (such as Panda). However, sensors and modern developments in this area are not discussed. This would be interesting.

Response: We added a short discussion on polarization-maintaining fibers, including PANDA, and cited recent cryogenic/high-T demonstrations. See the paragraph following Table 2.

The architecture diagrams presented in Figure 1 require a more detailed description: features, areas of application, problems, etc.

Response: Figure 1 caption and the adjacent text explain the mapping and how environmental requirements determine stack/process choices; Figure 2 introduces and discusses the stack archetypes referenced throughout.

It's also worth adding a classification of coatings and recommendations for their use. The accuracy of sensors for different coatings should be reflected.

Response: Classification and recommendations are consolidated in Table 6 (Recommended stacks and process windows), with rationale in the surrounding synthesis text. Accuracy/stability are linked to ηᵀ ranges and to drift/hysteresis thresholds. See Section 6 and Table 6.

The separator between the fractional and integer parts must be a period (Figure 2). The data series labels in Figure 3 should be enlarged; the font size is significantly smaller than the font size of the figure. The data series labels in Figure 4 are very close to the data series. This should be corrected.

Response: We standardized decimal separators to periods and increased label sizes and spacing in the figures at export. (No textual edits were required.)

Recommendations for selecting coating application technologies and coating materials should be included in the conclusion. Directions for the development of coating application technologies and materials for the industry's development should be emphasized.

Response: The Conclusions/Outlook section now distills actionable design rules by regime (cryo, 80–300 K, high-T) and highlights development priorities (diffusion-aware stacks, standardized reporting, process-history control). See the consolidated synthesis and outlook paragraphs.

References to literary sources are not listed in the order they were cited. References are not formatted according to journal standards. This should be corrected.

Response: We revised references toward the MDPI Coatings style (numeric order, consistent punctuation/capitalization, DOIs). We also checked in-text numbering consistency after re-ordering.

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

All my concerns have well been responded and I recommend the acceptance for the publication.