A Review of the Research Progress on Optical Fiber Sensors Based on C-Type Structures

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The introduction currently lacks quantitative comparisons between C-type fibers and traditional designs (e.g., D-type fibers). Add specific sensitivity data (e.g., "C-type fibers exhibit 5.4× higher sensitivity than conventional PCFs" ) and explicitly highlight how C-type structures address limitations like evanescent field exposure and mechanical stability. Clarify the research gap by linking challenges (e.g., temperature cross-sensitivity) to the unique advantages of C-type designs early in the text.

Author Response

About Reviewer #1:
Comment 1: The introduction currently lacks quantitative comparisons between C-type fibers and traditional designs (e.g., D-type fibers). Add specific sensitivity data (e.g., "C-type fibers exhibit 5.4× higher sensitivity than conventional PCFs") and explicitly highlight how C-type structures address limitations like evanescent field exposure and mechanical stability. Clarify the research gap by linking challenges (e.g., temperature cross-sensitivity) to the unique advantages of C-type designs early in the text.
Response 1:
We sincerely appreciate the valuable feedback provided by the reviewers. In response to this concern, we have added a quantitative comparison between the C-type and traditional D-type optical fiber designs in the introduction. Additionally, we have supplemented and clarified this comparison by referring to relevant literature, addressing the limitations of the original manuscript. Furthermore, we have explicitly highlighted the advantages of the C-type optical fiber in reducing evanescent field exposure risks and improving mechanical stability. The specific revisions can be found in lines 64-81.
(Razali et al. employed the COMSOL Multiphysics wave…for the design of other types of sensors)
In addition, in response to your suggestion to connect the application challenges with the unique advantages of the C-type structure, we have added new content in lines 56-64 and 75-81 of the manuscript. Specifically, we have integrated the issue of temperature cross-sensitivity with the structural characteristics of the C-type optical fiber, emphasizing its advantages in mitigating temperature cross-sensitivity. This
provides a stronger justification for the rationality of the design adopted in this study.
(Rex Xiao Tan et al. inscribed fiber Bragg gratings (FBGs)…the influence of thermal drift on sensing signals.)
(This structural optimization strategy…for the design of other types of sensors.)

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Authors provide a comprehensive and insightful review of C-type optical fiber sensors, covering their structural design, sensing mechanisms, and diverse applications. The authors have done an excellent job in synthesizing recent advancements and highlighting the unique advantages of C-type fibers, such as high sensitivity, robust mechanical properties, and resistance to interference. The comparison with traditional D-type fibers and the discussion on future directions are particularly valuable. The manuscript is well-organized and clearly written, making it a significant contribution to the field of optical fiber sensing.

1. The paper identifies femtosecond laser micromachining as the primary fabrication method, but a more detailed discussion of specific challenges—such as precision control and defect minimization—along with potential solutions would improve the reproducibility and scalability of the process.

2. The section on cross-sensitivity in multi-parameter sensing is somewhat brief. Expanding this analysis, perhaps by exploring mitigation strategies like polarization diversity or advanced signal processing techniques, would strengthen the manuscript.

3. While the theoretical foundations and lab-scale applications are well covered, including real-world deployment examples or industrial use cases would better demonstrate the practical relevance of C-type fiber sensors.

4. Some figures (e.g., Fig. 1, Fig. 5) could be improved with clearer labels and additional details. Enhancing their clarity and providing more descriptive captions would aid reader understanding.

Author Response

About Reviewer #2:

Comment 1: The paper identifies femtosecond laser micromachining as the primary fabrication method, but a more detailed discussion of specific challenges—such as precision control and defect minimization—along with potential solutions would improve the reproducibility and scalability of the process.

Response 1:

We sincerely appreciate the expert suggestions provided by the reviewers. In response to this feedback, we have supplemented Section 2.1 of the manuscript with a systematic overview of the challenges faced by femtosecond laser micromachining in practical applications, including issues such as precision control, thermal effect management, and defect formation. Additionally, we have explored potential solutions to these challenges, incorporating the latest research findings. The revised content, which aims to enhance the understanding of the repeatability and scalability of this process, can be found in lines 132-157.

(Although femtosecond laser micromachining…enhancing its stability and adaptability for practical engineering applications.)

 

Comment 2: The section on cross-sensitivity in multi-parameter sensing is somewhat brief. Expanding this analysis, perhaps by exploring mitigation strategies like polarization diversity or advanced signal processing techniques, would strengthen the manuscript.

Response 2:

We sincerely appreciate the constructive feedback provided by the reviewers. In response to this suggestion, we have made comprehensive revisions and extensions to Section 4.2 of the manuscript. We have reorganized the research approach in this section and added discussions on various cross-sensitivity suppression strategies, including polarization diversity and signal processing techniques. Additionally, we have updated Table 5 to summarize the structural core, measurement parameter ranges, and sensitivity performance corresponding to each decoupling strategy, thereby enhancing the systematic and practical nature of this section. The relevant revisions can be found in lines 579-686.

(In the ongoing evolution of optical fiber sensing technology towards multi-parameter,…multi-parameter collaborative optical fiber sensing systems.)

 

Comment 3: While the theoretical foundations and lab-scale applications are well covered, including real-world deployment examples or industrial use cases would better demonstrate the practical relevance of C-type fiber sensors.

Response 3:

We sincerely appreciate the valuable suggestions provided by the reviewers. Given the limited number of publicly available case studies on the deployment of C-type optical fiber sensors in practical industrial settings, we have reviewed and summarized existing deployment examples and application scenarios from the literature. The new content has been added to Section 5, with a focus on lines 689-723, to further illustrate the potential and prospects of C-type optical fiber sensors in real-world engineering environments.

(In recent years, C-type optical fiber structures have continued to make breakthroughs in integrated manufacturing precision, micro-scale control capabilities,…and system adaptation is expected to drive the large-scale application and commercialization of C-type optical fiber sensors in complex engineering environments.)

 

Comment 4: Some figures (e.g., Fig. 1, Fig. 5) could be improved with clearer labels and additional details. Enhancing their clarity and providing more descriptive captions would aid reader understanding.

Response 4:

We sincerely appreciate the constructive feedback provided by the reviewers. We have carried out a comprehensive formatting optimization for all the figures and tables in the manuscript, including enhancing image resolution and adjusting label font sizes and layouts to improve overall clarity and readability. Additionally, we have reorganized and refined the titles of all figures and tables to make them more descriptive, facilitating a better understanding of the illustrated information for the readers.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Summary of the paper:

This manuscript presents a comprehensive review of recent advancements in optical fiber sensors based on C-type fiber structures. The authors categorize the developments into interferometric and photonic crystal fiber-based sensors, discussing key mechanisms such as Fabry-Perot, Mach-Zehnder, and Sagnac interferometry, as well as SPR-based sensing using metal and novel plasmonic materials. The review also discusses structural innovations, sensitivity enhancements, multi-parameter detection, and future challenges in practical implementation. Tables and figures are well-utilized, and the manuscript is thorough in both depth and scope. However, some improvements are suggested:

• The tables in the review are well put together. Since there are multiple tables, recommend adding 2-3 line summary after each table to interpret the findings. This helps to draw comparative conclusions.
• It would be great to add the differentiating factor of this review versus existing literature. Recommend adding 2-3 lines highlighting the unique contributions. 
• There is only some discussions on fs laser process development and etching methods to fabricate these structures. It would be great to add a few lines containing latest developments including time for fabrication, cost and industrial adoption barrier. Few references to consider may be:
  - Wang, Hao, et al. "Two‐photon polymerization lithography for optics and photonics: fundamentals, materials, technologies, and applications." Advanced Functional Materials 33.39 (2023): 2214211.
  -Xiong, Cong, et al. "Optical fiber integrated functional micro-/nanostructure induced by two-photon polymerization." Frontiers in Materials 7 (2020): 586496.
  -Otuka, Adriano JG, et al. "Two-photon polymerization: Functionalized microstructures, micro-resonators, and bio-scaffolds." Polymers 13.12 (2021): 1994.
  -Singhal, Anuj, et al. "2-photon polymerized IP-DIP 3D photonic crystals for Mid IR spectroscopic applications." IEEE Photonics Technology Letters 35.8 (2023): 410-413.
• The multi-parameter sensing is discussed in depth. It is recommended to add a table summarizing different techniques, their challenges, sensitivity etc to enhance readability.
• Photonic crystals are finding real world applications rapidly. The C-type fibers can further penetrate the usability. It is recommended to highlight summary of real-world applications. Some references to look for are:
  - Kassani, Sahar Hosseinzadeh, et al. "Fast response in-line gas sensor using C-type fiber and Ge-doped ring defect photonic crystal fiber." Optics express 21.12 (2013): 14074-14083.
  - Kassani, Sahar Hosseinzadeh, et al. "Sensitivity enhancement of in-line chemical sensing device with C-type fiber and photonic crystal fiber." Fiber Lasers IX: Technology, Systems, and Applications. Vol. 8237. SPIE, 2012.
  -Example applications from reviews like:
  -- Singhal, Anuj, and Igor Paprotny. "Slow-Light Enhanced Liquid and Gas Sensing Using 2-D Photonic Crystal Line Waveguides—A Review." IEEE Sensors Journal 22, no. 21 (2022): 20126-20137.
  -- Tawfiq, Zainab H., Makram A. Fakhri, and Salah A. Adnan. "Photonic crystal fibres PCF for different sensors in review." IOP conference series: materials science and engineering. Vol. 454. No. 1. IOP Publishing, 2018.

 

Comments on the Quality of English Language

Although the technical content is strong, the manuscript requires minor language revisions for fluency and grammar. Examples:

• Line 92: "targeted research recommendations and an outlook" → consider rephrasing for clarity.

• Line 672: "play an increasingly significant and widespread role" → could be shortened or made more precise.

Author Response

About Reviewer #3:

Comment 1: The tables in the review are well put together. Since there are multiple tables, recommend adding 2-3 line summary after each table to interpret the findings. This helps to draw comparative conclusions.

Response 1:

We greatly appreciate the reviewers' recognition of the table structure and their suggestions for further improvement. In response, we have added brief summaries and comparative analyses of the data in the paragraphs following each table, which will help readers better understand the differences and advantages between the various parameters.

The specific revisions can be found in lines 315 to 334.

(Table 1 summarizes the key performance parameters of typical C-FPIs discussed in this section…and industrial process online analysis.)

The specific revisions can be found in lines 370 to 379.

(Table 2 summarizes and compares the key performance characteristics…the structural advantages of C-type optical fibers show great potential in efficient and precise sensing applications.)

The specific revisions can be found in lines 448 to 466.

(Table 3 summarizes and compares the key performance characteristics…offering greater potential for optical fiber sensors in complex environmental monitoring applications)

The specific revisions can be found in lines 565 to 577.

(Table 4 summarizes the impact of different metal layer…especially in high-temperature, high-pressure, and harsh environments.)

The specific revisions can be found in lines 669 to 686.

(C-type PCF SPR multi-parameter sensing technology is transitioning…in the future multi-parameter collaborative optical fiber sensing systems.)

 

Comment 2: It would be great to add the differentiating factor of this review versus existing literature. Recommend adding 2-3 lines highlighting the unique contributions.

Response 2:

We sincerely appreciate the valuable suggestions provided by the reviewers. In response, we have added a discussion in the introduction highlighting the differences between this review and existing literature in terms of topic perspective, structural classification, and focus on key issues. Additionally, we have clearly outlined the unique contributions of this study.

The relevant content can be found in lines 87 to 98 of the manuscript.

(This paper presents a comprehensive review of…new possibilities for their integration into next-generation intelligent sensing systems.)

 

Comment 3: There is only some discussions on fs laser process development and etching methods to fabricate these structures. It would be great to add a few lines containing latest developments including time for fabrication, cost and industrial adoption barrier.

Response 3:

We sincerely appreciate the expert suggestions provided by the reviewers. In response to this feedback, we have supplemented Section 2.1 of the manuscript with the latest research developments in femtosecond laser processing technology, particularly addressing the challenges related to manufacturing time and the barriers to its industrial application. Additionally, we have proposed relevant optimization pathways based on recent literature. These additions help to further enhance the completeness and practical relevance of this section from a real-world engineering perspective.

The relevant revisions can be found in lines 132 to 157.

(Although femtosecond laser micromachining…enhancing its stability and adaptability for practical engineering applications.)

 

Comment 4: The multi-parameter sensing is discussed in depth. It is recommended to add a table summarizing different techniques, their challenges, sensitivity etc to enhance readability.

Response 4:

We appreciate the constructive suggestions provided by the reviewers. In response, we have thoroughly reorganized and optimized Section 4.2, restructuring its logical framework and further enhancing the content of Table 5. We have categorized and compared various aspects, including technical classification, decoupling strategies, structural core, measurement ranges, and sensitivity performance, to improve readability and clarity. Additionally, we have added a comparative summary at the end of this section to help readers more intuitively understand the characteristics and differences of the various multi-parameter sensing technologies.

 

Comment 5: Photonic crystals are finding real world applications rapidly. The C-type fibers can further penetrate the usability. It is recommended to highlight summary of real-world applications.
Response 5:

We greatly appreciate your suggestions. We have made adjustments to the beginning of Section 4 of the manuscript by incorporating the research background and current status of C-type photonic crystals, to facilitate the development of the subsequent content.

The specific revisions can be found in lines 470 to 496.

 

Comment 6: Although the technical content is strong, the manuscript requires minor language revisions for fluency and grammar. Examples:

Line 92: "targeted research recommendations and an outlook" → consider rephrasing for clarity.

Line 672: "play an increasingly significant and widespread role" → could be shortened or made more precise.

Response 6:

We sincerely appreciate your valuable suggestions! We have thoroughly revised and polished the manuscript.

The content originally in line 92 is now located in lines 95 to 98 of the current manuscript.

(By analyzing the current technological bottlenecks,…for their integration into next-generation intelligent sensing systems.).

The content originally in line 672 is now located in lines 738 to 741 of the current manuscript.

(Despite these challenges, the ongoing efforts…industrial monitoring, and environmental sensing.)

Author Response File: Author Response.pdf