Fiber-Optic Hydraulic Sensor Based on an End-Face Fabry–Perot Interferometer with an Open Cavity

Round 1

Reviewer 1 Report

The article provides a comprehensive overview of the design and manufacturing process of a fiber optic microphone utilizing a macro cavity in an optical fiber. The development of a droplet-shaped macro cavity and its potential applications in various optical fiber sensors are well-described. The study also investigates the sensitivity of the macro cavity interferometer its potential for high-precision optical fiber measurements. While the article is well-structured and informative, it can be improved based on following comments:

1. The article is well organized with clear sectioning. The paper presents an innovative approach to create droplet-shaped macro cavity, derived from the formation of a quasi-periodic array of micro cavities. This unique approach contributes to the development of fiber-optic sensors and will appeal to wide range of readers of the journal.

2. The mix of experimental and theoretical data makes this paper top in its field.

3. The article creates pathway for various applications of the cavity in terms of temperature, pressure etc.

4. The fabrication, discussion and conclusions drawn are sound and will be enough to reproduce by anyone else.

5. Overall language and grammar are acceptable.

6. The tables, figures and equations are sound and are acceptable as is.

In conclusion, this article is a valuable contribution to the field of fiber-optic sensors. The article can be accepted as is.

Author Response

We thank the Reviewer for the careful review and the appreciation of our work.

Reviewer 2 Report

Comment 1

-The abstract could benefit from highlighting the potential drawbacks or areas that require further investigation for a more balanced overview of the research.

Comment 2

-In addition to the first negative point , it lacks a comprehensive exploration or discussion about potential challenges or limitations encountered during the fabrication process, or the real-world application limitations of this sensor.

Comment 3

- The negative aspect also lies in the lack of specificity regarding the methodology and evaluation metrics used to measure the sensor's sensitivity and performance.

Comment 4

- The extensive introduction presents an overwhelming volume of information on the different types and methods of constructing Fabry-Perot interferometers (FPI) for various pressure-sensing applications. Although rich in detail, the narrative lacks focus, resulting in an inundation of diverse and intricate technological methodologies without clear guidance or a central theme. The multitude of examples, materials, and techniques described in rapid succession can potentially confuse or overwhelm the reader, hindering the comprehension of the main research purpose and significance. The abundance of details, while informative, may dilute the impact and clarity of the introduction, making it challenging for a reader to discern the primary objective or innovative aspects of the study amidst the array of disparate information.

Comment 5

- The introduction tends to lack a coherent structure, where the information on varied FPI implementations and characteristics, ranging from diaphragm materials to different construction methodologies, could be difficult to follow due to the absence of a clear thematic thread.

Comment 6

- Emphasize the novelty of your work and its significance in the field. Clearly state how your study deviates or improves upon existing research, offering a more focused perspective.

Comment 7

- The omission of a detailed explanation or a descriptive legend accompanying Figure 2 hampers the understanding of the relationship between cavity size and shape concerning the discharge arc power. The inadequacy in providing critical information leaves the reader puzzled about the specific implications of varying arc power on the cavity characteristics. The lack of detailed labels or a comprehensive explanation within the context of the experiment or research setup significantly detracts from the paper's value, as it fails to complement and elucidate the information conveyed in the visual representation

Comment 8

-The conclusion barely touches upon potential drawbacks or challenges in the implementation of these sensors, particularly related to the gas diffusion and cavity alteration over time. A more comprehensive conclusion should discuss these potential pitfalls and offer insights on how they could be addressed in future iterations of the sensor design.

-It lacks a mention of real-world experimental  and simulation validations values.

Comments for author File: Comments.pdf

-The English language used in the document seems a bit convoluted in certain sections. The phrasing could benefit from simplification and a more consistent structure to enhance clarity and ease of understanding.

Author Response

The authors thank the Reviewer for the constructive comments and suggestions.

Comment 1

-The abstract could benefit from highlighting the potential drawbacks or areas that require further investigation for a more balanced overview of the research.

Thank you for the remark. The abstract has been edited as follows:

Abstract: The paper describes design and manufacturing process of a fiber optic ‎microphone based on a macro cavity at the end face of an optical fiber. The study explores the step-by-step fabrication of a droplet-shaped macro cavity on the optical fiber's end surface, derived from the formation of a quasi-periodic array of micro cavities due to the fuse effect. Immersing the end face of the optical fiber with macro ‎cavity in liquid leads to the formation of a closed area with gas, where interfacial ‎surfaces act as Fabry-Perot mirrors. The study demonstrates that the macro cavity can act as a standard foundational element for diverse fiber optic sensors, using the droplet-shaped end-face cavity as a primary sensor element. An evaluation of the macro cavity interferometer's sensitivity to length alterations is presented, highlighting its substantial promise for use in precise fiber optic measurements. However, potential limitations and further research directions include investigating the influence of external factors on microphone sensitivity and long-term stability. This approach not only significantly contributes to optical measurement techniques but also underscores the necessity for continued exploration of parameters influencing device performance.

Comment 2

-In addition to the first negative point , it lacks a comprehensive exploration or discussion about potential challenges or limitations encountered during the fabrication process, or the real-world application limitations of this sensor.

The abstract was edited to include potential challenges and limitations.

Comment 3

- The negative aspect also lies in the lack of specificity regarding the methodology and evaluation metrics used to measure the sensor's sensitivity and performance.

The authors have added more detailed description of the methodology and evaluation procedure used to define the performance. Figure 5 was added,  and Figure 4,b was changed. The following was added to the text:

Lines 186 – 196: “To visualize the internal volume of the cavity and liquid around the fiber, the fiber end with the sensitive element 1 was immersed in a Helle-Shaw cell 2 filled with distilled water and isopropyl alcohol, taken at a temperature of 23±0.5 ºС (Figure 5). Visualization was carried out using digital camera 3 interfaced with PC 7. The lens of the camera was aimed at the wide edge of the cell.

The detection of the optical signal was carried out using the setup presented in Figure 5. Laser radiation from a broadband source 4 (wavelength 1525-1570 nm, half-width 0.16 nm with a power of up to 5 mW) through port 1 and 2 of circulator 5 was directed into the fiber with the cavity under study. The circulator made it possible to simultaneously send a signal through the fiber and record the reflection spectrum through port 3 using an EXFO OSA20 6 spectrometer.”

Lines 215 – 229: “The data shown in Figure 4,b correspond to the reflection spectrum obtained when the end of the fiber was in the air (black line), when the cavity was immersed in the liquids under study (red line – isopropanol, blue line – water) in the first minute of contact of the sensor with the medium. The graph shows that the interference signal in isopropanol has a shorter period compared to the spectrum detected in water. At the same time, a more intense flow of isopropanol into the internal volume of the cavity was observed, which led to a decrease in the size of the gas inclusion and, as a consequence, a change in the length of the Fabry-Perot interferometer. It is obvious that in addition to capillary effects, the presence of two phases in the system leads to processes of diffusion and solubility of gas into the environment. It is known that the solubility coefficient of oxygen in water is 0.0279 at a temperature of 25 ºС and normal atmospheric pressure [42]. While the solubility coefficient of oxygen in alcohols can exceed this value by orders of magnitude. Due to the above, it was decided to limit the experiments on detecting the acoustic signal to the aqueous medium, since the processes observed in isopropanol require more detailed study.”

In addition, in line 353, the dimensions of the cuvette were specified:“The fiber end was immersed in the cuvette measuring 5x5x5 cm — 4 filled with water.”

Comment 4

- The extensive introduction presents an overwhelming volume of information on the different types and methods of constructing Fabry-Perot interferometers (FPI) for various pressure-sensing applications. Although rich in detail, the narrative lacks focus, resulting in an inundation of diverse and intricate technological methodologies without clear guidance or a central theme. The multitude of examples, materials, and techniques described in rapid succession can potentially confuse or overwhelm the reader, hindering the comprehension of the main research purpose and significance. The abundance of details, while informative, may dilute the impact and clarity of the introduction, making it challenging for a reader to discern the primary objective or innovative aspects of the study amidst the array of disparate information.

Comment 5

- The introduction tends to lack a coherent structure, where the information on varied FPI implementations and characteristics, ranging from diaphragm materials to different construction methodologies, could be difficult to follow due to the absence of a clear thematic thread.

Comment 6

- Emphasize the novelty of your work and its significance in the field. Clearly state how your study deviates or improves upon existing research, offering a more focused perspective.

The authors thank the Reviewer for constructive comments 4, 5 and 6. However, we would like to keep the introduction in this form, since it fully reflects the path along which we came to the idea of this work. In order to increase the clarity of the objectives of the current work, we have added a paragraph at the end of the introduction as follows:

“The current work focuses on two main points. The first one is the development of the technological process and the creation of an open drop-shaped cavity at the end of the optical fiber, similar to [35], elaborating on the repeatability of the cavity shape and the factors that influence it. The second one is a theoretical and practical study of the possibility of using an open drop-shaped cavity at the end of an optical fiber as a membraneless Fabry-Perot interferometer acting as an acoustic sensor that allows monitoring sound vibrations propagated in a liquid medium.”

Comment 7

- The omission of a detailed explanation or a descriptive legend accompanying Figure 2 hampers the understanding of the relationship between cavity size and shape concerning the discharge arc power. The inadequacy in providing critical information leaves the reader puzzled about the specific implications of varying arc power on the cavity characteristics. The lack of detailed labels or a comprehensive explanation within the context of the experiment or research setup significantly detracts from the paper's value, as it fails to complement and elucidate the information conveyed in the visual representation .

We thank the Reviewer for the remark. For clarification, we have added the dimension d in Figure 2,a, and a more detailed description of the visually presented information in the text (lines 169-182):

“Figure 2 shows photographs of cavities that were obtained on a Zeiss Axiovert 40 MAT microscope after the manufacture of cavities at a fixed time of contact of the discharge arc with the welding elements with varying power P. The fiber was welded to the dif-fuser with a two-second exposure. The P value is given relative to its peak value P0 = 1.5 W. Experiments have shown that, under equal manufacturing conditions, there is a 5% spread in the maximum transverse cavity size d. Data averaging was carried out based on the dimensions of the cavities obtained in five implementations with fixed welding parameters. As it can be observed, with increasing discharge arc power, the maximum transverse cavity size d increases, reaching a plateau already at values P = 0.3∙P0. The cavity reaches its largest size under a three-second exposure to a discharge arc at P = 0.2∙P0. Thus, using the data obtained, it is possible to select the optimal mode P and t for the manufacture of cavities of a given size. In the future, it is planned to conduct a series of experiments that will make it possible to construct a complete map of modes.”

Comment 8

-The conclusion barely touches upon potential drawbacks or challenges in the implementation of these sensors, particularly related to the gas diffusion and cavity alteration over time. A more comprehensive conclusion should discuss these potential pitfalls and offer insights on how they could be addressed in future iterations of the sensor design.

-It lacks a mention of real-world experimental  and simulation validations values.

For a deeper understanding of the processes, the article’s material was supplemented with experimental data obtained by immersing the sensor in isopropanol. In the Discussion, the processes of diffusion and solubility of gas into the environment were described, and the possible methods for eliminating these effects were proposed (lines 413-417):

“Today, there are liquid viscosity and surface tension sensors based on open cavities [44–46] based on partial filling of the cavity with liquid. Our task is to eliminate this effect as much as possible. At the moment, the issue of changing the resonator (the size of the air cavity) over time is being addressed by reducing the wettability of the internal walls of the macrocavity by applying water-repellent films.”

Reviewer 3 Report

The authors present an approach to fiber optic microphone design using a droplet-shaped macro cavity at the end of an optical fiber. The paper explores the fabrication of this cavity, highlighting its application in high-precision optical fiber measurements. Key elements include the utilization of a quasi-periodic array of micro cavities and the integration of the macro cavity as a base element for various optical fiber sensors. The research extends to examining the sensitivity of this interferometer and its potential in diverse applications, including temperature, pressure, and vibration measurements. Additionally, the manuscript explores various materials and methods for constructing FPI membranes, addressing the challenges and advancements in this field. The study suggests a technology for creating a universal element for fiber-optic sensors, with potential applications in diverse fields of knowledge.

In my opinion, the question is well-defined, and the results provide an advancement of the current knowledge. Furthermore, the the work fits the journal scope. The article is written in an appropriate way, and the data and analyses are presented appropriately.  Overall, it seems benefitial to publish this work.

Still, in my opinion, the following issues need to be addressed before further consideration of the manuscript:

Major comments:

Line 107:  Value is incorrect. Should be: 2239 nm/Pa

Line 121: Double-check value.

Eq. (9):  equation for the Wave number seems dimensionally incorrect. Correct it or clarify the meaning of the variables.

Line 314-316 “Based on 314 the mathematical model, the sensitivity limits of the sensor are investigated and the eigen 315 resonance frequencies are estimated.” The eigenresonances have not been studied in the manuscript. Please do the eigenresonance study explicitly, or modify this sentence.

The manuscript lacks a thorough discussion section. Please write discussion paragraphs to discuss questions such as the ones below, citing appropriate references to back up the arguments.

a.            What is the state-of-the-art in this area, both commercially and in academic literature?

b.            How do your results compare with those published in the literature? How do your results compare with alternative approaches to reach the same goal? What would be the three main articles you could use to compare your results?

c.            Did you encounter any surprises in relation to the expectations generated by literature, or by conventional wisdom? Add references when discussing.

d.            Can you comment on the ease of reproduction of the results in a different setup?

e.            What limitations does the approach have (findings, applicability of the findings, unresolved questions, limitations of scope of the work (what you have excluded from the study), potential biases, and reproducibility limitations.

f.            In your opinion, in which cases would you know in advance that the procedure will fail?

g.            In your opinion, what are the weak aspects of the methodology used and of your results?

h.            What are further improvements that could be done to improve the accuracy and robustness of the device

i.             What are the open challenges or unresolved issues remaining for further research?

Please adequately address the above comments, before returning the manuscript to the editor.

Fine

Author Response

The authors thank the Reviewer for the careful review and the valuable comments and suggestions.

Major comments:

Line 107: 

Value is incorrect. Should be: 2239 nm/Pa

Thank you for the careful review. The value was corrected.

Line 121:

Double-check value.

Thank you, the value was corrected and the reference has been changed, citing more relevant work of the same authors.

 Eq. (9): 

equation for the Wave number seems dimensionally incorrect. Correct it or clarify the meaning of the variables.

The meaning of the variables was clarified as follows (lines 299-301): “where, ε_i is the relative dielectric permittivity, μ_i is the relative magnetic permeability of the ‎substance, λ  is the wavelength of radiation, ε₀ is the permittivity and μ₀ is the permeability of free space”.‎

Line 314-316

“Based on 314 the mathematical model, the sensitivity limits of the sensor are investigated and the eigen 315 resonance frequencies are estimated.” The eigenresonances have not been studied in the manuscript. Please do the eigenresonance study explicitly, or modify this sentence.

Thank you for the remark. Indeed, the study of eigenresonances was not in the scope of the current work. Therefore, the sentence was edited as follows: “Based on the mathematical model, the sensitivity limits of the sensor ‎are investigated.”

The manuscript lacks a thorough discussion section.  

Please write discussion paragraphs to discuss questions such as the ones below, citing appropriate references to back up the arguments.

1. What is the state-of-the-art in this area, both commercially and in academic literature?
2. How do your results compare with those published in the literature? How do your results compare with alternative approaches to reach the same goal? What would be the three main articles you could use to compare your results?
3. Did you encounter any surprises in relation to the expectations generated by literature, or by conventional wisdom? Add references when discussing.
4. Can you comment on the ease of reproduction of the results in a different setup?
5. What limitations does the approach have (findings, applicability of the findings, unresolved questions, limitations of scope of the work (what you have excluded from the study), potential biases, and reproducibility limitations.
6. In your opinion, in which cases would you know in advance that the procedure will fail?
7. In your opinion, what are the weak aspects of the methodology used and of your results?
8. What are further improvements that could be done to improve the accuracy and robustness of the device
9. What are the open challenges or unresolved issues remaining for further research?

The Reviewer proposed an excellent plan, guided by which the dissertation work will be written based on the results of this research and further works. If we include all these suggestions of the reviewer in the article, then the volume of this text may be several times greater than the volume of a regular scientific article in a journal. When presenting the material, we tried to provide all the necessary numerical data so that our results could be repeated in almost any laboratory with the corresponding equipment. In our article, we emphasize two main points: 1) a drop-shaped cavity can be formed at the end of an optical fiber with high repeatability; 2) a drop-shaped cavity at the end of the fiber can become a universal component of a large number of different sensitive elements, and as an example they gave the simplest solution for recording sound. We chose an application example of sound recording due to the fact that it only requires monitoring the relative change in power, which can be realized without the calibration process. The sensing element in exactly the same configuration can, of course, be both a temperature sensor (at constant pressure) and a pressure sensor (at constant temperature) and a refractive index sensor, if pressure and temperature are constant. Hence a universal platform has been proposed, which will allow the formation of sensors of various physical fields on a single principle of measurement conversion. We also understand that measuring, say, pressure, will require a technical solution that involves the use of more than one sensor and a procedure for their calibration. Moreover, we already have experience with such solutions (Calibration of combined pressure and temperature sensors / A. Z. Sahabutdinov, A. Z. Kuznetsov, I. I. Nureev [et al.] // International Journal of Applied Engineering Research. – 2015. – Vol. 10, No. 24. – P. 44948-44957.) At the same time, we deliberately would not like to complicate the material of the article, so as not to lead the reader away from the two abovementioned main points, on which we have focused. At the same time, as the Reviewer rightly noted, it makes sense to add some clarifications to the text of the article, namely in the discussion section.

There are, of course, many different configurations of the fiber-optic Fabry-Perot interferometers. In the introduction, we tried to outline the path that we followed during the search for a universal solution. The prototype for the development of the proposed sensor platform was the article [M. de Fátima F. Domingues et al., “Liquid Hydrostatic Pressure Optical Sensor Based on Micro-Cavity Produced by the Catastrophic Fuse Effect,” in IEEE Sensors Journal, vol. 15, no. 10, pp. 5654-5658, Oct. 2015, doi: 10.1109/JSEN.2015.2446534], in which the implementation of a pressure sensor on a microcavity formed at the end of the fiber was proposed. At the same time, we did not encounter a similar single unified platform that could become a single technological base for various sensors. In the discussion section, we tried to outline the difficulties that we faced in the current research. Some of them, such as long-term stability, have yet to be resolved, and perhaps one solution will be found in the formation of a membrane on the surface of the cavity, blocking either a gas bubble or, for example, liquid crystals. For each type of sensing element designed to measure individual physical impacts, an engineering problem must be solved to determine the measurement range, dynamic range, and conditions of use. At the same time, it seems to us that the repetition of our results can be obtained in any technically equipped laboratory, following the material presented in this work. The production of the cavity can be easily repeated on another welding machine, taking into account the welding settings, namely the time and power of the discharge arc. Experiments have shown that, under equal manufacturing conditions, there is a 5% spread in the maximum transverse cavity size d. Data averaging was carried out based on the dimensions of the cavities obtained in five implementations with fixed welding parameters. We have tried to outline the existing limitations, unresolved issues and proposed ways for further research in this direction. Our research team will be happy to collaborate with teams interested in this project.

Reviewer 4 Report

The author presents a method to measure acoustic signals based on an end-face FPI with an open cavity in the manuscript. The paper might be published after the following questions are well addressed:

Q1: How to measure the cavity size based on Figure 2?

Q2: Pg. 5 Line 172, there’s a missed value for d.

Q3: Pg. 11 Line 295, what’s the optical wavelength the author used in the experiment?

Q4: Pg. 11 Line 307, the author should explain why FPI is capable of capturing a signal with a large frequency range.

Q5. The paper has grammar and language issues, which need to be addressed.

Author Response

The authors thank the Reviewer for the careful review and the valuable comments and suggestions. Please, find our replies below.

Q1: How to measure the cavity size based on Figure 2? 

In the Figure 2 caption, the maximum transverse dimensions of the cavity is presented. For clarity, dimension lines are shown in Figure 2,a. The actual dimensions were determined from images taken on a Zeiss Axiovert 40 MAT microscope. Data averaging was carried out over five implementations performed with fixed welding parameters.

Q2: Pg. 5 Line 172, there’s a missed value for d.

Thank you, the value of d was added.

Q3: Pg. 11 Line 295, what’s the optical wavelength the author used in the experiment?

The following was added to the text (lines 351-353 after revisions): “The experiments were carried out at a wavelength of 1550 nm with a radiation power of up to 5 dBm.”

Q4: Pg. 11 Line 307, the author should explain why FPI is capable of capturing a signal with a large frequency range.

Thank you for the suggestion. The following was added in lines 376-378: “The main factors enabling the wide range of detected frequencies are the geometry of the cavity and its opening, and the properties of the ambient liquid (water).”

Reviewer 5 Report

This paper presents a sensor of acoustic oscillations in liquid is proposed on the basis of an open cavity at the end of optical fiber.  The study explores the step-by-step fabrication of a droplet-shaped macro cavity on the optical fiber's end surface, derived from the formation of a quasi-periodic array of micro cavities due to the fuse effect.  It is shown that droplet-shaped macro cavity at the face end of an optical fiber has significant potential for application in high-precision optical fiber measurements. Overall, the manuscript is prepared with lots of useful details, which might be very helpful for the researchers working in this area. The manuscript could be considered for publication after addressing the following questions.

1.          The abstract need to be polished so that it is more informative and self-contained so that any reader can know the significance of this paper and obtain sufficient information to decide if one should look into the main text. In general, the later part of the abstract seems general. It should reflect the results and data presentation of this study. Please reshape it.

2.          The authors should describe in detail the design and manufacturing process of a fiber optic microphone based on a macro cavity at the end face of an optical fiber, as well as the materials used in membrane fabrication.

3.          Please add more details to the Figures, i.e., Photograph of a drop-shaped cavity on the end of an optical fiber immersed in water in Figures 4 (a), reflection spectrum in Figures 4 (b), Axisymmetric cavity at the end of an optical fiber in Figure 5 (a).

4.          The quality in Figure 7-10 is weaker. The X-axis and Y-axis title text sizes need to be adjusted.

5.          In Figure 12, experiments show that the studied FPI is able to capture signals in the frequency range from 1 Hz to 100 kHz. How about the reproducibility? How many times were each experiment repeated (n=?)? Please add this information to the documentation. This point is critical.

6.          The verification experiment is too simple. It is recommended to use complex samples other than water for measurement.

7.          In "Detection of Sound Waves in Liquids" or "Discussion and Conclusion", the author should provide experimental quantitative analysis data instead of simply explaining it. In addition, the authors should specifically state the focus of the study and highlight how this study compares with other existing methods. Please reshape it.

8.          Conclusions, Page 12, Line 347-352, “The proposed unified platform for serial development of fiber optic sensors of different types using an open drop-shaped cavity at the fiber end or a closed cavity inside the fiber has not only a great potential for application......”. The “Future studies .....” here is too general to understand the motivation of the future work. Please specify it.

Author Response

The authors thank the Reviewer for the careful review and the valuable comments and suggestions. Please, find our replies below.

1. The abstract need to be polished so that it is more informative and self-contained so that any reader can know the significance of this paper and obtain sufficient information to decide if one should look into the main text. In general, the later part of the abstract seems general. It should reflect the results and data presentation of this study. Please reshape it.

This text takes into account the Reviewer’s comments, providing a more balanced and informative overview of the study, its objectives and potential benefits, and points to areas for future research:

Abstract: The paper describes design and manufacturing process of a fiber optic ‎microphone based on a macro cavity at the end face of an optical fiber. The study explores the step-by-step fabrication of a droplet-shaped macro cavity on the optical fiber's end surface, derived from the formation of a quasi-periodic array of micro cavities due to the fuse effect. Immersing the end face of the optical fiber with macro ‎cavity in liquid leads to the formation of a closed area with gas, where interfacial ‎surfaces act as Fabry-Perot mirrors. The study demonstrates that the macro cavity can act as a standard foundational element for diverse fiber optic sensors, using the droplet-shaped end-face cavity as a primary sensor element. An evaluation of the macro cavity interferometer's sensitivity to length alterations is presented, highlighting its substantial promise for use in precise fiber optic measurements. However, potential limitations and further research directions include investigating the influence of external factors on microphone sensitivity and long-term stability. This approach not only significantly contributes to optical measurement techniques but also underscores the necessity for continued exploration of parameters influencing device performance.

2. The authors should describe in detail the design and manufacturing process of a fiber optic microphone based on a macro cavity at the end face of an optical fiber, as well as the materials used in membrane fabrication.

The current work is dedicated to the development of the sensor with open cavity. However, as the Reviewer rightly noted, the sensing element with membrane is a more promising solution in terms of long-term stability of performance. Therefore, the further works will be dedicated to the development of the sensor with membrane. In this paper, we have added more details in the description of the design and manufacturing process of the FPI sensor with an open cavity.

3. Please add more details to the Figures, i.e., Photograph of a drop-shaped cavity on the end of an optical fiber immersed in water in Figures 4 (a), reflection spectrum in Figures 4 (b), Axisymmetric cavity at the end of an optical fiber in Figure 5 (a).

Thank you for the suggestion. For clarification, we added scales in Figure 4 (a) and Figure 5 (a) (Figure 6 (a) after revisions). In addition, another spectrum was presented in Figure 4 (b).

4. The quality in Figure 7-10 is weaker. The X-axis and Y-axis title text sizes need to be adjusted.

Thank you for the remark, the quality of the figures was enhanced.

5. In Figure 12, experiments show that the studied FPI is able to capture signals in the frequency range from 1 Hz to 100 kHz. How about the reproducibility? How many times were each experiment repeated (n=?)? Please add this information to the documentation. This point is critical.

Thank you for the questions. The following was added to the text:

“Figure 13 shows graphs of the signal amplitude versus time and the frequency spectrum (FFT) normalized to the maximum amplitude, obtained with the input signal of different frequencies: 10 Hz (a, b), 100 kHz (c, d), and frequency varying from 500 Hz to 5 kHz in 500 Hz steps every 10 seconds (e, f).

To assess the reproducibility of the results, measurements (n=10) of the selected frequency (500 Hz) were carried out at fixed input parameters of laser radiation (5 dVm) for one of the sensors under study (maximum cavity diameter d = 109.4 µm, opening diameter d' = 62.5 µm). Experiments showed that, regardless of the amplitude of the detected signal, the accuracy of determining the input frequency was no less than 0.005% and amounted to 500.095 ± 0.028 Hz.”

6. The verification experiment is too simple. It is recommended to use complex samples other than water for measurement.

Thank you for the suggestion. We have added the spectrum of the sensor ‎immersed in isopropanol in Figure 4,b. In addition, corresponding comments have been added to the text (lines 219-229):

“At the same time, a more intense flow of isopropanol into the internal volume of the cavity was observed, which led to a decrease in the size of the gas inclusion and, as a consequence, a change in the length of the Fabry-Perot interferometer. It is obvious that in addition to capillary effects, the presence of two phases in the system leads to processes of diffusion and solubility of gas into the environment. It is known that the solubility coefficient of oxygen in water is 0.0279 at a temperature of 25 ºС and normal atmospheric pressure [42]. While the solubility coefficient of oxygen in alcohols can exceed this value by orders of magnitude. Due to the above, it was decided to limit the experiments on detecting the acoustic signal to the aqueous medium, since the processes observed in isopropanol require more detailed study.”

7. In "Detection of Sound Waves in Liquids" or "Discussion and Conclusion", the author should provide experimental quantitative analysis data instead of simply explaining it. In addition, the authors should specifically state the focus of the study and highlight how this study compares with other existing methods. Please reshape it.

The authors thank the Reviewer for the remarks. Chapter 4 Detection of Acoustic Waves in a Liquid has been supplemented with additional experimental quantitative data, including the assessment of reproducibility of the results.

8. Conclusions, Page 12, Line 347-352, “The proposed unified platform for serial development of fiber optic sensors of different types using an open drop-shaped cavity at the fiber end or a closed cavity inside the fiber has not only a great potential for application......”. The “Future studies .....” here is too general to understand the motivation of the future work. Please specify it.

The text has been edited to include the following: “Further research will be dedicated to the usage of the presented developments of the fiber cavities as part of combined fiber-optic sensors with addressed fiber Bragg structures, with the possibility of implementing microwave-photonic interrogation, which will allow to increase the interrogation frequency to dozens or hundreds of gigahertz.”

Reviewer 6 Report

In the peer-reviewed manuscript, the authors propose a production technology of a microphone made of an optical fiber based on the macro cavity at the end of the optical fiber. The study examines and describes the step-by-step fabrication of a droplet-shaped macro cavity on the optical fiber's end surface, derived from the formation of a quasi-periodic array of micro cavities due to the fuse effect.

Experimental results show that the macro cavity can be used as a unified basic element for various optical fiber sensors, using a drop -shaped cavity at the end of the fiber as a base sensing element.

The paper brings original novel information in the domain of the journal’s thematic focus. The research results are clearly distinguished from results adopted and used literary resources are mentioned properly. Credibility of published results is documented (experiments - simulations). Text readability and its linguistic correctness (even English texts, especially in the case of the technical terminology) is on the appropriate level.

I have the following comments on the scientific content of the article and I ask the authors to comment on them:

I have the following comments on the scientific content of the article :

1. Chapter 4 must be supplemented and extended to a more detailed description of the experiment with the addition of photo documentation with experiments.

2. I am not sure about measurement accuracy??? From how many experiments the authors got the result in the form of Figure 12 ???

3. What is the sensitivity of the sensor designed?

I have the following comments on the formal side of the article:

1. the keywords need to be arranged alphabetically,

2. the abstract needs to be edited: clearly distinguish what is the goal and what is the benefit of the work.

In general, after appropriate corrections and additions, I approve publication of this manuscript.

Author Response

The authors thank the Reviewer for the careful review and the valuable comments and suggestions. Please, find our replies below.

I have the following comments on the scientific content of the article :

1. Chapter 4 must be supplemented and extended to a more detailed description of the experiment with the addition of photo documentation with experiments.

Thank you for the suggestions. Chapter 4 has been extended to include additional details and the results.

2. I am not sure about measurement accuracy??? From how many experiments the authors got the result in the form of Figure 12 ???

Thank you for the remarks. The following was added to the text (lines 369-374):

“To assess the reproducibility of the results, measurements (n=10) of the selected frequency (500 Hz) were carried out at fixed input parameters of laser radiation (5 dVm) for one of the sensors under study (maximum cavity diameter d = 109.4 µm, opening diameter d' = 62.5 µm). Experiments showed that, regardless of the amplitude of the detected signal, the accuracy of determining the input frequency was no less than 0.005% and amounted to 500.095 ± 0.028 Hz.”

3. What is the sensitivity of the sensor designed?

Thank you for the suggestion. Currently, work is underway to determine the sensitivity of the sensor, which will make it possible to define, in the range of nominal parameters, the ratio of the signal change to the corresponding change in the signal amplitude at a given frequency.

I have the following comments on the formal side of the article:

1. the keywords need to be arranged alphabetically,

Thank you, the keywords were rearranged.

2. the abstract needs to be edited: clearly distinguish what is the goal and what is the benefit of the work.

Thank you for the remark. The abstract has been edited as follows:

Abstract: The paper describes design and manufacturing process of a fiber optic ‎microphone based on a macro cavity at the end face of an optical fiber. The study explores the step-by-step fabrication of a droplet-shaped macro cavity on the optical fiber's end surface, derived from the formation of a quasi-periodic array of micro cavities due to the fuse effect. Immersing the end face of the optical fiber with macro ‎cavity in liquid leads to the formation of a closed area with gas, where interfacial ‎surfaces act as Fabry-Perot mirrors. The study demonstrates that the macro cavity can act as a standard foundational element for diverse fiber optic sensors, using the droplet-shaped end-face cavity as a primary sensor element. An evaluation of the macro cavity interferometer's sensitivity to length alterations is presented, highlighting its substantial promise for use in precise fiber optic measurements. However, potential limitations and further research directions include investigating the influence of external factors on microphone sensitivity and long-term stability. This approach not only significantly contributes to optical measurement techniques but also underscores the necessity for continued exploration of parameters influencing device performance.

Round 2

Reviewer 2 Report

All queries raised by me have been addressed by the authors satisfactorily

Reviewer 4 Report

Thanks for resubmission. The questions have been addressed properly and this manuscript is recommended to be published.

Reviewer 6 Report

All my suggested comments have been taken care.

In general, I agree to the publication of this manuscript.