Submit to this Journal Review for this Journal Propose a Special Issue

Article Menu

Open AccessArticle

Peer-Review Record

Investigation of Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure Sensing

Photonics 2021, 8(5), 138; https://doi.org/10.3390/photonics8050138

by Jun Wang¹, Long Li¹, Shuaicheng Liu¹, Diyang Wu¹, Wei Wang^1,2, Ming Song¹, Guanjun Wang^1,2,3,*

and Mengxing Huang^1,2,*

Reviewer 1: Anonymous

Reviewer 2: Anonymous

Reviewer 3: Anonymous

Photonics 2021, 8(5), 138; https://doi.org/10.3390/photonics8050138

Submission received: 24 February 2021 / Revised: 8 April 2021 / Accepted: 9 April 2021 / Published: 23 April 2021

Round 1

Reviewer 1 Report

The authors presented interferometric systems based on two layers of thermo sensitive materials at the tip of a fiber. These were characterized by temperature and pressure. The topic is in the scope of the journal Photonics, however I consider that the work does not have a contribution since the multilayer systems have already been widely reported and analyzed both theoretically and experimentally. Moreover, these type of systems has been used as elements for sensing multiple physical variables. In addition, I consider that the article must be improved…my comments and suggestions are the following :

The analysis presented in the introduction is very poor, it does not address a more complete analysis on dual-parameter sensors to specifically measure temperature and pressure and those made with materials at the end of an optical fiber.

the temperature sensitivity is of the same order of magnitude for the single layer [7] and two layers sensors [authors´s work], so I do not consider that the reported device is of high sensitivity.

In Figure 1, the layers are named as glue 1 and glue 2, but these expressions are not mentioned in the manuscript.

It is not clear why formula 1 was introduced. Why is formula 2 not as a function of temperature and pressure? It is necessary to indicate how the formula is modified to take into account the properties of the materials. I suggest showing how the temperature and pressure affects to the reflected relative intensity.

Why was the thermo-optic coefficient not taken into account? I suggest taking into account it (paragraph after formula 3). Besides, I suggest showing the theoretical spectra at different temperatures and pressures, in order to validate what is mentioned (paragraph after formula 3).

As authors mention that it is obvious that the dual -layer cavity have three-beam interference characteristics, I suggest showing the theoretical interference spectra of each one of the interferometric systems using the theory that they present .

It is not clear, how the thicknesses of the layers were controlled. Besides, I suggest indicating the numbers of the steps in figure 2 as mentioned in the manuscript.

In the fabrication process, the authors mention that the materials are mixed and the mixture is used to form a layer, but in section 3 it is understood that authors used the materials separately to form the layers. It is important to clarify the fabrication of the interferometric systems.

I suggest opening the spectra of figure 3 to better appreciate the interference patterns. Why is the FSR (free spectral range) of the spectrum 3b larger than that of spectrum 3b? What are the FSR of each one of the spectra of the systems?

I suggest explaining the shape of the spectra of the three systems. Two spectra are alike and resemble single-layer ones.

If a sensor is presented then it is necessary to do a broader study, for example the repeatability, the minimum detectable temperature and pressure, hysteresis, to make a comparison with other dual-parameter sensors for measuring temperature and pressure in terms of sensitivity. How is the temperature value differentiated from the pressure value if the same monitoring variable is used to measure both parameters?

Author Response

Cover Letter for Reviewers

(1^st Apr. 2021)

Dear reviewer:

Thank you very much for providing us with your comments. We found the comments are very constructive and useful to improve our work. We have revised the paper along the lines suggested by you. Below are our responses to the specific comments from you.

Part Ⅰ: Comments and Suggestions for Authors

The analysis presented in the introduction is very poor, it does not address a more complete analysis on dual-parameter sensors to specifically measure temperature and pressure and those made with materials at the end of an optical fiber.

the temperature sensitivity is of the same order of magnitude for the single layer [7] and two layers sensors [authors´s work], so I do not consider that the reported device is of high sensitivity.

In Figure 1, the layers are named as glue 1 and glue 2, but these expressions are not mentioned in the manuscript.

It is not clear why formula 1 was introduced. Why is formula 2 not as a function of temperature and pressure? It is necessary to indicate how the formula is modified to take into account the properties of the materials. I suggest showing how the temperature and pressure affects to the reflected relative intensity.

As authors mention that it is obvious that the dual -layer cavity have three-beam interference characteristics, I suggest showing the theoretical interference spectra of each one of the interferometric systems using the theory that they present .

It is not clear, how the thicknesses of the layers were controlled. Besides, I suggest indicating the numbers of the steps in figure 2 as mentioned in the manuscript.In the fabrication process, the authors mention that the materials are mixed and the mixture is used to form a layer, but in section 3 it is understood that authors used the materials separately to form the layers. It is important to clarify the fabrication of the interferometric systems.

I suggest opening the spectra of figure 3 to better appreciate the interference patterns. Why is the FSR (free spectral range) of the spectrum 3b larger than that of spectrum 3b? What are the FSR of each one of the spectra of the systems?

I suggest explaining the shape of the spectra of the three systems. Two spectra are alike and resemble single-layer ones.

If a sensor is presented then it is necessary to do a broader study, for example the repeatability, the minimum detectable temperature and pressure, hysteresis, to make a comparison with other dual-parameter sensors for measuring temperature and pressure in terms of sensitivity. How is the temperature value differentiated from the pressure value if the same monitoring variable is used to measure both parameters?

Part Ⅱ: Our Responses to Reviewer’s Comments

Comment 1:

Reply 1:

We sincerely thank the reviewer for pointing out this. However, by analyzing the existing literature that focusing on the issue of measuring of temperature and pressure based on dual-Fabry-Perot Interferometer (FPI) comprehensively, we found that part of works reported ultra high sensitivity. While the endlessly pursuing of the high sensitivity will bring out several problems which should be paid more attention, that exactly is this kind of fiber sensor can only applied for ultra-precise monitoring applications. What’s more, it will result in a great challenge for the design of interrogator’s bandwidth. Few other reports’ sensitivity should be significantly enhanced or cost should be cut down. Additionally, the chemical etching method assisted FPI fabrication processes do provides good sensitivity, however, the corrosive effects of the chemical etching and precise etching time are difficult to control. We also found some easy fabrication methods, but their sensitivity and repeatability of fabrication for the FP cavity should be improved.

Though massive works had been reported the dual-parameters monitoring based on FPI structures, overall, it is still worth to investigate and demonstrate the design of sensor structure, selection of sensor materials and optimization of fabrication processes, continuously. Hence, in this paper, a novel FP cavity with composite structure for fiber sensing based on multiple transfer method is proposed to measure the temperature and pressure, which possess the advantages of proper sensitivity, simple fabrication, cost-effectively, controllable cavity length.

For the above reasons, we do think our work is important, significant and constructive for fields of dual parameter sensing based on fiber sensor. The contribution of the updated paper is distinctive, that just revised along the suggestion given by the reviewer.

Comment 2:

Reply 2:

Just followed your suggestions, we have re-write the Introduction section significantly with updating the reference lists, which highly focused on the issue of temperature and pressure measuring based on the dual FPI. In addition, the modified version also carefully compares the temperature and pressure sensing characteristics of sensors with different composite structures in Table 3. We do believe this revision can improve the Introduction.

Comment 3:

the temperature sensitivity is of the same order of magnitude for the single layer [7] and two layers sensors [authors´s work], so I do not consider that the reported device is of high sensitivity.

Reply 3:

Sincerely thanks to you for pointing out this, according to your advice, we seriously considered the reference [7] (in the revised version, reference [7] has been updated to reference [17]) , we found this article proposed a FPI based on pendant polymer droplet with the temperature sensitivity of 249 pm/℃, pressure sensitivity of 1130pm/MPa. In our work, the temperature and pressure sensitivity are enhanced 3-5 times and about 30 times, respectively. It is really a truth that our temperature sensitivity is of the same order of magnitude with reference [17]. However, the reference [17] didn't clearly point out the control method of diaphgram thickness. Nevertheless, our work have the advantages of proper sensitivity, simple fabrication, cost-effectively, controllable cavity length. Hence, the investigation of this paper is a great supplement for the dual parameters sensing.

Comment 4:

In Figure 1, the layers are named as glue 1 and glue 2, but these expressions are not mentioned in the manuscript.

Reply 4:

Thanks for your valuable suggestions and corrections. This is indeed an omission in our manuscript. We have appropriately modified the the annotations in Figure 1 to make the content of the figure correspond to the content of the article.

Comment 5:

Reply 5:

Thanks a lot for your suggestions. The purpose of introducing formula 2 is to explain the principle of double-beam interference (The formula 2 has been revised).Simultaneously, the formula 3 shows the principle of three-beam interference. In addition, we have supplemented the principle between the change in cavity length () and the change in temperature/pressure (/). The supplemented formula (6) and formula (7) clearly dispalys how the temperature/pressure affects to the cavity length, then, the change in cavity length is able to make a differenc on the refected relative intensity.

Comment 6:

Why was the thermo-optic coefficient not taken into account? I suggest taking into account it (paragraph after formula 3). Besides, I suggest showing the theoretical spectra at different temperatures and pressures, in order to validate what is mentioned (paragraph after formula 3). As authors mention that it is obvious that the dual -layer cavity have three-beam interference characteristics, I suggest showing the theoretical interference spectra of each one of the interferometric systems using the theory that they present .

Reply 6:

We sincerely thanks the reviwer to point out this. We have collected and read a large number of documents of the same type of temperature and pressure sensing to explain the content of the principle section more comprehensively. The revised paper has supplemented the formula (4), (5), (6), (7) and (8). The above formulas clearly demonstate the principle of temperature and pressure sensing, which considers multiple parameters of the material, the thermo-optic coefficient, thermal expansion, Poisson’s ratio, Young’s modulus, etc. are all included. We do think that the updated version has fully considerated the sensing sensing mechanism.

In addition, we have simulated the three-beam interference according the formula (2). In this part, we have supplemented the simulated interference spectrum of the three-beam interference which shown in Figure. 2a. What’s more, the Figure 2b clearly shows the theoretical wavelength shift of the interference spectrum that induced by temperature or pressure change. .

Comment 7:

It is not clear, how the thicknesses of the layers were controlled. Besides, I suggest indicating the numbers of the steps in figure 2 as mentioned in the manuscript.

Reply 7:

Thanks for your valuable suggestions. In terms of the control of cavity length, we do didn’t elaborate clearly in the previous version. The detail of the cavity length control is as follows: The multiple transfer method is utilized to make different film layers. According to the measurement requirements, the film thickness can be controlled by increasing or reducing the number of transfers: namely, the formation of each FP cavity requires mul-tiple transfer of the material. After the material of the previous transfer is cured, the transfer method can be used again to increase the film thickness. Consequently, we achieve controllable cavity length, which is generally about 10-30μm. Additionally, it is also helpful to increase the viscosity and degree of solidification by heating the materials. The relevant content has already been described in detail in the revised version of the paper.

Comment 8:

In the fabrication process, the authors mention that the materials are mixed and the mixture is used to form a layer, but in section 3 it is understood that authors used the materials separately to form the layers. It is important to clarify the fabrication of the interferometric systems.

Reply 8:

We are really sorry for we didn’t clarify this point. In this paper, a composite structure of dual FP cavities is proposed for temperature and pressure sensing. Each FP cavity is formed by one kind of material with certain diaphgram thickness, rather than a mixture of multiple materials. We have carefully updated the description of the production process in the revised one to avoid misunderstandings.

Comment 9:

I suggest opening the spectra of figure 3 to better appreciate the interference patterns. Why is the FSR (free spectral range) of the spectrum 3b larger than that of spectrum 3b?

Reply 9:

Thank you for your pertinent suggestions to Figure 3. Since the spectrum range of the ASE in the test is 1525nm~1610nm, Figure 3(b), 3(d) (have already updated to 4(a), 4(c), respectively)) can only display the interference spectrum of 1525~1610nm. In theory, Figures 4(c) and 4(c) can show better two-beam and three-beam interference characteristics by utilizing the broadband light source with a wider spectral range. However, constrained by the existing experimental conditions, the interference spectrum range of the experimental test is 1525nm~1610nm, which cannot be expanded. Nevertheless, we also have adjusted the Figure 4 by supplementing the Fast Fourier Transformation.

The single cavity structure in 4(b) forms the double-beam interference spectrum in 3(b), and the dual cavities structure in 4(d) forms the three-beam interference spectrum. However, we defines the wavelength range corresponding to the interval between adjacent peaks or valleys of the sensor interference spectrum as a free spectral range (FSR). The FSR is closely related to the length of FP cavity and the refractive index of different diaphgrams, that is, the increase of the film thickness will reduce the FSR. The refractive index of different materials will also affect the FSR. Therefore, the FSR in 4(a) is greater than the FSR in 4(c)

Comment 10:

What are the FSR of each one of the spectra of the systems? I suggest explaining the shape of the spectra of the three systems. Two spectra are alike and resemble single-layer ones.

Reply 10:

Thank you for your comments. We define the wavelength spacing between the adjacent peaks or valleys of the sensor interference spectrum as the free spectral range (FSR). Thus, FSR₁≈27nm; FSR₂≈31nm, FSR₃≈24nm. The interference spectrum of dual-FPI that formed by Ecoflex0030 silicone rubber/PDMS, PDMS /Ecoflex0030 silicone rubber, repectively, which do exhibits the features just look like a single layer interference. It is the reason that the selected PDMS, Ecoflex0030 silicone rubber have the approximately refractive index, that led to the three beam interference phenomenon was not so notable.

Comment 11:

Reply 11:

We strongly agree with your views, which inspire us to deeply think about this issue. Refer to the repeatability of the sensor, this modified paper also set up repeatability experiments to verify the repeatability and stability of the sensor characteristics of the composite structure sensor. We have employed a re-made composite structure sample to test its response characteristics by adjusting the rise and fall of temperature, respectively. The Figure 13 and 14 clearly elaborate that the fabricated sensor has good repeatability. What’s more, we have also defined the ratio of the minimum resolution (20pm)of the spectrometer (OSA) to the sensor sensitivity S is the minimum measurement accuracy (MMA) in formula (8).

Simultaneously, the revised vision has seriously adjusted the Reference list and conducted an in-depth survey of the literature on optical fiber temperature and pressure sensing. In addition, we have also compared and summarized the temperature and pressure sensing characteristics of different structures in detail with the research in this paper through Table 3. However, the hysteresis problem may involve dynamic detection. According to the existing detection scheme and experimental equipment, it is temporarily impossible to detect the dynamic parameters.

Comment 12:

How is the temperature value differentiated from the pressure value if the same monitoring variable is used to measure both parameters?

Reply 12:

From theoretical analysis, simultaneous temperature and pressure detection can be achieved. However, we have found that it is temporarily impossible to realize the simultaneous detection of the dual parameters according to the existing experimental equipment. To realize the simultaneous detection of two parameters, a light source with a wider spectral range is needed to display more obvious three-beam interference characteristics, thus demodulating the high-frequency and low-frequency components. This paper focuses on the specific effects of different combinations of adhesives on the pressure and temperature sensitivity characteristics, the simplification of the production process, controllable cavity length, etc.

Part Ⅲ: Conclusions

Few of the previous works characterize the influence of diaphragm materials on the temperature and pressure sensitivity of Fabry-Perot Interferometer-based dual-parameters fiber sensor. In this paper, the multiple transfer method was used to fabricate the dual Fabry-Perot cavities, respectively consist of following combinations: Epoxy resin AB/polydimethylsiloxane (PDMS), Ecoflex0030 silicone rubber /PDMS and PDMS/Ecoflex0030 silicone rubber. The proposed composite structure possesses the advantages of proper sensitivity, simple fabrication, cost-effectively, con-trollable cavity length. According to the experimental results, the study found that the temperature or pressure sensitivity can be adjusted over a certain range within the test temperature range of 40~100°C and pressure range of 100kPa~400kPa, respectively, by optimizing the combinations and parameters of dual-diaphragms. This shows that the composite structure with dual-FP cavities designed in this paper has a proper sensitivity and can meet varied sensitivity-demand application scenarios.

For the above reasons, we do think our work is important, significant and interesting for the field of fiber optic sensing.

Yours Sincerely

Jun Wang，Guanjun Wang

(on the behalf of all authors)

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors present a FP fiber interferometer based on dual-layer structures formed by the combination of different materials. A simple fabrication method was demonstrated. The sensors were used to measure temperature and pressure. Clearly, the used materials contribute to the enhancement of the FP sensitivity to temperature and pressure.

The work is interesting but need some improvements. English must be fully revised.

Comments:

Page 3: it is not clear the passage from equation 2 to equation 3. L from (3) is not defined.

Line 126: “refractive index of the film layer is poor”, please clarify this statement.

Line 147: “the FP cavity with a dual-layer composite structure of three materials.”, rephrase sentence as it seems that the FP cavity is made of 3 materials.

Line 150: this paragraph must be English revised. Revise caption of figure 3.

About the fringe spectra on figure 3, the experimental free spectral range corresponds to which cavity (L2 or L2) ? FFT should give a clearer insight about this.

The fringe spectrum from S1 is very poor. Why?

Figures 6,7,and 8: pressure sensitivity is temperature sensitivity.

Caption from figure 7: S2 is S3.

Figure 6, 7 and 8: for dual layer FPs with similar thicknesses, (table 2), it was expected to have similar free spectral range. Why does this not happen?

It is possible to discriminate temperature and pressure with the proposed sensor?

Finally, a comparative table on the sensing characteristics of this type of sensors should be included, either for temp and pressure, indicating the place of your sensor in them. If your sensor is not the best, no big deal, the most important thing is to show that it is not the worst.

Author Response

Cover Letter for Reviewers

(1^st Apr. 2021)

Dear reviewer:

Thank you very much for providing us with your comments. We found the comments are very constructive and useful to improve our work. We have significantly revised section of Introduction, Abstract, and Conclusion. Additionally, the paper also revised along the lines suggested by you. Below are our responses to the specific comments from you.

Part Ⅰ: Comments and Suggestions for Authors

Page 3: it is not clear the passage from equation 2 to equation 3. L from (3) is not defined.

Line 126: “refractive index of the film layer is poor”, please clarify this statement. Line 147: “the FP cavity with a dual-layer composite structure of three materials.”, rephrase sentence as it seems that the FP cavity is made of 3 materials. Line 150: this paragraph must be English revised. Revise caption of figure 3.

About the fringe spectra on figure 3, the experimental free spectral range corresponds to which cavity (L2 or L2) ? FFT should give a clearer insight about this. The fringe spectrum from S1 is very poor. Why?

Figures 6,7,and 8: pressure sensitivity is temperature sensitivity. Caption from figure 7: S2 is S3.

Figure 6, 7 and 8: for dual layer FPs with similar thicknesses, (table 2), it was expected to have similar free spectral range. Why does this not happen?

It is possible to discriminate temperature and pressure with the proposed sensor?

Part Ⅱ: Our Responses to Reviewer’s Comments

Comment 1:

Page 3: it is not clear the passage from equation 2 to equation 3. L from (3) is not defined.

Reply 1:

Thanks a lot for your valuable advice. The L (L has been defined in the revised manuscript.) is the length of FP cavity, which makes the difference of intensity of the interference spectrum of equation 2. The purpose of introducing equation 3 is to explain that the peak shift is related to the cavity length L. Therefore, the principle of three-beam interference can be analyzed by the peak shift. Additionally, we have updated the principle section based on the in-depth investigation of relevant literature.

Comment 2:

Page 4: clarify the first paragraph because, from table 1, Ecoflex00-30 silicone rubber has the best coefficient of thermal expansion. Please see: Line 125, “Table 1 clearly shows the thermal expansion coefficient of PDMS is better” and Line 128, “Ecoflex00-30 silicone rubber had the best coefficient of thermal expansion”. Line 126: “refractive index of the film layer is poor”, please clarify this statement. Line 147: “the FP cavity with a dual-layer composite structure of three materials.”, rephrase sentence as it seems that the FP cavity is made of 3 materials.

Reply 2:

We are very sorry for the unclear presentation of the paper and thank you for your suggestions. We have carefully modified that paragraph in the revised version to avoid some misunderstandings and supplemented the Young's modulus and Poisson's ratio parameters of the materials.

Comment 3:

Line 150: this paragraph must be English revised. Revise caption of figure 3. About the fringe spectra on figure 3, the experimental free spectral range corresponds to which cavity (L2 or L2)? FFT should give a clearer insight about this. The fringe spectrum from S1 is very poor. Why?

Reply 3:

Thank you for your opinions and comments. We reorganized the relevant pictures and re-written the content of this paragraph. We have also introduced the fast fourier transform in order to better display the characteristics of the interference spectrum.

The single cavity structure in Figure 3(a) forms the double-beam interference spectrum in 3(b), and the dual cavities structure in Figure 3(b) forms the three-beam interference spectrum. The characteristics of the fringe spectrum is closely related to the cavity length and the properties of the materials.

Comment 4:

Figures 6,7, and 8: pressure sensitivity is temperature sensitivity. Caption from figure 7: S2 is S3. Figure 6, 7 and 8: for dual layer FPs with similar thicknesses, (table 2), it was expected to have similar free spectral range. Why does this not happen?

Reply 4:

Just followed your comments, we have already corrected the error of the figure annotation. What’s more, the free spectral range (FSR) is closely related to the length of the FP cavity and the refractive index of different materials. Namely, the increase of the cavity length will reduce the FSR and the refractive index of different materials will also have an impact on the FSR. The composite structures proposed in this article utilize different material with different refractive index. Although the double-layer FP cavity has similar thickness, the difference in refractive index still affects the FSR.

Comment 5:

It is possible to discriminate temperature and pressure with the proposed sensor?

Reply 5:

Thanks a lot for your questions about our work, which is a great inspiration to us. We have also carried out deep thinking and research on this issue. From theoretical analysis, simultaneous temperature and pressure detection can be achieved. However, we have found that it is temporarily impossible to realize the simultaneous detection of the dual parameters according to the existing detection scheme and experimental equipment. To realize the simultaneous detection of two parameters, a light source with a wider spectral range is needed to display more obvious three-beam interference characteristics, thus demodulating the high-frequency and low-frequency components. The above content is also stated in the revised paper.

Comment 6:

Reply 6:

Thank you for your constructive suggestions for us. Just as follows your comments, we have significantly updated the Reference list and conducted an in-depth survey of the literature on optical fiber temperature and pressure sensing. In addition, we have also compared and summarized the temperature and pressure sensing characteristics of different structures in detail with the research in this paper through Table 3.

Part Ⅲ: Conclusions

We have done significant rewriting and restricting of the paper, taking into account the valuable comments we received from the reviewers. We also proof-read the paper, we hope the modified version is significantly improved and acceptable.

Yours Sincerely

Jun Wang，Guanjun Wang

(on the behalf of all authors)

Author Response File: Author Response.pdf

Reviewer 3 Report

In this paper, Wang et al. proposed a composite two layer cavity structure using an optical fiber to detect two parameters (temperature and pressure) at the same time. The work has some novelty and fit the journal mandate, but there are some major revisions necessary before final verdict:

- The most important issue of this paper is the writing component. The paper needs stringent proofreading and editing. I think some parts of the manuscript are even difficult to understand for experts in the field due to lack of clarity in writing.

- I think authors should expand their rather short introduction and discuss the following: - Why fiber-optic sensors are interesting for various applications? - What type of fiber-optic sensors are out there? The authors can for example look at the following paper to see various type of fiber optic sensors which used for temperature/pressure or refractive index sensing: https://doi.org/10.1016/j.rinp.2019.102297- Why fiber-optic cavity sensors can be better than the other types for specific applications? - What are the disadvantages of cavity optical fiber sensors?

- Figure 5.a or 7.a: How the authors can distinguish between various peaks? There are at least three peaks (or even more!) in the intensity spectra, and some of these peaks are even stronger than the identified peak. If you use the sensor for a real world application, it's possible to detect a wrong peak and therefore report a wrong value for temperature or pressure. How can you identify the right peak?

- If you change the temperature and pressure at the same time, can you still detect the variations in both parameters (at the same time)? I think authors need to elaborate on this point.

- Can author compare the performance of their sensor to similar reported sensors in the literature in a table?

Author Response

Cover Letter for Reviewers

(1^st Apr. 2021)

Dear reviewer:

Thank you very much for providing us with your comments. We found the comments are very constructive and useful to improve our work. We have significantly revised the sections of Introduction, Abstract, and Conclusion. Additionally, the paper also revised along the lines suggested by you. Below are our responses to the specific comments from you.

Part Ⅰ: Comments and Suggestions for Authors

The most important issue of this paper is the writing component. The paper needs stringent proofreading and editing. I think some parts of the manuscript are even difficult to understand for experts in the field due to lack of clarity in writing.

I think authors should expand their rather short introduction and discuss the following: Why fiber-optic sensors are interesting for various applications? What type of fiber-optic sensors are out there? The authors can for example look at the following paper to see various type of fiber optic sensors which used for temperature/pressure or refractive index sensing: https://doi.org/10.1016/j.rinp.2019.102297- Why fiber-optic cavity sensors can be better than the other types for specific applications? - What are the disadvantages of cavity optical fiber sensors?

Figure 5.a or 7.a: How the authors can distinguish between various peaks? There are at least three peaks (or even more!) in the intensity spectra, and some of these peaks are even stronger than the identified peak. If you use the sensor for a real world application, it's possible to detect a wrong peak and therefore report a wrong value for temperature or pressure. How can you identify the right peak?

If you change the temperature and pressure at the same time, can you still detect the variations in both parameters (at the same time)? I think authors need to elaborate on this point.

Can author compare the performance of their sensor to similar reported sensors in the literature in a table?

Part Ⅱ: Our Responses to Reviewer’s Comments

Comment 1:

Reply 1:

We have comprehensively modified significant part of the manuscript, including the abstract, introduction, principle, experimental methods and conclusions. We have also modified the sentence grammar and the confusing narrative. We do hope that the revised paper would become concise and easy to understand.

Comment 2:

Reply 2:

We sincerely thank you for your constructive and valuable comments and opinions, which is helpful to us to reorganize the introduction. Just as follows your comments, we have carefully rewritten introduction section. First of all, we start with the application scenarios of optical fiber sensors, then in-depth investigation of various types of optical fiber sensors. After that, we have focused on the analysis and comparison of the characteristics of FPI temperature and pressure sensors. Finally, we summarized the defects existing of dual FPI-based fiber temperature and pressure sensors, more details, please refer to the Introduction part of the paper.

Comment 3:

Reply 3:

The equipment for temperature and pressure testing in this paper is static testing equipment. Therefore, when reading data, take the peak with the largest deviation as the measured value. In practical applications, dynamic spectrum monitoring equipment should be used to dynamically track the peak changes to avoid detecting false peaks in the spectrum.

Comment 4:

If you change the temperature and pressure at the same time, can you still detect the variations in both parameters (at the same time)? I think authors need to elaborate on this point.

Reply 4:

Thanks a lot for your questions about our work, which is a great inspiration to us. We have also carried out deep thinking and research on this issue. From theoretical analysis, simultaneous temperature and pressure detection can be achieved. However, we have found that it is difficult to realize the simultaneous detection of the dual parameters according to the existing experimental equipment. To realize the simultaneous detection of these two parameters, a light source with a wider spectral range is needed to display more obvious three-beam interference characteristics, thus demodulating the high-frequency and low-frequency components. This paper focuses on the specific effects of different combinations of adhesives on the pressure and temperature sensitivity characteristics, the simplification of the production process, controllable cavity length, etc.

Comment 5:

Can author compare the performance of their sensor to similar reported sensors in the literature in a table?

Reply 5:

Part Ⅲ: Conclusions

In conclusion, the major revisions in Introduction, Abstract and Conclusion, etc. are done, the carefully proof-read is also done. We hope the modified version is significantly improved and acceptable.

Yours Sincerely

Jun Wang，Guanjun Wang

(on the behalf of all authors)

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

What are the thermo-optic coefficients of the materials? The response to temperature is described in the manuscript, mentioning the thermo-optic coefficient; Furthermore, according to theory, these coefficients were taken into account to obtain the theorical spectra of Figure 2b.

The wavelength shift of the spectrum is understood to be due to changes in temperature or pressure. In Figure 2b, the shifted spectra must be presented separately because the authors do not present a sensor that simultaneously measures temperature and pressure. Furthermore, there is insufficient information to indicate how these theorical spectra were obtained.

Author Response

Cover Letter for Reviewers

(8^th Apr. 2021)

Dear reviewer:

Thanks a lot for providing us with your comments and suggestions. And we would sincerely show our respects and appreciate to the reviewer for your time and hardwork in handling our manuscript. We found the comments are very constructive and helpful to improve our work. We have revised the paper along the lines suggested by you. Our responses to the specific comments from you are as follow:

Part Ⅰ: Comments and Suggestions for Authors

Part Ⅱ: Our Responses to Reviewer’s Comments

Comment 1:

Reply 1:

Thanks for your valuable comments. Just followed your suggestions, We once again carefully searched for some cross-field research literature research, such as the field of polymer, the field of soft robots, etc. We have added data on the thermo-optical coefficient of materials through the supplementary literature [31-33] into Table 1. Simultaneously, we have supplemented some of the previously vacant data into Table 1. As a result, we have demonstrated the properties of related materials more comprehensively.

Comment 2:

Reply 2:

We sincerely thank the reviewer for pointing out this. We agree that the shifted spectra should be presented separately because the sensor detects temperature and pressure separately. What’s more, the schematic diagram of the theoretical interference spectrum is a simulated by MATLAB, which obtained by comprehensively considering the relevant parameters of the material, such as thermo-optical coefficient, thermal expansion coefficient, Young's modulus, Poisson's ratio, etc., combining with the formula proposed in the theoretical part, and utilizing the principle of three-beam interference. We have already modified the description of this part. And Figure 2(a) and Figure 2(b) respectively show the schematic diagrams of the interference spectrum movement at different temperatures and pressures.

Part Ⅲ: Conclusions

In conclusion, the minor revisions in the Figure 2 and Table 1 have been completed. And we have carefully addressed the comments received from the reviewer. Thanks again for your time and review work of our paper.

Yours Sincerely

Jun Wang，Guanjun Wang

(On the behalf of all authors)

Author Response File: Author Response.pdf

Reviewer 2 Report

Authors made a significant effort to improve the manuscript. It can be accepted in the present form.

Author Response

Cover Letter for Reviewers

(8^th Apr. 2021)

Dear reviewer:

Thank you very much for recommending publication of the manuscript. And we would sincerely show our respects and appreciate to the reviewer for your time and hardwork in handling our manuscript, as well as the constructive comments and suggestions provided, which is of great help to the revision of our works.

At last, we sincerely hope everything goes well with you.

Yours Sincerely

Jun Wang，Guanjun Wang

(on the behalf of all authors)

Author Response File: Author Response.pdf

Reviewer 3 Report

I think the authors addressed most of my previous comments.

Author Response

Cover Letter for Reviewers

(8^th Apr. 2021)

Dear reviewer:

At last, thanks again for your review work. We sincerely hope everything goes well with you.

Yours Sincerely

Jun Wang，Guanjun Wang

(on the behalf of all authors)

Author Response File: Author Response.pdf

Article Menu

Investigation of Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure Sensing

Further Information

Guidelines

MDPI Initiatives

Follow MDPI