Microscopic Temperature Sensor Based on End-Face Fiber-Optic Fabry–Perot Interferometer

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this work, a fiber-optic temperature sensor based on a Fabry-Perot interferometer on the end-face of the fiber has been developed. The technique of polymer bridge has been implemented by using a photosensitive polymer that is transparent to visible light and the near-IR range and cured under the influence of UV radiation. However, the potential advantages with respect to existing fiber optic temperature sensors like FBGs are not discussed. Temperature sensitivity does not result improved with respect to them and, moreover, the geometry and size of the fiber tip introduce more fragility. For these reasons, the paper is not suitable for publication.

Author Response

In this work, a fiber-optic temperature sensor based on a Fabry-Perot interferometer on the end-face of the fiber has been developed. The technique of polymer bridge has been implemented by using a photosensitive polymer that is transparent to visible light and the near-IR range and cured under the influence of UV radiation. However, the potential advantages with respect to existing fiber optic temperature sensors like FBGs are not discussed. Temperature sensitivity does not result improved with respect to them and, moreover, the geometry and size of the fiber tip introduce more fragility. For these reasons, the paper is not suitable for publication.

We thank the Reviewer for the careful review. In the Introduction, we have added a table (Table 1) to compare the proposed sensor with the existing ones, and have added the following paragraph to highlight the aim of the current work: “In this work, we propose to form a Fabry–Perot interferometer at the end of an optical fiber from an affordable and widespread photopolymer material using a simple manufacturing method. The proposed sensor features significantly higher sensitivity in comparison with fiber Bragg gratings and the quartz glass based FPIs, as well as smaller size and simpler manufacturing process compared to the Bragg gratings, silicon pillar and hollow fiber based FPIs, which can be beneficial in the areas where the usage of minimally invasive sensors is necessary, such as medical and biological applications.”

As the Reviewer rightly noted, the proposed sensor design introduces more fragility, however, in certain applications, such as biological research, the minimal dimensions of the sensor are of higher priority than the sensor durability. In such applications, the sensing element is displaced with high precision using micropositioners, which is controlled under a microscope, therefore, the problem of accidental sensor breaking is minimized.

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, the authors have proposed a simple technology for fabricating a miniature fiber-optic endpoint temperature sensor based on a Fabry-Perot interferometer formed from a transparent UV-curable resin.  I think this manuscript seems interesting for sensing applications. Hence this manuscript is suitable for publication in this journal. 

 

 

However, for publication of this manuscript, several reconsiderations are required. Hence, I recommend to this manuscript is major revision

 

I strongly hope to reconsider several points as follows:

 

1. I recommend that the authors add a table in the introduction section to compare their work with others' existing works.

 

2. What are the repeatability of the temperature sensing and the standard deviations of the tests if the same process is used to fabricate several temperature sensors based on End-Face Fiber Optic Fabry-Perot Interferometer? The repeatability of the sensor is very important, and I doubt it if the characteristics of the sensitive element are different in each sensor fabricated.

 

3. The authors must describe the characteristics of the photopolymer (optical properties, physical properties)

 

4. What is the sensor's response if the column's length increases or its diameter increases?

 

5. In Figure 4 a), the word laser appears; however, in the previous paragraph of this figure, one can read superluminescent fiber source. The authors must specify whether it is a laser or a superluminescent source.

 

6. In Figure 6, the fit obtained is incorrect y=-0.04457x+1531. If we consider x=50, we obtain y= 1528.77. This value falls outside the fit line.

Author Response

In this paper, the authors have proposed a simple technology for fabricating a miniature fiber-optic endpoint temperature sensor based on a Fabry-Perot interferometer formed from a transparent UV-curable resin.  I think this manuscript seems interesting for sensing applications. Hence this manuscript is suitable for publication in this journal. 

However, for publication of this manuscript, several reconsiderations are required. Hence, I recommend to this manuscript is major revision

I strongly hope to reconsider several points as follows:

 

1. I recommend that the authors add a table in the introduction section to compare their work with others' existing works.

We thank the Reviewer for the suggestion. A table has been added in the Introduction with comparison between the existing sensors and the proposed one.

 

2. What are the repeatability of the temperature sensing and the standard deviations of the tests if the same process is used to fabricate several temperature sensors based on End-Face Fiber Optic Fabry-Perot Interferometer? The repeatability of the sensor is very important, and I doubt it if the characteristics of the sensitive element are different in each sensor fabricated.

Thank you for the concern. Temperature tests of six manufactured sensors showed that the sensitivity and the shape of their spectra is maintained with high accuracy. The repeatability of the sensor’s sensitivity was determined using the t-Student’s criterion and is (0.0441 ± 0.0086) nm/℃.

 

3. The authors must describe the characteristics of the photopolymer (optical properties, physical properties)

Thank you for the suggestion. Several relevant properties of the photopolymer are listed in Table 2: refractive index is 1.59, thermo-optic coefficient is −1.87×10⁻⁴ °C⁻¹, thermal expansion coefficient is 1.6×10⁻⁴ °C⁻¹. Additional properties include: viscosity in liquid form (3500 – 4500 cps), hardness in cured form (95 Shore D).

 

4. What is the sensor's response if the column's length increases or its diameter increases?

According to experimental investigations of sensors having various dimensions of the column, the increase of the column’s length leads to the decrease of the interferometer’s free spectral range. The dimensions of the column have no noticeable effect on the sensor’s sensitivity.

 

5. In Figure 4 a), the word laser appears; however, in the previous paragraph of this figure, one can read superluminescent fiber source. The authors must specify whether it is a laser or a superluminescent source.

Thank you for the remark. Indeed, the optical source used in the setup was a superluminescent fiber source. Therefore, the designation in Figure 4,a has been corrected.

 

6. In Figure 6, the fit obtained is incorrect y=-0.04457x+1531. If we consider x=50, we obtain y= 1528.77. This value falls outside the fit line.

Thank you for the remark. The equation of the linear fit has been corrected as y=−0.04457x+1530.536.

Reviewer 3 Report

Comments and Suggestions for Authors

The comments to the authors on aspects missing in the manuscript that need to be solved to publish the article are listed below:

The manuscript lacks a clear description of how the column of the proposed FP was cut in the fabrication process. This detail is crucial for a comprehensive understanding of the research.

There are omitted references from where the equations were taken them. 

The mathematical model does not follow a simple development sequence

The dynamic range of the proposed sensor is small.

In line 190, it seems like it is cited the wrong figure.

For the "Ensuring High Interrogation Speed at Low Cost of the Sensor System" section, the authors did nothing experimental; they only suggested using another proposed method from another article. 

The conclusion section states that the geometry of the sensing element and the polymer material allows for the control of both the temperature range at which a linear shift of the spectrum occurs and the sensitivity of the sensor formed at the end of the fiber. However, nothing of that is presented in the experimental section. 

 

Comments on the Quality of English Language

The English in the manuscript is acceptable. 

Author Response

The comments to the authors on aspects missing in the manuscript that need to be solved to publish the article are listed below:

The manuscript lacks a clear description of how the column of the proposed FP was cut in the fabrication process. This detail is crucial for a comprehensive understanding of the research.

Thank you for the comment. After the epoxy “column” is cured, the optical fibers are moved apart using the micropositioners, which results in cleavage of one end of the “column”. Although such approach leads to rather high deviation of the resulting “column” length (±10 μm), it allows to obtain sensing elements with similar characteristics, and subsequently can be improved using modified optical fiber cleavers and automated micropositioners, enabling mass production of the proposed sensors.

 

There are omitted references from where the equations were taken them. 

Thank you for the remark. The references have been added.

 

The mathematical model does not follow a simple development sequence

The mathematical modeling of the Fabry – Perot interferometer is carried out in the following sequence:

Firstly, the scattering matrix of uniform continuous medium is formulated for the polymer “column” (eq. (1) and (2)).

Secondly, the scattering matrices for two stepwise changes of the medium parameters are formulated (eq. (3)) for the interface between the optical fiber and the polymer “column”, and between the polymer “column” and the ambient air.

Thirdly, the scattering matrices are transformed into the transfer matrices according to the eq. (4), after which the transfer matrices are sequentially multiplied according to eq. (5) to form a resulting transfer matrix of the whole sensing element.

Finally, the resulting transfer matrix of the sensing element is derived from the transfer matrix according to the eq. (6), which is used to calculate the reflection spectrum of the interferometer.

The authors deliberately excluded the derivation of the scattering matrices, as it would encumber the text of the article.

 

The dynamic range of the proposed sensor is small.

Thank you for the concern. The temperature range during tests was 25 °C, however, the total dynamic range of the sensor is significantly higher and is limited by the operating temperature of the epoxy adhesive used as the material of the sensing element (−60 – 200 °C).

 

In line 190, it seems like it is cited the wrong figure.

Thank you for the remark. The citation has been corrected (Figure 8).

 

For the "Ensuring High Interrogation Speed at Low Cost of the Sensor System" section, the authors did nothing experimental; they only suggested using another proposed method from another article. 

Thank you for the remark. The Section 5 has been expanded to include the results of the artificial neural network application to the temperature determination using the modeling of the FPI interrogation process in accordance with the ref. [30].

 

The conclusion section states that the geometry of the sensing element and the polymer material allows for the control of both the temperature range at which a linear shift of the spectrum occurs and the sensitivity of the sensor formed at the end of the fiber. However, nothing of that is presented in the experimental section. 

According to experimental investigations of sensors having various dimensions of the column, the increase of the column’s length leads to the decrease of the interferometer’s free spectral range, which can be seen by comparing spectra in Figures 4 and 8. The sensor’s sensitivity (spectrum shift with temperature variation) remains unchanged with the change of the column’s dimension and depends only on the properties of the polymer material (thermo-optic and thermal expansion coefficients). For clarification, the sentence has been reformulated as follows (lines 291 – 294): “During the experimental studies, it was concluded that the choice of the dimensions of the sensing element and the polymer material makes it possible to control both the free spectral range of the interferometer and its temperature sensitivity.”

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors have added some discussion and comparison with respect to other sensors.

Reviewer 2 Report

Comments and Suggestions for Authors

I appreciate the revisions the authors made and I am satisfied with the current version.