Numerical Analysis of Midinfrared D-Shaped Photonic-Crystal-Fiber Sensor Based on Surface-Plasmon-Resonance Effect for Environmental Monitoring

Round 1

Reviewer 1 Report

Dear Colleagues,
It was very nice to get acquainted with the material of your article.
A very competently constructed and verified article containing a deep introduction, with a good overview; clear statement of the problem for mid-infrared region; a description of the mathematical methodes; very attractive analysis of different sensor parameters and, finally, a clear analysis of various metrological technologies, reduced to criteria that determine the quality of the final sensor construction.

I believe that the article can be published, but with minor editing.

Blame all the fig. 5d. It is not clear why the numerical values of the FOM are connected by straight line segments with different slopes. Either it is necessary to explain such FOM law, or leave the points separately and approximate the curve, if approximation is really needed.

And, additionally, please check the punctuation of your sentences, for example, on lines 34, 67.

Author Response

Dear Colleagues,

It was very nice to get acquainted with the material of your article.
A very competently constructed and verified article containing a deep introduction, with a good overview; clear statement of the problem for mid-infrared region; a description of the mathematical methods; very attractive analysis of different sensor parameters and, finally, a clear analysis of various metrological technologies, reduced to criteria that determine the quality of the final sensor construction.

I believe that the article can be published, but with minor editing.

Comment 1: Blame all the fig. 5d. It is not clear why the numerical values of the FOM are connected by straight line segments with different slopes. Either it is necessary to explain such FOM law, or leave the points separately and approximate the curve, if approximation is really needed.

Response: First of all, thank you for your approval. We are sorry that fig. 5(d) has caused your ambiguity. In our works, FOM is calculated point by point according to formula 7, there is no obvious approximate relationship between these points. We have redrawn the fig. 5(d) to make the result more rigorous. Thank you for your suggestions again.

Comment 2: And, additionally, please check the punctuation of your sentences, for example, on lines 34, 67.

Response: Thank you for your comments. We are sorry for our negligence here. We have re-read the paper and corrected the improper punctuation. All revisions have been marked in red.

Reviewer 2 Report

The authors present a numerical study on the performance of a D-shaped photonic crystal fiber sensor. In particular, the simulated sensor uses the surface plasmon resonance effect and accordingly with the specifications and obtained results, it is presented as having potential application in the environmental area for monitoring purposes.

In general, the paper is well structured, with clear images complementing the text. The introduction correctly addresses the “problem” and prepares the reader for the following sections. The theory and methodologies for the numerical implementation are enough for having a correct understanding without unnecessary information. Conclusions are well supported by the results and its analysis. However, a few points must be addressed by the authors, making mandatory a revision of this paper:

1. The title is somehow misleading since its numerical “nature” it is not clearly stated. Something like “Numerical analysis of (…)” would be more appropriate.
2. In line 22, the authors refer a “huge FOM”. First, qualitative hyperboles like “huge” should be avoided (high should be “enough”). Second, the acronym FOM appears without explanation. Only in line 50 it is clear that means “figure of merit”. The general rule is to explain it the first time it appears. Or, in this case, in the abstract it can be mentioned by its complete name.
3. In lines 70-72, the authors comment the gaps in the current state of the art regarding the D-shaped PCF-SPR sensors. It is pointed out that these sensors operate “almost in the visible or near infrared bands” (sic). However, there are indeed some works where mid-infrared regions are considered, for example:

Xin Chen, Li Xia, Chen Li, “Surface Plasmon Resonance Sensor Based on a Novel D-Shaped Photonic Crystal Fiber for Low Refractive Index Detection”, IEEE Photonics Journal 10(1), 2018. DOI: 10.1109/JPHOT.2018.2790424.

Chao Liu, et al., “Mid-infrared surface plasmon resonance sensor based on photonic crystal fibers”, Optics Express 25(13), 2017. DOI: 10.1364/OE.25.014238

These and/or other papers should be referred and added to the state of the art. The sensor being studied must also be compared with those tested by that authors.

4. Since there are several types of Sellmeier equations, in lines 94-94, the authors should add a “a”: “(…) can be characterized by a Sellmeier equation (…)” instead of “(…) can be characterized by Sellmeier equation (…)”.
5. The sentence between lines 142 and 146 is too long. Please divide it in shorter sentences for better understanding.
6. Is there any base for the affirmation that “the gold layer will certainly be consumed”? Is this a “feeling” of the authors, or is it based in some experience? Some explanation (or a reference) would improve the understanding of the subject.
7. The authors mention the software and the properties of the FEM simulations. However, there is no mention to the hardware (characteristics, performance…).
8. In the conclusions there is no mention to future work. Will the authors test their simulation/study experimentally (with a real sensor)? Although a simulation (as this one seems to be) can be implemented in a sound manner, nothing replaces experimental validation.

Besides these issues, the authors must check (again) the English language being used. Just a few examples:

- Line 13: use “sensors” instead of “sensor”

- Line 14: use “gold layers” instead of “gold layer”.

- Line 29: use “simple” instead of “simply”

- Line 41: use “(…) is a promising candidate which is known (…)” instead of “(…) is a promising candidate which known (…)”

- Line 42: remove “now”.

Author Response

The authors present a numerical study on the performance of a D-shaped photonic crystal fiber sensor. In particular, the simulated sensor uses the surface plasmon resonance effect and accordingly with the specifications and obtained results, it is presented as having potential application in the environmental area for monitoring purposes.

In general, the paper is well structured, with clear images complementing the text. The introduction correctly addresses the “problem” and prepares the reader for the following sections. The theory and methodologies for the numerical implementation are enough for having a correct understanding without unnecessary information. Conclusions are well supported by the results and its analysis. However, a few points must be addressed by the authors, making mandatory a revision of this paper:

Comment 1: The title is somehow misleading since its numerical “nature” it is not clearly stated. Something like “Numerical analysis of (…)” would be more appropriate.

Response: First of all, thank you for your approval. The title has been modified and revisions have been marked in red. Thank you for your suggestions.

Comment 2:  In line 22, the authors refer a “huge FOM”. First, qualitative hyperboles like “huge” should be avoided (high should be “enough”). Second, the acronym FOM appears without explanation. Only in line 50 it is clear that means “figure of merit”. The general rule is to explain it the first time it appears. Or, in this case, in the abstract it can be mentioned by its complete name.

Response: Thank you for your comments. Based on your suggestions, we have changed “high” to “enough” and corrected the statements that have been marked in red on L22 in Abstract.

Comment 3:  In lines 70-72, the authors comment the gaps in the current state of the art regarding the D-shaped PCF-SPR sensors. It is pointed out that these sensors operate “almost in the visible or near infrared bands” (sic). However, there are indeed some works where mid-infrared regions are considered, for example:

Xin Chen, Li Xia, Chen Li, “Surface Plasmon Resonance Sensor Based on a Novel D-Shaped Photonic Crystal Fiber for Low Refractive Index Detection”, IEEE Photonics Journal 10(1), 2018. DOI: 10.1109/JPHOT.2018.2790424.

Chao Liu, et al., “Mid-infrared surface plasmon resonance sensor based on photonic crystal fibers”, Optics Express 25(13), 2017. DOI: 10.1364/OE.25.014238

These and/or other papers should be referred and added to the state of the art. The sensor being studied must also be compared with those tested by that authors.

Response: Thank you for your guidance. We have added the corresponding statements about DOI: 10.1109/JPHOT.2018.2790424. that have been marked in red on L62-65 in Introduction. The statements about DOI: 10.1364/OE.25.014238 have been added on L235-237 in section 3.

Comment 4:  Since there are several types of Sellmeier equations, in lines 94-94, the authors should add a “a”: “(…) can be characterized by a Sellmeier equation (…)” instead of “(…) can be characterized by Sellmeier equation (…)”.

Response: We are very sorry for improper expression. We have added a “a” on L95 in Section 2. Thank you for your suggestions.  

Comment 5: The sentence between lines 142 and 146 is too long. Please divide it in shorter sentences for better understanding.

Response: Thank you for your suggestions. We have divided the original sentence for easy understanding and revisions have been marked in red on L143-147 in Section 3.

Comment 6: Is there any base for the affirmation that “the gold layer will certainly be consumed”? Is this a “feeling” of the authors, or is it based in some experience? Some explanation (or a reference) would improve the understanding of the subject.

Response: Thank you for your comments. We consider that in practical applications, the gold thin layer is in direct contact with the analytes. When the sensor is in a high temperature environment or chemical reaction will cause the consumption of the gold sensing layer. So we think that the gold layer will certainly be consumed after a period of use. Your suggestions are very good. We will strictly consider these factors in future research.

Comment 7: The authors mention the software and the properties of the FEM simulations. However, there is no mention to the hardware (characteristics, performance…).

Response: Thank you for your comments. We have added statements about hardware that have been marked in red on L138-139 in Section 2.

Comment 8: In the conclusions there is no mention to future work. Will the authors test their simulation/study experimentally (with a real sensor)? Although a simulation (as this one seems to be) can be implemented in a sound manner, nothing replaces experimental validation.

Response: Thank you for your suggestions. We have added the statements about experimental configuration that have been marked in red on L271-277 in Conclusion.

Comment 9: Besides these issues, the authors must check (again) the English language being used. Just a few examples:

Line 13: use “sensors” instead of “sensor”.

Line 14: use “gold layers” instead of “gold layer”.

Line 29: use “simple” instead of “simply”.

Line 41: use “(…) is a promising candidate which is known (…)” instead of “(…) is a promising candidate which known (…)”.

Line 42: remove “now”.

Response: We are very sorry for these wrong expressions in English. We have re-read the paper and have corrected the English expression. Revisions in the full paper have been marked in red. Thank you for your suggestions again.