Experimental Analysis of Bragg Reflection Peak Splitting in Gratings Fabricated Using a Multiple Order Phase Mask
Round 1
Reviewer 1 Report
Review report of the paper entitled " Experimental analysis of the Bragg reflection peaks splitting in gratings fabricated using a multiple order phase mask”
Journal: Sensors
Manuscript ID: sensors-414596
Type of manuscript: Article
The authors have performed an experimental analysis of the effect of phase mask alignment on the Bragg gratings reflection spectra around the wavelength of λB = 1560 nm fabricated in polymer optical fiber by using a multiple order phase mask, which obtained a very good match between the measured and calculated peak positions for the second order reflection peaks. Generally speaking, this paper meets the journal’s publication requirements, and can be considered for publication in the Journal of “sensors” with major corrections.
A few general suggestions are listed below:
1. The quality of the figures is low, such as Figure 6, Figure 7 and Figure8, can you contribute vector diagrams like Figure 9 in this paper.
2. The author said “…… which describes the rotation of the phase mask in the plane of incidence of the laser beam (Figure 4a) and the angle θ which describes the rotation of the phase mask around phase mask normal, in the plane perpendicular to the plane of incidence of the laser beam (Figure 4b)”, but we can't see these clearly from Figure 4，so we suggest that the author take convincing pictures again.
3. Dimensions and materials can be marked in Figure 3 as in Figure 5.
4. The author only uses a few sentences to illustrate Figures 7 and 8. Such an analysis is very inadequate. The author should make more explanations on this part in order to increase the persuasion of the article.
5. The author should pay attention to the format of the references.
Conclusion: This paper can be considered for publication in sensors after including all these clarifications and major changes.
Author Response
Review 1
List of changes and replies to the reviewers comments
English language and style
( ) Extensive editing of English language and style required
( ) Moderate English changes required
(x) English language and style are fine/minor spell check required
( ) I don't feel qualified to judge about the English language and style
Yes | Can be improved | Must be improved | Not applicable | |
Does the introduction provide sufficient background and include all relevant references? | (x) | ( ) | ( ) | ( ) |
Is the research design appropriate? | ( ) | (x) | ( ) | ( ) |
Are the methods adequately described? | ( ) | (x) | ( ) | ( ) |
Are the results clearly presented? | ( ) | ( ) | (x) | ( ) |
Are the conclusions supported by the results? | ( ) | ( ) | (x) | ( ) |
Comments and Suggestions for Authors
Review report of the paper entitled "Experimental analysis of the Bragg reflection peaks splitting in gratings fabricated using a multiple order phase mask"
Journal: Sensors
Manuscript ID: sensors-414596
Type of manuscript: Article
The authors have performed an experimental analysis of the effect of phase mask alignment on the Bragg gratings reflection spectra around the wavelength of λB = 1560 nm fabricated in polymer optical fiber by using a multiple order phase mask, which obtained a very good match between the measured and calculated peak positions for the second order reflection peaks. Generally speaking, this paper meets the journal’s publication requirements, and can be considered for publication in the Journal of “sensors” with major corrections.
A few general suggestions are listed below:
1. The quality of the figures is low, such as Figure 6, Figure 7 and Figure 8, can you contribute vector diagrams like Figure 9 in this paper.
Response 1:
Figures 6, 7 and 8 have been replaced with high quality images.
2. The author said “…… which describes the rotation of the phase mask in the plane of incidence of the laser beam (Figure 4a) and the angle θ which describes the rotation of the phase mask around phase mask normal, in the plane perpendicular to the plane of incidence of the laser beam (Figure 4b)”, but we can't see these clearly from Figure 4，so we suggest that the author take convincing pictures again.
Response 2:
The photos of the stages from Figure 4 were taken again. Moreover, we added descriptions and arrows to clearly indicate the rotation angles of the phase mask relative to the fiber.
We have added the sentence to the figure caption (Figure 4):
“The directions of rotation of the phase mask relative to the fiber are marked with red arrows.”
3. Dimensions and materials can be marked in Figure 3 as in Figure 5.
Response 3:
Relevant descriptions in Figures 3 and 5 have been added.
4. The author only uses a few sentences to illustrate Figures 7 and 8. Such an analysis is very inadequate. The author should make more explanations on this part in order to increase the persuasion of the article.
Response 4:
We have added comments to clarify this issue (section 3, paragraph 2):
“We studied the evolution of the Bragg peaks during the inscription process and after switching off the He-Cd laser. Precise values of distinct peaks position are plotted against irradiation time in Figure 7. Figure 7a shows the relative position of the peaks with respect to the initial position of the central peak λB0, Figure 7b shows the relative position of the peaks with respect to the initial position of each peak and in Figure 7c we present the peak separation during and after the inscription process. All the peaks shifted towards shorter wavelengths with increasing irradiation time, Figure 7a,b. During the irradiation with UV beam, the fiber heated up. As a consequence, a stress frozen in the fiber partially relaxed and the gratings’ periods decreased. After the He-Cd laser was turned-off, all the Bragg peaks quickly moved towards longer wavelengths, what is most probably caused by reorientation of molecular chains to the initial positions, they had before the writing process was started [6]. This effect is not completely reversible, because the grating shrinks permanently, since the part of the frozen stress is released at increased temperature. The dynamics is the same for all the observed peaks, and as we can see in Figure 7b, and the peaks’ separation remained unchanged during the whole process (Fig. 7c).
The relative position of the peaks depends on phase mask orientation. We analysed the spectra of several FBGs inscribed with different value of tilt angle ϕ. As shown in Figure 8a,b,c,d,e, the peaks become more distant when ϕ increases. For few tilt angles (for example Figure 8e) the outer side peaks were not observed. The prominence of the five peaks is related to the efficiency of the interfering diffraction orders. The most of UV beam power (about 72%) is present in +1st and -1st diffraction orders (about 36% in each). Consequently, the central reflection peak dominates. The inner pair of side-peaks, associated with interference of 0th and ±1st diffraction orders, is significantly weaker than the central peak, because only 1.6% of laser beam power diffracts into the 0th order. Moreover, the outer pair of side peaks, associated with the 0th and ±2nd diffraction orders, is barely visible, due to the weakness of the 0th diffraction order and low diffraction efficiency of the ±2nd diffraction orders (5.5%).”
5. The author should pay attention to the format of the references.
Response 5:
We have removed space between the initials and have corrected the symbol “-” to the symbol “ -”, so the references format is now preserved.
Conclusion: This paper can be considered for publication in sensors after including all these clarifications and major changes.
Response 6:
To improve English language and style we will submit the manuscript to MDPI English editing service.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
It is clear that the authors have dedicated a lot of effort to the introduction, which pays off, since it is very informative and complete. I would like all manuscripts to contain an introduction such as the one in this manuscript. The objectives of the research are, thus, clearly explained in the introduction as well as what the reader could expect from the results. As the reader progresses through the manuscript, however, the same quality observed in the introduction is not maintained, unfortunately. It is the opinion of this reviewer that the results are indeed of interest to the research community, but several aspects should be improved before acceptance. The following are the points that should be revisited by the authors.
1) The authors claim that theoretical studies have been conducted even though the results presented are strictly experimental and the theoretical description is borrowed from previous research documents. If the authors have in fact contributed to the theoretical description of the reflection peaks, they should make it extremely clear what the contribution is.
2) The experimental analysis corroborates the predictions that the authors have discussed in section 2. However, little to no detail is given about how the results were acquired. It may be a usual method in the specific field of FBG inscription, but the broader audience of Sensors would have a hard time understanding the steps. These are:
2.1) How are the transmission/reflection spectra measured during irradiation?
2.2) Why are the dynamic ranges of the peak so low? (10db)
2.3) What is the wavelength resolution of the measured spectra profile.
2.4) What is the modification introduced with respect to [6] to allow precise control of angle phi?
2.5) It would be extremely helpful if the actual experimental apparatus images could be edited to depict the angles of interest.
3) An 18.5% slope drift is attributed to a poor quality of the peaks. Are the authors referring to the low SNR of this measurement? Or to the quality of the inscription? Furthermore, an almost 20% slope drift seems to be very high, how well does the theoretical analysis predict either of these slopes? Which one is most likely to be correct?
4) It would be interesting to know how well the prediction of figure 2 has performed with respect to the experimental results of figures 6 and 8.
5) Is there an experimental limitation to running FBG inscriptions with a major number of different values of the angle phi? The authors have limited their experimental analysis to four angles even though their setup has been modified to allow for more precise control. Also, a greater number of values of phi could make the data fitting and analysis much more informative.
6) Several grammatical and typing errors have been found in the manuscript.
Author Response
Review 2
List of changes and replies to the reviewers comments
English language and style
( ) Extensive editing of English language and style required
(x) Moderate English changes required
( ) English language and style are fine/minor spell check required
( ) I don't feel qualified to judge about the English language and style
Yes | Can be improved | Must be improved | Not applicable | |
Does the introduction provide sufficient background and include all relevant references? | (x) | ( ) | ( ) | ( ) |
Is the research design appropriate? | ( ) | (x) | ( ) | ( ) |
Are the methods adequately described? | ( ) | ( ) | (x) | ( ) |
Are the results clearly presented? | ( ) | (x) | ( ) | ( ) |
Are the conclusions supported by the results? | ( ) | ( ) | (x) | ( ) |
Comments and Suggestions for Authors
It is clear that the authors have dedicated a lot of effort to the introduction, which pays off, since it is very informative and complete. I would like all manuscripts to contain an introduction such as the one in this manuscript. The objectives of the research are, thus, clearly explained in the introduction as well as what the reader could expect from the results. As the reader progresses through the manuscript, however, the same quality observed in the introduction is not maintained, unfortunately.
Response:
Thank you very much for your kind words. Indeed, we put a lot of effort into writing the introduction and a clear explanation of the problem. We hope that the responses to valuable comments of the reviewers will improve the readability of the rest of the article.
It is the opinion of this reviewer that the results are indeed of interest to the research community, but several aspects should be improved before acceptance. The following are the points that should be revisited by the authors.
1) The authors claim that theoretical studies have been conducted even though the results presented are strictly experimental and the theoretical description is borrowed from previous research documents. If the authors have in fact contributed to the theoretical description of the reflection peaks, they should make it extremely clear what the contribution is.
Response 1:
Indeed, it does be the experiment, what forms the core of this work. However, the understanding we get with the application of the theoretical model is also important. We agree, that in the previous version of manuscript, we did not indicated clearly the border between the theoretical model, which was published before, and its extension developed here. In order to clarify this point, we introduced the following paragraph as the introduction to the section 2 (section 2, paragraph 1).
“To explain the position of observed peaks, we applied the theoretical description developed in our earlier work [23]. According to this model, we are able to calculate the periods of inscribed grating and positions of corresponding Bragg peaks. In this work, we also extend the model, by considering the higher reflection orders. For sake of clarity, we recall the model below and present its application for our experimental system.”
2) The experimental analysis corroborates the predictions that the authors have discussed in section 2. However, little to no detail is given about how the results were acquired. It may be a usual method in the specific field of FBG inscription, but the broader audience of Sensors would have a hard time understanding the steps. These are:
2.1) How are the transmission/reflection spectra measured during irradiation?
2.2) Why are the dynamic ranges of the peak so low? (10db)
2.3) What is the wavelength resolution of the measured spectra profile.
Response 2 (2.1, 2.2, 2.3):
We have added the paragraph to clarify this issues (section 3, paragraph 1):
“Due to high attenuation of the polymer fibers in the third telecommunication window, we used a silica fiber coupler a supercontinuum source (NKT Photonics – SuperK Versa) and an optical spectrum analyzer (Yokogawa AQ6370C) with resolution of 0.2 nm to monitor a reflection spectrum of the Bragg grating during the inscription process. The light was coupled into short piece (about 30 cm) of POF from silica fiber using index matching gel to reduce Fresnel reflection. In our current fabrication setup the distance between connection of POF with silica fiber and part of the POF where grating was inscribed, was about 10 cm and was longer, than before. The result is that the height of the peaks is low, about 10 dB and is much lower than FBGs reflection spectra presented in our previous works [6,25].”
2.4) What is the modification introduced with respect to [6] to allow precise control of angle phi?
Response 2.4:
We have added the comment to clarify this issue (section 3, paragraph 1):
“In our previous setup the POF was mounted in a specially designed base with V-groves. The phase mask was placed on the top of the base and the gap between the fiber and the phase mask was adjusted by the distance plates with precision of about 10 mm. In such case the tilt angle could be controlled with accuracy lower than 0.05°, however the tilt angle change involved the need to unscrew the base on which the phase mask was mounted. Moreover there was no possibility to verify the angle adjustment on the camera.”
2.5) It would be extremely helpful if the actual experimental apparatus images could be edited to depict the angles of interest.
Response 2.5:
The photos of the stages from Figure 4 were taken again. Moreover we have added descriptions and arrows that better illustrate the rotation angles of the phase mask relative to the fiber.
3) An 18.5% slope drift is attributed to a poor quality of the peaks. Are the authors referring to the low SNR of this measurement? Or to the quality of the inscription? Furthermore, an almost 20% slope drift seems to be very high, how well does the theoretical analysis predict either of these slopes? Which one is most likely to be correct?
Response 3:
To clarify this point, we revised the corresponding sentence in the manuscript (section 3, paragraph 3).
“The reason of this difference might have been poor quality of the outer side peaks (due to the low signal-noise ration of this measurement) which made it difficult to precisely indicate their spectral position.”
Comments to the Reviewer:
Similarly to the Reviewer, we are not fully satisfied with the quantitative agreement between experiment and theory for the outer side-peaks. Our first hypothesis, was that the difference is related to the dispersion of effective refractive index. Discussion of this effect, closes the section 3 and shows that for considered tilt angles the dispersion is not enough.
It seems, that theoretical model does not account some important factors or takes some approximation. For example, we calculate the projection of diffraction orders’ wave vectors difference on the fiber direction (the component parallel to the fiber) and neglect the transverse components. In fact, the Bragg gratings for (0, ±1) and (0, ±2) diffraction orders’ pairs have skew components. As a result some part of energy is coupled to cladding modes, what may modify the characteristics of observed reflection peaks. This skew component is almost zero for (-1, +1) pair, non-zero for (0, ±1) pair and the biggest for (0, ±2) pair.
However, as we do not have enough data to support this argumentation, we decided not to include this in manuscript.
4) It would be interesting to know how well the prediction of figure 2 has performed with respect to the experimental results of figures 6 and 8.
Response 4:
Figures 2 have been replaced and we have added the comment (section 2, paragraph 3):
“We also calculated the difference between position of each peak and the central one λ(-1,+1)(1). The comparison with experimental data will be easier with this quantity, as the positions of all peaks change slightly during inscription, due to polymer properties.”
and change the figure caption:
“Figure 2. Calculated positions of Bragg reflection peaks λ(m,q)(p), where m and q are the diffraction order number and p is the reflection order (p = 1 or 2). The main axis shows the difference between each position λ(m,q)(p) and position of central peak λ(-1,1)(1).”
Comments to the Reviewer:
As it is shown in Figure 7a,b the position of the peaks change during the inscription. So the comparison of the theoretical position of each peak with experimental data is difficult. In plots below you can see the position of the central peak and inner pair of side-peaks during the inscription process and after the inscription. However, as it is shown in Figure 7c, the peak separation during and after the inscription process has change only slightly, so we decided do presented the difference between position of each peak and the central one λ(-1,+1)(1).
5) Is there an experimental limitation to running FBG inscriptions with a major number of different values of the angle phi? The authors have limited their experimental analysis to four angles even though their setup has been modified to allow for more precise control. Also, a greater number of values of phi could make the data fitting and analysis much more informative.
Response 5:
Comments to the Reviewer:
The only experimental limitation is the limited amount of polymer fiber. For this reason, the grating was inscribed for seven different angles, which seemed to be enough points to show the trend. Unfortunately, due to the poor quality of outer peaks, the dependence of the position of these peaks with the tilt angle is in fact limited to only four angles.
6) Several grammatical and typing errors have been found in the manuscript.
Response 6:
To improve English language and style we will submit the manuscript to MDPI English editing service.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have considerably improved the manuscript and I recommend publication.
Reviewer 2 Report
The authors addressed most of this reviewer's concerns satisfactorily.
