Highly Sensitive Sensor Structure Based on Sol-Gel Waveguide Films and Grating Couplers

Round 1

Reviewer 1 Report

The paper claimed to develop a two-component SiOx-TiOy waveguide films having  a grating coupler with a period of 147 nm based on sol-gel and dip-coating fabrication methods. However, there are no clear description on fabrication procedures for readers to follow/reproduce and no experimental proofs for the claimed structures. Further details on fabrication process and characterization  as well as characterization results of films and structural features should be provided. 

In addition, it would be easier to follow if the authors follow the conventional paper structure with introduction, methodology ( experimental and/or theoretical), results and discussions, conclusions.

 

Author Response

I am grateful the careful and thorough reading of this manuscript and constructive comments that helped me to improve its quality. The comments are addressed in detail below. Herein, my response follows.

 

The paper claimed to develop a two-component SiOx-TiOy waveguide films having  a grating coupler with a period of 147 nm based on sol-gel and dip-coating fabrication methods. However, there are no clear description on fabrication procedures for readers to follow/reproduce and no experimental proofs for the claimed structures. Further details on fabrication process and characterization  as well as characterization results of films and structural features should be provided. In addition, it would be easier to follow if the authors follow the conventional paper structure with introduction, methodology (experimental and/or theoretical), results and discussions, conclusions.

The part of the manuscript describing the technological procedure of sol preparation has been extended. The composition of the manuscript has also been rebuilt. The substantial part of the content from page 13 has been moved to Introduction. The Section 3 has been extended of the subsection Materials and Methods. 

 

Reviewer 2 Report

The manuscript has reviewed the development of waveguide couplers particularly in the area of materials and methods for the film, and reported work of an evanescent wave planar waveguide sensor using nanoimprinted silica-titania film. 

Areas for improvement 

- Is there any shortcomings for using large refractive indices difference or high refractive index contrast in integrated optics? Is line 332 one of the reasons?

- The authors may want to consider reviewing the references list and the manuscript content again to avoid narrative heavily towards authors' own previous work and selected research groups. 

- Key publications: Should more recent papers already explained the course of development, the authors may wish to use it as that reference instead of repeating again. A generic narrative description may be more appropriate for a scientific paper reporting new information. 

Specific comments

- Line 40: An integrated optics consists of many components, is the film's materials the only limiting factor hindering the development? To facilitate rapid development will fabrication cost and yield output the other possible reasons?

- Line 92-97, a generic description may be more appropriate in literature review as reader may not be very familiar with authors work. A comparison table may also allow readers to understand the differences.

- Line 167, it appears that from Fig. 3 the grating period < ~450nm when r=1. Is my understanding correct?

- Page 13 is like another introduction, like section 1. Authors may consider to shift selected concise content to section 1 and selectively highlight the novelty of the data with a few closet relevant work.

Non-technical comments

- Line 44, 47, 48, etc., abbreviation in brackets like line 34 and 35.

- Line 107, 112, In section X

- Fig. 4, green colour line might be easier to see when change to like those in Fig. 2.

- Fig. 5, letters and numbers appears distorted and hard to read. Please use bigger font or black (for example) colour.

- Line 215, It is possible

- Line 222, 224, 225, 226, etc., subscript

- Fig. 9, is the unit of d correct? in nm?

- Line 343, etc. for spelling error, which may come

Author Response

I am grateful the careful and thorough reading of this manuscript and constructive comments that helped me to improve its quality. The comments are addressed in detail below. Herein, my response follows.

1. Is there any shortcomings for using large refractive indices difference or high refractive index contrast in integrated optics? Is line 332 one of the reasons?

The spread of technology parameters occurs in all technology processes. For this reason all structures fabricated in the same conditions will differ among their parameters. Structures with grating couplers may vary in refractive index and thickness of the waveguide film. It is for this reason that it is necessary to measure the refractive index and thickness of every individual waveguide film, instead of relying on values resulting from technology characteristics (Fig. 10). These parameters are determined in a manner described in Section 3. In the first step the effective refractive indices are determined from values of synchronous angles measured under conditions when air is the ambient (cover). Subsequently, the refractive index and thickness of the waveguide film are calculated from characteristic equations (6). The relation between the synchronous angle and cover refractive index is calculated in the next step (Fig. 14, solid line). The characteristic equations (6) are true for uniform waveguide films having homogeneous refractive index distribution. Therefore waveguide film uniformity and homogeneity of refractive index are of great importance.

2. The authors may want to consider reviewing the references list and the manuscript content again to avoid narrative heavily towards authors' own previous work and selected research groups.

We agree with this remark. The arrangement of the manuscript was rebuilt and the list of references revised.

3. Key publications: Should more recent papers already explained the course of development, the authors may wish to use it as that reference instead of repeating again. A generic narrative description may be more appropriate for a scientific paper reporting new information.

According to our research of literature sources, the development of technology of waveguide films applied in evanescent wave spectroscopy has so far not been studied. Therefore, we decided to address it in this work. It happens that there are articles presenting the results of the development of waveguide films in a biased way. For example, one can read about the development of waveguide films having optical losses of 0.3 dB/cm. However the optical loss is measured for the fundamental mode, while the waveguide films are bimodal. The discussion in such articles lacks the important piece of information. Namely the evanescent wave spectroscopy related applications require single mode films. In that case one cannot expect that optical losses will be equally low for single mode waveguide films. In this example, the attenuation of the fundamental mode can have value of up to several dB/cm. Considering planar waveguides, the optical losses decrease with waveguide film thickness if the latter is higher than the thickness for which homogeneous or surface sensitivity is maximum. Waveguide films presented and discussed in our work are single mode.

Specific comments

1. Line 40: An integrated optics consists of many components, is the film's materials the only limiting factor hindering the development? To facilitate rapid development will fabrication cost and yield output the other possible reasons?

We have mentioned in the manuscript that at the present time the main material platforms for the integrated optics rely on silicon and indium phosphide (transparent in the NIR wavelength range) as well as on silicon nitride (transparent in the Vis-NIR wavelength range). Integrated optics components and structures fabricated from these materials require application of complex and costly technological installations of an industrial nature. Those installation are so costly, not only in purchasing but also in maintenance. As a result they are inaccessible for many research groups. Frequently they must also serve the industrial needs. For these reasons only the telecommunication industry disposing of relevant financial resources was able to support rapid development of the integrated optics for NIR spectral range. For the same reason, the development of the integrated optics for applications in evanescent wave sensors is not so impressive.

The technical challenge arises from very rigorous requirements the waveguide films must met. The development of waveguides films complying with those requirements require intensive labour, which is also an element of the cost of the final solution. We use the fabrication method that does not require costly technological installations, however the development process of waveguides presented in our work required intensive labour. It should be noted that technological difficulties are increasing with the increase in refractive index, i.e. increase in the content of TiO₂. 

Recapitulating: in our opinion, the availability of waveguide films having appropriate properties and acceptable cost is a decisive factor in the development of integrated optics. Waveguide films are an intermediate product for the subsequent stages of a fabrication process of integrated optics systems. Frequently those processes and the apparatus used are the same as those applied in CMOS technologies which are inaccessible for many research groups. We believe that waveguide films developed in our research group will contribute to the development of the integrated optics in Vis-NIR spectral range, especially that the refractive of presented single mode silica-titania waveguide films is close to the refractive index of Si₃N₄ films.

2. Line 92-97, a generic description may be more appropriate in literature review as reader may not be very familiar with authors work. A comparison table may also allow readers to understand the differences.

In the revised manuscript the Introduction has been changed.

3. Line 167, it appears that from Fig. 3 the grating period < ~450nm when r=1. Is my understanding correct?

From the Eq. (3) comes the waveguide whose excited mode has effective index N=1.52, can be excited in the second order of diffraction (r=2) only if Λ>502 nm. Otherwise, the mode of such effective index can only be excited in the first order of diffraction (r=1).

4. Page 13 is like another introduction, like section 1. Authors may consider to shift selected concise content to section 1 and selectively highlight the novelty of the data with a few closet relevant work.

We agree with this remark. The extensive part of the content from the page 13 has been moved to Introduction.

Non-technical comments

1.Line 44, 47, 48, etc., abbreviation in brackets like line 34 and 35.

The relevant changes have been made.

2.Line 107, 112, In section X

The relevant changes have been made.

3. Fig. 4, green colour line might be easier to see when change to like those in Fig. 2.

The tone of the colour has been changed. The characteristics presented in Figs. 4, 5, 6 and 9 have been redrawn using dark green colour.

4. Fig. 5, letters and numbers appears distorted and hard to read. Please use bigger font or black (for example) colour.

The color of descriptions has been changed.

5. Line 215, It is possible

The text has been corrected.

6. Line 222, 224, 225, 226, etc., subscript

Subscripts have been corrected.

7. Fig. 9, is the unit of d correct? in nm?

The error in the name of the ordinate axis has been corrected.

8. Line 343, etc. for spelling error, which may come

The manuscript has been revised.

Round 2

Reviewer 1 Report

The authors have nearly addressed my comments. The paper could be accepted for publication after minor changes in English.

Author Response

We are grateful the Reviewer for constructive comments that helped us to improve quality of our manuscript. The manuscript has been revised again and grammatical errors have been corrected.

Reviewer 2 Report

We thank the authors for carefully revised the manuscript taking time to reply the comments made by all the reviewers. All of the comments raised are replied in the Authors' Responses to Reviewer's Comments. 

1. However, the manuscript has been selectively revised and authors avoided making the necessary new content directly into the manuscript. 
2. Moreover, the authors did not make ample effort to address all the three key areas of improvement directly into the manuscript for the broad comments and specific comments. 
   
   1. Page 13 content was largely shifted entirely inserted into the introduction with minor amendments and little effort to integrate to the introduction.
   2. The authors should also look again into the grammar part again in Page 13 content (original manuscript), now in the introduction section. 
   3. The lead author has high tendency to make reference to pieces of own work, especially the lead author own work, (for example, Embossable grating couplers for planar evanescent wave sensors, Opto-Electron. Rev. 2011, 19 (1), 10 - 21; Sensor properties of planar waveguide structures with grating couplers, Opto-Electron. Rev. 2007, 15 (3), 168 - 178) in recent published papers.
   4. In another recent work (Development of integrated photonics based on SiO2:TiO2 sol-gel derived waveguide layers: state of the art, perspectives, prospective applications) n ~1.8 and loss_p 0.2 dB/cm were reported. What are the key improvement or new insight available for reader since that work compared to this manuscript? 
3. Non-technical comments are addressed with spelling error still presented in the revised manuscript's new content. For example line 444: worka. 
4. Comments are not addressed clearly in the reply and in the manuscript. The authors should kindly answer them clearly.
5. It will also very helpful to highlight clearly what were shifted, what were added, what is removed in the Authors' Responses. 

I will recommend the authors to reconsider again all the comments made here and in the first Review Report to improve the quality of the manuscript and facilitate reviewing process. 

Author Response

We are grateful the Reviewer  for careful and thorough reading of this manuscript and constructive comments that helped us to improve its quality. The answers are given in the attached file Authors_Response_to_Reviewers_R2.pdf

 

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

We thank the authors for taking time to highlight the changes made in the manuscript between v1 and v2 into v3 manuscript. It become very clear what has been changed or shifted between v1 and v2.

All of the comments raised in review report v2 are replied in the Authors' Notes File. 

After careful reviewing the manuscript and authors' round 2 replies:

1. The authors have again in this review round v2 explained (in detail) in the attached Authors' Notes File, and previously in review round 1 explained in the Authors' Responses to Reviewer's Comments. Why are the authors' explanation still not documented and text edited into the manuscript? We see the authors are very resistance in any amending any content changes to their manuscript, which is why comment #1 in review round 2 wrote "However, the manuscript has been selectively revised and authors avoided making the necessary new content directly into the manuscript".
2. For the reply review round 2 comment #2c, the authors have insisted that they referred to those specific reference papers multiple times as the basis for their justifications. If we go back to see review report 1 comment #2 "The authors may want to consider reviewing the references list and the manuscript content again to avoid narrative heavily towards authors' own previous work and selected research groups.", the authors are doing exactly this. This is not appropriate.
3. For the reply review round 2 comment #2d, the authors failed to explain the key improvement or new insight going from refractive index 1.8 to 1.9. In that recent paper, they already mentioned that the SiI2:TiO2 film can be up to refractive index 1.94. Moreover, that paper has much better presentation of content and writing style compared to this manuscript.

I will strongly recommend the authors to reconsider again all the comments made in Review Report round 1, 2 and 3 to improve the quality of the manuscript.

Author Response

We are grateful the Reviewer for careful and thorough reading of this manuscript and comments that helped me to improve its quality. The detailed response is in the file answer_to_Reviewers_remarks-round_3.pdf

Author Response File: Author Response.pdf

Round 4

Reviewer 2 Report

We thank the authors for taking time to reach out again for us to reconsider the manuscript by preparing a new cover letter.

In the new cover letter, the authors replied to the three bullet points from Review Report 3. They have not attended directly to the last bold sentence in Review Report 3 "I will strongly recommend the authors to reconsider again all the comments made in Review Report round 1, 2 and 3 to improve the quality of the manuscript". This point again should be noted by all the the authors for future review responses and take note to read and response to each and every line. This similar remark was already written in Review Report round 2 and 3. Nevertheless, the responses required from earlier reviews can be indirectly found in-part in the latest cover letter.

 

Comments on Point #1 in cover letter

In the new text added to the manuscript (in red). The lines "Nevertheless, the differences is so big, optical losses in our waveguide films should be considered very low. The same physical effects are the source of optical losses in both these types of optical waveguides. The single mode waveguide films presented in our papers have thickness of 138 nm (Fig.12)" is not clear. The authors are advised to revise the sentences. They proposed to compare the optical fiber at 1550 nm (SMF-28-J9 @ 0.18 dB/km) verse their work at 632.8 nm (0.3 dB/cm) which makes the loss between them at ~10000 orders (at different wavelength). If we compare with optical fiber at 633 nm (SM600 @ 15 dB/km), the difference is only ~200 orders. Maybe the authors want to compare the thickness at 138 nm, so it would have very low loss at either 1550 nm or 633 nm, which is acceptable but for this is not sure if this is what the authors meant.

 

Comments on Point #2 in cover letter

The authors provided justification that by narrowing down to the experiment setup keywords on 'single mode waveguide films made of transition metal oxides, having high refractive index and low optical losses' there are lesser directly related articles. If we search in Google Scholar the phase 'optical loss in planar waveguides', there are over 200,000 returned results. If we search those 17 keywords above, Google Scholar returned over 17K results. There are just many opportunity there that could be gathered and summarised to present new insight for readers.

Another point to note is that the authors have history of over self-citing on multiple published papers (regular papers, not review papers). 

For examples on their recent published work:

1. https://doi.org/10.3390/ma14092290 7 out of 26 references: 7, 9, 12, 13, 14, 19, 20 
2. https://doi.org/10.3390/coatings10060509 5 out of 32 references: 11, 12, 17, 27, 28
3. https://doi.org/10.1117/12.2559059 7 out of 16 references: 5, 6, 7, 8, 13, 15, 16

The correct removal of citations in the manuscript will not bring harm to the authors or the publisher. For example, any reader could easily search 'Paweł Karasiński' and Google Scholar will return his profile first and over 500 related results. The proposed suggestion is to give external group one citation so that other new insight from other related optical loss in planar waveguides work can be shared and cited too.

 

Comments on Point #3 in cover letter

There is no detail description on the improved made on the manuscript when compare to their recent published work or past work. In this manuscript they reported n=1.9 and optical loss 0.3 dB/cm, and in https://doi.org/10.1117/12.2559059 they reported n=1.81 and optical loss 0.1 dB/cm. 

In the cover letter, the authors mentioned improved repeatability. The quality of homogeneous layer and surface roughness contributes a lot to the optical losses. In other words, (in general and in many photonic waveguide design) the refractive index is inversely proportional to optical loss. If we kindly assume now another batch of samples are made by improvement on some processes so that n = 1.94 (a higher refractive index) and we can largely predict the optical loss will be higher than 0.3 dB/cm. 

Explicitly, the authors need to have in the text of the manuscript how they 'overcome this problem' as written in section 3.3. The concise  information should also be integrated in the new conclusion and new abstract. Without this, there is little new significance of content as the author are simply reporting results without any method. Could it be on  engineering processes improvement?

In the new text added, the authors did not cite their recent published work and explain the new insight in the text. This is in contradiction to what they have mentioned in response #2 on 'substantial contribution to ... planar waveguides fabricated by the sol-gel method'. 

 

Ending

All the cover letters and review reports should be made open. The authors are highly recommended to submit another new review paper for their achievements and contribution in 'single mode waveguide films made of transition metal oxides, having high refractive index and low optical losses'. Wish you good health and research. 

Author Response

We are grateful the Reviewer for the comments. The answer is in the file answer_to_Reviewers_remarks-round_4.pdf

Author Response File: Author Response.pdf