Linear Fiber Laser Configurations for Optical Concentration Sensing in Liquid Solutions

Round 1

Reviewer 1 Report

The manuscript under review presents an investigation of linear fiber laser configurations used to measure paracetamol liquid solution concentration. In general, the article is reasonably structured, and the results are clearly presented. However, several issues need to be addressed before the manuscript could be considered for publication:

1. In the Introduction, it is necessary to add a comparative analysis of the existing fiber-optic sensors to measure the concentration of liquid solutions, in particular, paracetamol. Please include the comparison of performance between the existing sensors and the developed sensor, mentioning their benefits and drawbacks.

2. At the end of the Introduction, it is recommended to highlight the novelty and the advantages of the proposed sensing system in comparison with the state-of-the-art.

3. It would be more convenient for the readers if the authors presented all the data of Figure 5 in a single plot with different colors, moving the inserts in a separate plot, also in the same axes. This would make it easier to compare the data. 

Author Response

Authors appreciate all the comments and suggestions made by the reviewer. According to them, the manuscript was improved. All the comments were answered below and all the modifications in the text were highlight.

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript has to be supplemented by:

1)Introduction (description of different laser system sensor),

2)more detail explaination of Fresnell reflection and polarization, 

3) specify parameters of used laser system and sensing head 

Author Response

Authors appreciate all the comments and suggestions made by the reviewer. According to them, the manuscript was improved. All the comments were answered below and all the modifications in the text were highlight.

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors presents results using a linear fiber laser resonator, with an internal sensor head based on Fresnel reflection, for measurements of paraceptamole concentrations in liquid solution. The claimed sensitivity is 1.75 g/kg. The sensor configuration is original, however it is not clear whether this configuration has any benefits when compared to other refractometric techniques, especially considering the complexity of the setup. Using refractometric measurements for concentration measurements is not new. It is impossible to properly evaluate the significance of the results without more careful experimental studies combined with theoretical analysis of the sensor configurations. Therefore without proving real benefits over more conventional refractometric techniques, I can not recommend the manuscript for publication. 

The following are some questions that needs to be addressed:

1. Why do you need to operate at threshold? How close to threshold? How do the results depend on the exact pump power?

2. The sensor head, the left FBG reflector and the coupler represents a Michelson interferometer. How do the results depend on phase and/or polarisations changes in the sensing arm and laser cavity. 

3. What is the significance of the results in Figure 5 wrt. the sensor performance? What is the liquid used?

4. What is the averaging time used in Figure 6?

5. The results in Figure 7 shows large drift. E.g. in (b) the drift is approximately 50 pW/min while the reported standard deviation is Table 1 is 10 pW. Have you compensated for the drift? How drift affects the sensor performance must be addressed. 

6. The claimed resolution of 1.75 g/kg corresponds to about 2.6 10^-4 RIU. How does this compare to other refractometric methods?  

Author Response

Authors appreciate all the comments and suggestions made by the reviewer. According to them, the manuscript was improved. All the comments were answered below and all the modifications in the text were highlight.

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

There are some points, concerning the paper:

1. The underlying physics is not described. I can see the Michelson interferometer, formed by FBG_1 and sensing head (Fig. 3): does it plays any role in the observed effect? In fact, the scheme contains multiple interferometers.

2. Reflection spectra of optical blocks (i.e. left part of Fig. 3) are not provided.

3. There are no any analytical dependencies of the proposed method.  The problem of optimal loss for lasers has been already solved [Rigrod W W 1965 Saturation Effects in High-Gain Lasers J. Appl. Phys. 36 2487–90].

4.  (Figure 5) - All lines could be placed at one plot to reduce the space used.

5.  (Line 141) From Figure 5 it is no obvious what do you call ASE. Does the total noise power is comparable to laser power?

6. The R-squared parameter is hard to be interpreted. It is better to use standard deviation instead. 

7. As the proposed sensor is not specific for paracetamol, than it is better to call it RI-sensor.

Author Response

Authors appreciate all the comments and suggestions made by the reviewer. According to them, the manuscript was improved. All the comments were answered below and all the modifications in the text were highlight.

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I have not more comments for authors.

Author Response

Authors thank to the reviewer for all his work.

Reviewer 3 Report

See attached document.

Comments for author File: Comments.pdf

Only minor corrections are required. 

Author Response

Authors appreciate all the comments and suggestions made by the reviewer.

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

Dear Authors,

Your response is acceptable, except for the following:

1. "... However, this is not a Michelson interferometer. The difference in these arms is outside the coherence length of the laser. In this way, the difference between these arms is a lot higher than the coherence length of the laser. To illustrate this in a better way, Figure 3 was modified..." - At first, it would be great to provide the following values in the text: the path difference and  laser spectral width (or coherence length), as it is hard to estimate from the Figure 5a. Secondly, please, remove the fiber coil added to the FBG_1 arm at Figure 3, as it still looks like both paths have comparable length.

3. I need to clarify my hesitation. The proposed method looks unique, but the underlying effects is studied enough by now to be presented in the paper. In my opinion, the paper would gain a lot if it contained the analytical or numerical confirmation of obtained results and prediction for optimal coupling ratio in proposed scheme. The analytical expressions could be derived from [Barnard, G., Myslinski, P., Chrostowski, J., & Kavehrad, M. (1994). Analytical Model for Rare-Earth-Doped Fiber Amplifiers and Lasers. IEEE Journal of Quantum Electronics, 30(8), 1817–1830. https://doi.org/10.1109/3.301646] and References (especially [17]) with some simplifications.

Author Response

Authors appreciate all the comments and suggestions made by the reviewer.

Please see the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

See attached file.

Comments for author File: Comments.pdf

No special comments.

Author Response

Authors appreciate all the comments and suggestions made by the reviewer. Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

Dear Authors,

I'm looking forward for Your next paper concerning the proposed method.

Author Response

Authors thank the reviewer for all the suggestions.