Fiber Bragg Grating Salinity Sensor Array Based on Fiber Tapering and HF Etching
, Mengchao Yan 1,3, Tuanwei Xu 1,2, Ancun Shi 1, Yuanhui Liu 1, Xuechun Li 1,4 and Fang Li 1,2,*
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsAuthors report on salinity optical fiber sensor based on two Bragg gratings inscribed in a tapered fiber which is then etched. This is an unpractical system in terms of engineering that require the execution of many complex steps for its fabrication.
The response of the sensor is compared (table 1) with 4 other systems reported in the literature however 2 of them ref [13] and [22] do not correspond to salinity sensors. In the same table the authors provide a sensitivity value for their system (based on ref 29) that is not justified adequately. The discussion in the introduction about absolute salinity and study of seawater dispersion characteristics is not elaborated further in the manuscript.
Author Response
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Reviewer 2 Report
Comments and Suggestions for Authors
This article describes an innovative technique based on micro/nano fiber Bragg grating to measure seawater salinity variations. The theoretical concept of their technique is well explained in section 2. The fabrication technique is also very well described in section 3, and the experimental results are clearly exposed, but it remains a few points to be clarified or corrected. They are detailed below:
Line 32: replace “neutral component” by “non-ionic components”
Line 34-35: optical refractometry is just one technique to measure the refractive index. Interferometry is another one (see: https://www.frontiersin.org/articles/10.3389/fmars.2022.1031824/full). TEOS-10 manual doesn’t recommend any optical technique to measure absolute salinity. Optical techniques allow a better resolution to be accessed than acoustical techniques but that’s all.
Line 37: explain what you mean by “dispersion”.
Line 38: why do you take a huge change of 300 nm? Replace “can lead” by “can leads”.
Line 56: 0.0165 nm/M: M is not a standard unit. The unit of moles is mol. But, absolute salinity is rarely expressed in mol because it is composed of a various number of molecules, but in g/kg. Could you give the equivalent?
Line 121 and following: could you give a reference to explain where these relationships come from?
Line 134-135: by substituting Eq.(5) into Eq.(3) and then into Eq.(2)?
Line 175-176, how do you do to avoid breaking of the fiber with such small diameters (1.8 microm)?
Line 200: add “the” before sensor.
Figure 3 b): the red labels are not readable. Could you change the color?
Line 255-257: reference [4] doesn’t rely salinity to RI. It is only the Millard and Seaver relationship that do it: reference [29].
Line 265-266: 0.1 pm is not a lot! Isn’t it sensitive to temperature variations? And, that is the resolution of the demodulator, but can you measure the centre of the wavelength curves with the same resolution?
0.1/8.39 = 0.012 %0 in salinity.
Line 267: could you precise that it is RI resolution?
But, I think that there's something wrong with your calculation. 1x10-5 in RI gives 0.054 in salinity according to ref [29], and 0.1/8.39 = 0.012 %0 in salinity. Could you explain that?
Line 302: 8.34 pm with a sensitivity of 8.28 pm/°C makes a shift of 1 °C. That’s a lot, and it is not sufficient for most of practical applications! In oceanography, long term stabilities of 0.001 °C are required! Could you modify this sentence?
Lines 325-326: Doesn't this thinner structure alter the strength of the fibre and make this assembly more sensitive to impact?
Table 1: line 1, change “M” by “mol” which is the SI unit, and, if it is 5.7 mol, it doesn’t correspond to a range in %0 in salinity. Line 4, could you translate the range, given in RI, in salinity to be homogeneous?
Line 337: I don’t understand what you want to do. What is the relation between arrays of MNFBGs and the seawater state equation, and the relation with the dispersion characteristics?
Line 348-349: isn’t it the dilatation effect more than the elastic effect?
Line 352: what do you mean with constancy? The measurements you did at constant temperature show important variations.
Comments on the Quality of English LanguageI am not completely qualified to assess the quality of English in this paper.
Author Response
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Reviewer 3 Report
Comments and Suggestions for AuthorsAfter thoroughly reviewing the submitted manuscript by Gaochao Li et al., which describes the development and evaluation of a seawater salinity sensor array based on micro/nanofiber Bragg grating structures, I am pleased to recommend its acceptance, mainly due to the following topics:
a) Goog Reading Experience provided by the manuscript: The authors have presented their research in a well-structured and lucid manner. The sequence of content and the clarity in reporting and discussing results contributed to a gratifying reading experience.
b) The experimental results are presented with crystal-clear clarity. The mentioned sensitivities for both salinity (8.39 pm/‰ and 7.71 pm/‰) and temperature (8.28 pm/°C and 8.03 pm/°C) from the cascaded sensor arrays underscore the precision and repeatability of the proposed sensor. Additionally, the ability to measure the refractive index with values of 11.7 nm/RIU and 10.7 nm/RIU adds a significant feather to this work;
c) The detailing in the fabrication process also has a clear explanation, particularly regarding the influence of the surrounding refractive index;
d) The authors have done a sufficient job of discussing their findings in light of existing literature and, in doing so, highlighted the significant differences between their work and others;
e) The authors have emphasized the sensor's potential in seawater environmental monitoring and quantitative studies of seawater dispersion characteristics. The compactness, excellent linearity, and impressive repeatability of this sensor structure make it an invaluable asset in marine studies.
In conclusion, the authors have contributed a solid piece of research to the scientific community. Therefore, I endorse the acceptance of this manuscript as is, without further revisions.
Comments on the Quality of English LanguageMinor editing of English language required
Author Response
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Round 2
Reviewer 2 Report
Comments and Suggestions for Authors
Most errors were corrected, but it remains a few ones to correct:
Line 36: replace “non-conductive neutrals” by “non-conductive components”.
Line 37: remove the “.”.“and the seawater's RI is needed to indirectly determine the salinity”. Acoustic devices can be used too (See and cite https://www.frontiersin.org/articles/10.3389/fmars.2022.1031824/full), but the resolution and the reproducibility specifications of these devices are not sufficient to respond to oceanographic needs.
Line 59: the international symbol of “litre” is not “L” but “l” (see SI units on www.BIPM.org).
Line 309: add “of” after “1 °C”.
Line 350: you can speak to present and erase “will”.
Line 352: I don’t understand the meaning of this sentence. Perhaps, by replacing “of” by “by” will give the research meaning.
Line 351-356: replace these lines by something like: “we plan to use our device to make measurements that will allow the extension of the validity range of Millard and Seaver relations [29] in wavelength”.
Line 357: replace “calibrating” by “extending”.
Line 358: replace “of dispersion characteristics within seawater state equations” by “of the sweater absolute salinity”.
Line 372: replace “and provides exceptional consistency” by “and offers exceptional compactness”. I think it corresponds to what you mean.
Some questions still remain without answer:
Line 124 and following: could you give a reference to explain where these relationships come from?
Line 179: how do you do to avoid breaking of the fiber with such small diameters (1.8 microm)?
Could you give the resolution in salinity of your device?
Author Response
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