Accurate Measurements of a Wavelength Drift in High-Temperature Silica-Fiber Bragg Gratings

Round 1

Reviewer 1 Report

Work is an interesting and useful to the sensor community. In my opinion, the topic of the manuscript is fit with the Journal outline after address the minor following points.

1. Thermal expansion and photoelastic coefficient are two parameters that can't be separated in the optical fiber, So, how the authors differentiate if the draft appear from change in T or stress variations?
2. Why authors didn't use Ge doped fiber for FBG fabrication? why is SMF-28 where it has a low sensitivity to the UV light?
3. Why the results of Na at table 1 has a high drift? draft suppose to be linear due to the the  thermal expansion. But here is seems nonlinear! can they check it theoretically. Also, did the authors found any degradation in the FBG after 6 months?
4. Paragraph starting from line 164 needs a reference? as authors didn't study the performance of cladding at high T.
5. The results of Figure 4 were published in ref. [13], so authors need to add [13] to the figure caption, then readers can back to the reference to understand the difference between graphs. otherwise, provide more details about the difference here are needed.
6. Line 220,authors  mentioned that they can't "leave the experiment unattended overnight due to safety concerns", but I can see they measured for 50h, it means you leave it overnight!
7. Line 248, did the authors tried to measure the state of the polarization (SOP) using polarimeter device to see the change in the SOP over the T? 
8. Figure 9, for 90, why there are no measurement between 8-22 h? In that case, why authors use linear fitting line?
9. Line 271, But authors mentioned that they used SMF-18 where the dopant is very low. This might me right if they used Ge-doped fiber.
   930 C is close to melting point of fiber silica and might be the thermal expansion leads to permanent degradation of the fiber material.
10. Lines 288-291, the explanation is not clear. I think thermal expansion of the fiber materials play the major role for this issue. 

Author Response

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Reviewer 2 Report

In this paper, the authors report some of the experimental caveats that could lead to inconsistent results and confusion in measuring wavelength drifts by using packaged π-phase-shifted FBGs, including the influence of FBGs packaging, the variability of FBGs, birefringences of FBGs, and so on. But there are few explanations of the rationale for the study design and the research topic is not so novel. Some suggestions are as followed:
1.As an academic paper, the expression of words should be concise and to the point. But there’s some expression that needs to be improved in this paper to make the paper self-explanatory. For example, I suggest adding a schematic in the making and testing process to make it easier for readers to understand.
2.The authors put forward some conjectures but there are no further discussions nor experiments on these. And the conclusions are overstated. For example, between line 153 to line 156 “We speculate that the heating of the epoxy above the rated temperature limit resulted in its degradation and re-deposition of by-products everywhere inside the sheath including the FBG portion of the fiber. It is yet unclear how this re-deposition led to the subsequent FBG failure.” Between line 222 to 225 “The short time duration, in this case, was chosen because we were unable to leave the experiment unattached overnight due to safety concerns. As a result, it is entirely possible that the measured negative drift rate of -0.946pm·h-1 is due to insufficient time for it to stabilize”. Several variables may lead to the result, so suggest adding the experiment with sufficient stabilizing time to testify to the hypothesis. 
3. Suggest improve the quality of diagrams and pay more attention to details. For example, the form of abscissa name “Elapsed_Time_h” is not the same as the ordinate name “Wavelength change, pm” in figure 3. Moreover, part of the orange line is covered in figure 6. In figure 7, the data points the polarization state of 90 degrees is too discrete. Please check all diagrams.
In consideration of the above points, the manuscript can not accept for the journal at this moment.

Author Response

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Reviewer 3 Report

Dear Authors,

I read carefully your manuscript about the wavelength drift of FBG under high temperature. It is well written with a lot of very interesting information.

I have only some questions/remarks:

• P.2 L.78: is it a SMF28 with acrylate or polyimide coating ?
• Why to study the behaviour at mainly at 600°C ? (have you some special applications ?)
• Did you think that the reflectivity of the FBG influence the stability ?
• Is a special gas in the packaging can influence the stability ?
• P.9 L.256: If I understand correctly, the stress relaxation happened only at 1000°C but not at 600°C ?
• P.10 L.267: in the equation why do you have 72 ? I don't see this value in Table 1 
• P.12 L. 342 to 344: if the FBG can differ significantly, how can be they interchangeable ?
• Based on your results, do you think that it will be possible to cascade FBGs in the same fiber for high and precise temperature measurement . In the other case, it will be different to be competitive with PRT ? 
• Do you think that this drift also exists for regenerated fiber Bragg grating ?

Regards. 

Author Response

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Round 2

Reviewer 2 Report

Suggest accept it.