Review Reports - Temperature-Insensitive Ferrofluid-Clad Microfiber Bragg Grating for Magnetic Field Sensing

Round 1

Reviewer 1 Report

The research content of the manuscript is interesting, and I suggest the authors make the following improvements:

1. There are two major technologies available for fabrication of FBGs in microfiber: ultraviolet (UV) laser radiation and ultrafast laser radiation. Which method is used in this work?

2. Fig.5 and Fig. 7 just show the Bragg wavelength shift of temperature from 20゜C to 80゜C. What if outside this temperature range such as within the range of 0゜C to 20゜C?

3. Does the change of temperature affect the intensity of the reflection spectrum? What is the impact of temperature on intensity?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The authors reported a temperature insensitive magnetic field sensor based on ferrofluid clad microfiber Bragg grating. I would recommend this paper to be published in Photonics, which is a quite suitable journal for the subject. Nevertheless, the authors may like to answer the following questions:

1. How the Bragg gratings are written in microfiber? It should be introduced.

2. The authors say that “Finally, the Teflon coated MF-BG was placed in a FF filled capillary tube with length being 5 cm and inner diameter being 600 nm.” The capillary inner diameter is not acceptable since the diameter of MF-BG is ~ 2.94 μm.

3. From Fig.2, one can calculate that the thickness of Teflon film is not 600 nm mentioned in the manuscript but 605 nm.

4. The authors claim that “As the diameter increases to 2.8896 μm, the sensitivity to temperature changes almost linearly to 1 pm/゜C.” However from Fig. 3(b), the sensitivity is more likely to be -1 pm/゜C than 1 pm/゜C.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The Authors propose a microfiber Bragg grating sensor for the measurement of magnetic field. By changing the fiber dimensions, the sensitivity to temperature decreases substantially, enabling a temperature-independent magnetic field sensor. Overall, the manuscript is well structured. However, there are a few questions/comments to be addressed before being recommended for publication at MDPI Photonics.

1) There are a few typos and the English needs improvement. For example, in the abstract the term “responsibility” should be changed to “response” or “sensitivity”. In line 36, “tiled FBG” should be “tilted FBG”. Please check.

2) There are some acronyms in the introduction that are not used throughout the manuscript, which could be removed.

3) Please provide more details regarding the Teflon coating procedures.

4) Was the FF sealed inside the capillary tube?

5) I found the description of Eqs. 2-4 and Fig. 3 confusing. I would suggest to carefully revise this section, and to provide more details regarding the numerical calculations.

6) What is the r^2 of the linearization made in the temperature measurements?

7) Please comment on the sensor stability and reproducibility.

8) It would be interesting to have a comparison table with other sensors reported in the literature.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The authors have successfully designed a temperature insensitive magnetic field sensor that is based on MF-BG and coated with FF. The theoretical analysis shows a clear relationship between the MF-BG diameter and the thermal effect of the sensor, indicating that a diameter within the range of 2.889 μm to 2.997 μm can effectively suppress the response of the sensor to temperature. The experimental results are highly promising, showing that a MF-BG with a diameter of 2.94 μm has an ultra-low temperature sensitivity of 0.13 pm/°C. The sensor's Bragg wavelength is observed to have red-shifted by 146 pm when the magnetic field was increased from 0 Gs to 221 Gs, and its temperature cross-sensitivity is found to be as low as 0.2 Gs/°C. Overall, the proposed sensor is a highly accurate and reliable solution for magnetic field measurement.

The following are the reviews for the manuscript

1) The paper could discuss potential applications of the proposed sensor, such as in high-precision magnetic field measurements in various fields, to demonstrate its practical significance.

2) Also the paper could delve deeper into the mechanism behind the low temperature sensitivity and high accuracy of the sensor, to provide a deeper understanding of its underlying science.

3) The authors should provide clarification on the technical terms such as MF-BG and FF, which may not be known to all readers.

4) Providing more context on the significance of the results, for example, why the temperature sensitivity of 0.13 pm/°C is considered ultralow and why the Bragg wavelength shift of 146 pm is considered significant.

5) The text could benefit from a clearer and more concise writing style to improve readability and clarity.

6) Several typographical mistakes are present, for eg. In equation 1 the grating period is showing with different symbol. Eg. 3 must be properly oriented.

7) Authors need to add some of the important journals related to Bragg grating applications for refractive index sensing,

· Optics Communications (2017) 402:408–412. doi:10.1016/j.optcom.2017.06.051

· Compact SOI nanowire refractive index sensor using phase shifted Bragg grating," Opt. Express 17, 15330-15341 (2009).

· https://doi.org/10.1117/12.869644

· doi:10.1016/j.ijleo.2018.04.034

· Modelling of photonic wire Bragg gratings, Optical and Quantum Electronics, vol. 38, pp. 133-148, 2006

· High-Q and temperature stable photonic biosensor based on grating waveguides. Optical and Quantum Electronics (2018) 50:307. doi:10.1007/s11082-018-1578-x

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

The manuscript is written well and can be considered for publication in the present form.