Magnetostrictive Behavior of Metglas^® 2605SC and Acoustic Sensing Optical Fiber for Distributed Static Magnetic Field Detection

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper treats fiber-optic magnetic field sensor and provides some interesting experimental results for optical fiber sensor technology. The authors are requested to respond to the followings in the text:
1.    Against the authors’ claim (L364), the technology of dc magnetic filed measurement using magneto-strictive  materials is not new (see “Fiber-optic Fabry-Perot interferometer and its sensor applications,” IEEE Trans. Microwave Theory Tech. 30(10), 1612–1621 (1982), which presented the first experimental study of the relevant subject).
 
 2.  What do you mean by “DAS fiber”.  It looks to be the same as “FBG mirrored fiber-optic Fabry-Perot interferometer”. Clarify their relationship.  Obviously, the latter terminology is more popular and  clear for fiber sensor people than the former. 

  3 . Some explanation about the operation performance of your signal interrogation scheme based on Sentek’s picoDAS interrogator of a commercial product is requested for most readers.

4. The authors stressed distributed sensing ability of your sensor, but you made no actual study of this item. Considering this, your statement, say in your Title, is not suitable and causes misunderstanding to readers. Modify the associated expression into more suitable and moderate one.

Author Response

Reviewer 1:

1) Against the authors’ claim (L364), the technology of dc magnetic field measurement using magneto-strictive materials is not new (see “Fiber-optic Fabry-Perot interferometer and its sensor applications,” IEEE Trans. Microwave Theory Tech. 30(10), 1612–1621 (1982), which presented the first experimental study of the relevant subject).

Response: The authors appreciate the comment, and the wording used in the manuscript was misleading. The claim was changed (now L429) referring instead to a new method rather than new technology as the authors agree that was not the case. The method used described in the study for static magnetic field sensing relying on frequency shifting and intensity changes from added static magnetic fields detected via vibration imparted to a DAS fiber is the first of its kind.

2)  What do you mean by “DAS fiber”.  It looks to be the same as “FBG mirrored fiber-optic Fabry-Perot interferometer”. Clarify their relationship.  Obviously, the latter terminology is more popular and clear for fiber sensor people than the former. 

Response: The authors agree this relationship should be clarified and the following is included in the manuscript (lines 121-126):

• “This optical fiber also uses broad band FBGs that are inscribed every 2 meters (the spatial resolution of the sensing fiber). The gratings contain a periodic index variation, and the fiber itself is designed to transmit light at a wavelength of 1550 nm. The 2-meter spaced gratings form Fabry-Perot interferometer pairs allowing for distributed sensing capabilities, also referred to as DAS.”

 3) Some explanation about the operation performance of your signal interrogation scheme based on Sentek’s picoDAS interrogator of a commercial product is requested for most readers.

Response: The suggestion is appreciated, and the following was added (lines 136-139):

• “...Sentek’s picoDAS interrogator to characterize the strain imposed on the fiber. The system measures dynamic strain based on changes in optical path difference (OPD) between FBGs outputting a 3-dimensional matrix of strain, position, and time. The interrogator operates at a sampling rate of 37kHz with a sensitivity below 0.25 nano strain.”

Additional information is also referenced in the manuscript to two other papers  (line 144). However, some information cannot be given as it is proprietary.

4) The authors stressed distributed sensing ability of your sensor, but you made no actual study of this item. Considering this, your statement, say in your Title, is not suitable and causes misunderstanding to readers. Modify the associated expression into more suitable and moderate one.

Response: The authors agree as the distributed nature of the sensor is heavily emphasized. A new figure was added including data demonstrating the distributed capabilities of the fiber as to better support our claims (lines 292-319):

• “To demonstrate the distributed detection capabilities of the sensor, the DAS fiber was tested in a series of three air core solenoids following the same setup. A magneto-strictive ribbon was centered in each of the three solenoids with the sensing fiber laying on top of each one. Three separate “sensors” (all along the same fiber) were coupled with each ribbon respectively and then exposed to a uniform magnetic field. A software capture is shown in Figure 8 of this setup. A more in-depth discussion of the setup can be found in [13] for AC magnetic field measurements.

Figure 8. 3-dimensional software capture of 8 FBG pairs along 16 meters of fiber showing position, strain and time with three air core solenoids as magnetic field sources (centered at 6m, 10m, and 16m respectively).

 

Each colored signal represents a separate 2-meter sensing length from an FBG pair all along one DAS fiber. The three largest sinusoidal strain responses correspond to a dif-ferent solenoid and magnetic field source at varied frequencies and amplitudes (100 Hz is no longer solely used so that the unique magnetic fields at each source can be easily seen). The first is shown in red at the 6m position. The second solenoid is positioned such that the contained sensing length is centered at 10 meters, shown in pink, and the third at 16m in white. At the 10-meter position, both the driving and doubled frequency components (caused by a static magnetic field bias) can be seen, while the other two solenoids show a strain response composed of primarily one frequency component. It is clear from the capture that separate magnetic fields can be independently resolved across the sensing fiber. While this test only shows distributed magnetic sensing along 16 meters worth of fiber, it still demonstrates the distributed capabilities for the mag-netic sensor. The DAS system in question is capable of measurements along tens of kilometers with low loss while other commercial systems can even take measurements over 100 km of optical fiber.”

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents an approach for distributed static magnetic field sensing. By applying an initial known AC magnetic field, the authors demonstrate how static magnetic fields can be detected via the modulation of strain responses in both multi-material fiber designs and ribbon-based configurations. There are several points that require clarification and additional discussion before the manuscript can be considered for publication.

 

1. The proposed sensing principle relies on additional modulation, yet vibration, magnetic field, and temperature variations all contribute to the output optical signal. It is not clear how vibration and temperature variations may affect the accuracy of static magnetic field measurement. In the context of structural health monitoring, what level of accuracy is typically required for static magnetic field sensing, and to what extent could these environmental factors compromise the measurement precision? A quantitative analysis or at least an estimation of the expected error margins would strengthen the work.

 

2. The manuscript does not provide sufficient detail regarding the uniformity of the applied AC modulation magnetic field along the fiber propagation direction. Non-uniform field distribution may lead to uneven strain induction and could affect the accuracy of the magnetic field sensing. The authors should clarify how the uniformity of the applied field was ensured or discuss its possible effect on the measurement results.

 

3. While two types of optical fiber (multi-material inclusion fiber and ribbon-coupled fiber) were investigated, the relative advantages and limitations of these types are not discussed. A more explicit comparison of the trade-offs between the two types would be helpful for readers to assess the applicability of each type.

 

4. As shown in the results, the amplitude response relative to the magnetic field is a multi-valued function, and it is not clear how the authors resolve the ambiguity in determining the specific magnetic field strength. This issue is critical for ensuring unambiguous static magnetic field measurement, and the authors should clarify how the problem of multi-value mapping is handled in practice.

Author Response

Please refer to the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors' responses done to my comments 2)-4) are roughly acceptable for me. However, regarding my comment 1), your response is quite inacceptable. In your revised statement (L426-429), you still claim, “ While previous work has demonstrated a working prototype for AC magnetic field measurement, this is the first demonstration of static magnetic field detection using this method”. This statement obviously wrong as commented to you before; the method and technique you noted are the same meaning. So you are not allowed to claim for originality in your demonstration of the static magnetic field detection by magneto-strictive transducer. In spite of my previous suggestion, you will not correct this wrong misleading statement; the method and technique you noted are the same meaning. So I can’t recommend this  paper for publication in trustworthy quality journal.

Author Response

The authors have removed the claim entirely (lines 430-432) and hope that this is agreeable with the reviewer. The statement was meant to be in relation to our group's work and not the field as a whole. The intent was not to claim the first instance of static magnetic field detection using fiber optics and we apologize if that was implied. The introduction also includes a newly added statement referencing prior innovations in fiber based field detection to clarify this (Lines (48-40):

"Prior innovations in magnetic sensing using fiber optic strain sensors have successfully demonstrated static and dynamic field detection with bonded magnetostrictive materials..."

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made revisions and provided responses to most of my comments. The quality of the manuscript has improved, and I find it acceptable for publication.

Author Response

The authors would like to thank the reviewer for their time and meaningful suggestions to improve the quality of the manuscript.