Verdet Constant of Magneto-Active Materials Developed for High-Power Faraday Devices
Round 1
Reviewer 1 Report
This review will be of use for magneto-optc people because it presents a list of articles and characteristic data on recenty developed Faraday materials.
Author Response
The authors would like to thank Reviewer #1 for the work he/she spent on reviewing our manuscript and for highlighting the key points and benefits of the presented work for the scientific community. This truly provided us with positive feedback in the sense that the fundamental message of the manuscript should be clear to its potential readers.
In the response to the ranking of the English language, we have run a more detailed grammar check, which eliminated (as we hope) the vast majority of the typos and grammar mistakes we missed in the previous version of the manuscript.
Reviewer 2 Report
In general, the article is unclear purpose on which this work is focused. The title of the article indicates that these materials are suitable for high-power laser systems. However, no damage threshold data of these materials is not given. In the introduction, the fact of the negative effect of absorption on the operation of magneto-optical devices is indicated. Further, there is no graphic (for example, the dependence of the Verde constant obtained at the same radiation power in samples with different absorption) of a table material that would allow to evaluate this effect. A significant part of the work is also devoted to the effect of heating on the magneto-optical properties, but there is no data of temperature dependence of Verde constant. The part devoted to the review of materials used for magneto-optical devices is presented in a form inconvenient for analysis. The separation of parameters by spectral ranges is not very clear. In my opinion, specifying the working range and values of the constant Verde for typical wavelengths is more convenient than that represented in this paper. Also in the article, magneto-optical glasses are discussed, but any data of glasses are not presented.
Author Response
The authors would like to express their gratitude for the constructive criticism and hard work of Reviewer #2. His/her pieces of advice were included as far as possible and we hope that our revision has improved the paper to a level of his/her satisfaction. Number wise answers to reviewers’ specific comments are addressed below. For easier orientation in the revised manuscript, we highlighted changes to the manuscript with a red color.
A) In general, the article is unclear purpose on which this work is focused. The title of the article indicates that these materials are suitable for high-power laser systems. However, no damage threshold data of these materials is not given.
Our response: After careful reconsideration, the authors need to admit that the title of the manuscript may be a bit misleading or confusing for some readers in the following sense. The original title “Overview of magneto-active materials for high‑power Faraday devices” may seem to address a broader range of the key parameters of the magneto-active materials suitable for the high-power operation. One of these parameters is the LIDT data, as the reviewer #2 correctly comments, and the others are: the Verdet constant, thermal conductivity, absorption, Q and P thermo-optical constants and the optical anisotropy parameter (as described in the introductory part of the manuscript). In this review, we are focused on the characterization of the Verdet constant, as a function of wavelength and temperature, and on an overview of the Verdet constant characterizations of several materials in the yet-reported literature. From our understanding, characterization of the Verdet constant represents an initial step in the benchmarking of the new magneto-active materials developed for the high-power Faraday devices, providing valuable data for the scientific community dealing with the additional testing of these materials. Although we do not provide the LIDT data, we provide a reader with “Additional references” in the overview section, leading to additional studies of the listed materials (some of them containing the thermal properties or LIDT data as well).
In response to this comment, we have decided to adjust the title of the manuscript to a version more aligned with the content of this review. We have also included some minor changes to the text of the abstract, introductory part and overview part. Further, we have introduced sub-sections in the introductory part to make this part even more clear and readable. The authors firmly believe that these changes should avoid a possible confusion of a reader regarding the purpose of the manuscript and its content.
B) In the introduction, the fact of the negative effect of absorption on the operation of magneto-optical devices is indicated. Further, there is no graphic (for example, the dependence of the Verde constant obtained at the same radiation power in samples with different absorption) of a table material that would allow to evaluate this effect.
Our response: Yes, this is correct, the absorption has indeed a very negative impact on the operation of the Faraday devices. For the purposes of this manuscript, we believe that it is sufficient to present the cause-and-effect diagram, providing a reader with a brief explanation of the involved phenomena and their impact on the performance of high-power Faraday devices. The topic of the thermal effects is very broad and complex, hence, given the fact that this review focuses mainly on the Verdet constant characterizations, going deeper into the analysis and evaluations of the thermal effects would be beyond the scope. We provide an extensive amount of additional references for a reader interested in the thermal effects evaluations: either theoretically (e. g. the Refs. 15-20) or experimentally (many references in the overview section in the tables with materials). Because of these reasons, we have decided not to do any changes to the manuscript according to this comment.
C) A significant part of the work is also devoted to the effect of heating on the magneto-optical properties, but there is no data of temperature dependence of Verde constant.
Our response: Yes, in the introductory part, there is a brief description of the thermal effects and their impact on a Faraday device, explaining the motivation of the research around the magneto-active materials for high-power devices. The main part of the manuscript is, however, dedicated to the characterization method and to the material overview based on the room-temperature investigations of the Verdet constant. Concerning the temperature dependence of the Verdet constant, it influences the polarization rotation angle distribution particularly in the magneto-active media with a high absolute value of dV/dT, e.g. those operating at cryogenic temperatures (the Verdet constant is approximately proportional to ~1/T, see the equations 10 and 12). Further, the studies dealing with the temperature dependence of the Verdet constant are usually restricted to only a few wavelengths (most often only one). Hence, these studies already address a relatively narrow range of the Faraday devices, i.e. those operating at cryogenic temperatures on some specific wavelength. Because of this, we have decided to include the studies on the temperature dependence of the Verdet constant only as the “additional references” to the individual materials and focus more on the initial step: the examinations of the Verdet constant dispersion at room temperature. These reports became very frequent recently and that is why we decided to review them primarily.
D) The part devoted to the review of materials used for magneto-optical devices is presented in a form inconvenient for analysis. The separation of parameters by spectral ranges is not very clear. In my opinion, specifying the working range and values of the constant Verde for typical wavelengths is more convenient than that represented in this paper.
Our response: The authors have carefully reconsidered the current version of the material overview, and decided not to change the way how the material parameters are organized, as we will explain on the following lines.
Originally, we had considered the presentation in the form of working ranges of the individual materials, as the reviewer#2 suggests. This arrangement, however, had a serious drawback: the working ranges were found to be hardly presentable in a clear/lucid form. The first issue is that it is hard to define the working range of a material per se. The absorption/transmission spectra for the covered materials are often incomplete or missing, making the definition of their working ranges difficult. As an example, we may mention the fluorides, which were studied for the UV region (PrF3 or LiREF4) [56], but they also have transparency windows in the MIR. The magneto-optical properties, however, have not been studied for the MIR range yet for these materials. The second issue is that a great portion of the covered materials possesses some absorption lines within the UV, VIS, NIR or MIR range. Hence, defining the working range of material would eventually mean to specify its transparency windows, leading to a massive table, which would hardly be lucid for any reader.
To address this issue, we switched to a different grouping of the materials, which presents the materials’ Verdet constant data in a different way. The spectral ranges in the paper are selected in a way that is more application-based and reflects the current situation of the Faraday materials research in general. The main core is the 400 – 1100 nm range, which is traditionally “occupied” by the TGG crystal in the application area. This range corresponds, for example, to the emission lines of the Nd and Yb-doped laser gain media and their second harmonics. As we explain in the manuscript, there is a vast number of reports on new materials, which aim to supersede the TGG in any of the material properties relevant to the high-power Faraday device. It is, therefore, convenient to group these materials together, so that the potential of these materials’ could be more easily assessed by using the listed models of the Verdet constant dispersion or by comparing the values of the Verdet constant at the selected wavelengths. Additional testing of the materials will be of course needed for the full assessment by the figure-of-merit parameter.
Another group of materials is that for the spectral range above the 1100 nm wavelength. Although the Tb-based materials (the TGG and most of its potential successors are Tb-based) are transparent up to ~1600 nm wavelength, above the 1100 nm, the YIG material is already transparent and possesses magneto-optical properties superior to the Tb-based materials. That is why the YIG is widely utilized for Faraday devices in telecommunications, medical applications, etc. The variety of materials which could be used for the >1100 nm range is very small compared with the TGG-range and it is better to discuss it individually what could actually be done in the Faraday material research in this region. The same argument holds also for the UV region – not many materials are transparent in this range, as most of the compositions possess dominant absorption lines in this region. This is better to discuss it individually.
According to this response, we have modified the text in the introduction part of the overview section to explain to the readers our motivation regarding the overview structure. We believe that in this arrangement, the overview should be clearer and enable the analysis of the covered materials. We also refer to the other two reviewers, which ranked the structure as well organized.
E) Also in the article, magneto-optical glasses are discussed, but any data of glasses are not presented.
Our response: Yes, this is correct. We have added some of the glass-based magneto-active materials to the material tables in the overview part. In the UV region, we have switched from the previously listed 193 nm wavelength to 248 nm, enabling to compare more glass-based materials with the listed crystals. We have added some of the referenced glass materials to the VIS+NIR section as well.
Reviewer 3 Report
No suggestion. It's OK in its present form.
Author Response
The authors would also like to thank Reviewer #3 for the work he/she spent on reviewing our manuscript and for the positive feedback, we received from him/her.
Round 2
Reviewer 2 Report
In my opinion, this article is not an overview of magneto-active materials developed for high-power Faraday devices. It is a mixture of original research (part 2) with an overview (part 1 and 3). Part 2 of the Characterization of the Verdet constant as a function of wavelength and temperature does not describe the various methods for determining the Verde constant at different temperatures. The changes made by the authors after the first review do not answer key questions for high power laser application, for example, the dependence of constant Verde on power density. I believe that this article needs to expand the part associated with the experimental determination of Verde constant in powerful laser systems. At least the minimum requirements for radiation resistance and absorption at the working wavelength for such devices should also be described. From the article does not follow any specific application of magneto-optical materials specifically for high-power lasers.
Author Response
At first, the authors would like to thank the Reviewer #2 for his/her efforts giving a detailed review of our manuscript. We have tried to follow his/her suggestions where it was possible. Our answers to the reviewer’s specific comments are addressed below as in the previous round. For easier orientation in the revised manuscript, we have, once again, highlighted the implemented changes to the manuscript with a red colour.
A) In my opinion, this article is not an overview of magneto-active materials developed for high-power Faraday devices. It is a mixture of original research (part 2) with an overview (part 1 and 3). Part 2 of the Characterization of the Verdet constant as a function of wavelength and temperature does not describe the various methods for determining the Verde constant at different temperatures.
Our response: We have carefully reviewed the manuscript again in an attempt to find any major part presenting original research in the section devoted to the “Characterization of the Verdet constant as a function of wavelength and temperature”. To the best of our knowledge, the information in the manuscript is pure reviews of the yet-reported findings or small-scale analysis using standard calculus as we will describe in the following lines.
The experimental part of the characterization method has already been described in the Refs. [44 - 48]. The only expanded parts are:
a) The discussion over the detection systems, which provides readers with additional examples of how to adjust the experimental setup according to their current needs (e. g. adjusting the effective spectral range of the measurement). A standard Jones calculus had been used for the analysis of these detection systems.
b) The expression for the measurement error, which may be obtained by a standard error/uncertainty calculus. We added this expression for this review to underline the fact that there are different contributions to the error of the measurement, which is useful for a detailed analysis of the measurement error, providing useful inputs for further development of the presented method.
Concerning the section devoted to the model function for the Verdet constant dependence on the wavelength and temperature, its content had been merged from the theory of the Faraday rotation in solid-state compounds containing rare-earth paramagnetic ions described in the Refs. [1, 45, 48, 50 - 53].
However, from our understanding, this comment from the reviewer suggests that the section devoted to the characterization method lacks discussion of the additional methods which may be used for the Verdet constant characterization. To provide the reader with these missing information, we have extended the introductory part of the characterization section. In the updated version, we inform the reader about the presented method (described in section 2) in the context of the other commonly used methods for evaluation of the Verdet constant. We have also extended the discussion on the limitations and drawbacks of the method, providing additional references to the available literature describing the alternative methods, which could be used for the characterization of the Verdet constant when the presented method in section 2 may not be used. Finally, the authors believe that this extended introduction of the characterization methods should be satisfactory for giving a context to the reader interested in the additional Verdet constant measurement methods.
We, however, wish to be focused on describing the presented method in more detail since it represents a very general method for evaluation of the Verdet constant, which could be adjusted to the particular needs of individual Verdet constant studies.
B) The changes made by the authors after the first review do not answer key questions for high power laser application, for example, the dependence of constant Verde on power density. I believe that this article needs to expand the part associated with the experimental determination of Verde constant in powerful laser systems. At least the minimum requirements for radiation resistance and absorption at the working wavelength for such devices should also be described.
Our response: We have carefully reviewed the comments & answers from the first round of the responses to the reviewer’s #2 comments. We believe that the changes to the manuscript were adequate to the reviewer’s comments on the high-power laser benchmarking of the magneto-active materials. Specifically, reviewer #2 has asked about the LIDT data (comment A) and about the temperature dependence of the Verdet constant (comment C) of the reviewed materials. As we discussed in the answers to the previous round of comments, we provide a reader with “Additional references” in the overview section, leading to additional studies of the listed materials (some of them containing the temperature dependence of the Verdet constant or LIDT data, or additional parameters relevant to high-power Faraday materials benchmarking via the magneto-optical figures of merit). The high-power benchmarking related data are for the vast majority of the yet-reported magneto-active materials still unknown. That is one of the reasons why is this review focused mainly on the yet-reported Verdet constant investigations, highlighting which of the additional data for the material benchmarking are still missing and needs to be investigated.
Regarding the reviewer’s new suggestion (to include the topic related to the dependence of the Verdet constant on the power density), as far as we know, this area has not been very extensively investigated in the available literature. One of the reasons may be that the high-intensity lasers, for which the Verdet constant would exhibit the nonlinear behaviour, are experiencing a larger-scale boom just in recent years. Further, concerning the operation of high-power Faraday devices, it is desirable to avoid the nonlinear regime of the Faraday rotation, since it would raise another complication in addition to the thermal effects. The description and the experimental determination of the Faraday rotation in such a regime would be very complex and beyond the intended scope of this review article. The authors agree that such a topic would be very interesting to study in the future, but there is no reason to include it to this manuscript now.
Similarly, including the discussion over the requirements for radiation resistance and absorption is out of the scope of the manuscript. We would like to remind that this point was already discussed in the previous round in the points A) and B) of the responses. We have changed the title, abstract, and introductory part of the overview and of the characterization section (which is now new) in a way that the concept of the review should be more clear: The manuscript focuses on an introduction to the Faraday devices and the thermal effects associated with their high-power operation. Then, a method for characterization of the Verdet constant/Faraday rotation is described in detail and finally, we give a brief overview of the available materials based on their Verdet constant, which is of great importance for comparison between the materials and for their further research. Furthermore, the requirements on the LIDT and absorption greatly differ among the Faraday devices in the laser systems. It strongly depends on the particular application, beam parameters, cooling scheme, etc. Also, the LIDT testing of optical components for high power lasers focuses mainly on testing of the coatings as their resistance is usually much weaker than that of the bulk material. More information about this matter may be found in the following references:
· Mariastefania De Vido, P. Jonathan Phillips, Joachim Hein, Jörg Körner, Jodie M. Smith, Klaus Ertel, Paul D. Mason, Saumyabrata Banerjee, Oleg Cheklov, Thomas J. Butcher, Stephanie Tomlinson, Andrew Lintern, Justin Greenhalgh, Waseem Shaikh, Steve J. Hawkes, Cristina Hernandez-Gomez, Malte C. Kaluza, and John L. Collier "Influence of polishing and coating techniques on laser induced damage on AR-coated ceramic Yb:YAG", Proc. SPIE 9237, Laser-Induced Damage in Optical Materials: 2014, 92371M (31 October 2014); https://doi.org/10.1117/12.2068037
· Štěpán Uxa, Václav Škoda, Jan Vanda, and Mihai-George Muresan "A comparison of LIDT behavior of AR-coated yttrium-aluminium-garnet substrates with respect to thin-film design and coating technology", Proc. SPIE 10805, Laser-Induced Damage in Optical Materials 2018: 50th Anniversary Conference, 108051R (16 November 2018); https://doi.org/10.1117/12.2500189
Supported by the positive feedback from the other two reviewers, the authors would like to keep the present structure of the revised manuscript.
C) From the article does not follow any specific application of magneto-optical materials specifically for high-power lasers.
Our response: The specific applications of magneto-active materials, or, more specifically, of the Faraday materials, in the high-power laser systems are mentioned in section 1.1 dedicated to the “Faraday effect and its applications”: “The FDs are used in the laser systems particularly for the multi-pass amplification and regenerative amplifiers as well as for optical isolation of one part of the system from another by eliminating possible harmful back-reflections. This makes the FDs indispensable components of any high-power laser system.” Among these, the Faraday isolators are probably one of the most often researched Faraday-effect-based magneto-optical devices in the high-power laser systems. More information about these applications is given in the cited literature sources throughout the whole manuscript, e. g. [7 - 9, 12 - 16], or in the sources cited in the material overview part (section 3).
Round 3
Reviewer 2 Report
In my opinion the article can be published in present form. However, if you completely remove parts 1.3 and 2, then the practical usefulness will not change.
