Optical Accelerometers for Detecting Low-Frequency Micro-Vibrations

Round 1

Reviewer 1 Report

The paper gives a review of optical vibration sensors. In particular four types of sensors are discussed: Optical path modulation; Optical intensity modulation; Optical phase modulation; Optical wavelength modulation

Strengths and weaknesses of all listed types of sensors are discussed. The presentation is ordered and supported by insightful illustrations.   The review is well ordered and gives an updated account of the recent advances in the field; The figures help to understand operational principles of the sensors;  

The weaknesses are: Figure quality could be improved even though they are well though over;
Lack of a section with a comparative experimental study of all sensors;  

I have not noticed any significant issues with English or the presentation side in general but I have some suggestions for minor corrections:

1. Figs 5, 6, 8, 9 should be shifted to the right
2. In table 5 would it be possible to add a row with a cost estimate and som ecomments on robustness, i.e. can the sensor be deployed in a lab only or outside of a lab environment also?
3. There are fonts in some figures that are very difficult to read: In Fig.1 the plot on the far right, Fig.2 inset, Fig.5 the bit on the left, Fig.17, Fig.18b

Other than this the paper is well organised and fairly easy to read whilst giving the reader a lot of information on the state of the art in the field of optical vibration sensor. I therefore recommend publication of the paper subject to minor revision.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors wrote a brief review of selected types of accelerometers with optical readout. They classify them according to their light modulation mechanism to optical path-, intensity-, phase-, and wavelength-modulated devices. They erroneously (and repeatedly) claim that no other reviews on such devices have been previously published (see point 1 below). They chose rather specific types of optical accelerometers and presented them (mostly devices utilizing geometrical optics), skipping whole classes of devices (namely those based on wave optics and evanescent fields). Some erroneous general claims related to the state of the art are made (these are not scientific errors, to be precise, but solely incomplete information). Within the mentioned limitations, the text is rather well written, scientifically sound and relatively well organized.

The paper could be made acceptable for publication upon major revisions, after the missing parts are introduced in the text and after the erroneous claims have been dealt with. It could serve as a quite useful reference paper, if properly corrected and expanded.

Below I give a point-by-point list of my major concerns.

1. In Abstract the authors claim the lack of any “systematic evaluation of currently available optical accelerometers” and in Introduction they write “a systematic review of optical accelerometers has remained elusive”. Yet this reviewer has found some good quality systematic reviews of optical accelerometers in recent literature, mostly giving a broader, more comprehensive and more systematic surveys of the devices of interest than the present article. The authors are advised to look up the following sources: a) Q. Lu et al, “Review of micromachined optical accelerometers: from mg to sub-μg”, Opto-Electronic Advances, Vol. 4, No. 3, Art. No. 200045, pp 1-24, doi: 10.29026/oea.2021.200045, Aug 2021; b) B. Malayappan, P. K. Pattnaik, “Optical MEMS Accelerometers: A Review”, Ch. 6 in Microelectronics and Signal Processing: Advanced Concepts and Applications, ed. by S. Goel, doi: 10.1201/9781003168225, CRC Press, Boca Raton, U.S., June 2021; c) Malayappan, B., Lakshmi, U. P., Rao, B. P., Ramaswamy, K., Pattnaik, P. K. “Sensing Techniques and Interrogation Methods in Optical MEMS Accelerometers: A Review”. IEEE Sensors Journal, Vol. 22, Iss. 7, pp. 6232 – 6246, doi: 10.1109/JSEN.2022.3149662, 2022.
2. The present review skipped several important classes of optical accelerometers, mostly those making use of the wave optics. One of the missing classes are the devices based on photonic crystals. I believe the authors will not have problems to find appropriate references related to this class. Also, the review is missing the devices based on optical near fields/evanescent waves, for instance those based on surface plasmon polaritons and attenuated total reflection. Further, there are wave optics-based devices utilizing optical metamaterials, nanoplasmonics and generally subwavelength nanophotonics.
3. Generally, a more physics-based approach to the basics of operation of the devices would be more than welcome, elucidating the underlying mechanisms and principles rather than considering the devices as “black boxes”.
4. Generality and systematicity are lacking even among the quoted devices based on geometrical optics. From some reason, the authors chose to present only certain subclasses while avoiding the related ones. For instance, in the part on optical path modulation they insisted on surveying devices based on the pre-fab building blocks of voice coil motors and DVD pickup heads instead of analyzing in a comprehensive manner all MOEMS-based and generally microfabricated and nanofabricated accelerometers based on this approach.
5. Why did the authors quote and consider only a very narrow group of applications (inertial navigation, structural health monitoring and active vibration isolation systems) when there is an enormously broader field of accelerometer applications related to widely different subjects in practically all fields of human activity? To mention just a few, a majority of the existing accelerometers are used in vehicular technologies (e.g. air cushions in automotive) and telecommunication (smartphones), also in homeland defense. An important field would be green energy sources (wind turbines, vibration energy harvesters). Why avoid these? Please be consistent and present possible uses in a systematic and comprehensive manner. Mind that there is an enormous number of applications in these fields and that at least a small fraction merits to be mentioned if a systematic approach is a goal.
6. While writing about the light modulation methods in optical accelerometers the authors keep repeating “Optical” with all of them. They write “optical intensity-, optical phase-, optical wave-length” What wavelengths and phases there exist in optical accelerometers except the optical ones?
7. In several places deep subwavelength resolutions of 1 nm and lower are mentioned. I believe such deeply subwavelength resolutions strongly exceeding Abbe limit merit at least some physical/optical explanation. Kindly include it.
8. It is written that the accelerometers used in structural health monitoring “must achieve high accuracy with a small size”. This demands a clarification – why the sensor size must be “small” in these literally gigantic systems?
9. Could the authors check if the permission to reproduce the artwork of other teams includes the open Creative Commons license.
10. There is a slight mix-up with the terminology. The optical modulation scheme is described as the “optical measurement principle”. Also terminologically, what is the “the accuracy of photodetectors” that “can reach the nanometer or even sub-nanometer scale” – photodetectors have their standard parameters that include sensitivity, quantum efficiency, specific detectivity and NEP – how do you define the “accuracy” of a photodetector (which by definition does not represent a device trait, and may only denote a system property which you then must define).
11. In line 229 the authors mention a “nano-grating based on photolithography”. They are kindly requested to elucidate what kind of photolithography ensures nano-resolutions an what “nano” really means in this context.
12. In section 4 the authors describe fiber-based devices only. What is the situation with other types of phase-sensitive accelerometers?
13. It is it written that capacitive accelerometers have a “zero frequency response” – please explain the meaning of this. Depending on their size, capacitive sensors can be fabricated rather small, fast and responsive. A problem is their sensitivity to electrical interference.
14. It is incorrectly written that all electronic accelerometers (piezoelectric, capacitive, and piezoresistive) exhibit low sensitivity. In reality that property critically depends on the sensor geometry/dimensions and material choice (thinner membranes will exhibit higher sensitivities, with a peak sensitivities achievable with nanomembranes and optimized seismic masses)
15. I did not notice that the authors wrote that basically all optical accelerometers share a common trait with their electronic counterparts – they use some kind of proof mass (seismic mass), which influences their inertial properties and sensitivity in the identical manner to that encountered in conventional devices.
16. I also did not notice that the authors wrote about the inherent problems with optical devices, including the requirement for relatively complex and large readout blocks, which strongly reduce their applicability and increases the overall sensor dimensions in many cases. There are method to avoid this at least partially and I believe the authors should add an elucidation of this important subject.
17. The authors write “high buildings sway when strong winds blow through them” – I guess they wanted to say “when strong winds blow around them”. Also, there is a much more important factor, namely the earthquake proofing of buildings, bridges, highways, etc. Please elaborate.
18. For some types of accelerometers the authors quote as one of their deficiencies the thermal dependence of their parameters, while for other types, that are obviously as sensitive to temperature changes (if not more) nothing is written. A systematic approach would require to consider thermal dependences for all the investigated devices, probably with an intercomparison. Otherwise, it is rather nonsensical to consider a parameter in a single sensing device type and skip all the others.
19. There are some unclear or simply unwieldy formulations, for instance “mutation-tapered” or “external excitation acceleration”, “elastic structure made of elastic material”
20. There is an disproportionally large number of older references (even 30 years old or more) in a text dedicated to modern optical devices. Last time I was informed, a recommendation for MDPI reviews has been that at least 50% of references needed to be less than 5 years old. An advice to the authors is to try and increase the number of recent references, if possible by including as many as possible those published in 2021 and even in 2022 – it can only help in keeping the review up-to-date.
21. Also related with references: it is suggested to the authors to include doi numbers for all of their references and to remove the superfluous notation ([J], [D], [C] and [M]).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

In the revised version of their already good quality paper the authors have made substantial improvements, have removed most of my concerns by adding the missing parts, accepting my suggestions and clarifying their claims. In my opinion the paper is now acceptable for publication.

Only few minor omissions remained that could be corrected even at the page proof stage:

Line 313: “with a sensitivity as low as” should be “with an accelerometer sensitivity as low as” (because another sensitivity, that of photodetector, given in A/W, also appears in this same paragraph, only one sentence before the above quoted text)

Line 491: it still remains unclear what “mutation” means in the context of an accelerometer.

Line 538: the authors use the abbreviation “PhC” and in all other places they use “PHC”

Line 552: It is written “FHC” instead of “PHC”.

Line 556: It is written “FHC” instead of “PHC”.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf