Demodulation Technique Based on Laser Interference for Weak Photo-Acoustic Signals on Water Surface

Round 1

Reviewer 1 Report

The authors present a novel signal demodulation technique for interferometric measurements of underwater sound fields. The work is interesting and relevant.

Some minor issues could be improved:

- In Figure1 it would be intersting to zoom out the high frequency components (or maybe use a logarithmic scale), since they can hardly be distinguished from the line of the axis.

- The characteristic ratio should be explained in more detail, in order to bettwer gasp its significance. Also, the parameter m is not explained in Equ.2. J0 and J1 probably refers to the Bessel functions, but this should clearly be stated in the text.

- In the characteristic ratio based approach, the amplitude is estmated using a look-up table. The authors should give some details about this look-up table. What are the relevant parameters? How was the table generated?

-Figure3: The scheme as well as the experimental details should be given in more detail.

- The authors should aso comment some more, on the practical relevance of their results, which prove the principle quite nicely. However, extension to a measurement in a realistic environment seems not trivial to me.

- Some typos.

Author Response

Dear Reviewer:

We appreciate the valuable suggestions and comments from you. We studied them very carefully and tried our best to revise our manuscript. Detailed revision and explanations are in the file.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper presents a Demodulation Technique Based on Laser Interference for Weak Photo-acoustic Signals on Water Surface.
It is an interesting paper; however, I have some comments written below.

Abstract: acronym WSAW is used without writing water surface acoustic waves.
Always remember to include a spacing between a number and unit, 10kHz should be 10 kHz

Keywords: Include: Optical interference, water surface acoustical waves

The introduction is OK however a discussion of prior art with respect to signal to noise is lacking.

Materials and methods:
Eq. 1 directly describes interference signal from a perfect Michelson interferometer without describing the background of using this signal. One should start describing the surface vibrations + noise and thereafter use this signal as the phase modulation of the optical signal (two-beam interference). Consider use an illustrating figure.

Page 2 line 71: This sentence is not understood: The above formula receives a trigonometric function expansion and a Bessel function simplification in turn…..??

Figure 2: How is this figure generated (simulation parameters are missing)
Page 3 line 101: This sentence is not understood: Spatial modulation depth can be estimated from the light intensity distribution of different diffraction lights in their spatial frequency distribution pattern??

Page 3 line 105: What is precisely meant with P (amplitude of components in spectrum…)?
Page 4 line 116: The look-up table….? Not at all understood, please describe

Figure 3: provide more describing figure text (self explaining!).
Page 5, line 137, write the eq. tan(Theta)=U1s/U1c. In general: use equation instead of formula!

Figure 5: Nice with pictures, however all images are too small and blurred. Please improve significantly

Figure 6: the vertical axis has different ranges – therefore difficult to compare!

In general, a detailed explanation of the optical/mechanical set-up is lacking, e.g. laser type/wavelength?

Table 1 should be on the same page. I suggest making a plot of the frequency difference error with standard deviation as function of frequency (box plot) for better visualization of results.
Page 7, line 179. Why is e.g. 1 kHz and 1.5 kHz not tested? (I understand that 2 kHz is chosen as threshold frequency, however, the performance at these “lower” frequencies is not documented.)

Table 2: Title: Selected amplitude measurement results
The structure of the table is unclear and not well described. I need more explanation of all rows and columns and in figure 7 indicate the two sigmas, 1.63 and 1.31 with a circle or similar in order to connect table and figure. And in the updated figure 7 write that the two circles indicate the two sigmas from table 2….

Page 209 line 208. The fitted equation y=26.29sin()…..: This can be presented more elegant and scientific correct and what is x – symbol not described. Would it not be possible to use e.g. Hilbert transform for envelope detection?
Figure 8. Figure text: write what the amplitude depth is in the experiment.

Conclusion
It is from the conclusion not clear which improvement that has been specifically obtained in this work compared with previous/others work.
What is the signal to noise S/N ratio of the tested system?
Page 9 line 232: What filter performance are discussed?

General comments:

I do miss a simulation of the system based on figure 3, with known modulation frequency, noise amplitudes and noise frequency spectrum and of course known signal amplitude and frequency. In this simulation the S/N could also be tested (and compared with experiment). Why is 10 kHz chosen for modulation frequency?
Why is the reference modulation carried out in a water tank and not internally in the interferometer using an optical phase modulator?
What’s the similarities between your interferometer approach and laser Doppler interferometry where a high frequency reference modulated beam is also used?

Author Response

Dear Reviewer: We appreciate the valuable suggestions and comments from you. We studied them very carefully and tried our best to revise our manuscript. Detailed revision and explanations are in the file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thanks for the revised manus, which has improved significantly.

Regarging “respons 5”, i suggest that the deviations given in the letter could be placed in an appendix in order to clarify that issue.

Author Response

Dear reviewer：

Thank you very much for your attention and consideration.

We have already added an appendix in the paper.

Best wishes!

Author Response File: Author Response.pdf