Focusing of a Laser Beam Passed through a Moderately Scattering Medium Using Phase-Only Spatial Light Modulator

Round 1

Reviewer 1 Report

Dear authors

The authors conducted experimental investigations to improve the focusing of a laser beam passing through a medium. It is of interest because of the numerous applications to be developed in several disciplines.

The objective of the article is clearly presented, and the contents are well organized. Also I have enjoyed reading the article because of the well organized structure of contents and pedagogical approach, i.e examples in Figure 1, 2 and 3.

Thus I think that this paper may be published in Photonics. However, some minor changes are required previously

General comments:

1.Reference 28 is not cited in the paper.

2.In line 114 you write: “Based on the Montecarlo simulation performed, we found out that there is a big portion of the quasi-ballistic photons that passes through the turbid medium and reaches the detector”. But, please can you explain and write an analysis of the impact in your results of this simulation. Otherwise, what is the point of performing a set of numerical simulations?

3.Line 262: You write: “It can be noticed that the efficiency in focusing is much higher for higher concentration value”. I agree, but “it can be notices is very vague”. It would be better introducing a measurable parameter. I imagine that you can obtain much better parameters, but only as an example you may obtain:

4.Line 260: Why is not reduced the diameter of the focal spot from 6.8·10⁵ mm^-3 to 8.2 concentration?.

English and writing:___________________________________________________

Line 26-26: “If the medium absobs light – energy is lost, there is nothing to be done”

I understand the sentence, but the style might well be improved. Please, delete the hyphen and write a sentence in scientific style: Energy is lost, therefore ….

Line 57-60: You write 4 lines between hyphens. Please, could you write it as different sentences joined by prepositions in order to improve the scientific style.

Line 158-161 and Line 154 are exactly the same words. I imagine that you used copy nd paste. Please remove one of the paragraphs.

Line 165-169: Figure 4: Please include comments on figure c)

Line 218: Please give reference for the formula you employ

I hope my comments may help you to publish this work

Regards,

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

Thank you so much for your kind words, we really appreciate that!

Please, use the attached PDF file with our responses, which contains figures.

Comment: “1. Reference 28 is not cited in the paper.”

Response: We have added the reference to the text, thank you.

Comment: “2. In line 114 you write: “Based on the Montecarlo simulation performed, we found out that there is a big portion of the quasi-ballistic photons that passes through the turbid medium and reaches the detector”. But, please can you explain and write an analysis of the impact in your results of this simulation. Otherwise, what is the point of performing a set of numerical simulations?”

Response: We decided to provide the reference to the related work and not to explain in details the numerical analysis we made because it is already described in our previous paper about bimorph mirror usage. But probably it was a mistake.

So, the idea of the numerical simulation was rather simple. The implementation of the Monte Carlo model was the first necessary step, because based on this model we created our own model of Shack-Hartmann sensor to measure the distortions of scattered light (section 2.3 of the manuscript). And during the Monte Carlo simulation we found out an interesting result that we used further. It was discovered that there are relatively big number of quasi-ballistic photons reaching the sensor for the scatterers’ concentration range we used.

We have updated the figure 2b of the manuscript.

Comment: “3. Line 262: You write: “It can be noticed that the efficiency in focusing is much higher for higher concentration value”. I agree, but “it can be notices is very vague”. It would be better introducing a measurable parameter. I imagine that you can obtain much better parameters, but only as an example you may obtain:”

Response: Yes indeed, we totally agree with you. We provided the ‘before’ and ‘after’ values of integral intensities in the first version of the manuscript, but did not provide the ‘efficiency increase’ parameter. Though in the paragraph before we also provided the values showing how the diameter of focal spots decreased. But the parameter you suggested is a good one, we will use it.

Comment: “4.Line 260: Why is not reduced the diameter of the focal spot from 6.8·105 mm-3 to 8.2 concentration?.”

Response: Thank you for this comment. We have included the explanation of this phenomenon in the manuscript. It can be noticed that the decrease of the focal spot diameter for the concentration value of 6.8×10⁵ mm^-3 is lower than the one for the concentration value of 8.2×10⁵ mm^-3 (16.4% versus 13.2%). It is due to the impact of multiply scattered diffuse light. Figure 2b (we provided it here in the response to the second comment) clearly shows that the number of diffuse photons becomes higher than the number of quasi-ballistic and ballistic photons since the concentration value becomes higher than 8×10⁵ mm^-3. In other words, the integral intensity in the focal spot can become higher due to the optimization, but the diameter of the focal spot is decreasing more slowly because a diffuse light unavoidably broadens the focal spot.

Comment: “Line 26-26: “If the medium absobs light – energy is lost, there is nothing to be done” I understand the sentence, but the style might well be improved. Please, delete the hyphen and write a sentence in scientific style: Energy is lost, therefore ….”

Answer: Thank you, we have corrected this sentence.

Comment: “Line 57-60: You write 4 lines between hyphens. Please, could you write it as different sentences joined by prepositions in order to improve the scientific style.”

Answer: Thank you, we have corrected this paragraph.

Comment: “Line 158-161 and Line 154 are exactly the same words. I imagine that you used copy nd paste. Please remove one of the paragraphs.”

Answer: Yes indeed, thank you for finding this out. We have removed the duplicated paragraph.

Comment: “Line 165-169: Figure 4: Please include comments on figure c)”

Answer: It was a typo, thank you. We did not include the description of the figure (a). We have updated the comments on figure.

Comment: “Line 218: Please give reference for the formula you employ”

Answer: We have added the reference to the formula

Author Response File: Author Response.pdf

Reviewer 2 Report

The author reports on the compensation of phase by SLM to improve the focusing conditions in the special case of "moderately scattering medium". The paper is written clearly and smoothly.  The topic has been well investigated in the previous paper of the same author by other techniques such as bimorph mirror [Ref from 34, 35, 37, 38] etc, the methodology part is very well presented but brings no new information. The paper should offer at least the following information before it can be considered for publication.

Main problem:

The main problem is the lack of supporting experimental results. In the results part, I see very limited results presented, which might be very confusing for the readers. I strongly suggest to put more details, like:

1. Section 2.2: Where are the measured wavefront results by the Shack-Hartmann sensor? Fig. 4 is not well discribed, such as what are these black ground and white dot? What are the scale bar? 
2. Results part: a) What is the beam shape before/after the shaping? b) What is the phase pattern used for the SLM? c) What are the intermediate results during the optimization loop? d) 3.1×105 mm-3, 5.1×105 mm-3, 6.8×105 mm-3, and 8.2×105 mm-3  are mentioned, why the results only show 2 of them?

This information will make the results more abundant and convincing.

Author Response

Dear reviewer,

Thank you for your comments. Please, use the attached PDF file with our responses, which contains figures.

Comment: “Section 2.2: Where are the measured wavefront results by the Shack-Hartmann sensor? Fig. 4 is not well discribed, such as what are these black ground and white dot? What are the scale bar?”

Response: Thank you for this comment.

The wavefront measured by the Shack-Hartmann sensor are presented on the Figure 4a. This chart shows the dependence of Zernike coefficients of symmetric distortions (defocus and spherical aberrations) on the scatterers’ concentration value. Since the Mie scattering by spherical particles has symmetric nature, the distortions were also represented only by centrally symmetric Zernike polynomials. It is true both for the simulation (for obvious reasons) and for the experimental measurements. Of course, there were non-symmetrical distortions during the experiment due to misalignment of optical elements and other reasons. But these distortions were negligible.

We did not provide the 2D or 3D representation of the measured wavefront surfaces due to the fact that they look similar to each other (the only difference is the amplitude of the surface — we pointed it out in the text of the revised manuscript.

White dots on Figure are focal spots generated by the photons that passed through the scattering medium, fell on the microlens array and were registered on the detector plane. Black ground is just the area of the sensor plane that was not illuminated (the number of photons fell on this area was negligible).

As for the scale bar — we have added it to the figure 4b now. The diameter of each microlens of the Shack-Hartmann sensor was equal to 150 um.

Comment: “Results part: a) What is the beam shape before/after the shaping? b) What is the phase pattern used for the SLM? c) What are the intermediate results during the optimization loop? d) 3.1×105 mm-3, 5.1×105 mm-3, 6.8×105 mm-3, and 8.2×105 mm-3  are mentioned, why the results only show 2 of them?”

Response: Thank you for this thorough comment.

a) We have added the shape of the beam before and after optimization for the concentration values 3.1×10⁵ mm^-3 and 8.2×10⁵ mm^-3 on the figure 7 (a, b, c).

b) We have added the SLM phase patterns on the figure 7 (d, e, f). 

c) In the results and discussion section we presented only the results that corresponded to the minimal and maximal scatterers’ concentration values considered in this paper. In our opinion intermediate results actually brings no new information to the reader.

As an example, in the attached PDF you can find the encircled energy charts for all of the concentration values considered in this work (we provided (a) and (d) charts in the manuscript).

d) As we described in the paper, we used glass cuvette filled with the distilled water as a ‘container’ for the scattering medium. To make the scattering medium, we injected the drops of the prepared suspension of polystyrene microbeads with known concentration inside this glass cuvette filled with water. Thus, we have limited number of concentration values (equaled to the number of drops) of the suspension used in the experiments. And we displayed the results that corresponds to the minimal and maximal concentration values only.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, the authors propose a strategy to improve the focusing efficiency of laser beam propagating through a moderately scattering medium by using phase-only SLM. The wavefront distortion of the scattered beam is estimated based on Shack-Hartmann principle, and then compensated by optimizing phase patterns loaded on SLM. The modulated focal spot is demonstrated with increased integral intensity and decreased diameter. There are some issues to be addressed before this manuscript can be accepted for publication.     

1. The authors should explain the novelty of this work compared to their previous work in Reference 38. Does the only difference between them lie in replacing bimorph deformable mirror with SLM? What is the advantage of SLM compared with deformable mirror in compensating the wavefront distortion?
2. Some experimental details are missing, for example, the parameter of the sensor used in distortion measurement.
3. The caption of Figure 4a is missing. And what does PV in Figure 4c mean?
4. The authors should check whether the flow map in Figure 6 agrees with the description in the manuscript. According to Page 7, the value of Zernike polynomial is changed whether the merit function becomes better or worse until the best value is obtained, then the procedure is repeated for the next polynomial. But in Figure 6, the procedure goes to the next Zernike coefficient if the merit function becomes worse.
5. In Page 4, the texts in Line 158-164 already appear in the last paragraph.
6. Apart from the function of focusing improvement, SLM shows vast potential in various applications, like micro/nano fabrication [ACS Nano 14(5): 5233−5242 (2020)]. Brief introduction to the relevant applications of SLM can broaden the reading interests of this work.

Author Response

Dear reviewer,

Thank you for your comments. Please, use the attached PDF (that contains figures) with our responses.

Comment: “The authors should explain the novelty of this work compared to their previous work in Reference 38. Does the only difference between them lie in replacing bimorph deformable mirror with SLM? What is the advantage of SLM compared with deformable mirror in compensating the wavefront distortion?”

Response: Thank you for this comment.

Basically, there are a few differences between the current and previous work in Reference 38.

First of all, of course we made the separate experimental scheme with different optical elements due to the different apertures of the SLM and bimorph mirror.

Secondly, in the presented experiment we placed the cuvette with the scattering medium after the SLM. In the previous work we placed the cuvette before the bimorph mirror. It was necessary because the SLM ignored part of the scattered radiation with the “wrong” polarization. The bimorph mirror does not depend on the light polarization. From this point of view these two experiments are differ from each other, because SLM made pre-compensation of the wavefront of the coherent light while the bimorph mirror (in the experiment described in the previously published paper) compensates for the distortions of the averaged wavefront of partially coherent light.

Thirdly, in this manuscript we provided more thorough theoretical analysis and explanation of the place of this particular work amongst other related works (i.e., figures 1 and 2). In some sense, this work elaborates our previous paper.

Finally, as you stated in the comment, another difference between this manuscript and the manuscript published previously in Optics Express is indeed lies in replacing the bimorph mirror with the SLM.

Comment: “Some experimental details are missing, for example, the parameter of the sensor used in distortion measurement.”

Response: We have updated the manuscript with the sensor parameters information, thank you.

Comment: “The caption of Figure 4a is missing. And what does PV in Figure 4c mean?”

Response: It was a typo, thank you. We have updated the Figure 4a caption.

Comment: “The authors should check whether the flow map in Figure 6 agrees with the description in the manuscript. According to Page 7, the value of Zernike polynomial is changed whether the merit function becomes better or worse until the best value is obtained, then the procedure is repeated for the next polynomial. But in Figure 6, the procedure goes to the next Zernike coefficient if the merit function becomes worse.”

Response: Thank you for pointing this out. We have updated the figure 6 with the correct flow map.

Comment: “In Page 4, the texts in Line 158-164 already appear in the last paragraph.”

Response: It is a type, thank you. We removed the duplicated paragraph.

Comment: “Apart from the function of focusing improvement, SLM shows vast potential in various applications, like micro/nano fabrication [ACS Nano 14(5): 5233−5242 (2020)]. Brief introduction to the relevant applications of SLM can broaden the reading interests of this work.”

Response: This is a good note, thank you. We have added the proposed reference and improved the introduction section of the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors fixed my concerns.