Effect of Wavefront Distortion on the Performance of Coherent Detection Systems: Theoretical Analysis and Experimental Research
Round 1
Reviewer 1 Report
The authors studied theoretically and experimentally the effect of wavefront distortion arising from atmospheric turbulence on the mixing efficiency, bit error rate and mixing gain of coherent detection systems. Further, the improvement of the wavefront correction for tilt, defocus, astigmatism, coma, and spherical distortion on the performance of coherent detection systems have been discussed in detail. The content of the manuscript is quite clear and interesting. I recommend that the manuscript can be published in Photonics after it is revised according to the following comments.
1. How far does the signal light travel through the atmosphere in the experiment? How about the repeatability of the measurement results in the experiment? For example, In Fig.11, Is it the average result of multiple experimental measurements or not?
2. The analysis of Fig.7 and Fig.8 seems somewhat rough. The authors just provided some detailed information from the figures and the further analysis should be carried on.
3. The authors discussed the effect of the defocus-spherical distortion class, tilt-coma distortion class and astigmatism class of wavefront distortion separately. However, further comparison between them should be provided.
Author Response
I am very grateful to the reviewer for reviewing my article and carefully reading your review comments. Based on the review comments, I have provided corresponding responses in Word.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript has conducted theoretical analysis and experimental research on the impact of wavefront distortion in free space coherent optical communication on the performance of coherent detection systems, which is of guiding significance for the application of adaptive optics to correct wavefront distortion in coherent detection systems. The paper is clearly stated, and the simulation and experimental data are detailed. The following issues still need to be considered and supplemented by the author:
1. Figure 10 shows the Zernike coefficients of the wavefront after correction in different situations, and describes a transient amount. Please provide the correction process, such as the variation of the peak valley value and root mean square value of the wavefront with the number of iterations, to correspond to the results in Figure 11.
2. How the wavefront distortion caused by atmospheric turbulence is simulated in the experiment, such as how D/r0=2 in Figure 11 is generated？
3. The conclusions given are too lengthy and are requested to be edited and condensed appropriately by the author.
Author Response
I am very grateful to the reviewer for reviewing my article and carefully reading your review comments. Based on the review comments, I have provided corresponding responses in Word.
Author Response File: Author Response.docx
Reviewer 3 Report
This is a carefully done study, and the findings are a bit interesting. A few major revisions are list below.
This paper first presented the concept of mixing efficiency based on signal to noise ratio in coherent optical detection. A mathematical model between the mixing efficiency and phase error was proposed. This model was used to evaluate the quality of IF signals under influence of wave front distortion, which caused by atmosphere turbulence. The correlation between mixing efficiency and BER was briefly presented. Secondly, the relationship between mixing efficiency and distortion factors was simulated, which included defocus-spherical distortion, tilt-coma distortion and astigmatism distortion of X direction and Y direction. The relationship between correction order and mixing efficiency affected by wave front distortion was also simulated. The optimal correction order was analyzed when the wave front distortion relatively weak. At last, FSO coherent detection system with deformable mirror was established. The wave front distortion due to turbulence was expressed by Zernike coefficient. In the closed loop control experiment, the mixing efficiency was tested at different sampling points. The IF signal amplitude displayed by the oscilloscope was tested. The main innovations of this paper were as follows. (a). This article simulated the attenuation law of mixing efficiency under the influence of different types of wave front errors, and the effectiveness of correction. (b). Experimental verification of some optimal parameter solutions for correcting wave front errors corresponding to simulation under closed-loop control was done.
It is a topic of interest to the researchers in the related areas, but the paper needs very significant improvement before acceptance for publication. My detailed comments are as follows:
1. The error rate formula cited was valid under certain limitations and did not have universality. However, most of the simulation content of error rate in the article was only based on a simple transformation of mixing efficiency. It should be noted that the core research content of this article was based on the wave front distortion described by Zernike polynomials, and the direct and key impact parameter on the system was mixing efficiency. Therefore, the description of bit error rate was not necessary (theoretical and simulation results). We suggest deleting it.
2. After describing the impact of phase distortion in Section 3.1, the "correction" was suddenly introduced in Figure 5 in Section 3.2, but it did not specify what the correction was doing. On the other hand, it did not specify the initial distortion state from which the correction started (including all types of distortion, or a certain type of distortion, or a certain matrix parameter), and only the initial value of turbulence intensity was given in this paper. we suggest author to explain the differences and connections between the concepts of atmospheric wave front distortion generation and atmospheric wave front distortion correction, and to supplement the purpose of this section, as well as the initial state of Figure 5.
3. There was a liquid crystal spatial light modulator in b) of Figure 9, which was not shown in a), and the purpose of the device was not explained in this paper.
4. The description in Figure 10 was not clear enough, and we could not determine whether it was a corrected coefficient or an uncorrected coefficient. The comparison description before and after correction was not clear enough.
5. Figure 11 lacked an uncorrected mixing efficiency curve.
Author Response
I am very grateful to the reviewer for reviewing my article and carefully reading your review comments. Based on the review comments, I have provided corresponding responses in Word.
Author Response File: Author Response.docx
Reviewer 4 Report
This study establishes a mathematical model of wavefront distortion based on the coherent detection theory with mixing efficiency, BER, and mixing gain. It also analyzes the improvement limits of wavefront correction on mixing efficiency, BER, and mixing gain under different atmospheric turbulence conditions and experimentally measures them. The paper provides a thorough analysis of the effect of wavefront distortion on the performance of coherent detection system The authors establish a mathematical model and conduct both numerical simulation and experimental research to analyze the improvements in limits of wavefronts correction on mixing efficiency, BER, and mixing gain under different atmospheric turbulence conditions. This paper can be accepted if the following comments are addressed.
Comments and Suggestions for Authors:
[1]- Under the influence of atmospheric turbulence investigate all the atmospheric scintillation and the impacts of atmospheric turbulence and pointing errors, especially in all turbulence conditions such as weak, mild, moderate, and strong turbulences for the wavefront correction order required for homodyne detection and heterodyne detection, owing to the higher requirement for spatial phase matching. The changes in the effective beam width for the high-order wavefront correction orders and the influence of low-order wavefront correction on the mixing efficiency to become the mixing efficiency more stable and improve the quality of coherent optical communication in free space. Can the authors provide more detail regarding the experimental setup used to measure the improvement limits of wavefront correction?
[2]- The variation in the heterodyne detection mixing efficiency and the changes in the effective beam width and coherence length under varying atmospheric refractive index structure constants can explain in more detail the amplitude of the astigmatism class distortion for (a) evaluation of correction low and high order on mixing efficiency (b) evaluation of correction order on BER to achieve high system mixing efficiency and improve the bit error rate (BER) due to the periodic extension of wavefront distortion. Is there any potential for further optimization or improvement of the coherent detection system beyond wavefront correction, and if so, how might this affect the results presented in this study?
[3]- Clarification and justification are required for the effect of the low-order wavefront correction in the homodyne detection on the mixing efficiency and BER at the receiver side of the system is the same as that in the heterodyne detection. What are the improvement limits of the wavefront correction?
[4]- The results show that correction wavefront distortion can significantly improve the mixing efficiency and reduce the bit error rate. The authors also evaluate the effect of wavefront distortion on the mixing gain at the receiver and determine the best distortion correction for wavefront distortion that can be provided within the limits of receiver aperture size. The experimental results validate the theoretical analysis and demonstrate the effectiveness of wavefront correction. However, there are a few areas that could be improved. Firstly, the abstract does not enough the specific methods used in the experiments, which may limit the reproducibility of the results. The abstract of the manuscript should describe the main issues addressed by the researchers, the proposed technique, and a summary of the results. It would be helpful to include more details in the experimental setup and methods used to correct the wavefront distortion.
[5]- Are there any practical limitations or trade-offs in terms of cost, complexity, or size of the correction system, and how might these affect the overall performance of the detection system?
[6]- It is helpful when describing the experimental setup to include details about the equipment and used testing conditions (e.g., the distance between transmitter and receiver, and atmospheric conditions during testing).
[7]- It would be interesting to know how the optimal wavefront correlation orders were determined for each type of distortion.
[8]- The references should be of good quality and up to date. They need quality improvement, to be applicable and more effective.
[9]- Add more references that are useful and applicable and appropriate and may increase the importance and quality of the research. References should be improved to be more applicable and practical.
https://doi.org/10.3390/photonics9060426
https://doi.org/10.3390/photonics10020168
https://doi.org/10.1109/ACCESS.2019.2924531
Qian Guo, Shuang Cheng, and Xizheng Ke, "Experimental Study of Large-amplitude Wavefront Correction in Free-space Coherent Optical Communication," Curr. Opt. Photon. 5, 627-640 (2021)
Author Response
I am very grateful to the reviewer for reviewing my article and carefully reading your review comments. Based on the review comments, I have provided corresponding responses in Word.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
We recommend that authors work on image and text layouts to keep pages organized.
Reviewer 4 Report
The authors addressed all comments clearly. I recommend acceptance of the paper for publication.
