Phase Compensation Method in OPA System Based on the Linear Electro-Optic Effect
Krishna Murari
Round 1
Reviewer 1 Report
This work introduces a new method for phase compensation in nonlinear materials, with potential applications in high-power laser amplification. The results obtained in this work may be interesting for the photonics community. The field of the manuscript is quite attractive, and the experimental results are convincing. I would recommend the work for publication, after revision of some issues.
My major comments on the paper are as follows:
- When referring to polarization, I would suggest using “polarization axes” and “principle polarization axes” instead of “axes” and “principle axes”, according to the standard nomenclature.
- Figures 1, 2, 4, and 5 shows the conversion efficiency versus different parameters. However, the conversion efficiency has not been defined. Since this is a key value in this work, authors should explain which parameters has been taking into account to define the efficiency, and how is experimentally measured. This is a very important point, since I can imagine different manners to calculate the efficiency. I suppose that this efficiency is defined as I_signal/I_pump, but the normalization makes difficult to take these plots as conversion efficiency curves.
- Also related to last point: I understand that the use of normalizad efficiencies help to evaluate the overall performance of the samples. However, it would be a good idea to give the ration between maximum efficiency with and without compensation. Although the results show a promising improvement in terms of tolerance, the actual value of conversion efficiency is also an important value. In addition, experimental results (Figs. 4 and 5) are notably coincident with the predictions of the numerical calculations. A proper evaluation of the results should include the actual efficiency (without normalization).
- 3 can include more information, such as the properties of the picosecond laser (central wavelength, spatial/temporal width, peak intensity…). The DKDP can be also labeled. I cannot find information about the detection stage, and maybe it can be included in Fig. 3 (as well as in the text). For example: pump intensity is measured after or before the DKDP?
- The results show a linear dependence of the applied voltage. The required DC voltage is quite elevated (10 kV). Where is the limit of this compensation? Authors give some explanation about this idea in the Discussion section, but I would appreciate a deeper analysis.
As a summary, my overall opinion about the manuscript is good. The results are quite promising, and because of that, I think they deserve a rigorous analysis. Principally, I would like to know an actual value of the conversion efficiency, besides a deeper analysis of the results.
Author Response
Dear Reviewer：
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Phase compensation method in OPA system based on linear electro-optic effect”. The comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:
- We revised the description and replaced “principle axes” and “principle axes” with “polarization axes” and “principle polarization axes”.
2.and3. In the manuscript we added a definition of conversion efficiency. Since the injected signal light energy is relatively high, and in order to display the acceptable bandwidth, we define the conversion efficiency as . We have replot Figure 1.2.4.and 5, which cancels the normalization of conversion efficiency. The actual conversion efficiency can be seen directly.
4.We revised Figure 3 and added more information, such as pulse duration, wavelength, energy and a Power Meter. The DKDP have been labeled in Fig.3. Since the repetition rate of the laser is 1kHz, the corresponding single pulse energy can be calculated by measuring the laser power with a Power Meter.
- In the revised manuscript, we added a discussion of the limitations of this compensation method and ways to overcome the limitation.
Once again, thank you very much for your comments and suggestions.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The introduction rather gives the impression that the authors will present methods to compensate the phase match stability at 800 nm while in this work the signal wavelength is 1064 nm.
In section 2: The rotation of the principle axis depends on the amplitude of the electric field. It is not true that a constant field would generally rotate the axis by 45 degrees.
Could you please explain the conversion efficiency? What do you mean by OPA efficiency = 0 or 1?
In theoretical figures 1 and 2, what are the data points? I understand that the continuous curve should be the simulation, but I do not understand what the points are. In Fig. 1(b) there are bumps on the yellow curve at delta_T ~ -6 and +6 C, that are not associated with any points…
Line 86, base on --> based on.
Line 107: Compared with the 95%-deuteration DKDP the 70%-deuteration DKDP requires a higher voltage value to compensate phase-mismatch.
A figure with the compensation voltage of 95 % sample and the 70 % sample overlaid is welcome and it also clarifies your claim.
Line 110 and the rest: Your explanation is very qualitative and very ambiguously explained. You could at least write the values of the refractive index and the electro optic coefficient for each sample.
Section 3: Figure 3. This figure should be modified What are the angles theta and alpha? What is the orientation of the electric field? Please write down the pulse energy and duration on the optical paths.
Line 144: the T ranges are the same for the curves shown in figure 4 (a) and , (b) though.
Line 148 and similar places: replace the term bandwidth full width with --> Full width at half maximum.
Line 151: in order to show that the “voltage changed more evidently with the temperature” please show two curves overlaid.
Figure 4 (b), there exist a shoulder on the curve at about 5 degree C that does not correspond to any experimental point. How did you extract this behavior? The same question holds for figure 5 (b): on the yellow curve at 0.04 degree and on the blue curve on the interval of -0.06 to -0.04 degree.
According to my experience, for the phase matching angle a precision > 0.05 degree is really sufficient to have adequate conversion efficiency. I am curious to know how a phase mismatch of 0.01 degree would drop the conversion efficiency from 1 to almost 0.6 …
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript titled “Phase compensation method in OPA system based on the linear electro-optic effect” presents a phase compensation method for the phase mismatch occurring in an OPA due to a rise in temperature or mechanical deformation in nonlinear crystals. This method of phase compensation could be quite useful if the authors could implement this technology and present a comparison study for different nonlinear crystal. The article is very well written and the authors have given proper background of the problem. I would highly recommend publishing this article. My few minor concerns are as follows and I hope the authors can have them improved in the manuscript.
- In the introduction section the authors compare the size vs nonlinearity of DKDP and LBO. It would be better to have a rough comparison factor of nonlinearity difference in the two crystals and show how despite lower nonlinearity of DKDP it can be useful if it is grown in a bigger size. Nevertheless, the citations should be improved. There are more published work in literature in this direction which is not mentioned in the paper.
- The axis number and labels are two small. The voltage axis has too many numbers, it can be reduced. Also its not standard to have the units after “/” rather it should be in brackets.
- In the motivation, the authors say that they have chosen DKDP as it can be manufactured to size up to 300 mm but in the manuscript, they have only used 30 mm size crystal. Why didn’t they go for LBO which is highly used due to their large nonlinearity? If this method can be applied to LBO as they say towards the end of the manuscript, why didn’t they go for LBO in the first place?
Author Response
Dear Reviewer：
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Phase compensation method in OPA system based on linear electro-optic effect”. The comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:
- In the latest manuscript we cited more references about LBO and DKDP.
- The axis numbers and labels have been appropriately enlarged after adjustment. The number of voltage axis has been reduced. And we use “()” instead of “/” in the labels.
- The experimental content of this article is mainly to verify the feasibility of this method. Due to the constraints of our group, we do not have a 300mm diameter DKDP crystal, so we can only use the existing 20mm diameter crystal as the research object. However, it is sufficient to verify the feasibility of this method. Since the electro-optical coefficient of LBO is not very clear (few articles report specific data), there will be uncertainty when using LBO as the research object, we chose DKDP as the research object. However, we are sure that this method can be used in LBO amplifier for phase mismatch compensation.
Once again, thank you very much for your comments and suggestions.
Round 2
Reviewer 1 Report
Authors have correctly addressed al the suggestions made by the reviewers. The article can be accepted. However, I suggest a revission of the English language in the new sentences added to the current manuscript (i.e. caption of the new Fig 3, "Calculated the perfect compensation voltage (...)" should be rewritten).
Author Response
Thank you very much for your comments and suggestions. The caption of the new Fig 3 and fig 7 have been rewritten in the new manuscript.
Reviewer 2 Report
I thank the authors for having improved the manuscript. It is reads better now. Here below, you may find my comments:
Line 113: grammar à to the changes of the refractive index.
I also notice that the explanation has turned into an even more ambiguous one. In the previous version of the paper the discussion was as follows:
“When the electro-optic coefficient is a constant, materials with large refractive index are more sensitive to changes in refractive index with respect to electric field intensity. The refractive index of DKDP increases with the increase of the deuteration rate. Moreover, the electro-optic coefficient of 95%-deuterated DKDP is slightly greater than 70%-deuterated DKDP. Under the combined effect of factors, the 95%-deuterated DKDP requires a lower voltage for phase mismatch compensation”
First, I would write the first sentence in this way: The refractive index variation under the effect of the electric field is more important for materials in which this index is large. (or something similar…)
Second, if the refractive index and the EO coefficient are both larger for the 95% sample, I would conclude that the delta_n is larger for this crystal with respect to the 70 %-doped one. Therefore, I conclude that in order to compensate the effect of the delta_n, one should apply a higher voltage. In the paper, the opposite conclusion is drawn. I kindly ask the authors to clarify this statement or to simply remove it.
Figure 4, caption: You could improve the caption and the description of the figure.
Line 128: The diameter of the laser beam (and not the picosecond!).
Line 199: Miximum and not maxi-mum.
I repeat my previously asked question: I According to my experience, for the phase matching angle a precision > 0.05 degree is really sufficient to have adequate conversion efficiency. I am curious to know how a phase mismatch of 0.01 degree would drop the conversion efficiency from 1 to almost 0.6 … Is it because the DKDP crystal is such sensitive?
In the cover letter, the authors reply: In the figures, the dots on the curve are the data obtained by simulation calculation or experimental measurement. The connection lines between dots are just for aesthetics, it has no actual physical meaning. Even though such a precision in absolutely redundant for the conversion efficiency, the curves should be representative of the physical mechanism and should not be manipulated for the sake of “aesthetic”. I there kindly ask the authors to remove the bumps and show only the connection between the points.
Comments for author File: Comments.pdf
Author Response
Thank you very much for your comments and suggestions. Please see the attachment.
Author Response File: Author Response.pdf
