Sub-Nanosecond, High Peak Power Yb:YAG/Cr⁴⁺:YAG/YVO₄ Passively Q-Switched Raman Microchip Laser with the Emission of Multiple Pulses

Round 1

Reviewer 1 Report

PHOTONICS

Manuscript ID: photonics-2787260 Sub-nanosecond, high peak power Yb:YAG/ Cr4+:YAG/ YVO4 passively Q-switched Raman microchip laser operated in a burst mode.

Submitted to section: Lasers, Light Sources and Sensors,

Comments and Suggestions for Authors:

This paper presents Sub-nanosecond, high peak power Yb:YAG/Cr4+:YAG/YVO4 passively Q-switched Raman microchip lasers (PQSRMLs) at 1134 nm operated in a burst mode under quasi-continuous-wave (QCW) pumping. The results found are very good, because they are suitable for generating high repetition rate pulses due to a compact planar-planar laser cavity, resulting in a short round trip time in the cavity. However, the paper needs to consider the following comments:

1.     Introduction. It is suggested that the authors carry out an in-depth review of the state of the art. It’s important to reference relevant works to enhance understanding and acknowledge existing knowledge in the field. The state-of-the-art is very poor.

2.     Proposed method. It is understood what the challenge is posed in the manuscript, but it is suggested that the authors describe the proposed method in more detail to achieve the stated objectives.

3.     Experimental results. It is suggested that in each figure that includes subsections a), b), etc., each subsection be explained separately, this would give greater clarity to the explanation. See e.g. Fig. 2, 4, 5, 7, 8, 10, 11.

In general, I consider that it is necessary to improve some aspects related to the Introduction, state-of-the-art, Proposed Methodology, and Figures of the Results. The novelty and contribution are clear and excellent results.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The material presented in the manuscript  can be regarded as interesting and informative. However, in my opinion, the presented analysis of the results contains a number of unfounded and erroneous statements and requires a serious revision.

 The insets to Figures 1a,b show the temporal structure of the optical pump signal. R.R. is used for pump pulse repetition rate designation while the R.R. number is not available in the text. In the following, R.R.  denotes the repetition rate of the generated pulses during the pump pulse. This creates some confusion.

 It should be noted that, in accordance with the description of both experimental schemes, there is no resonator for the Raman laser at a wavelength of 1134 nm, since the rear mirror on the input surface of Yb:YAG is specified only up to 1100 nm (lines 91 and 113). Therefore, the YVO4 crystal works more like an intracavity Raman converter of the Yb:YAG/Cr:YAG microchip laser, rather than a Raman laser. Moreover, in this case, the 11 and 16% output mirrors simply attenuate the output radiation by ~9 and ~6, respectively.

 Obtaining “a perfect mode match between the laser mode and the pump beam” (lines 122-123) must be justified.

 Position of a longpass optical filter which “was applied to separate the Raman laser and the fundamental laser” should be shown on the schemes in the Figures 1a,b.

 The lasing threshold in Figures 1 and 6 is determined incorrectly (lines 170 and 242). There must be a value at the intersection of the graph with the x-axis.

 In my opinion, the authors use the term of “slope efficiency” not quite correctly. This usually means the ratio of useful losses to the total losses per round trip of the resonator. The authors provide values, but do not analyze them. While they actually include 1134 nm radiation losses on the mirror in the estimate, which leads to an underestimated value of the slope efficiency.

Also, in this case, the authors persistently emphasize the presence of a thermal lens, which introduces increasing losses with increasing pump energy. In conditions of changing losses, the concept of the slope efficiency loses its meaning.

 In statements “Raman thermal lens effect is proportional to the time-averaged power density of the first Stokes laser” (line 176-177) it is not clear what the authors meant.

 The statement “These instabilities are mainly attributed to the current fluctuations of squire pumping” (lines 233-234) looks unfounded.

 The meaning of the expression “The dependence of Raman crystal length on the Raman laser output…” (line 239) is not clear.

 Figure 11 looks wrong: the time scale at the top looks strange; the bottom picture does not correspond to the “window” highlighted in the top picture.

 It seems to me that the term “burst” is not appropriate in this case. It is mainly used to denote a sequence of pulses separated by fixed intervals, such as the resonator time travel.

English quality should be improved. The text contains syntax errors, missing words etc.

Line 138: "...fixed to 90%"

Line 139: "...fixed to 0.9ms"

Line 148: "residual fundamental laser... were detected"

Line 159-160.

Line 200: "...crystal can reduces..."

Line 250: "...total raman laser out energy..."

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

1. The scale in the upper plot of Fig. 11 seems to be wrong. The caption needs to be improved for clear description.

2. Some original references for using YVO4 crystal as Raman gain medium should be mentioned. 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The manuscript has been significantly improved by the authors and can be published if several comments are taken into account

1. To more clearly distinguish between the two types of repetition frequency, it would make sense in the inset to Figure 1 to designate the repetition frequency as QCW R.R. or something like that.

2. In Figure 1, it is better to replace "fundamental laser" by “fundamental wavelength laser”.

3. The text still does not indicate that the Yb:YAG input face reflects radiation at 1134 nm. Although, in their response to the reviewer, the authors confirmed this fact and even provided an explanatory drawing, which, however, was partially disavowed by the subsequent text. It is necessary to either include this explanatory figure as an inset to Figure 1, or in the text near line 124, or 146, or somewhere else to simply indicate the approximate reflectance value of the Yb:YAG input facet at a wavelength of 1134 nm.

4. The value T0 entered on line 230 must be defined at first mention.

5. The meaning of the word “dramatic” in the context of the changes in instability and duration described in this manuscript is not entirely clear. Replacing, for example, with “essential” would seem appropriate.

6. I would recommend replacing the red rectangle at the top of Figure 11, for example, with an oval. The rectangle appears to be a time window with a width of ~0.08 s, which does not correspond to the bottom figure, which has a range of 1 ms. In the case of an oval, such an association, in my opinion, would not arise

Sincerely,

Reviewer

The meaning of the word “dramatic” in the context of the changes in instability and duration described in this manuscript is not entirely clear. Replacing, for example, with “essential” through the whole text would seem appropriate.

Author Response

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Author Response File: Author Response.pdf