382 mW External-Cavity Frequency Doubling 461 nm Laser Based on Quasi-Phase Matching

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript reported a 461 nm external cavity frequency doubling laser based on a periodically polarized KTP crystal, with an output power of 382 mW and a frequency doubling conversion efficiency of 66.2%. The parameters of this laser are quite good as an external cavity frequency doubling blue laser. Meanwhile, in the manuscript, the mechanism of the thermal effect in crystal induced by frequency doubling blue laser was discussed in a new perspective. A multi-layer crystal model was proposed to analyze the thermal effects, and results were consistent with experimental data. Compared with the traditional calculation of crystal thermal effect in frequency doubling blue lasers, the model proposed in this manuscript is better agreed with the experimental situation. The thermal model in this manuscript is more accurate in theory for calculating the focal length of the thermal lens, which is quite important for the design of an external cavity frequency doubling lasers.

In summary, the article provided a more accurate theoretical model for calculating the thermal effects in crystal of external cavity frequency doubling blue lasers, and demonstrated a high power, high efficiency and high beam quality 461 nm laser. Therefore, this manuscript is recommended to be published in Photonics.

Before the manuscript is published, the authors are expected to explain and revise the following issues:

1. In the paragraph in line 257, on page 7, the article described the maximum output power of the laser was 460.5 mW when the oven temperature was reduced. What should be the maximum output power under ideal conditions, i.e., without taking into account the influence of thermal effects? The output power reduction due to thermal effects should be added to the text.

2. Some textual and grammatical problems:

 (1) In line 37 on the first page, in view of the following text, the author should have expressed several popular kinds of optical clocks, not several popular optical clocks.

 (2) In line 43 on page one, “method” should be in the plural form.

 (3) On page 2, the company name need not be italicized.

 (4) In line 127, on page 3, “n” should be subscripted and the types of lasers represented by corner symbols 1 and 2 should be stated.

 (5) The “-1” in “0.003 cm-1” in line 149, on page 4, should be superscript.

 (6) There should be a space in “28.6°C” in line 232, on page 6.

 (7) In line 323, on page 9, “Before the laser coupled into...” should be “Before the laser was coupled into...”.

 (8) In line 384, on page 10, “After the thermal stable established...” should be “After the stable thermal state was established...”.

 (9) In line 407, on page 11, “Figure. 9” should delete the dot between Figure and 9.

 (10) In line 408, on Page 11, “there is obvious...” should be “there are obvious...”.

 (11) The paragraph formatting of the references on page 12 should be modified according to templet.

Comments on the Quality of English Language

The language of the manuscript can be improved by a native English speaker.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript by Chen et. al reports the production of 382 mW of laser radiation at 461 nm by second harmonic generation (SHG) for ultracold Sr experiment applications, in particular optical lattice clocks (OLC). The manuscript presents a theoretical analysis of the SHG process and its thermal effects, followed by an experimental implementation of this system. The SHG strategy adopted is based on intra-cavity SHG by means of a PPKTP nonlinear crystal.

It is difficult to understand the originality of this work, or its advancement, with respect to Ref.[31]. Although the authors claim that their work can advance OLC technology beyond the state-of-the-art towards miniaturized and portable blue laser systems, they to not provide any hints from their work about how to reach this goal. Their theoretical work is not fully original, and as said before, their results are derivative and expected (probably a little more engineered than Ref.[31]). They do not take into consideration the advancements in direct blue diode laser production (see list of minor comments and questions below) and do not evaluate how to outmatch this very competitive solution. Finally, there is no account about the cavity stability, the power degradation over time, and the lifetime of the crystal, all issues that makes the resonant SHG very good in metrological (or high-profile research) laboratories, but not suitable for field deployable OLCs.

Regarding the quality of the presentation, the level of the English is below the minimum for acceptance of a scientific publication. The abstract should be revised to better indroduce the reader to the scope and the perspectives of the paper. It also presents grammar errors (see the list of minor comments and questions at the end of this report). The introduction and their references should be updated with more recent advances (see for instance  J. N. Tinsley et al., "Watt-level blue light for precision spectroscopy, laser cooling and trapping of strontium and cadmium atoms," Opt. Express 29, 25462-25476 (2021) ). As a general comment, the bibliography should be expanded and updated.

In view of the above criticism, I consider this manuscript not suitable for publication unless extensive changes are applied.

Minor comments and questions

- Abstract:

"For the rapidly improvement of strontium optical lattice clocks [...] is on demand."

> For the rapidly improving strontium optical lattice clocks [...] is highly required.

"can induce thermal effect" > can induce thermal effects

- pp. 2

"Toptica photonics has also commercialized a tunable 461 nm ... "

The quoted is not the only commercially available diode laser at 461 nm delivering 200 mW power or more.

", but the power still can not meet demand"

The authors should explicitly specify the power requirements for a transportable OLC. To see other transportable setup, the authors should refer to Yogeshwar B Kale et al 2022 Quantum Sci. Technol. 7 045004 and references therein.


- pp. 10

"and the frequency doubling efficiency was obtained by calculation." > This is obvious, so this sentence may be dropped.

Comments on the Quality of English Language

Regarding the quality of the presentation, the level of the English is below the minimum for acceptance of a scientific publication (see the list of minor comments and questions at the end of the main referral report).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors report on the realization of a setup for frequency-doubling in order to generate light at 461nm for use in strontium cold atoms experiments and optical clocks. The manuscript begins with an extended theoretical part aiming at modelling thermal effects, such as phase mismatch or lensing, in the doubling crystal due to absorption at 461nm. The authors argue that a careful characterization of the thermal phase mismatch is essential for reaching high conversion efficiencies. After that the authors present their setup and provide a brief characterization of the doubling efficiency as a function of temperature. Careful optimization of cavity parameters and crystal position allow for mitigation of thermal effects to some extend.

The theory and the experimental procedure are not new. The setup is standard in every respect, as well the way its characterization. At least, no point is made on innovation. Since the invention of the lasers many labs have mounted doubling cavities and characterized them in similar ways. The authors used a commercial laser (Toptica DLpro) and a commercial tapered amplifier (Toptica TApro) to seed the frequency doubling unit. They reach 382mW output power, which is less than the >1W reached in a Toptica SHGpro. 

 The paper has not been written with sufficient care. It reads mostly like a status report converted from a PhD thesis. Theory and experiment are not sufficiently connected. The necessity of the extensive theoretical model for the experiment is not sufficiently motivated. I am sceptic about its use for other groups aiming at constructing similar units. In  my opinion, in its present shape the paper should not be published.

Some specific remarks:

1) Blu-ray lasers have a typical wavelength of 405nm. What is the relationship to frequency-doubled 922nm lasers?

2) All figure captions are totally insufficient.

3) The authors present in Fig.7 a locking curve without saying what technology was used for locking, Pound-Drever-Hall, lock-in, Hänsch-Couillaud?

Comments on the Quality of English Language

The paper is not well written. It is full of grammatical mistakes making the lecture difficult and tedious. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript improved after the authors considered the reviewers' comments. Although the results are not earth-shattering, their publication might be envisaged after another round of thorough linguistic revision.

Comments on the Quality of English Language

I wish authors wouldn't leave the task of polishing their manuscripts to reviewers, who might really resent the burden. Nevertheless, there are still many grammatical mistakes. Maybe the editor will take care?!