Research Progress on Tunable External Cavity Semiconductor Lasers in Visible and Near-Infrared Wavebands

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The presented manuscript “Research progress on tunable external cavity semiconductor lasers in visible and near-infrared wavebands” is a comprehensive review of existing work and achievements on  cavity semiconductors and infra-red waveband technologies in electronics. In some sense, I believe it is also a historical review which provides all the details  of the research advances on the subject. 

 

The paper is useful for a broad readership representing all the important information in a relatively brief form. Therefore, I recommend this paper for publication in Coating provided the authors address the following comments and suggestions.

 

Most of my suggestions for improvement will concern the abstract.  When I read the abstract it was really difficult to understand what is the main message and what the authors are trying to do here.  Instead of using way too much space to explain the importance of the considered problems and physical phenomena,  you should start with a clear mention of what you actually do: a comprehensive review of the known important results. 

 

Do you have any original results on this paper?  Did you provide actual scientific research which is presented here?  If so it must be clearly presented and mentioned in the abstract. 

iThenticate shows quite a large percent of the coincidence with existing text.  The situation is pretty normal for a review paper but I urge the authors to paraphrase and not completely repeat existing materials.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this review, the authors have considered the main approaches to creating tunable laser sources. The review provides the main characteristics of tunable lasers of various designs, and within each approach, the authors consider several implementation options, which is undoubtedly useful for obtaining the most complete information in the area under consideration, since each of the proposed solutions has its own advantages and disadvantages related to stability, linewidth, spectrum tuning range, and output power. However, there are a number of comments related to the need to supplement the review with information that will allow readers to get a clearer idea of the features of the approaches under consideration. The work may be accepted for publication if appropriate corrections are made.

1. It makes sense to add information on the results demonstrated for powerful tunable semiconductor lasers to the Littrow type structure TECSLs section. It is important to note the physical features that occur when operating in continuous or pulsed modes. For example, in the work [https://doi.org/10.1364/AO.58.009089] the maximum continuous power of 13 W (line width of 0.15 nm) in continuous mode for a laser in a scheme with an external resonator was demonstrated, and the tuning range reached 100 nm. Limitations of the maximum power were associated with overheating of the active region. In the pulsed mode in the work [doi.org/10.1364/JOSAB.384971] the achieved powers are significantly higher (45 W at a current of 100 A, line width of 0.15 nm), however, the limitation is associated with the occurrence of generation in the modes of the intrinsic Fabry-Perot resonator, despite the existing enlightenment. I think that it makes sense for the authors to add information that would describe similar features of the operation of lasers in an external resonator for Littrow type structure TECSLs.
2. The review would benefit significantly if the authors added information on the technical characteristics of the key elements of the optical schemes under consideration. For example, the type and characteristics of diffraction gratings (e.g., reflection coefficients, losses, number of lines), requirements for optical schemes based on collimating lenses, requirements for semiconductor crystal designs (strip type, mirror reflection coefficients).
3. It would be clear if the authors explained the formulas used with pictures, i.e. showed the lengths, angles and other parameters from the formulas given in the article in the pictures.
4. It makes sense to check and correct the units of measurement: kilohertz to kHz, decibels to dB, nanometers to nm and so on
5. In the Filter type structure TECSL section, the authors provide various types of filters, and in order not to confuse readers, a more detailed description of the filters used should be given, since it is not clear from the information provided, for example, how the wavelength can be tuned in a wide range (tens of nm) using a Fabry-Perot filter. Fig. 4 shows the scheme for Filter type structure TECSL, where the wavelength is tuned by tilting the filter, and this is a different type. In Table 3, the type column should be supplemented with the filter type.
6. In the Fiber type structure TECSL section, the authors should add information on how to solve the problem of expanding the tuning range compared to the usual fiber Bragg grating (FBG) structure and clarify these filter types in the corresponding column of Table 4.
7. In the Waveguide type structure TECSL section, the authors provide descriptions of tunable lasers based on the use of planar waveguides with spectral selection elements. Just as for other sections, there is a lack of a quantitative description of the technical characteristics of the structures used for spectral selection, for example, the MRR diameters. In addition, in this section, the authors do not describe how SOA and a planar optical waveguide serving as the external resonator are matched, what approaches to light input, and what losses are acceptable to achieve the desired line width and tuning range. Also in Table 5, in the type column, you should indicate what type of spectral selection element was used.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper reviews the progress of different configurations of tunable external cavity semiconductor lasers operating in visible and near-IR.

It could be potentially interesting, but several issues must be considered before it can be accepted for publication. The most important one refers to the (lack of) drawings showing the operating principles of the TECSL configurations. More precisely, although the authors have included figures to illustrate different configurations, these figures are EXTREMELY sketchy and, as such, irrelevant. A representation of a roundtrip in each case could help, as can the inclusion in the figures of the laser parameters found in the formulae

When it comes to formulae, these are in short number and not very helpful since the parameters are not represented in figures and/or some notations are not explained. Examples: in (2) lambda_q is not defined, assuming its the same as lambda in (3), in (4) we do not know if n is a refractive index or an integer, the phase-matching factor is not defined, and so on....

Then, the authors use a lot of acronyms but sometimes these are not defined in the text. Examples: SMSR, IF, etc. Please make sure that ALL acronyms are explained.

In line 285 the reader assumes that l/mm means lines per mm, but actually it reads as liters/mm. Please use lines/mm. Then, in lines 394 and 435 it is indeed mold selection or mode selection??

Finally, the manuscript should be greatly improved if the authors would explain the reasons of the very large differences in the parameters of some laser of the same type in their tables. For instance, in Table 1, the tuning range varies between 8.6 nm and 360 nm. How are these very different values obtained (by what means) and what would be the advantage of a lower tunability designed if compensated by other meaningful parameters, for instance power? 

The reader should understand not only the basic principles of different configurations of TECSLs but also what to do to get the optimum configuration for his/hers specific needs. These different designs are very briefly compared in the Conclusion section, but the comparison should be extended

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This work presents a review of the research process regarding various TECSL type structures. They analyze the advantages and disadvantages of different external cavity structures. Additionally, they explore future deployment trends of TECSL. The authors confirm that TECSL has achieved significant improvements in terms of wide range, narrow linewidth, high SMSR, and high output power. They indicate that in the future, TECSL will continue to evolve towards high performance, miniaturization, and multifunctional integration to meet the demands of emerging applications.

• Most figure need improvements especially the resolution.
• The authors indicate that this structure is capable of generating 3.05 watts of power when a drive current of 7 A is applied. With watt level output power and outstanding thermal stability, it is ideal for high-power applications that demand precise wavelength control. Authors need to present and clearly articulate these phenomena that influence the performance of the system.
• Authors should explain the phenomenon of thermal stability well. This phenomenon is very sensitive for systems.

In conclusion, the work presented by the authors is very attractive for applications in the field of photonic integration.

 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I have carefully studied both the new version of the manuscript and the authors’ responses,  and I concluded that the authors have done a very good job improving the manuscript and addressing all the referees’ comments and suggestions.  Therefore, I believe the paper could be accepted for publication in Coatings and the present form.