Hybrid Photonic Integrated Circuits for Wireless Transceivers

Response to Reviewer 1 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions highlighted in the re-submitted files.

2. Point-by-point response to Comments and Suggestions for Authors

Comment 1: While the manuscript outlines the development of hybrid photonic integrated circuits (PICs) for wireless transceivers, the descriptionand discussion of key results (e.g., simulation outcomes, experimental performance metrics) is overly concise and lacks critical details. For instance, Section 2.1.2 Figure 4 shows the typical characteristics of an InP MZM, the corresponding textual description and discussion are overly brief and lack critical details, weakening the technical contribution.

Response 1: Thank you for pointing this out. We agree that the original discussion of the InP MZM results was too concise. In response, we have expanded the description and discussion of the experimental performance metrics associated with the InP MZM in Section 2.1.2. The revisions have been highlighted in red in the manuscript (page 6, line 200 to 209).

Comment 2: The manuscript contains inconsistencies and formatting errors in the reference citations, which compromise readability and academic rigor. For example, most of the journal titles are abbreviated (e.g., J. Lightwave Technol.), while others are written in full (e.g., Applied Sciences in Ref. [14][16]...). Other issues regarding the format include journal title duplication, e.g. Ref [46] contains a duplicate journal title “Optica”. The authors should adopt a consistent abbreviation style to enhance readability and compliance with academic standards. The authors should cross - check all references against a reliable abbreviation database.

Response 2: Thank you for pointing this out. We have double-checked with the citation tool database and made sure the abbreviation style is consistent (page 16, References section).

Comment 3: The resolution of some images are not high enough. For example, Fig 11(b), an augmented micrograph of the polymer - based optical phased array (OPA) chip, is excessively blurry. The low resolution and poor contrast obscure critical details, e.g. the MMI seems broken.

Response 3: Thank you for pointing this out. We acknowledge the issue with the resolution and clarity of Fig. 11(b). In response, we have replaced the blurred micrograph with a layout illustration that clearly presents the key components and structural details of the polymer-based optical phased array (OPA) chip, ensuring better visibility and understanding of the design. (Page 10, Figure 11)

Comment 4: Figure 12, which illustrates waveguide cross - sections and coupling simulations for InP, PolyBoard, and Si3N4 platforms, is structurally unclear and fails to effectively communicate critical design details. The authors should replace Figure 12(a)-(c) with high - resolution schematics that use bold lines and distinct colors to differentiate material layers to enhance visual clarity. In addition, structural parameters need to be included.

Response 4: Thank you for your helpful comment. We fully agree with your suggestion and have replaced Figure 12(a)–(c) with high-resolution, clearly structured schematics. In addition, we have included relevant structural parameters and expanded the accompanying description to better communicate the design details and simulation outcomes. These revisions are highlighted in red in the manuscript (page 11, line 328 to 353).

Comment 5: Page 9 line 282 “Numerical simulations were performed using ANSYS Lumerical MODE, optimizing the waveguide width for each material platform to achieve optimal mode overlap and maximize mode - field diameters, thereby enhancing coupling tolerance.”. It would be better for the authors to mention some key results or trends obtained from the simulations, rather than just stating the simulation method and purpose.

Another issue concerning this section is the use of ANSYS Lumerical software, since  the use of commercial software requires explicit acknowledgment of compliance with licensing terms. While the software is widely adopted, the absence of information about the license type (e.g., academic vs. commercial) and whether the authors obtained proper authorization to use it in this work raises potential copyright and ethical concerns.

Response 5: Thank you for your detailed and constructive comment. In response, following the response 3, we have expanded the discussion in the manuscript to include key results from the simulations (page 11, line 328 to 353). Additionally, we have added a proper acknowledgment of the use of ANSYS Lumerical MODE software, including clarification regarding licensing and authorization, in the Acknowledgment section (page 16, line 480 to 483).

5. Additional clarifications

According to journal editor’s following comments, we have also improved further in the manuscript.

Editor’s comment 1: Furthermore, please add a section of discussion/improve the discussion section. A well prepared discussion section will greatly improve the value of the paper.

Response 1: We have included the new discussion section (section 4), highlighted in red in the manuscript, to discuss the achieved results and address the remaining challenges.

Editor’s comment 2: During the technical check of your manuscript, we noticed that a high proportion of the cited references belong to you or your co-authors Refs.

28,29,30,65,73,15,57,59,60,61,63,64,66,27,19,21,36,42,53,55,62,16, which is a self-citation rate of about 30%. Could you please check whether the inclusion of each of these references is appropriate?

Response 2: We have added several additional references, bringing the total number of references to 84 and reducing the self-citation rate to below 30%. However, with regard to the references related to THz emitters and receivers (e.g., [61, 62, 64, 65, 66, 67, 68]), we believe it is appropriate to include work by colleagues from Fraunhofer HHI, as the institute is a recognized leader in this field. These references are essential for providing context and acknowledging foundational contributions to the research area.

Response to Reviewer 2 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions highlighted in the re-submitted files.

2. Point-by-point response to Comments and Suggestions for Authors

Comment 1: In the laser section, the authors mention different types of ECDLs, such as MRR, distributed Bragg reflectors, SLRs, and MZIs, along with the corresponding references. Then, the authors state that Fraunhofer HHI mainly uses hybrid InP/polymer DBR lasers. Providing more discussion here is necessary.

First, more details about the hybrid InP/polymer DBR laser should be included, such as a schematic diagram of its structure and its working principle.

Second, explaining why this method was chosen over the other mentioned methods would be helpful. A more detailed comparison, such as presenting the information in a table, would be very useful.

Response 1: Thank you for your valuable comment. We fully agree with your suggestion and have accordingly revised the manuscript to include a schematic and a representative photo of the hybrid InP/polymer DBR laser, now shown in Figure 2. We have also added a detailed description of its structure, working principle, and clarified the rationale for selecting this laser design over other types. These revisions have been highlighted in red in the manuscript (Page 4, Lines 136–150; Page 5, Line 165).

Regarding the request for a comparative table, while we recognize the value such a summary could provide, we believe a comprehensive and balanced comparison across various ECDL designs and integration platforms would require a more in-depth discussion beyond the scope of this manuscript. In our view, this topic is substantial enough to warrant a dedicated review paper. Nonetheless, we have provided additional contextual explanation in the revised text to clarify our design choice.

Comment 2: It is better to add the abbreviation of thermo-optical phase shifters (TOPS) in the explanation of Figure 11.

Response 2: Agree. We have added the abbreviation in the Figure 11 (page 10, line 300).

Comment 3: The authors demonstrated that their OPA can achieve 20-degree beam steering. I suggest adding more details to discuss the limitations of angle control and the current research frontiers.

Response 3: Thank you for point this out. We totally agree with this comment. We have further discussed the limitation of angle control and included more references on the other methods such as the use of dielectric rod waveguide and metasurface for beam steering. The changes can be found in the manuscript highlighted in red (page 10, line 305 to 311).

5. Additional clarifications

According to journal editor’s following comments, we have also improved further in the manuscript.

Editor’s comment 1: Furthermore, please add a section of discussion/improve the discussion section. A well prepared discussion section will greatly improve the value of the paper.

Response 1: We have included the new discussion section (section 4), highlighted in red in the manuscript, to discuss the achieved results and address the remaining challenges.

Editor’s comment 2: During the technical check of your manuscript, we noticed that a high proportion of the cited references belong to you or your co-authors Refs.

28,29,30,65,73,15,57,59,60,61,63,64,66,27,19,21,36,42,53,55,62,16, which is a self-citation rate of about 30%. Could you please check whether the inclusion of each of these references is appropriate?

Response 2: We have added several additional references, bringing the total number of references to 84 and reducing the self-citation rate to below 30%. However, with regard to the references related to THz emitters and receivers (e.g., [61, 62, 64, 65, 66, 67, 68]), we believe it is appropriate to include work by colleagues from Fraunhofer HHI, as the institute is a recognized leader in this field. These references are essential for providing context and acknowledging foundational contributions to the research area.

Response to Reviewer 3 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions highlighted in the re-submitted files.

2. Point-by-point response to Comments and Suggestions for Authors

Comment 1: The main contributions of this manuscript need to be clearly stated in the abstract.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have reformed the abstract and highlighted the key contributions. They have been highlighted in red in the manuscript’s abstract section (page 1, line 25 to 43.)

Comment 2: The title of Figure 1 should be simplified. A detailed description can be included in the main text.

Response 2: Agree. We have simplified the title of Figure 1 accordingly, and provide the detailed description, highlighted in red in the main text (page 3, line 119 to 124)

Comment 3: “This integration scheme offers 85 fully integrated PICs with building blocks such as tunable lasers, on-chip optical isolators, optical phased arrays,”, “Moreover, the combination of an optical phased array (OPA)”, “Beam steering with optical phased array (OPA) and optical beam forming network (OBFN)”, “2.3 Beam steering with Optical Phased Array (OPA) and Optical Beam Forming Network (OBFN)”, “Figure 10. Illustration of the schematic design of optical phased array (OPA) targeted for THz beam steering.”, The abbreviations of the OPA and OBFN should be given the whole expression when they first appear. Moreover, the abbreviations should be used in the following text.

Response 3: Thank you for point this out. We totally agree with this comment. We have checked through the manuscript and removed the unnecessary duplicated definition of “optical phased array” and “optical beam forming network” in the manuscript. They have been highlighted in red in line 92, 96, 109, 283, 284, and 297.

Comment 4: “These PMs are independently controlled by sinusoidal RF signals, enabling the generation of an 113 optical frequency comb (OFC)”， “There are several different ways of generating optical frequency combs (OFCs) using 152 integrated photonic approaches”, “Here, we choose two cascaded InP-based electro-optic modulators which are modulated with sinusoidal RF driving currents and thus generate an optical frequency comb 160 (OFC).”, “optical frequency comb” is repeatedly abbreviated.

Response 4: Thank you for pointing this out. We totally agree with this comment. Same as response 3, we have checked through the manuscript and removed the unnecessary duplicated definition of “optical frequency comb”. They have been highlighted in red in page 5, line 170, 178.

Comment 5: Figure 5, Figure 6, Figure 7, and Figure 8 miss the title of the whole figure.

Response 5: Thank you for pointing this out. We have included the title (highlighted in red) for Figure 5 (page 7, line 223), Figure 6 (page 7, line 235), Figure 7 (page 8, line 247), Figure 8 (page 9, line 277).

5. Additional clarifications

According to journal editor’s following comments, we have also improved further in the manuscript.

Editor’s comment 1: Furthermore, please add a section of discussion/improve the discussion section. A well prepared discussion section will greatly improve the value of the paper.

Response 1: We have included the new discussion section (section 4), highlighted in red in the manuscript, to discuss the achieved results and address the remaining challenges.

Editor’s comment 2: During the technical check of your manuscript, we noticed that a high proportion of the cited references belong to you or your co-authors Refs.

28,29,30,65,73,15,57,59,60,61,63,64,66,27,19,21,36,42,53,55,62,16, which is a self-citation rate of about 30%. Could you please check whether the inclusion of each of these references is appropriate?

Response 2: We have added several additional references, bringing the total number of references to 84 and reducing the self-citation rate to below 30%. However, with regard to the references related to THz emitters and receivers (e.g., [61, 62, 64, 65, 66, 67, 68]), we believe it is appropriate to include work by colleagues from Fraunhofer HHI, as the institute is a recognized leader in this field. These references are essential for providing context and acknowledging foundational contributions to the research area.