A Theoretical Sub-0.1 dB Loss Single Mode Fiber-To-Chip Edge Coupler for Silicon Nitride Waveguides
Round 1
Reviewer 1 Report
The authors propose a manuscript that tackles optical coupling into an integrated chip.
The proposed solution is interesting but some aspects of the work need to be improved to be publishable.
In equation 1 the area A is missing.
the coefficient eta2 is not defined properly.
On page 3 line 107, it would be useful to show the effective index against length to see the evolution.
Also, as in [1] the author should try to comment on High Order Modes.
Figure 3 is interesting but a Monte-Carlo calculation showing the expected distribution could provide more insight in particular regarding possible "cross-connections" between parameters.
Finally, the figures should be improved in terms of captions and labels.
The authors should provide a summary of the main results in the introduction to give readers an overview of the paper.
Also, the authors should comment on the likeliness of a possible fabrication.
Overall, the structure is interesting but it might be difficult to achieve in a real-life foundry.
[1] L. Lever, Z. Ikonić, and R. W. Kelsall, "Adiabatic mode coupling between SiGe photonic devices and SOI waveguides," Opt. Express 20, 29500-29506 (2012)
Author Response
The authors propose a manuscript that tackles optical coupling into an integrated chip. The proposed solution is interesting but some aspects of the work need to be improved to be publishable.
- In equation 1 the area A is missing. The coefficient eta2 is not defined properly.
Response:
Thank you for the comments. We have clarified the definition of the terms in the calculation formula.
Action taken:
In the section " Device structure and operation mechanism ", we added following sentence: “Where E1 and E2 represent the electric field of the fiber mode and the complex electric field amplitude of the edge coupler end facet. A is the Mode field distribution area at fiber end face and edge coupling end face. ” in Page2 line 84-86.
In the section " Device structure and operation mechanism ", we added following sentence: “We define η2 as the the mode conversion efficiency of the edge coupler. The total coupling efficiency η is multiplied by η1 and η2. ” in Page2 line 89-91.
- On page 3 line 107, it would be useful to show the effective index against length to see the evolution.
Response:
Thank you for the comments. As shown in Figure 1. We plot the distance from the end face versus the effective index of refraction of both TM and TE modes.
Figure 1 The effective index for TM and TE modes of the waveguide as a function of the distance from the endface
Action taken:
In the section " Device structure and operation mechanism ", we added following sentence: “In this process, the effective refractive index is gradually changed. ” in Page3 line 128-129.
In the section " Device structure and operation mechanism ", we added following sentence: “Fig .2(f) shows the relationship between the effective index and the distance from the end face in TE and TM modes. ” in Page3 line 131-133.
In the section " Device structure and operation mechanism ", we revised Figure 2 in Page 4, line 134:
Figure (a) TM mode profile of the SMF-28. (b) TM Mode field at the end face of the edge coupled structure. (c) TM mode field distribution at 200 nm from the end face. (d) TM mode field distribution at 400 nm from the end face. (e) TM mode evolution along the coupler (0-400 μm) in the XOY plane. (f) The relationship between the effective index and the distance from the end face in TE and TM modes.
- Also, as in [1] the author should try to comment on High Order Modes.
Response:
Thank you for the comments. As shown in Figure 2, we plot the end faces of the structure for the 1st order modes. TE1 modes can be observed in the structure, as shown in Figure 2 (a)(b).However, for these higher-order modes, the calculation using the overlap integral formula is always 0. Therefore these modes cannot be excited.
Figure 2
Action taken:
In the section " Device structure and operation mechanism ", we added following sentence: “For this end-face structure, no higher-order modes are excited.” in Page3 line 124-125.
- Figure 3 is interesting but a Monte-Carlo calculation showing the expected distribution could provide more insight in particular regarding possible "cross-connections" between parameters.
Response:
Thank you for the comments. Monte-Carlo calculation is a good way to observe the relationship between various parameters. However, for the six parameters in this work, it will be very complicated to calculate using the Monte Carlo method. Meanwhile, each of the two parameters can regulate different effects of the mode field distribution. w1 and h2 regulate the distribution of the mode field near the central SiN waveguide. w5 and h3 regulate the distribution of the mode field near the upper and lower waveguides. h4 and n regulate the overall mode field distribution. Therefore, the images drawn using these three pairs of parameters in Figure 3 can clearly see the effect of each parameter on the coupling loss.
- Finally, the figures should be improved in terms of captions and labels.
Response:
Thank you for the comments. We have improved the labels in Figure 1-3 to make the figures more clearly presented.
Action taken:
In the section " Device structure and operation mechanism ", we revised Figure 1 in Page 3 line 116. We added a front view of the structure to make this coupled structure diagram clear.
In the section " Device structure and operation mechanism ", we revised Figure 2 in Page 4, line 134:
In the section " Device performance ", we revised Figure 3 in Page 5 line 171:
- Also, the authors should comment on the likeliness of a possible fabrication. Overall, the structure is interesting but it might be difficult to achieve in a real-life foundry.
Response:
Thank you for the comments. We have added a discussion section, mainly discussing the feasibility of the experimental preparation of the structure and the specific preparation steps in the future. Although the preparation process of this structure is relatively complicated, since the minimum preparation process of this structure is 200 nm, it meets the minimum preparation line width using DUV lithography. A possible fabrication process is described in the discussion section.
Action taken:
In the section "Discussion", we added the following sentence: “The critical dimension of the preparation process is 200 nm. It can be fabricated using 193 nm lithography. A possible fabrication process is as follows: On the SiO2 substrate, sputtering/ plasma enhanced chemical vapor deposition (PECVD)/ low pressure chemical vapor deposition (LPCVD) is first used to prepare a layer of 65 nm SiOxN. Then, the SiOxN layer is patterned by photolithography and etch to obtain the desired structure. A second PECVD step is used for SiO2 growth, followed by sputtering/ PECVD/ LPCVD to deposit a 400 nm SiN film, which is then etched twice to make into the shape of the central waveguide. The fabrication process of the upper layer SiOxN is the same as that of the lower layer.” in Page7 line 217-227.
Author Response File: 
Author Response.docx
Reviewer 2 Report
well written paper
Author Response
Thank you for the comments.
Reviewer 3 Report
Manuscript No.: photonics-2206175
Di Wu et al.; reported a research manuscript titled “A Sub-0.1 dB Loss Single Mode Fiber-to-Chip Edge Coupler for Silicon Nitride Waveguides”. They have reported a multilayer edge-coupler using SiOxN materials with different indices to allow efficient edge coupling between SMF-28 fiber and SiN single-mode waveguides for silicon photonic integrated circuits. They have also proposed a structure that attributes the advantages of large fabrication tolerance, large alignment tolerance, wide operation bandwidth, and low polarization-dependent loss.
This theoretical work is exciting and can be considered for publication. However, the author needs to address the following minor points carefully.
1. Provide the expansion of CMOS on Page 1 and line 25; it will be more convenient for readability.
2. Authors should be highlight the reasons for having low coupling loss and the total loss for the entire device in the results and discussion section.
Author Response
Di Wu et al.; reported a research manuscript titled “A Sub-0.1 dB Loss Single Mode Fiber-to-Chip Edge Coupler for Silicon Nitride Waveguides”. They have reported a multilayer edge-coupler using SiOxN materials with different indices to allow efficient edge coupling between SMF-28 fiber and SiN single-mode waveguides for silicon photonic integrated circuits. They have also proposed a structure that attributes the advantages of large fabrication tolerance, large alignment tolerance, wide operation bandwidth, and low polarization-dependent loss. This theoretical work is exciting and can be considered for publication. However, the author needs to address the following minor points carefully.
Provide the expansion of CMOS on Page 1 and line 25; it will be more convenient for readability.
Response:
Thank you for the comments. We have added an explanation of the CMOS process.
Action taken:
In the section "Introduction", we added following sentence: “Complementary Metal Oxide Semiconductor (CMOS) ” in Page1 line 25-26.
- Authors should be highlight the reasons for having low coupling loss and the total loss for the entire device in the results and discussion section.
Response:
Thank you for the comments. We have added the reason why this structure achieves such low coupling loss in the section "Conclusion".
Action taken:
In the section "Introduction", we added following sentence: “There are two reasons why this structure can achieve low coupling loss. One reason is that we increase the overlap of the electric field distribution between the coupling modes of the spot size converter and the SMF-28 fiber by changing the size of the central SiN waveguide; another one is that we improve the degree of matching between the coupling modes of the spot size converter and the SMF-28 fiber by adjusting the size, refractive index and spacing of the SiOxN layer.” in Page7 line 232-238.
Author Response File: Author Response.docx
Reviewer 4 Report
The title of the article is not enough accord with the content. May be include the word " teoretical analisys" for avoid more expectatives to the readers
The structure described is not clear, figure 1 must be improve
Figure 6 isn't a good comparative. The text says that the devices has different longer, but don't explain how it affect to the comparative
I think the study is interest but it isn't good explain
Author Response
I think the study is interest but it isn't good explain.
- The title of the article is not enough accord with the content. May be include the word " theoretical analisys" for avoid more expectatives to the readers.
Response:
Thank you for the comments. We have revised the title of this work to Design of a Sub-0.1 dB Loss Single Mode Fiber-to-Chip Edge Coupler for Silicon Nitride Waveguides
Action taken:
The title is revised to “Design of a Sub-0.1 dB Loss Single Mode Fiber-to-Chip Edge Coupler for Silicon Nitride Waveguides”
- The structure described is not clear, figure 1 must be improve.
Response:
Thank you for the comments. We have added a front view of the structure to make this device better illustrated.
Action taken:
In section " Device structure and operation mechanism ", we revised Figure 1 in Page 3 line 116 as follows:
Figure 1 (a) 3D-view of the edge coupler. (b) the front view of end-coupling structure. (c) the coupler geometry of the end facet. (d) the geometry of the SSC. (e) The Ez field distribution at the end facet for the fundamental TM mode along z axis compared to the TM0 mode of the SMF-28.
- Figure 6 isn't a good comparative. The text says that the devices has different longer, but don't explain how it affect to the comparative.
Response:
Thank you for the comments. For the end-face coupling structure, the most important part affecting the overall coupling efficiency is the coupling efficiency at the end face. The overall length of the device mainly depends on the dimensions of the transmission waveguide. The device lengths for different transmission waveguide sizes are different due to adiabatic tapering. The total length of different devices reported previously may not be a good indication of the structure performance. Therefore, Figure 6 mainly compares the different results of the overall coupling efficiency of different previous reports.
Action taken:
In the section " Device performance ", we added following sentence: “The overall length of the device mainly depends on the dimensions of the transmission waveguide. The device lengths for different transmission waveguide sizes can be different. Therefore, we focus on the coupling loss of the end-face coupler.” in Page6 line 206-209.
Author Response File: Author Response.docx
Round 2
Reviewer 4 Report
The new redaction way is better
