An Efficient Silicon Grating Coupler for a 2 μm Waveband Based on a Polysilicon Overlay
Round 1
Reviewer 1 Report
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The authors designed a dual-layer grating coupler based on raised polysilicon to address the low directionality which was designed on 220 nm-thick silicon-on-insulator. They reported an effective, and high-degree-of-freedom method for the development and optimization of double-layer gratings in their design methodology, and they apply an inverse design to assist with the arrangement of grating structures. This study offers a technique for increasing efficiency that applies to 2 μm wavelength high-efficiency grating couplers, which is compatible with the silicon photonic manufacturing processes. I only have some minor comments as follows:
- The authors should have explained in their paper that the MPW means multi-project wafer (line 197).
- The sensitivity of the structural design to errors in fiber placement and set angle can be discussed after the discussion of the tolerance of grating couplers to manufacturing variations.
None
Author Response
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Author Response.pdf
Reviewer 2 Report
The researchers of this paper have done an interesting working on the grating couplers for applications of integrated photonic chips in the wavelength band around 2μm. And the work is meaningful for extending optical fiber communication bandwidth. Based on the research work stated in the manuscript I have a few questions listed below for discussion.
1. As for the GC1 design scheme, it is a bit like increase the top silicon layer thickness of the SOI, did the researchers do some comparison work between performances of their GC1 design and other researchers’ or even their own work based on thicker top silicon layer?
2. In the manuscript’s design scheme, the researchers added top cover Silica layer on their proposed grating coupler. I wonder, what the effect of this cover silica layer would be.
3. What is its polarization related performance?
4. The conditions for simulation like the mode of light, light beam’s Gaussian distribution, light input angle, one dimensional simulation or two dimensional simulation, computational field description, material parameters used, etc. should be made clear in the context.
5. Should better give a full scheme diagram of your GC。
6. How do you realize your GC2 and GC3?
Author Response
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Reviewer 3 Report
The article describes how two grating coupler layers can be used to improve coupling efficiency between a waveguide and an optical fiber at 2 um. The design utilizes 2D FDTD and optimization. The article claims advantages in terms of fabrication tolerance. The article is well written in terms of English, except a few mistakes. The article might be good for publication after making the following corrections.
11) Line 15, remove “to”.
22) Line 51, replace “was” by “is”.
33) Please state clear definitions for the following terms: coupling efficiency, directionality, Up diffraction percentage and down diffraction percentage.
44) For figures 2 and 3, add to the caption the corresponding dimensions such as allowed periods, filling factor, grating coupler shifts, polysilicon height, etc.
55) Please add a table with the optical properties of the materials used in the design. These are silicon, silica and polysilicon. The table should show the real and imaginary parts of the refractive index for each material in the range of wavelength of the study.
66) Define the mode of light in the study. Is it TE-like or TM-like?
77) Why did you use polysilicon instead of silicon for the top grating?
88) Please add a discussion on the effect of absorption loss of polysilicon which is known to be way more lossy than silicon or silica.
99) Please explain how the optimizer was integrated with FDTD. State the settings and the time required to achieve the results using the available processing power which should also be mentioned.
110) Please state the ranges of the input parameters to the optimizer. For example, the range of the allowed periods, filling factor, grating coupler shifts, polysilicon height, etc.
111) What are the settings of the FDTD simulations (mesh size, window size, boundary conditions, etc.)?
112) In Figure 5, the boundaries look too close to the core of the structure. This way, the boundary conditions might cause reflections which will affect the results. Also, the buried oxide layer is supposed to block leakage of the power down to the substrate at wavelength of 1.55 um. In such technology, this box layer is 2 um thick. The authors need to repeat these simulations with the boundaries at y=-6 um and y=10 um to make sure about the leakage losses and possible reflections, specially that the wavelength is 2 um which is greater than the regular 1.55 um.
Comments for author File: Comments.pdf
Author Response
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Round 2
Reviewer 2 Report
All my concerns have been addressed. I think it is suitable for publication in Photonics now.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Here are my comments on the 2nd round of review of the article titled “ Efficient Silicon Grating Coupler for 2 μm Waveband based on 2 Polysilicon Overlay ”.
In general, I see the article not fit for publication as explained in my second comment.
11- The sentence with the definition of the coupling efficiency includes a wrong statement which is “ Coupling efficiency is a critical metric for evaluating the performance of a grating coupler, ”. In this work, the coupling efficiency is an input or design parameter and not a metric to evaluate the performance of the device. This mistake shows either lack of understanding of the role of the parameters or issues with the English of the authors. However, this mistake is minor and can be corrected.
22- Table 1 shows an unsupported claim by the reviewers that all three materials are lossless at 2 um (k=0). This goes against physics and casts strong doubts on the main results of this work.
I advise the authors to have a look at the following reference which show that polysilicon is a lossy material at 2 um:
Ghosh et al, “ Low-temperature polycrystalline silicon waveguides for low loss transmission in the near-to-mid-infrared region, ” Vol. 31, No. 2 / 16 Jan 2023 / Optics Express.
For the other materials, silicon and silicon dioxide, the losses are small and could be negligible but one cannot just claim that k = 0 for these materials.
Because this is a fundamental mistake in the work, I see this work not fit for publication.
Should the authors choose to re-submit this work, they should take into consideration the loss effects which could totally alter the results of the work.
The English can be improved. There are minor mistakes here and there in the article.
Author Response
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Author Response File: Author Response.pdf
Round 3
Reviewer 3 Report
Accept.
I detected many minor language errors.
If the article is accepted, the journal will need to make a thorough English review during the proof stage before publication.
