Review Reports - Grazing-Angle Fiber-to-Waveguide Coupler

Round 1

Reviewer 1 Report

In this paper, Shin et al proposed a new strategy of fiber-array fiber-to-chip couplers, which has a complementary metal-oxide semiconductor-compatible silicon structure. An ultra-high numerical aperture fiber is polished at a grazing angle and positioned on a taper-in silicon waveguide. A coupling efficiency of more than 90% over hundreds of nanometers and broad alignment tolerance ranges, ensuring the use of a fiber array for the packaging. Authors only show the numerical results. However, according to Fig. 2 (b)(c), the mode fields difference of fiber and waveguide chip is obvious. Then what is the physical origin of coupling strategy that can achieve 90% efficiency as shown in Fig. 4?

Author Response

Dear Referee A,

Thank you very much for taking the time to read our manuscript and providing us with your thoughts and invaluable feedback. As requested by the editor, we respond to your comments and queries in a point-by-point fashion. Please see the attached file for the details.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper presents a design work of a fiber-to-waveguide coupler with high coupling efficiency over a broadband. By optimizing the taper tip width W and half angle of the taper θ, the designed coupler attains 94% coupling efficiency over 245 nm wavelength bandwidth. The simulation results show that high coupling efficiency can still be maintained within the tolerance of ±14.4µm in the x-axis ,1.4µm in the y-axis and 80nm in the z- axis.

The following should be answered properly before it can be recommended to be published on Photonics.

1. The reason for the selection of 88 degrees of the fiber should be proved. Also, the angle tolerance of angle on the coupling efficiency should be analyzed.

2. The core diameter of the ultra-high numerical aperture (UHNA) fiber is about 2.2 μm, which is much smaller than the general single mode (SM) fiber. How about the coupling loss between the UHNA fiber and SM fiber?

3. In order to reduce the coupling loss, the index matching oil (IMO) is utilized. For the real applications, does the IMO affect the height in the z-axis? Can it be distributed evenly between the waveguide and the fiber?

4. Some details are suggested to modified.

(1) In Figure 4, it is recommended to add some auxiliary lines to illustrate what is mentioned in the paper, for example: "The maximum coupling efficiency occurs at 1535 nm, 119 and it appears to have a width of 245 nm for 0.5dB bandwidth with a center wavelength 120 of 1507.5 nm.

(2) In the conclusion of the article, tolerance ranges are not presented. Page 6 line 166 and 167: [[[xx]]] µm x-axis, [[[xx]]] µm y-axis, [[[xx]]] nm z-axis.

Author Response

Dear Referee B,

Author Response File: Author Response.pdf

Reviewer 3 Report

Y Seong et. al. propose a fiber-array to chip coupling strategy with robust and high coupling efficiency. Optical silicon photonics is a critical field of research for optical classical and quantum computing. As the authors describe, one of the key issues in these technologies is mitigating photon loss - this manuscript addresses the issue of loss, robustness, and usability of fiber-to-chip coupling. The authors propose a grazing angle fiber-to-chip coupler and show its validity using finite element numerical simulations. This manuscript has useful and novel contributions but there are a few issues that need to be addressed before I can fully recommend its publication -

Why was only the transverse-electric-like mode considered in the simulation? In surface effects, the other components could potentially be important.
The coupling robustness results are quite sensitive (80 nm) in the z-direction. Since the fiber surface roughness can be 10’s of nm, it starts to be a significant factor. How can one overcome this issue?
In line 66, the y-direction tolerance is 1.5 um but in line 134, it is specified to be 1.4 um. Which one is correct?
The efficiency numbers are missing in lines 166-167.
The section on patents seems to be missing in line 172.

Author Response

Dear Referee C,

Author Response File: Author Response.pdf

Reviewer 4 Report

The authors proposed a new fiber to silicon nanowire waveguide coupler based on an ultra-high numerical aperture fiber polished at a grazing angle and a taper-in silicon waveguide. The coupling efficiency and alignment tolerance are theoretically analyzed, and a high calculated efficiency of 94% with 0.5-dB bandwidth of more than 200 nm is achieved. The paper is well-organized and can be published provided that the following comments are properly addressed.

1) In the numerical calculation section, the authors fixed the length of silicon inverse taper, which might be not an optimum length. It would be good to show the effect of taper length on coupling efficiency. ˚

2) The authors suggested using a fiber array block to polish the fiber in the Discussion section. It is still very challenging to achieve the desired grazing angle (e.g., 88˚) since the sharp angle (2˚) is very easy to be damaged during grinding process. It would be good if the authors can give some simulation or discussion regarding the effect of the broken fiber top on coupling efficiency, as close to silicon waveguide.

3) A silicon waveguide is typically covered by an upper-cladding layer of silicon oxide. Could the authors add some simulation on this case?

Author Response

Dear Referee D,

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Reviewer 2 Report

The authors have answered the question properly in the revised paper. I would like to recommend it to be published in Photonics.