Optical Design of a Wavelength Selective Switch Utilizing a Waveguide Frontend with Beamsteering Capability

Round 1

Reviewer 1 Report

The paper explores a novel optical design for wavelength selective switches (WSSs) that enhances the spectral resolution of null-steering optical phased arrays (OPAs). The OPA with the proposed WSS can achieve ±4.9° and 13 dB side-lobe level. The paper is clearly written and coherent. The architecture and the analyses for it look promising. However, there are a few minor concerns that need to be addressed:

1. In line 166-169, the manuscript discusses the beam profile of the end-fire silicon nitride (SiN) OPA but does not present the far-field profile, which would provide a clearer visualization of the beam's full width at half maximum (FWHM). If the far-field profile of the end-fire SiN OPA can be shown in figure 1, it would enhance the readers' understanding of the beam profile and its characteristics.

2. In section 2.1, the author mentions that N is the number of SiN end-fire waveguides. In Section 2.2, the author proposes 4xN_o with N_o representing the number of available output channels (or output waveguide?). Is the N_o equivalent to the N mentioned before? Also, what is the meaning of 4 channels? Is it indicated how many OPA on the PIC? Clarification on whether No is equivalent to N and the significance of the "4 channels" would help dispel confusion. A more precise explanation of these terms and their roles in context is needed.

3. The principal section lacks a discussion on the function and importance of the channels (number of OPA?). Current OPA designs often focus on increasing the number of output waveguides to decrease the divergence of the output beam and increase the steering angle. The state-of-the-art OPA can achieve 8192 output waveguides in one OPA. But in this paper, 32 output waveguides are discussed, which is far away from the real OPA design. Why does the author discuss multiple OPAs instead of a single OPA with a higher number of output waveguides?

4. As the author mentioned, the WSS setup is operated under ITU 200 GHz. Usually, multi-wavelength scanning is used for OPA with a 1D grating coupler to achieve 2D scanning capabilities. However, for an end-fire emitter, it only achieve 1D scanning. Does the multiwavelength work cooperate with diffraction grating to achieve 2D scanning? Could the author provide more details about the function of multi-wavelength?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The article introduces a novel hybrid WSS architecture that integrates end-fire SiN OPAs into the front end of optical WDM networks, aiming to enhance routing capabilities within these networks. The architecture aims to improve spectral resolution on the LCoS panel by introducing beamforming and steering features to WSS systems. By utilizing 3D-FDTD and ray tracing, the authors indeed demonstrate increased spectral resolution and the capacity to handle higher input channel numbers. A few minor revisions would strengthen the manuscript.

1. Please explicitly mention AF in equation 3.

2. For reference in section 2.1, including a small schematic with the direction of the beam in Figure 1A for better understanding.

3. Figures showing the angular FWHM should be included in the context of lines 159-170. 

4. Labels should be included in Figure 2C, including the angle between the L1 plane and the DG.

5. Please define Numerical Aperture (NA) in line 170 instead of in line 179.

6. Paragraphs from lines 398-417 as well as 435-449 should be relocated to an integrated discussion/conclusion section as these do not directly describe the results.

1. In line 139, please change to “with respect to”.

2. In line 140, the authors possibly imply, “As a matter of fact”. Please change if so.

3. Please change “results to” to “results in” in lines 226, 303, 565 and elsewhere.

4. Please be consistent with tenses while describing results throughout the manuscript.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The authors propose a hybrid wavelength-selective shifter architecture that utilizes the beam shaping and steering capabilities of uniform silicon nitride-based end-fire optical phased arrays. The achieved performance has been evaluated through 3D-FDTD and ray optics simulations. The manuscript is well-written and warrants publication after addressing the following comments:

1. The novelty of the proposed device needs to be emphasized in terms of both performance and architecture, as demonstrated in prior work such as Kim et al.'s compact solid-state optical phased array beam scanners based on polymeric photonic integrated circuits (Compact solid-state optical phased array beam scanners based on polymeric photonic integrated circuits. Scientific Reports, 11(1), 10576, (2021)).

2. Regarding the choice of SiN material for WFE, the authors should provide justification for this selection and detail the primary propagating losses considered (see Ultra-low-loss high-aspect-ratio Si 3 N 4 waveguides. Optics express, 19(4), 3163-3174, (2011).). Additionally, they should discuss how these waveguides could be excited, potentially through butt coupling (see i.e., Silicon Nitride Spot Size Converter With Very Low-Loss Over the C-Band. IEEE Photonics Technology Letters, (2023); Study on inverse taper based mode transformer for low loss coupling between silicon wire waveguide and lensed fiber. Optics Communications, 284(19), 4782-4788, (2011)).

3. For beam-steering applications, the authors need to elucidate the tuning technique, conducting simulations and comparing results with simulated Phase Shifters (PS) documented in existing literature (see i.e. Design of an ultra-compact graphene-based integrated microphotonic tunable delay line. Optics express, 26(4), 4593-4604, (2018); Silicon graphene reconfigurable CROWS and SCISSORS. IEEE Photonics Journal, 7(2), 1-9, (2015))

4. Given the significance of Optical Phased Arrays for current and future applications, the proposed structure's compatibility with integration solutions is crucial. Therefore, the authors must clarify whether integration into a single chip is feasible.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The Authors have modified the manuscript according to the Reviewer comments. However, the delay lines overview has to be enlarged according to the Comment #3 of the 1st revision.

Author Response

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Author Response File: Author Response.pdf