Tailoring Spectral Response of Grating-Assisted Co-Directional Couplers with Weighting Techniques and Rational Transfer Functions: Theory and Experiment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript investigates the application of grating-assisted co-directional couplers (GADCs) in fiber-to-the-home (FTTH) networks, proposing a design method that leverages coupling profile weighting techniques and rational transfer functions to optimize spectral response. The study models the spectral response of GADCs through coupled-mode theory and verifies the design’s effectiveness experimentally. The manuscript discusses various weighting techniques (such as Hamming, Kaiser, and Blackman weighting) and methods for flattening the spectral response (including Chebyshev, Butterworth, and elliptic approximations). Experimental results align closely with simulations, validating the design's accuracy. This study shows innovation in WDM device optimization, particularly in the application of techniques to flatten the spectral response and suppress side lobes, providing new approaches to enhance optical device performance with practical application value. However, the connection between design methods and the experimental section lacks coherence. It is recommended to separate the design and experimental sections, explaining each design's rationale and validation method step-by-step, and to expand on the experimental details and comparative discussion of results to enhance the manuscript’s logical flow. Additionally, the manuscript has the following issues:

1.The writing structure is somewhat unclear, and the experimental results lack visual clarity. It is recommended to introduce the optimization methods and explanations first, presenting the resulting simulations in a single comparative figure. Then, introduce the experimental process and display the experimental results in another comparative figure. This structural improvement would make the manuscript’s logic more coherent and help readers better understand the findings.

2.While discussing the experiments involving different weighting techniques and spectral response flattening, it is recommended to use charts or tables to more clearly compare the experimental results for various weighting techniques, making the results more visually accessible.

3.In the experimental section, it would be beneficial to include details about the fabrication process and the instruments used, along with real images and electron microscope images of the samples, which would improve the completeness and credibility of the experimental section.

4.The manuscript does not specify the simulation software used. It is recommended to add details about the simulation software, parameter settings, and experimental steps for better reproducibility.

5.The manuscript lacks data comparing the results with previously published research. It's suggested to provide a comparative display of the optimized filter’s performance data and size against similar filters reported in the literature, highlighting its advantages.

Author Response

please see attachment

Author Response File: Author Response.PDF

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents innovative design methods for spectral response engineering, combining apodization-based weighting techniques and more sophisticated rational transfer function fitting to align with a predefined spectral window template. The authors apply coupled mode theory modeling to the design and experimental fabrication of InGaSP/InP Grating-Assisted Directional Couplers (GADCs) aimed at 1.3+/- μm diplexer applications in Fiber-to-the-Home (FTTH) access networks. While this research has potential, certain issues must be addressed to strengthen the study:

1. The literature review requires enhancement, as several studies have already addressed waveguide coupling in the presence of a grating. Relevant works should be discussed to provide a comprehensive background (see, e.g., “Comprehensive mathematical modelling of ultra-high Q grating-assisted ring resonators. Journal of optics,” 22(3), 035802, 2020; “Analysis of grating-assisted backward coupling employing the unified coupled-mode formalism,” JOSA A, 16(6), 1303-1311, 1999; “A unified approach to coupled-mode phenomena,” IEEE journal of quantum electronics, 34(7), 1109-1116, 1998).

2. One of the main challenges with the diplexer design is fiber-chip coupling. The absence of spot-size converters could lead to increased coupling losses. The authors should consider adding this activity to future work to reduce these losses (see, e.g., “Silicon nitride spot size converter with very low-loss over the C-band,” IEEE Photonics Technology Letters, 35(22), 2023; “Ultra-lowloss inverted taper coupler for silicon-on-insulator ridge waveguide,” Optics Communications, 283(19), 2019; “Fiber-chip edge coupler with large mode size for silicon photonic wire waveguides,” Optics express, 24(5), 5026-5038, 2016; “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. In Figure 4, the SLSR is only enhanced toward the first resonance on the left of the main lobe in apodization cases, while ideally, all side lobes should be suppressed. The authors should justify this selective suppression effect observed.

4. A section detailing the fabrication processes is essential to improve the clarity and reproducibility of the study.

5. The authors should provide theoretical explanations for all presented results to support the experimental data with robust modeling insights.

In conclusion, the paper is well-written and presents valuable contributions. Addressing these points would enhance the manuscript and prepare it for publication.

Author Response

please see attachment

Author Response File: Author Response.PDF

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have answered all the questions and revised their manuscript accordingly.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have satisfactorily addressed all the reviewers' questions.