Impact of Grating Duty-Cycle Randomness on DFB Laser Performance

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper presents the impact of the grating DCR on DFB laser performance based on a modified TMM method.

1.     The authors state that DCR is inevitable even with the state-of-the-art technologies, but there is insufficient evidence to support this claim. Please provide more evidence or references.

2.     Figure 1(a) showes some fabrication images, but it is unclear if these images were acquired by the authors or sourced from other references. Please include detailed information about the fabrication process and the imaging equipment used. Additionally, there is no information about the feature size.

3.     Figure 1(a) and 1(b) do not match. Please ensure consistency between the figures.

4.     In the simulation, lm and lm’ vary randomly, but their summation A is fixed. Does this make sense in fabrication? Please verify in SEM or TEM images what a reasonable range of variation should be. In equation 6, it seems like there is not limitation for lm and lm’.

5.     What’s the meaning of comparing the simulation results from TMM and SWM? SWM often miss the true root or find the false root in dealing with complicated grating structures.

6.     Please explain the meaning of “The grating is assumed to have a uniformly distributed random variation of its duty-cycle by ±25%.” in details.

7.     In figure 5-11, what does the legend “Duty cycle: 25%-75%” mean? I assume it means that the duty cycle is a random number between 25%-75%. If so, how many simulation runs were performed? Are the results reliable with an increased number of runs? Medium and average values are used in the figures, but neither of them can sufficiently represent the overall behavior of all simulations. Please adopt appropriate statistical methods in the analysis. 

8.     What are the best and worst cases in your simulation? Can you get some information about the relationship between randomness and device quality? Will a Duty cycle: 25%-75% perform worse compared to 35%-65%? There is a comparation in figure 10 and 11, but not others. 

Author Response

Thank you very much for taking the time to review this manuscript. Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript investigates the impact of grating duty-cycle randomness on the performance of distributed feedback (DFB) lasers through numerical simulations. The results indicate that grating duty-cycle randomness (DCR) leads to a reduction in the side mode suppression ratio (SMSR), an increase in linewidth, and heightened relative intensity noise (RIN). The study concludes that DCR significantly affects DFB laser performance, particularly in uniform grating DFB lasers.

 

Given that simulations are grounded in addressing real-world problems, the manuscript needs to answer the following questions:

1. What causes grating duty-cycle randomness in the manufacturing process of DFB diodes? This does not appear to be explained.

2. What is the typical range of DCR deviations caused by manufacturing processes? Does it exceed 35%-65%?

3. In the grating fabrication process, is the production of DCR decoupled from other parameters? Does randomness in the fabrication process that leads to DCR also affect other parameters such as grating depth and period?

 

There are also issues within the simulation process that need clarification:

1. The manuscript introduces DCR in simulations using Equation 6, assuming a linear distribution between [R 50%] and [50% R]. Does this representation accurately reflect the DCR variations observed in production? Is the DCR distributed linearly or would another distribution, such as Gaussian, be more appropriate? Have the results of these stochastic simulations undergone any further processing, such as averaging?

2. How was the time step in the simulation determined?

 

Regarding the novelty of the paper:

1. The manuscript emphasizes that compared to other simulation methods, it achieves "sub-pitch" resolution. A paper by Bao, S., Song, Q., & Xie, C. (2018) titled "The influence of grating shape formation fluctuation on DFB laser diode threshold condition" in Optical Review also achieved sub-pitch resolution. How does the simulation methodology in this manuscript compare or differ from that study in terms of advantages or innovations?

2. The manuscript provides an extensive review of DFB simulation technologies, but it seems to lack a discussion on how various simulation parameters impact the output characteristics of DFB lasers.

Author Response

Thank you very much for taking the time to review this manuscript. Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper entitled Impact of Grating Duty-Cycle Randomness on DFB Laser Performance could be accepted for publication after addressing the following concerns.

1.       How the grating coupling coefficient was optimized such as 2, 2.5, 3, etc? (Refer. Fig. 2)

2.       Why the phase-shifted grating was fixed to 1/4λ?

3.       Why with increasing the duty cycle, the SMSP is improving?

4.       What does abrupt phase change stand for?

5.       On the basis of Fig. 10, what should be the optimized cavity length?

Comments on the Quality of English Language

minor editing of English language required

Author Response

Thank you very much for taking the time to review this manuscript. Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed most the of the questions. However, there are still some minor changes that need to be made before publication.

1.     The Figure 1 is unacceptable in a scientific paper. Please replace it with the figures provided in the response to the reviewer document. It is crucial to present an unedited TEM image to accurately depict the feature size.

2.     The number of independent simulations conducted for the “Duty cycle: 25%-75%” remains unclear. The figures suggest that there may be seven independent simulations. If the authors have indeed conducted only a few independent simulations, it would be beneficial to detail the specific simulation parameters used in each case. This information is essential for reproducibility and for understanding the robustness of the simulation results.

Author Response

Thank you for your feedback. Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for your detailed responses to my review comments. While your replies are comprehensive, I believe some of my concerns have not been fully addressed.

Firstly, when conducting simulations for a physical problem, it is essential to introduce the physical background of the problem to the readers. This includes explaining how the issue arises in reality and its significance. This context is crucial to demonstrate the necessity and contribution of the simulation work. Additionally, the parameter choices in the simulations need to match real-world conditions. For instance, the range of DCR deviations. The introduction section should be enriched with background information to highlight the importance of this work.

Secondly, some issues are likely to be of significant concern to readers and need to be addressed in the manuscript. For example, the assumption of a linear distribution for DCR should be explained thoroughly, ideally with supporting references or experimental data to back up this claim, rather than relying solely on statements like "we observed" or "from our experience."

Author Response

Thank you for your feedback. Please see the attachment.

Author Response File: Author Response.pdf