Mode-Conversion-Based Chirped Bragg Gratings on Thin-Film Lithium Niobate
, Xingrui Huang 1,2,†, Yuxiang Yin 1,2, Hang Yu 1,2, Zhiguo Yu 1, Huan Guan 1 and Zhiyong Li 1,*
Round 1
Reviewer 1 Report
This manuscript demonstrated a mode-conversion-based chirped Bragg grating on thin film lithium niobate. The proposed device uses an ADC-based mode de-multiplexer to guide the reflected light to the drop port. This work is very interesting to the community and well presented. The manuscript is qualified for publication with the current version.
Author Response
We are grateful for your review and affirmation of our work.
Reviewer 2 Report
The manuscript proposes an integrated mode-conversion-based chirped Bragg grating on thin-film lithium niobate (TFLN). It utilize a adiabatic directional coupler to couple out the refelction, instead of using an off-chip circulator. Its main contribution is to realize a fully integrated Bragg grating and can be compatible with other devices on TFLN. The following comments should be addressed.
1. In Figure 6, the ripple is two large. From figure 6, the ripple is over 3 dB. The authors should comment on the reason in their manuscript.
2. Is the device very sensitive to the polarization of the input light? Please clarify that if the reflection and the transmission are polarization sensitive?
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 3 Report
In the manuscript titled “Mode-conversion-based chirped Bragg gratings on thin-film lithium niobate”, the authors demonstrate TFLN chirped Bragg grating waveguide with adiabatic directional coupler (ADC) integrated at its input to drop the reflected light without using an optical circulator or using a 3-dB Y junction with extra 6dB loss. To work with the adiabatic directional coupler, the waveguide width is selected to support at least two TE modes, TE00 and TE01, and the Bragg grating periods are chosen to meet the phase-match condition between the TE00 and TE01 modes. The authors also chirp the grating periods over the waveguide to obtain a large group delay and dispersion slope. In this manuscript, the authors provide a complete device research flow including design, simulation, experimental results, and discussion. However, this manuscript does not show very strong novelty. Using ADC and multi-mode Bragg waveguide has been demonstrated in SOI platform for quite some days, as listed in those reference articles in this manuscript. Using chirped gratings on either straight or spiral waveguides has also been extensively studied. It turns out that the most novelty here is that the authors simply apply the above method/approach on a different photonic platform, i.e., from SOI to TFLN, without any difficulty or block. Besides, there are some issues in this manuscript that need to be corrected and addressed. My comments for this manuscript are listed below:
1. In Fig2(b), what is the wavelength of the effective indices drawn for the TE00 and TE01?
2. What is the spectral response of the ADC used in this manuscript? It is very strange that the authors on line 80, 81 state: “The coupling efficiency between the two modes is 97.6% and the transmission efficiency of the TE0 mode is 99.6% at 1570 nm wavelength.” However, as shown in Fig3, the Bragg waveguide is designed to operate exactly away from 1570nm which should be optimized to match the performance of the ADC. So, what is the design methodology behind this? Why do the authors choose such design for the ADC that is not optimized for the Bragg wavelengths? Please clarify.
3. If the grating periods are selected to meet the phase match condition between TE00 and TE01 mode, why do the authors need to use asymmetric Bragg gratings instead of symmetric Bragg gratings? Please clarify.
4. In the discussion, the statements for the discrepancy from the simulation and experimental results are too general. From Fig6.(b), the ER of the stop-band in the transmission are ~20dB across the whole band. This cannot fully explain the extra high loss seen in Fig6(a) from 1565nm to 1580nm in the reflection spectrum. Please add some discussion on this issue.
5. Given that the sidewall angle is 67 degrees, is there any polarization diversity risk? Does the gratings and sidewall excite any TM polarization?
6. Table 1, the second design (Ref.[18]) is also spiral chirped as the first one.
Other minor error: Line 92, should be 4.7mm not 4.7 um.
Author Response
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Author Response File: Author Response.pdf
Reviewer 4 Report
This paper reports an interesting mode-conversion-based CBG on thin-film lithium niobate. I have some comments.
1. Introduction is missing some critical words about CBG in optical fiber for many application including optical communications and sensing. Please read and add some words about such importance: Optics express 26 (4), 4411-4420, 2018; Opt Quant Electron 54, 429,2022; Optics letters 43 (20), 5106-5109, 2018.
2. How about the the reproducibility of data from Fig. 6? Can we get very similar performance and approach? Please add some details.
3. How can the ripple in transmission can be avoided?
4. Please add some words about the option of tunable version? Can we easily produce it with tunable version. What are the parameters we need focus? Please add some words since it can be useful for many applications and interested for the research community.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 4 Report
The paper is ready for publication
