Generation of Millimeter Waves and Sub-Terahertz Waves Using a Two-Wavelength Tunable Laser for a Terahertz Wave Transceiver
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis paper reports the generation of millimeter waves at approximately 17 GHz and sub-THz waves at approximately 300 GHz by converting the frequency difference of a two-wavelength tunable laser fabricated using silicon photonics into an optical-electrical signal. This device is expected to be used as a two-wavelength tunable light source for THz wave transceivers. This paper could be published in Photonics, after the following questions and comments are addressed.
1. The advantage of generating RF signals through down-mixing of dual-wavelength lasers is the independent control over their wavelengths, enabling the production of signals at any frequency, even up to the high-frequency THz range. However, according to the results in this work, this dual-wavelength laser seems to be affected by thermal crosstalk, which could impact its independent control. Could you provide a detailed analysis of the range and limitations of independent control for this dual-wavelength device?
2. For subsequent RF generation applications, a single dual-wavelength output beam should suffice. Why then, in the work, is there a need to introduce an MZI to adjust the output light signals in the Psoa and Psi directions? What is the original intention of this design? Additionally, could you provide experimental values for the reflectivity and loss of the DBR?
3. Regarding the experimental results of 43.6mW and 54.6mW in Fig.9, the wavelength shift of Filter2 does not appear to be 'slightly shifted' as described by the authors. This could potentially cause a frequency change in the RF signal by tens of GHz. Are the tests in Fig.9 stable and repeatable? How might this be addressed in the future?
4. The authors stated, "The minimum wavelength interval obtained from this measurement was 0.14 nm, which corresponds to a frequency difference of approximately 17 GHz." What are the limiting factors that prevent further reduction in the dual-wavelength interval?
5. In Fig.12, it is shown that although the MZI can control the power variation of the two outputs, the overall power on the SOA side is significantly lower. What is the reason for this?
6. The authors mentioned, "Beyond 80 mA, the emission spectrum became unstable, and multimode oscillations were observed at currents exceeding 180 mA." What is the specific reason for the generation of multimodes with the increase in current?
7. What is the cause of the peak observed at the edge of the RF signal in Fig.15b?
8. If new optical frequency components are generated by four-wave mixing, is it possible to observe and verify the other mixing components in the microwave frequency range? And can we quantitatively analyze how much the target frequency mixing efficiency has been reduced as a result?
9. Could you provide a detailed explanation of the physical reasons and mechanisms behind the linewidth variation caused by the change in optical power as depicted in Fig.16b?
10. In the experiment shown in Fig.17, why was the original device not used, and instead, a device without an MZI was adopted? What are the performance differences between them? Additionally, why was the SOA side with a lower output power selected?
11. Why isn't the linewidth of the generated sub-THz as narrow as it is at lower RF frequencies?
12. Regarding Fig.19, what limits the tuning range? What would happen if the input power of ring2 was decreased or further increased? Would there be a sudden change in the signal frequency?
13. Some key device parameters in the manuscript are missing and need to be supplemented. For instance, parameters for the utc-pd, such as bias and responsivity, and parameters for the mixer, like conversion efficiency, the presence of a local oscillator input, and its corresponding power, among others.
Author Response
The reply haa been uploaded as a pdf file.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsYuga and co-author have done nice work by employing two-wavelength tunable laser (TWTL) to generate millimeter and sub-THz waves. Their experimental data are solid and well presented. I recommend this paper for publishing after minor revision. Here I also provide some additional questions for the authors:
1. The generated sub-THz waves show power below 30 dBm, which is quite low. Could the authors explain why the conversion efficiency is low and provide some proper solutions to enhance the output power towards practical applications?
2. By the way, the generated millimeter waves are also less efficiency. The authors should provide reliable solution for enhancing the conversion efficiency (from the perspectives of device fabrication, system coupling efficiency, and many other things).
3. I’m wondering if some new integrated on-chip photonics could be helpful for highly efficient THz generation (please cite “Progress In Electromagnetics Research, Vol. 174, 33-42, 2022”). Could the authors give some comments?
Author Response
The reply haa been uploaded as a pdf file.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsI am satisfied with the authors' corrections and willing to accept the paper.
