Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this manuscript, the authors explore the performance of an electro-optical up-converter using a semiconductor optical amplifier Mach-Zehnder Interferometer (SOA-MZI). Driven by a 19.5 GHz optical pulse clock, the SOA-MZI up-converts a signal which carries quadratic phase shift keying (QPSK) data, producing mixing frequencies 𝑛𝑓𝑠±𝑓𝐼𝐹. The authors characterize the sampled QPSK signals by evaluating conversion gains, error vector magnitudes (EVMs), and bit error rates (BERs). The authors also compared the simulations with experiments, which demonstrate consistent performance across a 195.5 GHz frequency range. The authors also suggest the feasibility to extend the frequency range to 1 THz by adjusting the optical pulse clock repetition rate and achieving terahertz duplicates at 6 Tbit/s with a 24% EVM at 195.5 GHz. The study contributes to understanding electro-optical up-converters for high-frequency and terahertz applications.

I think this technique is good since it provides a way to convert GHz RF signal to a terahertz signal. Besides, this manuscript is also well-organized and well-written, which could be published if the authors can address the following questions.

Major comments:

(1)     Figure.2 seems a bit confusing. Is the MZM part on the upper left used to provide the QPSK data, which would be measured by PD1 so that it can provide a modulated current with frequency f_IF for SOA1?

(2)     Still about Figure 2. Is the Optical Domain (PM) on the right side of the figure used to beat the optical signal after OBPF and eventually detected by PD2? The arrows on the PD1 and PD2 are a little confusing, where it looks like there are some optical signals outputting from them.

(3)     What’s the advantages of standard modulation approach, compared to the differential modulation approach you previously did? From table1, it seems that the differential modulation approach is much better than the standard approach.

Minor comments:

(1)     In section 3.A, line 166, “3f_s+f_{IF} = GHz”, a number is missing.

(2)     In fig.3 and fig.4, the vertical axis are electrical powers, but the unit are GHz.

 Thank you!

Author Response

Kindly find attached my response.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In the manuscript are the results of another application, Ultra-High Data Rate Transmission, of SOA-MZI system available to the authors. As in more experiments before, they have proven that the system is of a great value to the modern and high-frequency electro-optical mixing area of research. The simulations and are proven by the experimental data which is for me satisfying for proof of concept.

At the beginning I have been lost a bit in the principal of the method and had to check some explanations from other sources, but at the end I have fully comprehended the concept and the results. This experience brings an idea for authors maybe for the future manuscripts, that the full comprehension of the content requires higher expertise and it can lead to lower citation rate of the published paper. The principle of the SOA-MZI system has been published before, but at least the new elements in this paper, such as quadratic phase shift keying and the principal how it is embedded in the signal could have been explained more in detail, as it is quite interesting topic.

Anyway, the manuscript is well written and does not need many corrections. I would suggest to better distinguish the Figure 3 and Figure 4 in the text, as it is quite confusing, especially for Figure 4, if authors are describing simulated or experimental data. I had to go through it more times to realize if the figure 4 shows really experimental data or just more realistic simulations as it does not explicitly raises from the content at page 5 (it is mentioned only very vaguely in the first sentence at row 176) and it is not written even in the figures descriptions. Please add some more hints in the captions and for example in content at row 195 by adding "...of the mixed signal are presented due to the experimental setup limitations.". There is number missing before GHz at row 166. In Table 1 I would suggest to replace the term Frequency range for Frequency limit.

After considering my comments I will suggest to accept the manuscript for publication.

Author Response

Kindly find attached my response.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

1.     The innovation of the paper lies in the use of SOA-MZI to achieve electro-optical mixing in the frequency range of up to 1 THz, and successfully achieve high data transmission rates. This technology has application prospects in fields such as broadband communication.

2.     The article provides a detailed description of the experimental setup and simulation process, including the configuration and performance evaluation indicators of the electro-optic mixing system (such as EVM and BER). The optical conversion gain (OCG) can reach 44.5 dBm, and the electric conversion power is given in detail. When the mixed QPSK data is connected to the tenth harmonic of the sampling signal at 5 Gbit/s, the EVM reaches a minimum of 24% at 195.5 GHz. Suggest further elaborating on the impact of experimental conditions on the results (such as OCG values in different spread spectrum ranges), and how to optimize performance by improving experimental design.

3.     The article discusses the limitations of current research to some extent, but the description of future work directions is not detailed enough. Suggest further exploration on how to address the limitations in existing research and possible future research directions.

4.     The paper cites multiple relevant literature. To ensure the forefront of the research, it is recommended that the author check and update the latest relevant studies to strengthen the literature review section.

5.     In addition, there are some minor errors in the article that have not been corrected, such as the last paragraph of section A in the third section:

the output mixing frequency values ranging from f_S + f_IF = 20 GHz to 3f_S + f_IF = GHz is considered. (The triple frequency plus intermediate frequency signal lacks specific values.) Also, 44.5 dB in Part B should be in dBm units.

Author Response

Kindly find attached my response.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript is well revised and can be considered for publication.

Author Response

Thank you

Author Response File: Author Response.pdf