Microwave Frequency Dissemination over a 212 km Cascaded Urban Fiber Link with Stability at the 10⁻¹⁸ Level

Round 1

Reviewer 1 Report

This paper reports high precise fiber-based microwave frequency transmission. Through the 212 km cascaded urban fiber link, microwave frequency dissemination stabilities are 6.2×10^-15 at 1s and 6.4×10^-18 at 40000s. The paper gives a detailed and comprehensive description of the experiment. The results of this experiment are better than the performance of the current microwave atomic clock. This research is of great significance for the study of long-distance fiber-optic microwave frequency transmission over commercial communication fiber links. Overall, this manuscript is organized well. I would recommend to accept the paper on Photonics. However, there are some minor issues should be addressed before the paper is published.

1. In line 94，that should be "V1=" not "φ0="  ,according to author’s description.
2. In figure 1, please add the annotation of PD in the caption. The PD appear in the figure 1, but PD did not appear in the caption.
3. In section 3 line170-171, “the wavelength difference of optical carriers between local site and remote site was 0.8 nm, which can” How the 0.8 nm wavelength difference of optical carriers between local site and remote site prevent coherent Rayleigh scattering. In addition, what is the influence of the 0.8 nm wavelength difference to the experiment results.
4. Please give a reference to formula (2).
5. Though the reference [10] may in-press, If possible you can give a published reference [10].

Author Response

Dear Reviewer：

Thank you for taking time out of your busy schedule to review the manuscript. Those comments are all valuable and very helpful for improving our paper. Now we studied reviewer’s comments carefully and replied the manuscript for this revision. The main corrections are underlined in the paper. 

Author Response File: Author Response.docx

Reviewer 2 Report

The authors proposed a microwave freuqency transfe method by casscading two sub-links. The method has been widely adopted and experimentally demonstrated by other groups and also the authors’ group. The topic is important and worth to publish. However, the theoretical analysis is not enough for supporting the experimental results.  Before recommendation to publish this article, the authors should be addressed the following comments.

 

Major comments:

The most important contribution of this article is that the phase noise suppression can be enhanced by a factor of 2 as depected by Eq. (3). This hase been widely demonstrated in optical frequency transfer domian as [Olivier Lopez, et al. Opt. Express 20(21) 23518-23526 (2012), Nicola Chiodo, et al. Opt. Express 23(26) 33927-33937 (2015)]. In the optical freuqnecy transfer domain, due to the high carrier frequency of the transferred signal (~193 THz) the phase noise of the system is mainly dominated by one introduced by the fiber link.  I am suprising of the results. Here the transferred singal is below 10 GHz. In this way, the phase noise introduced by the fiber link between optical freuqency transfer and 10 GHz microwave frequency difference has a factor of 19300 difference. The phase noise value shown in Fig. 6 is low. I think the system is partially limited by the system noise floor. Once the system is not dominated by the phase noise introduced by the link, the factor of 2 could not by achieved.  At the same time. from Eq. (3), I couldn’t see the N times improvement by dividing the whole link into N sub-links. The authors should lay the correct equation for clarifying the point.

 

Minor comments:

1) why adopting the same freuqncy for the two systems will introduce the crosstlk? There is not any optical signal and microwave singal leakages between two links.

2) line 170: rayleigh should be “Rayleigh”.

3) line 174: what is structure of the Bi-EDFA? Does the author conside the ASE noise?

4) line 191: the author decleared that  the freuqency counter 5125A couldn’t satisfy the requirements. As far as I know,  the noise floor of 5125A is enough for the results presented in this article.

5) between value and unit should have a space.

6) Engligsh sholud be sigificanlty polished.

 

 

Author Response

Dear Reviewer：

Thank you for taking time out of your busy schedule to review the manuscript. Those comments are all valuable and very helpful for improving our paper. Now we studied reviewer’s comments carefully and replied the manuscript for this revision. The main corrections are underlined in the paper. 

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript presents and demonstrates a method enabling long-distance optical dissemination of reference microwave signals using cascaded fiber links.

The presented experimental using several techniques to reduce technical noises of such microwave compensated fiber links:

• local and remote lasers have slightly different wavelengths to avoid coherent Rayleigh scattering.
• the microwave frequency is shifted between the two cascaded links to avoid cross-talk.

The experimental results are in-line with the state-of-the-art in terms of fractional frequency stability and length of the link.

The work presented here is clearly explained, both from the theoretical and experimental point of views. The results are highly relevant for TF metrology and RF reference dissemination, and a number of technical aspects will be useful to the community.

I only have a few minor requests and some typographical comments on the manuscript:

comments and questions:

• p. 2, l.86, Vr should be defined.
• p. 2 l.94: to follow Fig. 1 the "received signal at remote" should be noted V_1 instead of phi_0.
• Fig. 1, the error signal Ve is directly fed to the OCXO, the loop filter is ommitted. That could be added or mentionned in the text.
• If I understand correctly, Fig 8 shows the residual instability of the two fiber segments and of the 10 to 3.6 GHz conversion. The quadratic sum of these instabilities is 5.3 10^-15 at one second, close the measured cascaded link stability (6.2 10^-15 at 1 s). Maybe the authors could comment on that?
• The authors mention the deterious effect of crosstalk between cascaded segments. It would be interesting to know to what level this crosstalk is problematic, or to provide references about this.

typos:

• p.1, 32 "geodetic measurement" -> "geodetic measurements"
• p.1, l. 33 ""satellited-based" -> "satellite-based"
• p. 2 l.62 "was not attracted" -> "has not attracted"
• p. 3 l. 104: "effective means, which" -> "effective means, with which"
• p. 4 l. 130 "10GHz" -> "10 GHz"
• p. 5 l. 153 "was 3.6 GHz, the" -> "was 3.6 GHz. The"
• p. 5 l. 166 "100MHz"-> "100 MHz"
• p. 9 l. 282 "larger correct bandwidth comparing to" -> "larger correction bandwidth compared to"

Author Response

Dear Reviewer：

Thank you for taking time out of your busy schedule to review the manuscript. Those comments are all valuable and very helpful for improving our paper. Now we studied reviewer’s comments carefully and replied the manuscript for this revision. The main corrections are underlined in the paper. 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Although the author edits the equation for estimating the cascaded stability as shown in Eq. (3) in the revised manuscript. The equation is estimated in the case that the system is mainly constrained by the fiber phase noise. This condition is usually not existing in the microwave frequency transfer system. At the same time, the results are different from the previous results from the authors' group. So recheck the relevant conclusions. 

Author Response

Thanks a lot for your comments

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

The present content is more reasonable.