Peer review of: Vernier frequency division with dual-microresonator solitons (2020)

 

REVIEWER COMMENTS 

Reviewer #1 (Remarks to the Author): 
 
In this paper, the authors propose and show a new method for generating an RF signal of the repetition rate 
of a microcomb divided by a large number. The method involves interfering two pairs of comb lines of one 
microcomb with a similar comb having a slightly different repetition rate, pumped by the same seed laser. 
The signal can be used to stabilize the repetition rate of the microcomb. The authors compare the repetition 
rate measured by their method with a known method of using an EOM to measure the spacing between two 
comb lines, and find the two methods to be similar in quality. I am not aware of previous work that uses this 
method. 
 
It is true that microcomb work is often interested in slowing down the repetition rate to maintain 
compatibility with electronics. In a lab scenario, this method might not be much different than using an 
EOM, but for practical applications of chip-level systems, this should be quite helpful, as multiple resonators 
can be built onto a chip. 
 
I can imagine this could be helpful for pushing microcomb work towards THz repetition rates. For a wide 
audience, this can be meaningful in the sense of making THz spaced combs more accessible, which may find 
new applications. For the general (low repetition rate) comb community, the technique may not be 
particularly useful. This type of beat mixing is fairly common for noise reduction or passive stability, and 
interfering individual comb lines generally requires an intermediate CW laser. There may be some use in 
dual comb experiments, but otherwise repetition rate measurement is trivial at lower repetition rates. 
 
The writing and available detail is good. Figures are quite detailed, well-labeled, and legible. 
 
One question, there is a line on page 3 using two detectors to improve signal to noise, but it is not clear to 
me what this is (a balanced detection technique?). Maybe a few more words could be added to the 
description. 
 
I recommend this manuscript for publication in Nature Communications. 
 

 
 
Reviewer #2 (Remarks to the Author): 
 
Microersonator frequency combs (Kerr combs) have seen remarkable progress in the last decade and are 
now considered for a variety of cutting edge applications, ranging from optical frequency metrology to 
ultrafast optical ranging. 
 
However, one common challenge is that the repetition rates are so high that they cannot be readout directly 
using electronics. This challenge needs to be overcome to enable many of the interesting applications. 
 
In this paper the authors show that the readout of a large FSR comb can be done using a Vernier scheme 
instead of the conventional dual-comb method which relies on a secondary comb with lower, electronically 
detectable repetition rate. The work is novel and convincing, and the write-up is concise and straight to 
point. 
 
The authors argue and show that the photodetected beatnote of the first sideband comb line pairs can be 
electronically mixed with beatnote at the Vernier overlap point divided by an integer factor, resulting in a 
divided version of the repetition frequency (fr1/N). This division of fr1 was analyzed by comparing it with an 
out-of-loop measurement using an electro-optic modulator (EOM). 
 Next the authors show that they can stabilize the comb’s repetition rate by phase locking the detected 
beatnote (fr1/N) to a rubidium reference. The authors then use an EOM division / downconversion method to 
assess the deviations of the comb’s repetition rate. 
 
The experiments reported in this paper are of high quality and are likely to have an important impact in 
advancing Kerr comb applications. I am happy to recommend publication in Nature Communications. 
 
I do have just a few minor comments for authors to address. 
 
In the manuscript the authors mention “see Methods section for EOM configuration,” but I couldn’t find 
details there, especially on using the EOM to assess the Allan deviations of the repetition rate. 
In the conceptual picture they proposed using the first sideband beatnote the overlap beatnote, but in the 
experiment they’ve used Δ_9 & Δ_11 instead. This is probably done to obtain some favorable choice of 
electrical frequency that fits the BW of their electrical dividers and equipment, however; they should clarify 
and comment on it. 
Was the achieved Allan deviation limited by the Rubidium reference or by other sources (e.g. noise from 
EOM sidebands)? It might be instructive to include the Allan deviation of the Rubidium clock for reference. 
 

 
Reviewer #3 (Remarks to the Author): 
 
Wang and colleagues present a new method of measurement for frequency comb repetition rates of 
hundreds of GHz commonly observed in Kerr microresonator frequency comb generation. Using an auxiliary 
Kerr frequency comb with a comparatively simple degenerate dual pumping setup and a repetition rate 
frequency mismatch of 10%, they manage to divide the original repetition rate by a factor of 200. They 
demonstrate coherent division of the 200 GHz repetition rate to below 20 GHz. 
Established techniques for high rep rate locking are the use of an auxiliary comb, either a RF rep rate Kerr 
comb as in Ref. 15 or electro optical comb generation as for example in ref. 37. Both require a low noise 
independent microwave frequency source and several independent phase locks and are thus at a 
disadvantage w.r.t. the presented method, which performs rep rate locking with a single phase locking loop. 
The paper is well written and illustrated with sufficient detail. The extensive literature on dissipative Kerr 
solitons and is well cited. However, I would like the authors to address the following points, which would 
improve the readers understanding of the method: 
• I suppose the term “free-running” in the caption of Fig.3 means that the laser – cavity detuning is not 
locked. Please clarify in the caption. 
• The frequency division scheme is non-trivial. The authors should explain why the 9th and 11th line where 
chosen instead of the 1st and 10th line as indicated in Fig. 1. Furthermore, the method section should be 
expanded and the frequency division scheme (analog/digital) should be explained in more detail. 
• “It is worth noting that the Vernier solitons are free-running in the entire measurement, and 
the rep-rate of Vernier solitons does not need to be stabilized.” I guess the laser detuning is meant and/or 
the repetition rate of the 2nd Kerr comb. Please clarify. 
• The authors quote the quality factors of the rings in the text with specifying whether or not the intrinsic or 
loaded Q factors are referenced and whether the cavities are operated in the under- critically- or 
overcoupled regime. 
• The term “rep-rate”, while common in the vocabulary of pulsed laser and frequency comb scientists, 
should be either formally defined as an abbreviation or avoided. 
• The authors should also reconsider the ordering of the figures. The experimental setup figure might be 
placed 2nd or attached to Figure 1 and the other figures moved down accordingly. I found myself jumping 
forward in the paper often when reviewing the experimental results in Figs. 2 and 3. 
 
In summary, I support the publication of the presented manuscript in Nature Communications provided that 
the authors address the raised points.