Peer review of: High-performance coherent optical modulators based on thin-film lithium niobate platform (2020)

Reviewers' comments: 
Reviewer #1 (Remarks to the Author): 
 
 
This is a paper by the same authors who published a paper in nature photonics on a very similar 
device(“High-performance hybrid silicon and lithium niobate Mach–Zehnder modulators for 100 Gbit s−1 and 
beyond”). The improvement is that they only use the larger multi-mode LN waveguide instead of using 
evanescent coupled small LN waveguides, and the device shows a very low insertion loss. At the same time, 
the authors use coherent equipment to do the coherent modulation test at a higher baud rate (maximum 
110 GBaud). The results are pretty good. There are some technical questions about the manuscript, which 
follow below. 
 
What are the modal properties of the LN waveguide? It looks like a multimode waveguide. Why didn't the 
authors choose a single mode waveguide? Only to have a lower optical loss? What is the loss of the MMI? 
What do they give up by going multimode? 
The caption of Fig.2 is wrong. The EO line is green and the TO line is blue. 
The Vpi description in Fig.3 caption is wrong. It should be 3.1 V instead of 7.5 mm. The wavelength data is 
(c) not (b). 
The intrinsic extinction ratio is around 25 dB. Is this a general phenomenon or a singular case? What is the 
limitation for this performance? 
Regarding the s21 response, there are two large ripples at around 40 GHz and 56 GHz for both the 7.5-mm-
long and 13-mm-long devices. What is the reason for this? 
In fig.5, it is mysterious that constellation diagram at 80 GBaud is better than that at 60 Gbaud, but the BER 
is worse? The constellation diagram is swapped or something else affects this? 
For the 16-QAM measurement, the device is over driven from the constellation diagram. Lower driving 
voltage is suggested to exploit the performance of the device, especially for Fig.5(h). 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
This paper presents what is claimed to be the first demonstration of an IQ modulator in thin film lithium 
niobate. The structure is traditional, consisting of two parallel Mach-Zehnder interferometers nested within a 
further Mach-Zehnder structure with a 90 degree phase shift introduced before combination. 
 
The Mach-Zehnder structures are very traditional similar to [1] with velocity and impedance matched 
electrodes in a push-pull configuration. The device length in this submission are 7.5mm and 13mm. The 
Loncar demonstration [1] has longer electrodes (10mm and 20mm), but with similar geometry. Loncar 
shows a Vpi of 1.4V with a bandwidth over 40GHz, while this submission demonstrates 1.9V with a 
bandwidth over 60GHz. So the drive voltage is not the highest reported. 
 
The bandwidth is also not the highest reported. The publicaiton [2] by Prather shows bandwidths can exceed 
100GHz (approaching 1 THz) - however with a degraded Vpi of 3.8V. 
 
The submitted paper quotes an optical insetion loss of 0.5dB/cm, while [1] quotes 0.5dB for the entire 
structure. The submitted paper does not report on the total insertino loss of the chip and in particular does 
not report the insertion loss due to propagation in proximity of the modulation or bias electrodes - it is 
important that these numbers be quoted as it is important that the electrodes do not interact strongly with 
the light for practical operation. 
 
The submitted manuscript devotes a significant portion of the paper to reporting on the thermo-optic phase 
controllers. After a quick scan of the literature, I did not find any other report of thermo-optic switches on 
LNOI, so this may be innovative. However, the analysis done by the authors is not convincing. While electro-
optic behaviour can be assessed based on the voltage required, the thermo-optic switches cannot be 
compared on voltage alone. The current must also be considered in order to assess the power draw. 
 
I also do not agree with the authors about the 'LN is well-known for its drifts in the DC bias point upon the 
application of a static electric field, which is a phenomenon that originates from the piezoelectric nature of 
the material". The authors must provide a reference for this statement. Bias control is necessary for 
interferometric lithium niobate devices due to the pyroelectric effect (changes in temperature cause a 
change in surface charge which in turn changes the refractive index). I know of many lithium niobae devices 
that do utilise an electro-optic bias and are able to maintain stability. Perhaps the authors are effectively 
controlling the temperature of the LNOI film with their TO heaters? I do not believe that their analysis of the 
instability of their own device when driven electro-optically provides any scientific insight and hence, I would 
encourage presenting the thermo-optic switches in a more technical journal where it can be the focus and 
referencing this in a higher profile paper describing the 'world first' IQ measurement. 
 
Overall, the LNOI technology presented here does not, in my view, represent any significant advancement in 
the state of the art; however, the implementatino of a IQ modulator may indeed be a world first, and 
perhaps this is significant enough to warrant publicatino in a high profile journal such as Nature 
Communications. However, in order to achieve this level, I think the focus needs to be placed more on the 
system demonstration that harnesses the IQ modulator (rather than the technology used to realise the chip 
itself) and it would need to be demonstrated that a record breaking transmission characteristic was achieved 
(such as modulation density, or power efficiency?). 
 
I also note a number of typographical errors throughout the manuscript (for example "both branches of the 
13-mm and 7.5-mm devices as a function of the applied voltage, showing Vπ of 1.9 and 7.5 mm, 
respectively." - the last number should be the Vpi of the 7.5mm electrode, not its length). There are several 
others. 
 
[1] Wang, C., Zhang, M., Chen, X. et al. Integrated lithium niobate electro-optic modulators operating at 
CMOS-compatible voltages. Nature 562, 101–104 (2018) 
 
[2] Mercante AJ, Shi S, Yao P, Xie L, Weikle RM, Prather DW. Thin film lithium niobate electro-optic 
modulator with terahertz operating bandwidth. Optics express. 2018 May 28;26(11):14810-6. 
 
 


Reviewer #3 (Remarks to the Author): 
 
Lithium niobate is one of the most important materials for optical modulators. With the LNOI (lithium 
niobate on insulator) platform, new types of optical modulators were introduced in recent years. This kind of 
modulators has the advantages of small volume and high performance, such as low driving power, high 
bandwidth, etc. However, LNOI based modulators still face several important questions. 1. Can this 
modulator be used on short reach links, such as metro and data-center interconnects? Modulators for short 
reach links account for the most amount of the modulator industry production. In such application, the basic 
requirements for the modulator are small volume, low optical loss, low power consumption, and low cost. 
Currently the standard choice is the semiconductor (InP, for example) based modulators. InP modulator has 
the drawbacks of high optical loss and non-linearity. LNOI waveguide showed an ultra-low loss down to 0.03 
dB/cm (Optica 4, 1536, 2017), and conventional LN modulator showed a good linearity. Therefore, LNOI 
modulator has a potential to be a strong competitor with InP modulators. 2. For LNOI modulator, which 
fabrication routine will be adopted? In other words, what is the future technology roadmap? Two 
complementary fabrication routine were used to make LNOI based modulators: monolithic and hybrid. 
Monolithic means direct etching of LN to form the waveguides (Nature 562, 101, 2018). This kind of 
modulator theoretically has a better device performance. An example of hybrid is SOI-LNOI modulator (Nat. 
Photon. 13, 359, 2019). This modulator has the advantage of the mature fabrication technology, and 
easiness to integrate with photo detectors (such as epitaxy of SiGe detector). Which fabrication routine will 
be the mainstream? This question is still not clear to the academy and the industry. The people in industry 
especially care about this question, because they concern the R&D inputs. 3. For conventional LN modulator, 
a major problem is DC drift. This phenomenon is attributed to the piezoelectric or pyroelectric properties of 
the LN material. People spent a lot of efforts to solve this problem. For LNOI modulator, because it is a new 
research field, there is very few data or public report on the DC drift problem. 
This manuscript reported the first monolithic IQ (in-phase/quadrature) LNOI modulator used for short reach 
links, solved the DC drift problem using TO (thermal-optic) phase shifters. The device dimension is small (15 
mm), and the performance is excellent, such as low loss (1.45 dB), low VπL (2.5 V cm), high data rate (320 
Gbit s−1), and large bandwidth (> 67 GHz). 
The drawback of this work is that TO phase shifters needed electric power to operate, which increased the 
power consumption. 
This work provided the strong evidences to answer the three questions mentioned above. It will be very 
interesting to the people working on the modulators. 
A publication of the manuscript is highly recommended. However, the authors should response the following 
comments before the publication. 
1. The authors can give some comments on the comparison on the modulators fabricated by hybrid or 
monolithic. Since the authors had the experience and insight on SOI-LNOI hybrid modulator (Nat. Photon. 
13, 359, 2019), and this manuscript was about monolithic modulator, the authors’ comments would be very 
useful to the readers. 
2. The authors can give some comments or data on the multimode interference couplers (MMIs) in this 
work. There is few report about MMI coupler on LNOI. Any information about it will be useful. 
3. The authors used TO phase shifter to suppress the DC drift, but the TO effect would increase the power 
consumption of the modulator. Can EO effect be used for DC drift control in LNOI modulator? 
4. The authors have to check the content of the manuscript carefully to avoid any miswritting, for example, 
in line 73, “Fig. 2b” should be “Fig. 1b”.