Peer review of: Attosecond electronic timing with rising edges of photocurrent pulses (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
Review of “Attosecond electronic timing with rising edges of photocurrent pulses”, by M. Hyun et al. 
 
This manuscript reports measurements on the timing jitter of electrical pulses from high-speed 
photodetectors. Rising and falling edge jitter are measured (with very high precision), as opposed to 
the “center of mass” jitter that is more often reported in the literature. As the authors note, the 
distinction between the rising edge jitter and center of mass jitter is quite important for some 
applications. The very low rising edge jitter measured here could be useful, even with substantially 
higher falling edge jitter. Relative intensity noise is identified as the leading cause of the jitter, and the 
authors employ strong RIN suppression that greatly reduces the measured jitter. Moreover, the 
authors design and characterize a scheme to reduce the falling edge jitter with balanced 
photodetection. 
 
The paper is well written and the measurements are thorough and quite interesting. I think the paper 
would of interest to the broad readership of Nature Communications. I recommend it for publication, 
provided the authors address the comments below. 
 
My main request is that the authors give a bit more recognition to previous work that shows the power 
dependence on the photocurrent pulse shape. Examples include S. Diddams et al, Opt. Expr. vol. 17, 
p. 3331 (2009), W. Zhang et al, Appl. Phys. B, vol. 106 p. 301 (2012), Y. Hu et al, IEEE Photonics 
Journal, vol. 9, art. 5501111 (2017), and J. Davila-Rodriguez et al, Opt. Expr. vol. 26, p. 30532 
(2018). All these references show and/or discuss the strong dependence on the falling edge of a 
photocurrent pulse with input optical power – much stronger dependence than on the rising edge. I 
would like the authors to consider the inclusion of one or more of these references. In light of these 
papers, it is not so surprising that amplitude noise on the optical pulse train has a strong impact on 
the falling edge. 
 
Under short pulse illumination, MUTC detectors have a photocurrent where the bandwidth is 
maximized and the impulse response is minimized (J. Davila-Rodriguez et al, Opt. Expr. vol. 26, p. 
30532 (2018), among others). This point is a minimum in the conversion of amplitude noise to center 
of mass jitter. Since most of this center of mass jitter would manifest as a falling edge jitter, I would 
think measurements at the minimum-width impulse response point would give lower jitter numbers on 
the falling edge. Did the authors look for this point? It appears that the authors only report on 
photocurrents above this conversion minimum point, where the noise conversion tends to be 
significantly higher. 
 
Lastly, the terms amplitude to timing coefficient (ATC) and amplitude to phase coefficient (APC) are 
both used. Please clearly distinguish the difference between ATC and APC. This may not be 
immediately clear to the uninitiated. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
In their manuscript entitled "Attosecond electronic timing with rising edges of photocurrent pulses", 
Hyun et al. report on a study where a train of picosecond electrical pulses is generated by coupling a 
modelocked Er:fiber laser into various broadband photodetectors. Exploiting the electro-optic effect, 
the authors characterize the timing jitter between the optical and electrical pulse trains and find 
remarkably low values down to the range below 100 as. The information that the timing jitter is 
especially low in the rising edge of the electrical pulse enables generation of broadband periodic 
electrical waveforms which are important for various tasks in contemporary high-speed electronics, 
data processing and information technology. The authors explore the best conditions for low jitter in 
terms of photodiode biasing and optical pulse energy. They demonstrate that the timing jitter 
predominantly arises from amplitude fluctuations of the optical pulse train. This finding is exploited to 
further enhance the quality by active noise control in the Er:amplifer of the optical system. Also, the 
authors demonstrate that elimination of the trailing edge of the optical pulse by generation of the 
differente between two wave forms with adjustable amplitude and time delay almost readily 
suppresses the timing jitter in the trailing edge. The paper is scholarly written and reports interesting 
results which are relevant for front-end contemporary optoelectronics and its applications. Therefore, 
this referee suggests publication of the study in Nature Communications. Beforehand, the authors 
should further enhance their manuscript by working on the following list of questions and comments: 
 
I) One of the main points motivating the present contribution is the fact that most previous work has 
focussed on characterizing the stability of a single frequency out of the harmonics spectrum of the 
electrical pulse train. Naively, from the results of the present paper, one would expect changes in the 
phase noise of each harmonic component depends on how much contributions are coming from the 
rising (mostly high-frequency components) or falling (fundamental and other low-order harmonics?). 
The authors might consider to briefly mention this context together with some analysis that should be 
easy for them to access. 
 
II) The authors convincingly show that the amplitude noise on their optical pulse train represents the 
main source for phase noise in the electrical output. From this, it seems straightforward to at least 
qualitatively understand that the rising edge contains less jitter than the falling one: Basically, the 
ratio is given just by the ratio between the temporal slopes of the amplitude. Is this simple picture 
really correct or is there more physics behind it? Please comment briefly in the paper. 
 
III)The brief sketch in the Methods section and Refs. 21 + 22 are not really adequate to explain the 
principle of the characterization method to a general audience. For an example of a tutorial 
introduction to the principle of ultrafast electro-optic sampling, please see Eur. J. Phys. 38, 024003 
(2017). 
 
IV) The variations in electronic pulse shape the authors observe upon changes of the biasing voltage 
on the photodetectors and number of photons absorbed per pulse are demonstrated to directly result 
in the behaviour of the timing jitter in the electrical pulse train. This entire context is simulated based 
on a model exploiting effective models for each device. Nevertheless, the physical origin for those 
effects is fairly clear when considering high-field transport in III-V semiconductors in combination with 
a finite screening of the biasing electric field due to the separation of photogenerated electron-hole 
pairs. Specifically, there is a sharp transition from ballistic to diffusive transport as soon as electrons 
are gaining energy beyond the intervalley threshold which is fairly advantageous in InGaAs. Please 
have a look at Phys. Rev. Lett. 82, 5140 (1999) and Phys. Rev. B61, 16642 (2000). In case the width 
of the high-field zone of their photodetectors is known, the authors would be able to qualitiatively 
discuss the resulting dependences. This would further enhance the value of the paper for a general 
audience. 
 
V) Interestingly, the data for the MUTC diode under 16 V of reverse bias are not included in Fig. 2(c). 
Nevertheless, a consistency check with the lower degree of change of noise expected from the pulse 
shape in Fig. 2(a) might be worth to mention. 
 
VI) At the bottom of p. 6 of the manuscript, the autors discuss changes in the noise performance 
when varying the optical pulse duration between 150 fs and 1 ps. In a linear regime, those effects are 
surprising given the fact that both pulse durations are far below the the temporal response of the 
elctronic system. Have the authors considered nonliner processes such as e.g. two-photon absorption? 
 
VII) In the lower half of p. 9, the authors state that "the shaped photocurrent pulse does not possess 
negative value in voltage". Why is is important to avoid negative voltages in the pulse form? 
 
VIII) The second sentence of the discussion on p. 10 is almost identical to the abstract. Please avoid 
unnecessary repetitions to save space for further enhancement of the scientific content. 
 
IX) In the first section of the Methods section, the authors state that "The RIN of optical pulse train 
can be stabilized by feedback to the pump laser diode current driver in the EDFA.". Actually, this fact 
is surprising because usually, such femtosecond Er:fiber amplifiers are pumped close to saturation, 
thus eliminating amplitude noise and rendering active stabilization with the pump power a highly 
nonlinear endevour. Can the authors comment on the specific operational setting of their laser 
system? 
 
X) In Fig. 4(a), please include the dark noise of the detection system in order to give the reader an 
impression in how far the stabilized result is limited by this contribution. 
 
 

 
Reviewer #3: 
Remarks to the Author: 
Attosecond electronic timing with rising edges of photocurrent pulses, submitted to Nature 
Communications by Minji Hyun et al. 
 
This work describes how to generate precise timing signals from a femtosecond pulse train. This is an 
important technology because the clock output of a modern optical atomic clock is of this type. 
Moreover, extremely stable radio frequency signals are obtained with this technology. This work sheds 
more light on the limitations of the fidelity to extract these stable timing signals and describes how to 
surpass these limitations. Therefore, I believe it is suitable for being published in Nature 
Communications. The manuscript is very well written. I have only a few comments that might help to 
improve it: 
 
- How is the noise floor measured, that is presented in fig. 1c? By turning off the laser? By blocking 
either the optical or electrical path? In both cases the path length jitters is no included in the noise 
floor. 2nm path length fluctuations correspond to 60as. 
 
- Amplitude to timing jitter is claimed to be the dominant noise contribution. However, it seems like 
this conclusion is obtained only from modelling together with the measured pulse response of the 
photodiode. Instead of potting the distribution of the “amplitude noise” and (computed) “timing error” 
separately in fig.3a one might plot the measured "amplitude noise" vs. the measured "timing error" to 
verify the correlations. 
 
- I suppose the measured RIN reduction presented in fig.4a is measured on loop. As in-loop 
measurements usually don’t allow to draw conclusions on the measured system, this should be clearly 
stated. 
 
- Page 4: a 40dB reduction of the PSD means a 20dB reduction of the timing jitter. Maybe this should 
be said explicitly to not mislead the reader. 
 
- fig1a: inspecting the arrows at the Sagnac interferometer I would conclude that there are only 
exiting arrows. One may wonder how it is possible that the Sagnac has only light leaving it but no 
inputs. 
 
- Fig.1b: it is impossible to see the 60as timing error on a ns time axis. The resolution obviously exists 
as the spectral domain data in fig.1c shows. It might be helpful to see a magnified time domain 
picture that shows the 60as timing jitter. 
 
- Fig2b is a mystery to me. The caption says the pulse position is plotted against the dependence on 
timing, but the axis read timing jitter vs. normalized amplitude. Why is the timing jitter of the order of 
20fs instead of the claimed 60as? What is the meaning of the dashed box at the lower left corner and 
the arrow? I have no clue what this all means and strongly recommend to rework this graph. 
 
- abstract: “… using low-cost and silicon-photonic-integrable …”. I don’t think low cost is a scientific 
argument. This is even more so when we speak about the cost of a photodetector as compared to the 
cost of a frequency comb. 
 
- Equation (1) on page 14: It might help to define the function F, i.e, say that it is the Fourier 
Transform.