Peer review of: Reconfigurable structured light generation in a multicore fibre amplifier (2020)

 

REVIEWER COMMENTS 
Reviewer #1 (Remarks to the Author): 
 
This paper reports a new method to generate high-power reconfigurable structured light, including arbitrary 
vortex and/or vector beams. It is based on the multicore-fiber amplification of a set of “beamlets” 
distributed in a given pattern. The independent control of amplitude, phase and polarization of each beamlet 
allows one a large freedom in controlling the resulting superposition occurring in the far field. A feedback 
control is then used to finely adjust these quantities in order to obtain a given target structured field. This 
proposed method is particularly interesting to achieve structured light of very high power, as other methods 
to structure an incoming beam of light (such as SLMs or q-plates) are usually not viable for very high 
powers. 
 
There is no very fundamental innovation in this work, as the approach based on combining a set of beamlets 
for structuring light was already known and multi-core fiber amplifiers are also well known. But the overall 
method itself is novel and efficient, it seems also convenient and flexible, and could prove very valuable for 
certain applications. The paper is also well presented and convincing. Overall, I think this work satisfies the 
criteria of importance and interest needed to be published in Nature Communications. 
 
 
Reviewer #2 (Remarks to the Author): 
 
The authors present a study and experimental demonstration of beam shaping using a 7-core fibre (with 
hexagonal arrangement of the cores) employing tiled aperture coherent combination. In general, this paper 
is interesting, and the results are of good quality, so I would like to recommend it for publication. The only 
thing that refrains me from doing so is that the manuscript, in its present form, is marred in some passages 
by a lack of details and, sometimes, (slight) inaccuracies. These should be addressed and corrected before I 
can give my approval to the publication of the paper, since the lack of information makes it difficult to judge 
whether the results are good enough for a journal such as Nature Communications. Let me share with you 
some of my observations (in the other of appearance in the paper). 
 
In the introduction the authors should be a little bit more exhaustive on their references to other works. For 
example, there has been some work on CV beams using an all-fiber setup that, in my opinion, should be 
referenced here. The references are: 
 
*Christoph Jocher, Cesar Jauregui, Christian Voigtländer, Fabian Stutzki, Stefan Nolte, Jens Limpert, and 
Andreas Tünnermann, "Fiber based polarization filter for radially and azimuthally polarized light," Opt. 
Express 19, 19582-19590 (2011) 
 
*Christoph Jocher, Cesar Jauregui, Martin Becker, Manfred Rothhardt, Jens Limpert and Andreas 
Tünnermann, “An all-fiber Raman laser for cylindrical vector beam generation,” Laser Phys. Lett. 10 125108 
(2013) 
 
Additionally, there has been some work on adaptive beam shaping using photonic lanterns that should also 
be cited, since the physical underlaying principle is somehow similar to the one used in the paper. This 
references are: 
 
*Juan Montoya, Chris Aleshire, Christopher Hwang, Nicolas K. Fontaine, Amado Velázquez-Benítez, Dale H. 
Martz, T.Y. Fan, and Dan Ripin, "Photonic lantern adaptive spatial mode control in LMA fiber amplifiers," Opt. 
Express 24, 3405-3413 (2016) 
 
*G. Lopez-Galmiche, Z. Sanjabi Eznaveh, J. E. Antonio-Lopez, A. M. Velazquez Benitez, J. Rodriguez 
Asomoza, J. J. Sanchez Mondragon, C. Gonnet, P. Sillard, G. Li, A. Schülzgen, C. M. Okonkwo, and R. 
Amezcua Correa, "Few-mode erbium-doped fiber amplifier with photonic lantern for pump spatial mode 
control," Opt. Lett. 41, 2588-2591 (2016) 
 
*Sergio G. Leon-Saval, Nicolas K. Fontaine, Joel R. Salazar-Gil, Burcu Ercan, Roland Ryf, and Joss Bland-
Hawthorn, "Mode-selective photonic lanterns for space-division multiplexing," Opt. Express 22, 1036-1044 
(2014) 
 
Even though the authors mention this afterwards, I would like that they say right up front that they use a 
tiled aperture combining approach in the description of the working principle. This would make the operating 
principle easier to understand. 
 
It would be nice if the authors could include the theoretical combining efficiency of the usable (central) part 
of the beams presented in Fig. 1. Just for the reader to get an idea of whether more complex beams incur in 
higher combining penalties or not… 
 
Please include more information about the SLM: number of pixels, pixel size, aperture, operating wavelength 
region, etc. Likewise, please describe in the main text in more detail the excitation scheme (some of these 
details can be found in the long figure captions, but the reader should not be forced to look for the 
information there). This would improve readability. 
 
Also include more information about the MCF: fiber outer diameter, polymer coating or not, doping 
concentrations, etc. Additionally, from Fig. 2 it seems that the cores are surrounded by a low-index trench. 
If this is the case please mention and describe it. Additionally, if the low index tranches are there, why do 
the authors use such an enormous (in relation to the core diameter) core-to-core distance? Is it really 
necessary to avoid core-to-core cross-talk? 
 
Which are the insertion losses of the beam-shaper? How high is the coupling efficiency of the shaped beam 
into the MCF? 
What is the bandwidth of the feedback loop? 
 
Why do you have the power unbalance seen among the lobes of the beam shown in Fig.4 (for the LP31)? 
 
Which are the correlation values of the shaped beams with the ideal beam? Please provide these values for 
all the measurements. 
 
Do you have any problems with self-lasing in the configurations in which you left some cores unseeded? 
Would it be possible to increase the output power further or those self-lasing (or any other effects) limit 
this? Please elaborate on these topics in the manuscript. 
 
Please include how long does your system need to reach the optimized output beam. 
 
Even though the authors claim that their measured 44% combination efficiency value is “close” to the 
expected theoretical value of 61%, I do not agree. I think that both values are very far away. So the authors 
should tone down their claim, explain why they didn’t reach the theoretical expectation and what can be 
done to improve this result. I am also surprised about the “high expected” combination efficiency. 60% is 
about the maximum that you can get when generating a Gaussian beam in the far field. Thus, I would have 
expected that the combination efficiency of more complex beams is somewhat lower. Please, to give readers 
like me a better feeling of what can be expected in terms of combining efficiency, include the theoretical 
combining efficiency in all your examples. 
 
The authors claim that their system is essentially operating in open loop. If this is the case then, how 
sensitive is the situation to changes in temperature and/or the exact position of the fiber? How stable are 
the optimum solutions in time? When you go to the lab in the morning do you need to look for a new 
optimum solution each day or does the one from the previous day still work as intended? 
 
The authors mention that conventional CBCs always require an active stabilization and control loop. This is 
inaccurate. If you only want to have the temporal stability shown in Fig.7, then you so not need any kind of 
feedback loop with CBC either. However, if you want to improve on that, then you would need a feedback 
loop both in conventional CBC and in the system presented by the authors. In essence, I do not think that 
their system is intrinsically more stable than a conventional CBC. Please justify or remove this sentence. 
 
Provided that all these points are addressed I would have enough information to judge whether the results 
are good enough for Nature Communications (and in principle they certainly look like 
they are). 
 
Best regards,