Influence of Magnetic Field on Amplification without Inversion Induced by a Femtosecond Pulse Train
Round 1
Reviewer 1 Report
The manuscript entitled “Influence of magnetic field on amplification without inversion induced by optical frequency comb” (corresponding author Luo Bin) addresses a mechanism to control the absorption without inversion in a three-level lambda system obtained between chosen Zeeman levels of 5S1/2 and 5P1/2 states of 87Rb isotope. The manuscript proposes a theoretical model based on the density matrix equations, that is solved approximately using an analytical approach. The influence of different parameters on the gain and population inversion obtained in such a system is discussed using different numerical simulations. The manuscript, interesting and original, demonstrates the influence of the magnetic field in the above phenomena. However, the calculations are not associated with any result that may be expected with different experimental setups for 87Rb spectroscopy. Upon correction of this disregard, I recommend the publication of the manuscript.
Precisely, the propagation of laser pulses through the alkali sample is not addressed and the analysis is provided only for the transverse velocity class v=0 m/s. Any possible Doppler, collisional and transit time broadenings are not addressed, for example in the case of spectroscopy in alkali vapour cell or using cold atoms. Please address the question if population inversion and gain may be obtained in presence of these broadening mechanisms.
In addition, there are some minor corrections that I suggest:
Line 58 and figure 1 : use the same notation for DB
Line 64 : Rabi frequency depending on time is not generally used, it may be useful to separate the temporal dependence from the Rabi frequency.
Eq. (4) : explain the notation {} and define Γ
Line 92-93: is not clearly stated if the repetition rate of the absorption is the repetition rate of the femtosecond laser or it depends on the detuning
Also, the manuscript contains many comments of the plots that are not clearly explained and that are valid only for ranges for the parameters in the plots that are not specified:
Lines 98-99: the assertion in not valid for all detunings
Line 100: the mirror symmetry in Fig. 1 is valid only for the B=0 G curves
Line 107: the assertion may be assumed valid on limited regions of the detuning dependences
Fig. 4 : (a) the maximum gain is reached by 0.05 GHz, not by 0.4 GHz, as it is indicated. (d) idem, make right plot at the position of the maximum gain.
In addition, due to the dependence of the Zeeman shift, the effects of the detuning and of the magnetic field are coupled. Please discuss conjointly these effects using the results of the plot.
Lines 140-141: the assertion is not valid for small number of pulses because of the oscillatory dependences, it is valid after ~100 pulses
The references should be verified to be edited with a valid doi number.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
I am not a theoretician and can thus not really check the equations in detail and the found calculation results. But it seems to be reliable that the absorption behaviour of Rb is influenced by a magnetic field under repetited influence of light pulses, as long their time distance is smaller than the relaxation time.
Nevertheless, the paper needs a thorough revision. I made some suggestions in the attached pdf file, and send also a suggestion for extending Fig. 1 (jpg file).
1) What the authors describe is the interaction of a pulse train with Rb vapor. This is NOT a frequency comb, generated by interaction of a pulse train with a non-linear medium, resulting in a wide spectrum of different frequencies separated by the pulse repetition frequency.
Thus in the whole text the wording "optocal frequency comb" must be avoided.
2) Today ultra-short is devoted to pulse durations lower from ca. 10 fs down to the attosec. regime
3) Fig. 1 should be extended, since not all readers are well familiar with the Rb hf splitting. I=3/2 must be mentioned, as well as the relaxation time T1 for the selected Rb transition
4) Lot of small corrections to the language are made in the attached pdf file, without claim for completeness.
5) In Figs. Detuning should be replaced by Detuning Delta (Greek letter)
6 In Figs. where TR is a parameter, the T1 value of Rb should be given.
Comments for author File: Comments.zip
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I would like to thank the authors for carefully addressing in their response letter all comments made by Referee 1 on the manuscript applsci-1529490. Actually, it seems to me that the way deriving eq. (5) from eq. (4) in the manuscript remains still unclear, because Γ should be a matrix expressed in terms of the 87Rb relaxation times. I suggest to provide the expression of Γ, to precise the derivation of eq. (5) for the final version of the manuscript, that I recommend for publication in the Applied Sciences review.
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
In general, the authors followed all my suggestions and the MS is quite improved.
Nevertheless, some additional remarks have been added in the attached pdf file (lines 54, 67, 70, 101).
The authors should also give a statement on the detuning. The Fourier limit shows that for forming the described pulse train a very broad amplification gain of the laser, generating the pulse train, is necessary (in the MS is given 2.2 THz, I estimated 800 GHz, see comment in the MS file). With respect to this value the Detuning Delta is very small: +- 0.4 GHz. Thus as an experimentalist I ask me how to realize such small detuning with a broadband radiation. Eventually it is necessary to re-define what the authors have meaned by "Detuning". I hope they have not treated one pulse as having a sharp frequency. However, a comment to this discrepancies is necessary.
Comments for author File: Comments.pdf
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
