Relocking and Locking Range Extension of Partially Locked AMLL Cavity Modes with Two Detuned RF Sinusoids
Round 1
Reviewer 1 Report
An investigation of actively mode-locked fiber ring lasers is presented. The authors use multiple RF inputs to extend the locking range of cavity modes and develop a theoretical model to understand the effect more deeply. As a result the locking range for 10th mode was extend to 6 kHz. It is quite interesting approach that can be applied in many applications, however, the article should be improved in a few points before publication:
1. Harmonic mode-locking [1*] term must be added to keywords, because it is exactly that the authors meant on an Nth cavity mode and it is quite difficult to understand in the first reading. This link must be established clearly in the introduction. In fact HML of different orders has been investigated but this fact remained veiled.
2. Line 27, "... AMLL occurs leading to broadening of pulses due to lock-loss of higher modes". All longitudinal cavity modes have a very high order because we are talking about a number of wavelengths that are in the cavity. It is better to write about carrier wavelength and about the modes that are more detuned from it.
3. AML phenomena has a wide range of application in terms of lasers operation modes which is not revealed in the introduction. In particular it allows to obtain a pulsed generation in Raman fiber lasers. So I offer to additionally discuss in the introduction a list articles related to AML [2*-4*].
4. Line 130, what is exactly meant the mode number n=19-55 = -36? According to my previous comment it should be HML order.
5. Line 169. "Note that pedestals in the traces could arise due to the averaging of pulse traces" Why you need to perform an averaging? Why not make single-shot measurements to show clear pulse shape?
6. Fig.4c. It would be interesting to show the pulse trains in much more wide range and add the control signal (two sinusoids) passed to the modulator for better understanding the influence of detuning to mode-lock regime.
Misprints:
line 40, double "to"
line 141, double "signal"
line 189, wrong index in R_10
References:
[1*] https://www.rp-photonics.com/harmonic_mode_locking.html
[2*] N. A. Koliada et al., “Generation of dissipative solitons in an actively mode-locked ultralong fibre laser,” Quantum Electron., vol. 43, no. 2, pp. 95–98, Feb. 2013.
[3*] X. Yang, L. Zhang, H. Jiang, T. Fan, and Y. Feng, “Actively mode-locked Raman fiber laser,” Opt. Express, vol. 23, no. 15, p. 19831, Jul. 2015.
[4*] A. G. Kuznetsov, D. S. Kharenko, E. V. Podivilov, and S. A. Babin, “Fifty-ps Raman fiber laser with hybrid active-passive mode locking,” Opt. Express, vol. 24, no. 15, p. 16280, Jul. 2016.
[5*] A. G. Kuznetsov, S. I. Kablukov, Y. A. Timirtdinov, and S. A. Babin, “Actively Mode Locked Raman Fiber Laser with Multimode LD Pumping,” Photonics, vol. 9, no. 8, 2022.
Author Response
The Authors thank the Reviewer for their valuable comments. Incorporating them has increased the strength of our work. By adding Raman lasers as potential application our work will reach others who might be able to adopt our work for their research, which is the ultimate goal of this endeavor. Their comment has also opened the avenue for future investigation of the temporal behavior of partially detuned AMLL with relocking RF signal in understanding pulse shape control. Please find our responses to the comments here.
Response to comment 1 – The Authors thank the Reviewer for identifying the role of the harmonics arising from the fundamental locking frequency contributing to the Nth cavity mode. Please note that in this research article only the relocking when AMLL is detuning of the RF signal around the fundamental resonant frequency f_0 has been addressed. An extension of the detuning for RF signal at any harmonic can be found in Chapter 3 of SK’s thesis (S. Krishnamoorthy, “Mode unlocking and relocking in a detuned actively mode locked fiber ring laser,” Ph.D. thesis, Indian Institute of Technology Madras (2020)). The AMLL driven at a harmonic RF frequency only excites a subset of the cavity modes. The effect of relocking of a detuned harmonically mode-locked with a second detuned RF signal is beyond the scope of current research article. [Lines 25-28]
Response to comment 2 – The Authors thank the Reviewer for this comment. We have modified the text to clarify this point. [Lines 23 – 25, 78 - 81]
Response to comment 3 – The authors thank the Reviewer for this valuable addition to the MLL applications. This has been incorporated into the introduction. [Lines 28-30]
Response to comment 4 – Authors mean modes with mode numbers n = 19 to 55 have been represented. The text has been modified to convey this explicitly. [Lines – 139, Fig 2 legend]
Response to comment 5 – The Authors recognize the suggestion to make single-shot measurements and showing pulse trains as a valuable addition to understanding the relocking process, especially in time domain. Kindly note that the work presented in this article focuses on the cavity modes of the AMLL. Here, we demonstrate the relocking and the extension in locking range of the AMLL. Due to the focus on the cavity modes in both the theory and the experiments in this article, the Authors have focused on the spectral content of the pulses where the locking state of the cavity modes is readily accessed. The time domain traces were collected with the hope that it presented a summarizing picture as shown in Fig. 4 of the nature of the pulses with slowly varying process that manifests as pulse bunches within which the pulses have different shapes and pulse widths. To elaborate the nature of varying pulse shapes we added Fig. 4 (c) where the constituent pulses in a pulse bunch are shown.
The individual pulse profiles within the bunch would surely be a very interesting future investigation into the time-domain nature of relocking. At the time of preparation of this manuscript, the Authors did not have access to equipment where single-shot measurements, like FROG, could be made. Since the writing of this article, the setup has been dismantled and repurposed for other scientific explorations. The first Author has moved on to other career positions. [Fig. 4(c)]
Response to comment 6 – Please see Authors’ notes for comment 5. [No change]
Again, we thank the Reviewer for such a valuable input to strengthen the manuscript with their expert insight.
-Regards
SK and AP
Reviewer 2 Report
Comments for author File: 
Comments.pdf
English language required to be improved.
Author Response
The Authors thank the Reviewer for their valuable comments. We have addressed the comments provided by the Reviewer
Response to comment 1 – The Authors thank the Reviewer for this comment. We have included a sub-paragraph highlighting the novelty of the work presented. We have also expanded on the applications of this technique to include Raman fiber lasers. The comment has helped highlight the impact of our work. [Lines 28-30, 42-48]
Response to comment 2 – The Authors thank the reviewer for the comment. We have ensured that all the parameters and the relevant assumptions are accessible to the reader. In addressing this comment we have increased the rigour of the work presented here. [Lines – 62 – 69, 72, 80-81]
Response to comment 3 – The Authors thank the Reviewer for this comment. The text has been modified to convey this explicitly. [Lines – 139, Fig 2 legend]
Response to comment 4 – The Authors thank the Reviewer for highlighting this issue. We have made suitable modifications and it has greatly improved the readability of the figures. [Fig. 4(a)]
Response to comment 5 – The Authors thank the Reviewer for the suggestion. We have made suitable modifications and it has greatly improved the presentation of our results. [Fig. 4(b)]
Response to comment 6 – The Authors thank the Reviewer for the suggestion. We have made suitable modifications and added a new subfigure. This modification has helped us to better convey the intricacies of the pulsetrain generated by our new method. [Fig. 4(c) - new, 4(d)]
Their input has strengthened our research and helped us for which we are grateful.
-Regards
SK and AP
Reviewer 3 Report
The authors reported the actively mode-locked Yb-doped fiber ring lasers modulated by two RF sinusoidal signals. It was demonstrated that the two inputs method has an advantage of extending the locking range of cavity modes, leading to a decrease of pulse width. I believe that this manuscript is suitable for publication after minor revision. My comments on the matter are given below:
1. In Figure 2, is the case with single input marked in blue? According to the line 131, the authors said that the case of one input is present in Fig. 2a-left.
2. Figure 4 shows that compared with the case of one input, the generated pulse of the AMLL with two inputs has a central peak with reduced temporal width on the wide pedestal. However, the physical mechanism behind the result should be more detailedly discussed.
3. Figure 4(c) shows the occurrence of pulse bunches with time periods of 3.77, 2.8 and 4.73 μs. Is the operation state related to switched Q state?
Author Response
The Authors thank the Reviewer for their valuable comments. Incorporating them has increased the strength of our work. In addressing the Reviewers comments, a nuanced explanation regarding the pulse shapes has been incorporated which has surely strengthened the manuscript.
Response to comment 1 – Corrections made to the legend of the figure to represent the cases correctly. [Lines – 140, 142, Fig. 2 legend]
Response to comment 2 – The time varying pulse shape is predicted by the slowly time varying phase-lock condition as encompassed in the D_nrk term in equation (20). The averaged pulse with the pedestal is a summarizing picture of the same. This argument to the time-varying nature of the phase-locking resulting in the changes in pulse shape and pulsewidth has been included in the manuscript. [Lines 186 - 190]
Response to comment 3 – The repetition rate of the pulse bunches corresponds to the difference in frequencies as indicated in the slow phase term D_{nrk} in equation (20). This is not a Q-switched state, but a partial mode-locked state with two-inputs which is a novel technique of operating a fiber AMLL as demonstrated in this paper. This has been elaborated in the text in the paper. [Lines 186 - 190]
Again, we thank the Reviewer for such a valuable input to strengthen the manuscript with their expert insight.
-Regards
SK and AP
Round 2
Reviewer 2 Report
The manuscript has been sufficiently improved. It can be accepted for Photonics.
