Tunable 60 GHz Multiwavelength Brillouin Erbium Fiber Laser

Round 1

Reviewer 1 Report

The paper report the experimental study of tunable 60 GHz multiwavelength Brillouin erbium fiber laser. Four sextuple Brillouin Stokes signals with high power of 10 dBm and more than 55 dB as an optical signal to noise ratio are achieved. The obtained Brillouin Stokes signals can be tuned over 30 nm 23 (1560-1590 nm). The work is interesting and have potential application in dense wavelength division multiplexing in optics communication systems. The manuscript can be accepted for publication after several points have been addressed.

1. What about the coherence of the Stokes signals in Figures 3, 4 and 7?

2. In Figure 6, why didn’t the pump power increase with equal space? Why did the pump power between 150 mW and 350 mW missing?

3. Line 203, “Figures 6a and 6f” should be “Figures 7a and 7f”.

4. Figure 8 provides the short-term stability of the Stokes signals. What about the long-term stability of the Stokes signals, for example several days or weeks?

Author Response

Reviewer #1

The paper report the experimental study of tunable 60 GHz multiwavelength Brillouin erbium fiber laser. Four sextuple Brillouin Stokes signals with high power of 10 dBm and more than 55 dB as an optical signal to noise ratio are achieved. The obtained Brillouin Stokes signals can be tuned over 30 nm 23 (1560-1590 nm). The work is interesting and have potential application in dense wavelength division multiplexing in optics communication systems. The manuscript can be accepted for publication after several points have been addressed.

Thanks to the reviewer for his constructive and important comments

1. What about the coherence of the Stokes signals in Figures 3, 4 and 7?

The generated Stokes signals in figures 3, 4, and 7 are coherent since it is generated according to stimulated Brillouin scattering (SBS) phenomenon inside the Brillouin gain medium spools (DCF1, DCF2, DCF3, and SMF). After the inserted optical power reach the threshold of SBS, the coherent Stokes signal generated which experiences Brillouin gain and propagated as a stimulated photons from the end of the fiber towards the inserted fiber edge.

2. In Figure 6, why didn’t the pump power increase with equal space? Why did the pump power between 150 mW and 350 mW missing?

The below paragraph is added into the revised version.

At 150 mW the first sextuple Stokes signal was appeared (6th Stokes signal). With the increment of pump power, no higher order sextuple Stokes signal (12th Stokes signal) was generated until the pump power value of 350 mW. In another word, no change in the number of the generated Stokes within the range of 150 to 350 mW. Increasing the pump power further, the third sextuple Stokes (18th Stokes signal) was effective at pump power value of 450 mW. 

3. Line 203, “Figures 6a and 6f” should be “Figures 7a and 7f”.

Thank you it is corrected in the revised version.

4. Figure 8 provides the short-term stability of the Stokes signals. What about the long-term stability of the Stokes signals, for example several days or weeks?

We added an additional figure (figure 8b) in the revised version included the stability within 12 hours for each 60 minutes. Unfortunately, TSL and OSA are common for all researchers in the laboratory. Therefore, we are not able to use them for multiple days or weeks. The below explanation and figure are added into the revised version.  

 

Figure 8a, b shows the peak power stability of the generated Stokes signals (Bp, S6, S12, and S18) within a short (one hour: each five minutes) and long (12 hours: each one hour) period respectively. The stability was recoded at the maximum pump powers of pump and Bp for these two periods. It is clear that S6 and S12 were stable within the periods. The reason for that can be attributed to the gain saturation of both of S6 and S12 which makes them stable. On the other side, a fluctuation of almost 2dB from 6.5 to 8.5 dBm was observed within the two periods for S18 since this Stokes signal did not reach the saturation level.

 

 

 

 

 

 

Author Response File: Author Response.docx

Reviewer 2 Report

1.The figure needs to amplify the image and more clearation

2. the references need to add the doi of each article write

3. English language needs to improve

4. I suggest adding this reference for support to that article

Author Response

Reviewer #2

Comments and Suggestions for Authors

Thanks for your important notes

1.The figure needs to amplify the image and more clearation

It is improved in the revised version.

2. the references need to add the doi of each article write

The doi for each reference is added in the revised version.

3. English language needs to improve

The revised manuscript edited by MPDI editing services (https://www.mdpi.com/authors/english)

Author Response File: Author Response.docx

Reviewer 3 Report

A multiwavelength fiber laser source is the most fundamental blocs in dense wavelength division multiplexing in optics communication systems. Indeed, the world-first multiwavelength fiber laser source was demonstrated by using four wave mixing (FWM), nonlinear optical mirrors or stimulated Brillouin scattering (SBS). Laser source experiments using point-defect photonic crystal cavities have been also reported. However, compared to other types of ring cavities, the quality factor of the point-defect photonic crystal cavities is rather higher, which has hindered the further applications of ring cavities. In this study, the authors present an experimental study of the tunable 60 GHz multiwavelength Brillouin erbium fiber laser. Two unidirectional ring laser cavities and two pre-amplification laser cavities are used in the design of the multiwavelength fiber laser source.

The manuscript is well-organized and nicely written. But I believe the manuscript could be further strengthened if the authors could revise the manuscript by considering the following points (although all are major points).

1. English usage in this manuscript must be substantially improved. There are many grammatical errors and vague descriptions.

2. It is thought that the novelty of the proposed optical multiwavelength Brillouin erbium fiber laser has not been discussed (deeply and in detail) enough by using references in the introduction section. In addition to the existing references, some up-to-date references must be given in this section.

3. For the proposed multiwavelength Brillouin erbium fiber laser, the authors choose a unidirectional cavity. In fact, the characteristics of the PhC can be also used for the design of the point-defect PhC cavities with ultra-high Q-factors and ultra-small mode volumes (Kassa-Baghdouche, L. (2019). Physica Scripta, 95(1), 015502, Kassa-Baghdouche, L., & Cassan, E. (2020). Optical and Quantum Electronics, 52(5), 1-13). The difference between these structures and the proposed unidirectional cavity should be included in the manuscript.

5. In the second section (Experimental Setup), the authors discuss the experimental setup of the proposed cavity. The cavity structure should be more explained by the authors. Why do the authors choose such geometrical cavity?

6. In the third section (Results and Discussions), it is better to have a table when making a comparison with some other similar tunable multiwavelength Brillouin erbium fiber.

7. In the third section (Results and Discussions), the discussion needs to be presented in a more clear manner, presenting a summary or results and their relation to published literature. The authors do not clearly present their conclusions and more text should be dedicated to this section.

8. Since this is a experimental result, it would be nice if the authors present more results. Some kind of a more general prediction for the values of the quality factor and volume modal of the proposed cavity? The quality factor and volume modal of cavity used in this paper should be calculated and their values should be compared with those of the point-defect photonic crystal cavity.

Author Response

Reviewer #3

A multiwavelength fiber laser source is the most fundamental blocs in dense wavelength division multiplexing in optics communication systems. Indeed, the world-first multiwavelength fiber laser source was demonstrated by using four wave mixing (FWM), nonlinear optical mirrors or stimulated Brillouin scattering (SBS). Laser source experiments using point-defect photonic crystal cavities have been also reported. However, compared to other types of ring cavities, the quality factor of the point-defect photonic crystal cavities is rather higher, which has hindered the further applications of ring cavities. In this study, the authors present an experimental study of the tunable 60 GHz multiwavelength Brillouin erbium fiber laser. Two unidirectional ring laser cavities and two pre-amplification laser cavities are used in the design of the multiwavelength fiber laser source.

The manuscript is well-organized and nicely written. But I believe the manuscript could be further strengthened if the authors could revise the manuscript by considering the following points (although all are major points

Our thanks to reviewer for his important comments to improve our paper further.

1. English usage in this manuscript must be substantially improved. There are many grammatical errors and vague descriptions.

The revised manuscript edited by MPDI editing services (https://www.mdpi.com/authors/english)

2. It is thought that the novelty of the proposed optical multiwavelength Brillouin erbium fiber laser has not been discussed (deeply and in detail) enough by using references in the introduction section.

The novelty of our proposed multiwavelength Brillouin fiber laser is discussed more in the revised version. The below paragraph is added into the revised version:

 

In terms of real time applications of optical communication systems, quintuple Brillouin frequency shift (50 GHz) can achieve data rate of about 200Gb/s per single channel. Increasing the channel spacing to 60 GHz is required for high data bit rate of 300 Gb/s per channel. To the best of our knowledge, no multiwavelength Brillouin erbium fiber laser with 60 GHz spacing was conducted.  In this paper, the experimental results on sextuple Stokes lines (60GHz) are proposed. Four Brillouin Stokes lines with sextuple frequency shift (0.48 nm) with large OSNR of more than 50 dB and large Stokes power of 10 dBm are achieved. The achieved signal can be tuned over a 30 nm from 1560 to 1590 nm. Up to 62 channels with 60GHz spacing could be implemented within the achieved 30 nm tuning range. In addition, most of the useful sextuple signals lie between 1570 to 1590 nm in the L-band region of the DWDM communication systems. The wide tuning range are obtained due to two parameters; high Brillouin pump (Bp) power and pre-amplification cavities that provide two factors: increase Brillouin gain and reduce the self lasing cavity modes competition.

 

 

In addition to the existing references, some up-to-date references must be given in this section.

Actually, no sextuple multiwavelength fiber laser paper is published yet. Therefore, we couldn’t add up to date reference related to the 60GHz channel spacing.

3. For the proposed multiwavelength Brillouin erbium fiber laser, the authors choose a unidirectional cavity. In fact, the characteristics of the PhC can be also used for the design of the point-defect PhC cavities with ultra-high Q-factors and ultra-small mode volumes (Kassa-Baghdouche, L. (2019). Physica Scripta, 95(1), 015502, Kassa-Baghdouche, L., & Cassan, E. (2020). Optical and Quantum Electronics, 52(5), 1-13). The difference between these structures and the proposed unidirectional cavity should be included in the manuscript.

 

These two references are added into the introduction part under the optical sensor field under numbers [6, 7]. Unfortunately, no bit error rate analyzer is available at our laboratory to measure the Q factor and the electric field of the intensity of the light. Therefore, we couldn’t compare our results in terms of Q factor and mode volume with these two references.  

 

4. In the second section (Experimental Setup), the authors discuss the experimental setup of the proposed cavity. The cavity structure should be more explained by the authors. Why do the authors choose such geometrical cavity?

More explanation is added into the revised version related to cavity structure as below:

 

Two open edge cavities (DCF3 and SMF) are used to enhance the tuning range by extracting the unwanted self lasing cavity modes that increase the competition with the Brillouin gain out of the cavity. In addition, the optical signal to noise ratio was improved with this kind of geometrical cavity.

 

5. In the third section (Results and Discussions), it is better to have a table when making a comparison with some other similar tunable multiwavelength Brillouin erbium fiber.

Actually, there is no similar sextuple Stokes signals published. instead, we made a comparison with lower Brillouin frequency shift papers in the revised version. The below table is added to the revised version:

Summary of different Brillouin frequency shift in MBFL with our work

Ref. No.	No. of Stokes	Tuning             range (nm)	Channel Spacing GHz	Description
[24]	2	22	30	· Three cascades long single mode fiber. · Change the cavity manually · Low output Stokes power of -3.5 dBm.
[25]	10	35	30	· Long SMF length of 50 km. · High pump power of 600 mW. · Low Stokes power of -4 dBm
[26]	4	not covered	30	· 70 km SMFs length · 1.2 W pump power
[28]	9	only at the center wavelength	30	· Two sections of erbium-ytterbium- doped fiber amplifier (EYDFA) · Long two SMF spool with length of 45 km.
[29]	3	40	30	· Long DCF spool with length of   12 km added an additional loss inside the cavity
[26]	3	not covered	40	· Four cascade SMFs for long length of 112 km. · High EDF pump power of 1.1 W.
[31]	1	not covered	40	· High- finesse ring filter · Two cascaded double Brillouin- frequency-spacing cavities
[32]	4	60	40	· Two ring cavities                   · Two DCF (6 Km) for each one    · Two EDF (5 m)
[33]	4	40	50	· Two ring cavities · one open end cavities by DCF                                                                                 · Three DCF (6, 6 & 3 Km) · Pre-amplification
our work	4	30	60	· Two ring cavities · Two open end cavities by (DCF & SMF) · Three DCF (6, 6 & 3 Km) · Three EDF (10 m)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6. In the third section (Results and Discussions), the discussion needs to be presented in a more clear manner, presenting a summary or results and their relation to published literature. The authors do not clearly present their conclusions and more text should be dedicated to this section.

We took this consideration in the revised version. The below paragraph is added into the revised version in the conclusion section:

Up to 62 sextuple channels can be used within the achieved tuning range and high data bit rate of about 300 Gb/s per individual Sextuple Stokes signal. In addition, large part of the tuning lies in the long band (L-band) optical communication region (1570-1590) therefore, most of the useful channel is out of the C-band region which is fully utilized in optical communication system.

 

7. Since this is a experimental result, it would be nice if the authors present more results. Some kind of a more general prediction for the values of the quality factor and volume modal of the proposed cavity? The quality factor and volume modal of cavity used in this paper should be calculated and their values should be compared with those of the point-defect photonic crystal cavity.

Unfraternally, due to our laboratory limitations (no bit error rate analyzer in our laboratory), we are not able to measure Q factor and the electric field of the light intensity subsequently we are not able to measure the volume mode.    

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors have implemented my suggestions.