Sub-kHz Narrow-Linewidth Single-Longitudinal-Mode Thulium-Doped Fiber Laser Utilizing Triple-Coupler Ring-Based Compound-Cavity Filter

Round 1

Reviewer 1 Report

the manuscript Photonics-2193019 proposes and demonstrates a  SLM TDFL, which can oscillate laser at 1941.28 nm with an high OSNR to 74.1 dB. Within one hour, the fluctruations of wavelength and outpower are less than 0.01 nm and 0.47 dB, respectively. It also has an ideal slope efficiency. I think this paper can be accepted for published in present form. Some comments are as follows:

1. Authors should be more careful. An example is the year of reference 2, it should be 1993;

2. The name of journal should keep unified. In this manuscript, some reference names are full-name, while some others are abbreviations.

Author Response

Thank you very much for your suggestions. We have made point-to-point responses based on your comments. See the word document for details. Looking forward to your reply.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper of Biao Guan, et al. tells about single-longitudinal-mode thulium-doped fiber laser. In the manuscript mode filtering is arranged via usage of a fiber Bragg grating and an interferometric filter. The authors achieved interesting results and the work could be published in Photonics in case the following comments are duly considered.

1.            Line 103 α is introduced as a fiber loss, line 124 α declared as coupling ratio. Two variables named with one sign is a bit confusing. I also did not found any information concerning coupling ratios of coupler 3x3.

2.            According to Fig 5b, the laser experiences so called mode hopping. Have you tried to solve this problem?

3.            In Fig 8 only one point corresponding to the shortest integration time have the linewidth of 910 Hz. Authors explain it by vibrations and other environmental effects but it seems that the external influence is responsible for a sharp linewidth broadening at an integration time about 1s but not at 0.001 s. Additionally, in the range from 0.01 to 0.1 s the linewidth changes rather slowly. I would recommend remeasure laser bandwidth since the value 910 Hz looks like a measurement error.

4.            Since there are other works on single frequency lasers in the region 1900-2000 nm, a comparison of the results achieved in the manuscript with analogous characteristics demonstrated in already published papers is highly recommended.

5.            Line 196 Apparently a typo: “Fabry rotator mirrors”

6.            Line 215 “The reason of low output power is the mode field mismatch loss” What is the loss of splicing between TDF and SMF?

Author Response

Thank you very much for your suggestions. We have made point-to-point responses based on your comments. See the word document for details. Looking forward to your reply.

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript demonstrated a single longitudinal mode thulium-doped fiber laser using a fiber Bragg grating combined with a triple-coupler based filter. The experimental results indicated the laser operated at SLM state with good stability. The manuscript was well drafted with comprehensive experimental details and convincing results. I would recommend publication after the following revision:

Results in fig 5(b) shows laser wavelength variation is 0.01nm. The plot indicates only two values 1941.28/1941.27nm detected. Is this limited by the OSA resolution? Could authors explain the root cause for the wavelength fluctuation?  

Author Response

Thank you very much for your suggestions. We have made point-to-point responses based on your comments. See the word document for details. Looking forward to your reply.

Author Response File: Author Response.docx

Reviewer 4 Report

In this manuscript titled “Sub-kHz narrow-linewidth single-longitudinal-mode thulium- doped fiber laser utilizing triple-coupler ring-based compound-cavity filte”, Guan et al, proposes and demonstrates a single-longitudinal-mode thulium-doped fiber laser using a passive triple-coupler ring-based compound-cavity filter (TCR-CC) and a uniform fiber Bragg grating. The paper is well written and adds significant knowledge towards field of fiber lasers. I recommend paper for publication in Photonics after minor revision.

The comments are as follow:

1.      Line # 73, the author claimed that 3.5 m thulium-doped fiber was used as an active medium. Why this particular length of doped fiber was used and what is the ionic concatenation of doped fiber and core/clad diameter of doped fiber and FC?

2.      A line width data for each case including cavity configuration TCR-CC filter and without the TCR-CC filter should be compared.

3.      How the length of L₄ was chosen L₄ = 0.73 m. The reason to choose this particular lengths? What happen if we used similar length as L₃?

4.      I would like to ask that can we calculate the integrated timing jitter from Fig. 8?

5.      What’s the reason behind frequency fluctuations appears in the phase noise trace (Fig. 8) after 10 Hz frequency?

 

 

 

Author Response

Thank you very much for your suggestions. We have made point-to-point responses based on your comments. See the word document for details. Looking forward to your reply.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I am satisfied with all the authors replies except for the response to comment 3.

The fact that the linewidth of 910 Hz was measured just in case of 0.001 integration time may indicate that the lasing is not stable in terms of the lasing wavelength (kind of wavelength jitter). That is why at longer integration times there is a sharp bandwidth broadening. Since in the range 0.01-0.1 the bandwidth is rather constant, the value of ~20 kHz is likely closer to the correct one.

If I'm wrong, please provide an explanation in the manuscript.

Author Response

Thank you very much for your suggestion again. We are sorry for the mistake and have made corresponding modifications to it. Looking forward to your reply.

Author Response File: Author Response.docx