Single-Longitudinal Mode Ytterbium-Doped Fiber Laser with Ultra-Narrow Linewidth and High OSNR Using a Double-Ring Passive Subcavity

Round 1

Reviewer 1 Report

The manuscript describes and characterizes a single-mode Yb fiber laser.

The experimental setup is clearly described and some simulations are included. The results are clearly detailed. The manuscript may be useful for a small community, but the applications are really interesting.

Author Response

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Reviewer 2 Report

The authors of the manuscript (MS) “Single longitudinal mode ytterbium-doped fiber laser with ultra-narrow linewidth and high OSNR using a double-ring passive subcavity” report the results of experimental characterization of an ultra-narrow linewidth single-longitudinal mode ytterbium doped fiber laser based on narrowband fiber Bragg grating and dual-ring passive subcavity. I believe that this MS can be suitable for publication in MDPI photonics after major revision. My comments on the matter are given below.

Section 1. Introduction:

-          The authors should improve the abstract by including a discussion on operation regimes of single-frequency fiber lasers: CW, gain-switch, and Q-switch operation modes. The following paper can be cited as an example of Q-switch operation: “Transform-limited pulses generated by an actively Q-switched distributed fiber laser”, Opt. Lett 33 (22), pp. 2590-2592 (2008).

Section 2. Experimental setup and principles:

-          The authors should include the FBG reflection spectrum in this section of MS and also its FWHM in picometers.

-          The authors should discuss the reason of election the laser wavelength 1030 nm.

-          The authors should discuss the laser output power vs. pump power and present the correspondent graph.

-          The authors should discuss the laser cavity Q-factor (this parameter was mentioned in the Abstract).

-          The authors should discuss polarization effects that can influence the laser operation.

Section 3. Experimental results and discussion:

-          The paragraph below Fig. 5: The OSA resolution of 20 pm is in the Erbium window only, in the ytterbium window it is about 34 pm, see the OSA manual.

-          The paragraph below Fig. 8: the authors should present the setup of the delayed self-heterodyne used for measuring the laser bandwidth and include some reference on the matter, for instance, “On the delayed self-heterodyne interferometric technique for determining the linewidth of fiber lasers,” Opt. Exp. 14(9), pp. 3923–3928 (2006).

Ref. [21]: the publication year is 1991, not 199

Author Response

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Reviewer 3 Report

The authors reported experimental and theoretical results on an ultra-narrow linewidth single-longitudinal mode ytterbium doped fiber laser based on narrowband fiber Bragg grating and dual-ring passive subcavity. The principle of the proposed scheme are explained and theoretically analyzed. The experimental laser scheme was implemented. The optical signal to-noise ratio is as high as 73 dB and the linewidth is as narrow as 355 Hz at the central wavelength of about 1030 nm. The good long-term stability is measured. I believe that the results are solid and interesting. I have only a few comments and suggestions for the authors.

1) With power scaling, is excellent laser performance expected to be maintained, or is performance expected to degrade?

2) What is the achieved slope efficiency?

3) The authors report the lengths of the fiber pieces in the experimental scheme without discussing why such values were chosen and whether they are optimal. I suggest adding a brief explanation.

4) The authors wrote the OSA resolution is 0.02 nm. So, it is not clear why some values are given with higher accuracy. For example, the central wavelength of 1030.052 nm; or in lines 240-241 “The fluctuation range of the output wavelength and power of the laser is in the range of 0.008 nm m and 0.19 dB…”

5) Some values are given several times. For instance, lines 130-131 and 162-163 duplicates the information that c = 3 10^8 m/s and n=1.47.

Author Response

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Reviewer 4 Report

The authors claim as original a SLM Fiber laser approach that has never been demonstrated before (so far). Assuming that this triply-filtered by a FBG and two nested interferometers is original of them; and in spite of the obvious complexity, that they claim not to exist, as well as the also obvious low conversion efficiency, that they should mention and describe; the results are interesting and useful for future higher power systems. 

I recommend a major revision of some sections of the manuscript before a final recommendation regarding its publication. Here my comments:  

In the Introduction section, please re-write most of the section in accordance with the following highlights:

The description of the (three) methods for realizing SLM Fiber lasers are no clear. Two of them are described with too much non-precise and confusing details. Please re-write with clarity. In particular the first one, the linear cavity, is extremely confusing. The third one that is supposed to be described within the same paragraph, is described in the next paragraph. 

The system that the authors are proposing, clearly is quite inefficient and has a lot of intracavity lossy components and a certain degree of complexity; then, please do not mention that it is simple. As a matter of fact, it is complex and the optical-to-optical efficiency probably is not higher than 1%, taking into account the total insertion loss of all the devices: WDM, CIR, FBG, PC, OC1(90 to 10%), OC2((90 to 10%) and OC3(also 90:10). In the best scenario, the system is providing some milliwatts of average power, while pumping with some hundreds of milliwatts. It is important to know this data, since it is not mentioned within the manuscript.

On the other hand, YbDFLS tend to oscillate naturally at 1035 nm. Before these, the Nd:YAGs and Nd:YVO4s crystal lasers established the 1064-nm wavelength as the more efficient and used in the one-micron band. Hence, when Yb doped materials were used to develop lasers, due to the higher efficiency, the were “forced” to oscillate out of their natural oscillations, 10 64 nm, the 1035 that is only 29 nm below was not useful for the uses of 1064; anyway, 1035 do not specially offers any advantage over 1064 nm. Please also do not highlight 1035 nm as a special wavelength that has been ignored, unless there is a strong argument other that being just different (Line 72: “Lasers with 1030 nm wavelengths can be used in the field of spectral synthesis to broaden the spectral range of spectral synthesis”?????; Line 75-78: “However, high OSNR and ultra-narrow linewidth single-fre- quency fiber lasers at this wavelength are rarely reported. The stable single longitudinal mode laser source at about 1030 nm is indispensable in the above applications. Therefore, it is of great significance to realize SLM YDFL with high performance at 1030 nm wave-length.” ????).

The Discussion section is redundant as discussions were included in the Experimental-results one. Besides, it reads as Conclusions.

Author Response

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Round 2

Reviewer 2 Report

The authors of the manuscript (MS) “Single longitudinal mode ytterbium-doped fiber laser with ultra-narrow linewidth and high OSNR using a double-ring passive sub cavity” report the results of experimental characterization of an ultra-narrow linewidth single-longitudinal mode ytterbium doped fiber laser based on narrowband fiber Bragg grating and dual-ring passive sub-cavity. I believe this MS can be suitable for publication in MDPI photonics after minor revision. My comments on the matter are given below.

Abstract:

-          The authors should improve the abstract by removing the phrase “the output laser is in continuous-wave (CW) state” and adding “continuous wave (CW)” to the beginning of the first sentence. The word “designed” should be replaced with “studied”. So, the first sentence should be as “A continuous wave (CW) ultra-narrow linewidth single-longitudinal mode (SLM) ytterbium-doped fiber laser (YDFL) based on narrowband fiber Bragg grating (NB-FBG) and dual-ring passive sub-cavity (DR-PS) was studied.

Section 2. Experimental setup and principles:

-          The authors should include “Fig. 2 Laser output power vs. pump power” shown in reply to my notes to the MS, as, for example, Fig. 1(c), and the corresponding discussion on the matter.

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Reviewer 4 Report

The authors have attended mi suggestions satisfactorily along with other reviewer´s suggestions. The manuscript is now more solid. I only recommend a final text editing. In my opinión, the manuscript might be published. 

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