Switchable Dual-Wavelength Thulium-Doped Fiber Laser Based on Polarization-Maintaining Fiber Bragg Grating and Compound Cavity Filter

Round 1

Reviewer 1 Report

1.Why not just use a FBG to change the wavelength, but PM-FBG and PC? Is it because this doesn't achieve SLM operation? I think authors should state the advantages of doing this clearly in your article.
2.It is a good and effective method to combine the signal flowchart and Matlab to design the TC-DRC cavity filter. However, I think the authors should have explained the signal flowchart in more detail, such as the nodes and electric fields(Fig. 1b).
And why not draw port 4 to port 6 and port 6 to port 8?
3.How to determine the proper length of all four sections of fiber for the filter?
4.Fig.7: Does the fiber under the FRM2 and SMF have any effect on the experiment?
5.Formula (9), authors should pay attention to the formatting of the corner markers.

Line 136, I guess "and" is redundant in that sentence and should be deleted.
Formula (9), authors should pay attention to the formatting of the corner markers.

Author Response

1.Why not just use a FBG to change the wavelength, but PM-FBG and PC? Is it because this doesn't achieve SLM operation? I think authors should state the advantages of doing this clearly in your article.

SLM operation is also possible with FBGs, but wavelength switching is not possible. Compared with the replacement of FBG to realize wavelength switching, the use of PM-FBG reduces the step of disassembling and assembling the device, thus reducing the insertion loss and fusion bonding loss, and enables the laser to realize the stable switching of different wavelengths under the operation state, and frequent disassembling and assembling of the device affects the length of the main cavity of the laser, which affects the stability of the laser. If cascaded FBGs are used, only dual-wavelength output can be realized, but single-wavelength switching is not possible, and insertion loss and fusion bonding loss are increased.

2.It is a good and effective method to combine the signal flowchart and Matlab to design the TC-DRC cavity filter. However, I think the authors should have explained the signal flowchart in more detail, such as the nodes and electric fields (Fig. 1b). And why not draw port 4 to port 6 and port 6 to port 8?

Signal flow graphs were originally proposed by Mason to obtain the cause and effect relationships of signal transformations and transmissions in circuits. This method is very helpful in understanding the operation of a CRC filtering system through graphical representations as compared to conventional methods. It also allows one to easily and systematically manipulate the variables of interest to accurately design and study a new CRC filter. In addition, the transfer function of the filter system can be derived directly using Mason's rule without the need for complex mathematical operations. The exact calculations are given by the newly added cited literature and are not given in detail in the article considering the word count. (Feng, T.; Wei, D.; Bi, W.; Sun, W.; Wu, S.; Jiang, M.; Yan, F.; Suo, Y.; Yao, X.S. Wavelength-switchable ultra-narrow linewidth fiber laser enabled by a figure-8 compound-ring-cavity filter and a polarization-managed four-channel filter. Optics Express 2021, 29, 31179-31200.)

Regarding the question of why port 4 is not plotted to port 6 and port 6 is plotted to port 8. Port 4 to port 6 is there, why not port 6 to 8, due to the 8 port signal light has been output, not in the compound cavity for the loop, combined with the formula 5 and 6, the coupler is double-armed, the nodes that form the loop constitute the signal flow diagram, the nodes that are not in the loop are not reflected. Thank you for your valuable comments!

3.How to determine the proper length of all four sections of fiber for the filter?

We simulate the subcavity coupler with different coupling ratios as well as different lengths of the uncoupler arm lengths, and finally determine the lengths and coupling ratios that have high side-mode rejection ratios and high transmittance, and recently we would like to use algorithms and perform calculations to derive the optimal coupling ratios and lengths, which are currently being designed, and we hope to be able to reflect this in a subsequent paper. Thank you for your valuable comments!

4.Fig.7: Does the fiber under the FRM2 and SMF have any effect on the experiment?

As shown in Fig. 7, we we set up an unbalanced Michelson interferometer (MI) consisting of a 3 × 3 coupler and two Faraday rotating mirrors (FRM) for laser phase noise demodulation. A 50 m length of standard single-mode fiber (SMF) was incorporated into the interferometer instead of the excessively long delay line that must be used in conventional delayed aberration linewidth measurement methods. Due to the high transmission loss of the 2 µm light. A 50-meter-long SMF is used as the delay line, which introduces a time delay between the two arms τ. The light in the arms is reflected by the two FRM's, returns to the coupler, undergoes interference, and then passes through the coupler into PD1 and PD2. The received interference fringes contain information about differential phase fluctuations accumulated during the MI delay time τ, which is uploaded by the OSC to a computer that calculates the laser's instantaneous phase fluctuations Sφ(f) and the power spectral density (PSD) of the frequency fluctuations SV(f).

5.Formula (9), authors should pay attention to the formatting of the corner markers.

the subscripts have been modified in the manuscript. Thank you for your valuable comments!

Reviewer 2 Report

Both scientifically and methodologically this paper appears correct and suitable for publication in this journal. I propose to add a section explaining more carefully why this is original research and how it differs from previous published results – even if occasionally in different wavelength regimes - by other authors. 

Furthermore, I suggest asking a native English speaker to check the language used in the paper.

Author Response

1.Both scientifically and methodologically this paper appears correct and suitable for publication in this journal. I propose to add a section explaining more carefully why this is original research and how it differs from previous published results – even if occasionally in different wavelength regimes - by other authors.

The compound cavity used is optimized for dual-wavelength switching and single longitudinal mode election using only the compound cavity, while maintaining a stable single longitudinal mode output at different wavelengths. In the previously published article, the wavelength was selected to be FBG with FP cavity, the 3dB bandwidth is 0.05nm, and the optimized compound cavity can still narrow down the linewidth to 8KHz under the condition of using PM-FBG only.

2.Furthermore, I suggest asking a native English speaker to check the language used in the paper.

I have asked a native English speaker to check the paper and correct any grammatical and tense errors in the paper, as requested. Thank you for your valuable comments!

Reviewer 3 Report

This manuscript presented a dual-wavelength thulium-doped fiber laser (TDFL) utilizing polarization-maintaining fiber Bragg grating. Wavelength switching can be achieved using a polarization controller. A three-coupler double-ring compound cavity was proposed for single longitudinal mode selection. The performance of the TDFL and the linewidth are performed in the experiment.

The content of the manuscript is relatively good, but there are so many grammatical errors, formatting errors and typos. The authors should read their manuscript carefully before submitting it for review.

Here are some questions to the author:

1. Which author belongs to the sixth affiliation “Shanxi Provincial People’s Hospital”?

2. Please provide more details about the PM-FBG, such as period, duty cycle, and is it made by UV laser or femtosecond laser?

3. The author should pay attention to the format. There should be a space after the symbol on lines 134-135, and there is a lack of content after the “and” on line 136.

4. The node between Y1 and D1 in Figure 3 should be 4 instead of 3.

5. In line 145, the author mentions that the frequency range is 7.56 GHz. In line 177, it changes to 12.8 GHz. What is the difference?

6. "Figures. (c)" on line 223 should be changed to Figure 6 (c).

7. The title of Table 2 is the same as Table 1.

Too many grammatical errors, please correct them carefully.

Author Response

1. Which author belongs to the sixth affiliation “Shanxi Provincial People’s Hospital”?

Author Yuping Suo belongs to Shanxi Provincial People's Hospital. The subscript has been added in the manuscript to link the author to the affiliation.

2. Please provide more details about the PM-FBG, such as period, duty cycle, and is it made by UV laser or femtosecond laser?

PM-FBG mask version of the period of 1347.3nm, duty cycle of 1:1, the use of UV laser, machine model: Germany Mlase-500. production process: 1. Turn on the laser preheating for 30 minutes, the laser frequency of 100hz, and the energy of 7.38mj. 2. Place the fiber in the fiber optic fixture, applying a tensile force of 0.2N. 3. The laser passes through a 1950nm Phase mask version of UV exposure for 80 seconds, and then other unchanged 2050nm phase mask version, repeats the above operation to inscribe the second part of the grating. Thank you for your valuable comments!

3. The author should pay attention to the format. There should be a space after the symbol on lines 134-135, and there is a lack of content after the “and” on line 136.

The missing space and content have been added in the manuscript.

4. The node between Y1 and D1 in Figure 3 should be 4 instead of 3.

The node between Y1 and D1 in Figure 3 has been changed to 4 in the manuscript.

5. In line 145, the author mentions that the frequency range is 7.56 GHz. In line 177, it changes to 12.8 GHz. What is the difference?

Changed to main cavity longitudinal mode spacing of 12.7 MHz. This is because both the main cavity longitudinal mode spacing and the grating reflection bandwidth are important factors in determining the TC-DRC mode selection. Thank you for your valuable comments!

6. "Figures. (c)" on line 223 should be changed to Figure 6 (c).

This has been modified in the manuscript

7. The title of Table 2 is the same as Table 1.

The title of Table to has been changed to “Laser linewidth measured at two wavelengths at different times”

Round 2

Reviewer 3 Report

The author solved most of my concerns. This manuscript can be considered for publication.