Novel Bidirectional Output Ytterbium-Doped High Power Fiber Lasers: From Continuous to Quasi-Continuous

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. In this work, the authors theoretically modeled the bidirectional output fiber laser and introduced the bidirectional output fiber laser in both CW and QCW operating states, including CW BOFL-OS, CW BOFL-OA, QCW BOFL-OS, and QCW+CW BOFL-OS. Then, the authors proved that BOFL has better SRS and TMI suppression capabilities than the UOFL. It is a complete work with potential applications. This article is clear, concise, and suitable for the scope of the journal. Several suggestions are supplied:

   1). It is suggested that the author add some references to explain the phenomenon of TMI in the introduction part.

   2). Does the BOFL have good SRS and TMI suppression caused by the reflectance of FBG in BOFL is lower than that of UOFL, in which the light intensity involved in oscillation in the laser cavity is relatively small?

Author Response

Response to Reviewer 1 Comments

Comments of Reviewer1：

In this work, the authors theoretically modeled the bidirectional output fiber laser and introduced the bidirectional output fiber laser in both CW and QCW operating states, including CW BOFL-OS, CW BOFL-OA, QCW BOFL-OS, and QCW+CW BOFL-OS. Then, the authors proved that BOFL has better SRS and TMI suppression capabilities than the UOFL. It is a complete work with potential applications. This article is clear, concise, and suitable for the scope of the journal. Several suggestions are supplied:

1. It is suggested that the author add some references to explain the phenomenon of TMI in the introduction part.
2. Does the BOFL have good SRS and TMI suppression caused by the reflectance of FBG in BOFL is lower than that of UOFL, in which the light intensity involved in oscillation in the laser cavity is relatively small?

 

Point 1: It is suggested that the author add some references to explain the phenomenon of TMI in the introduction part.

 

Response 1: Thank you for your valuable suggestions. According to your suggestion, we have added the principle of TMI generation, main features, and potential hazards to the laser in the introduction section. The specific content is as follows: “TMI in fibers originates from thermal effects caused by quantum defects, photon darkening, and other factors in active fibers [16, 17, 23, 24]. Under the influence of mode interference pattern and thermal optical effect, long-period refractive index fiber gratings appear in the fiber core. When the phase matching condition is met between the mode interference pattern and the long-period refractive index grating, mode coupling occurs between the fundamental mode and higher-order mode, resulting in TMI. The emergence of TMI is often accompanied by beam quality degradation and chaotic output timing, seriously endangering device safety [22, 25].”

 

Point 2: Does the BOFL have good SRS and TMI suppression caused by the reflectance of FBG in BOFL is lower than that of UOFL, in which the light intensity involved in oscillation in the laser cavity is relatively small?

 

Response 2: Thank you for your professional review work on our article. The only structural difference between an UOFL and a BOFL is the change in reflectivity of high reflectivity fiber Bragg gratings, and all characteristic changes are caused by changes in reflectivity. In the article, we also simulated the power and temperature distribution in UOFL and BOFL under the condition of consistent total pump power and pump configuration (bidirectional pump, 2000 W at both ends). The results are shown in Figure 1 and Figure 2 (Figures 4 and 5 in the article). It is evident from the figure that the power density in the fibers in BOFL is lower and there is also a weaker thermal effect. Therefore, we believe that the suppression of SRS and TMI by BOFL is due to the lower power distribution in the fiber. Meanwhile, the more comprehensive theoretical research of BOFL is also our next research focus.

Figure 1 Comparison of power distribution in BOFL and UOFL under bidirectional pumping configuration. (a) Simulated power distribution. (b) Total power distribution inside the cavity.

Figure 2 Temperature distribution in UOFL and BOFL under different pump configurations. (a) Temperature distribution under bidirectional pump (2000W×2). (b) Temperature distribution under unidirectional pump (total pump: 4000W). UOFL-FP: forward pump UOFL. UOFL-BP: backward pump UOFL.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Nice results.  There are just a few grammatical corrections suggested in the attachment.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

There are just a few grammatical corrections suggested in the attachment.

Author Response

Response to Reviewer 2 Comments

Comments of Reviewer2：

Nice results.  There are just a few grammatical corrections suggested in the attachment.

 

Response: Thank you for your valuable feedback on the article. According to your suggestion, we have made detailed revisions to the article. The revisions to the manuscript have been marked up using the “Track Changes” function.

 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

This paper presents interesting results and progress on bi-directional high power fiber lasers. Various configurations are shown to improve power, reduce TMI and SRS. All in all, I find the results quite impressive.

I have the following minor comments;

1- Please specify the meaning of the acronyms TMI and SRS when they are introduced

2- line 30: “Thanks to their convenient…”

3- The authors should briefly describe the mechanism and effect of TMI

4- line 58 “making it difficult to utilize and processing more complex” : this sentence is not clear

5- Can the authors discuss briefly the reasons to use wavelength-stabilized and non-wavelength stabilized pump diodes?

 

Apart from those minor corrections, I believe that the paper merits publication.

Comments on the Quality of English Language

See previous comments

Author Response

Response to Reviewer 3 Comments

Comments of Reviewer3：

This paper presents interesting results and progress on bidirectional high power fiber lasers. Various configurations are shown to improve power, reduce TMI and SRS. All in all, I find the results quite impressive.

I have the following minor comments;

1- Please specify the meaning of the acronyms TMI and SRS when they are introduced

2- line 30: “Thanks to their convenient…”

3- The authors should briefly describe the mechanism and effect of TMI

4- line 58 “making it difficult to utilize and processing more complex” : this sentence is not clear

5- Can the authors discuss briefly the reasons to use wavelength-stabilized and non-wavelength stabilized pump diodes?

Apart from those minor corrections, I believe that the paper merits publication.

 

Point 1: Please specify the meaning of the acronyms TMI and SRS when they are introduced.

Response 1: Thank you for your valuable comments.TMI represents transverse mode instability, SRS represents stimulated Raman scattering. We have improved the corresponding description in the article. The content is as follows: “Before the discovery of transverse mode instability (TMI), nonlinear effects represented by stimulated Raman scattering (SRS) were the main limiting factors for the increase in output power of high-power fiber lasers [13].”

 

Point 2: line 30: “Thanks to their convenient…”

Response 2: Thank you very much for pointing out the inadequacies in the article.We have revised the text to“Thanks to their convenient thermal management, flexible transmission, high efficiency, and good beam quality, high-power fiber lasers have been widely used, especially in industrial, medical, and scientific research fields[1-3].”

 

Point 3: The authors should briefly describe the mechanism and effect of TMI

Response 3: Thank you for your valuable suggestions. According to your suggestion, we have added the principle of TMI generation, main features, and potential hazards to the laser in the introduction section. The content is as follows: “TMI in fibers originates from thermal effects caused by quantum defects, photon darkening, and other factors in active fibers [16, 17, 23, 24]. Under the influence of mode interference pattern and thermal optical effect, long-period refractive index fiber gratings appear in the fiber core. When the phase matching condition is met between the mode interference pattern and the long-period refractive index grating, mode coupling occurs between the fundamental mode and higher-order mode, resulting in TMI. The emergence of TMI is often accompanied by beam quality degradation and chaotic output timing, seriously endangering device safety [22, 25].”

 

Point 4: line 58 “making it difficult to utilize and processing more complex” : this sentence is not clear

Response 4: Thank you for your nice comments on our article. In the article, what we want to express is that the processing of the idle arm of the high reflectivity grating is complex. For UOFL, there is only one laser output end. The other side of the high reflectivity fiber Bragg grating (HRFBG) is often not connected to any devices. However, because the reflectivity of HRFBG cannot reach 100%, there will always be a portion of the laser output from here. The end face treatment at this position can easily have a serious impact on the output characteristics of the laser, especially for SRS. For a fully packaged laser, it is necessary to always maintain the cleanliness of the position and avoid touching it, which increases the design difficulty. For BOFL, if both sides are laser output ports, there is no such processing difficulty. To avoid confusing readers, we have revised the original text as follows: “For oscillators, because the reflectivity of actual high reflectivity gratings cannot reach 100%, there is always a portion of light leaking from one side of the high reflectivity grating. These leaked light are difficult to utilize and the processing is complex.”

 

Point 5: Can the authors discuss briefly the reasons to use wavelength-stabilized and non-wavelength stabilized pump diodes?

Response 5: Thank you for your valuable comments. The optimizations of the wavelength of LDs is mainly based on the TMI and SRS in the laser. The center wavelength of the non-wavelength stabilized LDs used in the article is 976 nm, accompanied by a wider spectrum. Compared with the wavelength stabilized 976nm LDs with the same central wavelength, the non-wavelength stabilized LDs have a lower equivalent absorption coefficient and a higher TMI threshold (experimental verification has been conducted, see Ref. [1]). Similarly, a decrease in the equivalent absorption coefficient results in the need to increase the fiber length to ensure sufficient pump absorption, which leads to SRS enhancement and is not conducive to high peak power QCW fiber lasers. Therefore, QCW fiber lasers typically use a wavelength stabilized 976 nm LDs to achieve high power output. The main purpose of replacing wavelength stabilized 976 nm LDs with non-wavelength stabilized 976 nm LDs in the study of CW BOFL in the article is to suppress TMI. Based on your suggestion, we have added corresponding descriptions in the article as follows: “According to Ref [57], non-wavelength stabilized 976 nm LDs has a higher TMI threshold than wavelength stabilized 976 nm LDs, so in order to avoid the decrease in TMI threshold caused by the increase of fiber core, we use a non-wavelength stabilized 976 nm LDs as the pump source of the laser and replace the combiner with two (18+1)×1 pump and signal combiner.”

 

[1] Wan, Y.; Yang, B.; Xi, X.; Zhang, H.; Wang, P.; Wang, X.; Xu, X. Comparison and Optimization on Transverse Mode Instability of Fiber Laser Amplifier Pumped by Wavelength-Stabilized and Non-Wavelength-Stabilized 976 nm Laser Diode. Ieee Photonics J. 2022, 14, 1-5.

 

Author Response File: Author Response.docx