Double-Clad Bismuth-Doped Fiber with a Rectangular Inner Cladding for Laser Application

Round 1

Reviewer 1 Report

In this paper, the author fabricated a double clad Bi-doped gain fiber with a rectangular inner cladding. The fiber performance is well presented and the manuscript is clearly organized. The result may be interesting to the researchers working on fiber lasers. I have some questions that list below. If the author can reply my above concerns, I would recommend the publication of this work

1. I don’t think the multi-wavelength operation is induced by the intermodal coupling. The wavelength intervals are different for the 100m case (about 1nm) and 200m case (about 2nm). The author can also measure the beam quality to verify the guess.

2. What is the core size of the pump combiner and FBG? The mismatch of these components with the gain fiber would also decrease the optical efficiency.

3. What is the gain fiber bend diameter? Multimode laser is sensitive to the bend diameter in optical fiber.

Author Response

Author’s reply

We would like to thank all the reviewers for their valuable remarks, which helped us to improve the paper, for their interest and time.

Reviewer 1

In this paper, the author fabricated a double clad Bi-doped gain fiber with a rectangular inner cladding. The fiber performance is well presented and the manuscript is clearly organized. The result may be interesting to the researchers working on fiber lasers. I have some questions that list below. If the author can reply my above concerns, I would recommend the publication of this work

C1. I don’t think the multi-wavelength operation is induced by the intermodal coupling. The wavelength intervals are different for the 100m case (about 1nm) and 200m case (about 2nm). The author can also measure the beam quality to verify the guess.

R1. When analyzing data on multiwavelength lasing, please note that the lasers operated without FBGs, with two cleaved fiber ends providing feedback. Thus, the lasing wavelength in the multimode fiber laser was due to a number of factors, mainly total insertion losses, gain coefficient, reabsorption effect etc., which are not the same for different modes and different fiber lengths. We agree with the reviewer that more experiments are needed in order to obtain conclusive evidence regarding the intermodal coupling. Unfortunately, it is not possible for us to carry out such experiments promptly. This is why we do not give affirmative conclusions but only suggestion on the origin of multiwavelength lasing. Similar experiments of this phenomenon will be studied in detail and published elsewhere.

С2. What is the core size of the pump combiner and FBG? The mismatch of these components with the gain fiber would also decrease the optical efficiency.

R2. We used a commercially available pump and signal combiner (2+1)x1, where pump ports are the coreless fibers, while signal port is fiber with a core diameter of 10 um. The used FBG was written into a core of SMF-28 fiber (core diameter of ~8 um) (line 136 in previous version). We controlled the splicing loss of active fiber with other optical components. We think that the splicing loss was appropriate in all the cases and they cannot significantly affect the output parameters of the lasers.

C3. What is the gain fiber bend diameter? Multimode laser is sensitive to the bend diameter in optical fiber.

R3. The active fiber was wound on a steel coil with a diameter of ~20 cm. We believe that the diameter is sufficient to eliminate negative effects associated with the fiber bending. That is why we did not observe any changes.

Reviewer 2 Report

The authors report the fabrication of double clad bismuth-doped fibers with a rectangular inner cladding and demonstrate the laser lasting performance using such fibers, where the pump and emitting wavelengths are 808 nm and 1.46 μm, respectively. The rectangular design of inner cladding enhances the pump absorption and improves the slope efficiency. The multi-wavelength free-running lasting with this type of fiber is also studied. Overall, this paper is properly arranged, and the technical contents are sound and reasonable. My recommendation is that the paper can be accepted provided that the authors address a few comments I enumerate below.

1.      I have not understood well throughout the paper why the rectangular inner cladding outperforms the cylindrically symmetric structure. Please specific it. I wonder whether there is better inner cladding design than the rectangular shape?

2.      What’s the main advantage of the double clad bismuth-doped fiber lasers compared with the Er-doped fiber lasers? Can it output higher power? Or higher slop efficiency?

3.      We can see from Fig. 1(b) that the structures of F2 and F3 fibers have defects. The authors should discuss the effects of these defects on the lasting performance.

4.      Which fiber sample is used in the measurement of Fig. 2(a)? Are the results for all the three samples the same? The authors have claimed that the cut-off wavelengths are 1.4-1.45 μm for F1 and F2, and 1.8 μm for F3. However, the emitting wavelength is 1.4615 μm, which acutally has left the single-mode regime of F1 and F2 fibers. In addition, what’s the solid blue line in Fig. 2(b)?

5.      The manuscript should be carefully edited before publication, in particular the language.

Author Response

Author’s reply

We would like to thank all the reviewers for their valuable remarks, which helped us to improve the paper, for their interest and time.

Reviewer 2

The authors report the fabrication of double clad bismuth-doped fibers with a rectangular inner cladding and demonstrate the laser lasting performance using such fibers, where the pump and emitting wavelengths are 808 nm and 1.46 μm, respectively. The rectangular design of inner cladding enhances the pump absorption and improves the slope efficiency. The multi-wavelength free-running lasting with this type of fiber is also studied. Overall, this paper is properly arranged, and the technical contents are sound and reasonable. My recommendation is that the paper can be accepted provided that the authors address a few comments I enumerate below.

C1. I have not understood well throughout the paper why the rectangular inner cladding outperforms the cylindrically symmetric structure. Please specific it. I wonder whether there is better inner cladding design than the rectangular shape?

R1. In fibers with a radially cylindrically symmetric structure, there are transverse modes of the inner cladding (related to helical rays), which hardly overlap with the core, so that some part of the pump light exhibits incomplete absorption (lines 29-34). As a result, the gain and power efficiency are reduced. Modes with poor overlap with the core can be avoided by using a modified design with a lower symmetry, in particular, designs with an off-centered core or a non-circular (e.g. elliptical, D-shaped or rectangular) inner cladding. This approach was applied to a new optical fiber, namely Bi-doped fiber. This is a main purpose of this paper.

C2. What’s the main advantage of the double clad bismuth-doped fiber lasers compared with the Er-doped fiber lasers? Can it output higher power? Or higher slop efficiency?

R2. Bismuth-doped fibers are being developed for new optical bands, where there are no efficient rare-earth-doped fiber lasers. In this work, we developed Bi-doped fiber laser operating at a wavelength of 1460 nm, which is unavailable for Er-doped fiber lasers. That is why the comparison of the studied fiber lasers with Er-doped fiber lasers is not appropriate.

C3. We can see from Fig. 1(b) that the structures of F2 and F3 fibers have defects. The authors should discuss the effects of these defects on the lasting performance.

R3. The observed defects in the cross-sectional images of the fibers were associated with non-perfect fiber cleavage but not the fiber itself. The non-ideal cleaved fiber end could affect the laser operation in the case the fiber end is used as a mirror, but the defects are located only in the cladding, since reflection in free-running lasers is important for the core, such defects do not affect the laser parameters.

C4. Which fiber sample is used in the measurement of Fig. 2(a)? Are the results for all the three samples the same? The authors have claimed that the cut-off wavelengths are 1.4-1.45 μm for F1 and F2, and 1.8 μm for F3. However, the emitting wavelength is 1.4615 μm, which acutally has left the single-mode regime of F1 and F2 fibers. In addition, what’s the solid blue line in Fig. 2(b)?

R4. The output spectra of the lasers based on the tested fibers were similar, when we used the FBG with high reflectivity at a wavelength of 1460 nm. In the case of the free-running lasers, the situation is more complicated. As one can observe, various regimes of lasing, including multiwavelength took place. This is described in the text. Solid line in Fig. 2(b) was given for a reader for estimation of average efficiency of lasers. However, we decided that it should be deleted since it is not discussed in the text.

C5. The manuscript should be carefully edited before publication, in particular the language.

R5. Polishing of the text was done.

Reviewer 3 Report

Manuscript ID: photonics-1972084

Review of the manuscript “Double Clad Bismuth-Doped Fiber with a Rectangular Inner Cladding for Laser Application” by A. Vakhrushev et al. submitted for consideration to Photonics.

In this manuscript, the authors have fabricated three different geometries of BDFs as an active medium for construction of a laser. The generated output wavelengths of the laser are around 1.46 \mum when the BDFs were pumped by laser diodes at \lambda=808 nm. They have studied the peculiarities and performance of the laser by geometries, analyzing the dependencies of the slope efficiency of the lasers, active fiber length and core-to-inner-cladding area ratio.

I believe the manuscript is original, the approach completely described, well written, and has a scientific merit for publication. I do support the publication of the manuscript but some points need to be clarified as part of the changes for the manuscript.

- For clarity, I recommend that the authors add some sentences why the BDFL lases at the wavelength 1.46 \mum. What are the physical phenomena?

- According 2o Fig. 2a, the authors must discuss why the intensity of output is larger than the intensity of the input.

Author Response

Author’s reply

We would like to thank all the reviewers for their valuable remarks, which helped us to improve the paper, for their interest and time.

Reviewer 3

Review of the manuscript “Double Clad Bismuth-Doped Fiber with a Rectangular Inner Cladding for Laser Application” by A. Vakhrushev et al. submitted for consideration to Photonics.

In this manuscript, the authors have fabricated three different geometries of BDFs as an active medium for construction of a laser. The generated output wavelengths of the laser are around 1.46 \mum when the BDFs were pumped by laser diodes at \lambda=808 nm. They have studied the peculiarities and performance of the laser by geometries, analyzing the dependencies of the slope efficiency of the lasers, active fiber length and core-to-inner-cladding area ratio.

I believe the manuscript is original, the approach completely described, well written, and has a scientific merit for publication. I do support the publication of the manuscript but some points need to be clarified as part of the changes for the manuscript.

C1. For clarity, I recommend that the authors add some sentences why the BDFL lases at the wavelength 1.46 \mum. What are the physical phenomena?

R1. Lasing at 1.46 um is determined by, on the one hand, the configuration of laser cavity, where there is a high-reflecting mirror at this wavelength, and, on the other hand, the active fiber with a gain band in spectral region of 1400 – 1500 nm. The appearance of the gain band is due to bismuth-related active centers, which have an optical transition in this wavelength range. More general information on Bi-doped fiber can be found in earlier works for instance in [12, 13].

C2. According 2o Fig. 2a, the authors must discuss why the intensity of output is larger than the intensity of the input.

R2. Taking into consideration the scheme presented in Fig. 1(c), the laser has a backward-pumped configuration. In this case, in the output spectrum of the laser, a line of total output laser radiation and reflected unabsorbed pump radiation is observed. As a rule, in this case the level of pump is noticeably lower than that of laser radiation.