Narrow-Linewidth Diffraction-Limited Tapered Er-Doped Fiber Amplifier with 2 mJ Pulse Energy

Round 1

Reviewer 1 Report

Authors present a tapered fiber amplifier geometry for high energy amplification of narrow linewidth pulses. The paper is well written and results are relevant in the field of study.

I recommend publication after these comments and questions are addressed:

 

Authors should add this very relevant reference for the introduction section:

Akbulut, M.; Kotov, L.; Wiersma, K.; Zong, J.; Li, M.; Miller, A.; Chavez-Pirson, A.; Peyghambarian, N. An Eye-Safe, SBS-Free Coherent Fiber Laser LIDAR Transmitter with Millijoule Energy and High Average Power. Photonics 2021, 8, 15. https://doi.org/10.3390/photonics8010015

 

The energy measurement is conducted with a photodiode setup rather than a dedicated pyroelectric energy meter, which is the preferred measurement method for these kind of laser systems. When a photodiode or power meter is used for energy measurement, it is usually done with a time-gating AOM in front of it to reject in-band ASE and CW leakage, plus an optical filter to reject out-of-band ASE. Authors do not use any of this, and it is not clear how they convert photodiode measurements to energy measurements. Considering the 1kHz repetition rate, a significant amount of in-band and out-of-band ASE is expected, and can be seen in the inset of Figure 2.c. For the energy measurements to make sense, it is critically important for the authors to clearly validate their energy measurement method with all its details.

 

DM angle in Figure 2. is not specified. It is well known for these kind of lasers that coated optics placed at large angles at the output can act as beam and/or polarization cleanup filter. It would be important for authors to verify and mention that the beam quality and/or polarization purity is not artificially improved upon reflection from the DM.

 

In terms of SBS mitigation, this laser/amplifier system benefits significantly from counter-pumping, in addition to the tapering. The level of SBS mitigation may be comparable for both effects. Authors should include a discussion section comparing these effects.

 

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The paper "Narrow-Linewidth Diffraction-Limited Tapered Er-Doped Fi-2 ber Amplifier with 2 mJ Pulse Energy" by M.M. Khudyakov, A.E. Levchenko, V.V. Velmiskin, K.K. Bobkov, S.S. Aleshkina, T.S. Zaushitsyna, M.M. Bubnov, M.V. Yashkov, A.N. Guryanov, L.V. Kotov and M.E. Likhachev reports a comprehensive experimental study of high-energy pulse amplification in a diffraction-limited Er-doped fiber amplifier.

 

The paper is well-written and provides a rather detailed experimental treatment of a tapered fiber design of high-energy diffraction-limited Er-doped fiber amplifier, as well as a profound analysis of different approaches for inter-pulse ASE measurement. I believe that the paper might be interesting for the readers of MDPI Photonics and I'd like to recommend it for publication after a minor revision according to following concerns:

 

1. It's not clear from the text, how the data for Fig.1(a,b) was measured / retrieved. Since the aim of MDPI Photonics [https://www.mdpi.com/journal/photonics/about] is "to encourage scientists to publish their experimental and theoretical results in as much detail as possible", I'd like to suggest the authors to say few words about that or/and to cite some previously published paper about it.

 

2. It would be useful to show the correspondence of curves and vertical axes in Fig.1a with arrows (similar to Fig.3a and Fig.4a,b). (Although such correspondence is already shown with color, additional arrows might be useful for those readers who use b/w hard copies of the paper or those who are color blind.)

 

3. There's a misprint in line 115: instead of 1 μm it should be certainly 1 μs.

 

4. The abbreviation "DC" (see "DC EDFA" in line 128) should be explained in the text. It's worth highlighting the difference between first two EDFA cascades and DC EDFA, see the scheme in Fig. 2a.

 

5. It would be convenient if the abbreviations used in Fig. 2 were explained in the caption to the figure. This would make the scheme in Fig.2 understandable without having to read the whole text of the article.

 

6. It's stated in lines 133-134 that "inter-pulse power was below accuracy of our measurements (~ 2%)". Is it a misprint, should one read "energy" instead of "power" here? (If it's not a misprint, this measurement is not informative. Indeed, since pulse repetition rate is 1 kHz and pulse duration is about 1 μs (see the line 115), one can estimate duty cycle as 0.001. Thus if inter-pulse power is about or less than 2% of pulse peak power, inter-pulse energy is less or equal than 20*pulse_energy. Certainly this inequality says almost nothing about neither inter-pulse energy nor pulse energy: both of them can vary by an order of magnitude!)

 

7. Text captions in Fig.2a and in the inset in Fig.2b are too small and difficult to read. Please consider increasing the font size.

 

8. The authors mention pulse duration of 1 μs in line 115 and later 5 μs in line 167. Which of these two values is correct? Or pulses of different width were used? This should be either corrected or discussed.

 

Despite these issues, I believe that the paper might be interesting for readers of MDPI Photonics and I’d like to recommend it for publication after a minor revision.

 

Author Response

Please see the attachment

Author Response File: Author Response.docx