Ultra-Broadband NPE-Based Femtosecond Fiber Laser

Round 1

Reviewer 1 Report

In this paper, the authors report a dissipative soliton mode-locked fiber laser delivering ultra-broadband (over 60 nm) ultrashort pulses of energy up to 2.2 nJ and pulse duration of 149 fs without dispersion compensation, as well as possible further scaling up to 3.5 nJ and 100-nm spectral bandwidth by numerical simulation. The results could be beneficial to improve the performance of femtosecond fiber laser systems. In my opinion, the manuscript would be more indicative and meeting the publication criterial of this Journal upon the following revisions. In achieving this ultra-broadband emission spectrum, can the authors identify what they have done in special for their success? Or in other words, what have they identified as the most influential parameters for their performance in this rather common fiber laser configuration, be it the SMF length, dispersion, high-power pump, the PC adjustment, or some other fiber parameters?

Author Response

We thank the Reviewer for his valuable comments, which resulted in improving the paper quality. In fact the result has been obtained due to a combination of such parameters as high available pump power, short SMF length and gaussian-shaped spectral filter which facilitates nonlinear broadening without pulse distortion. The high value of Pcrit is provided by the short SMF length of about 1 m according to [37]. High pump level is needed to support a stable operation regime. Both parameters take a place only together. However, we believe that the most important is the value of rho_min which prevents a transition to a multi-pulse operation regime with increasing pump power. To highlight this fact we rewrite the next sentence in conclusion as follows:

“… reducing the low power transmission coefficient of the SAM, which prevents a transition to multi-pulse operation regimes with increasing pump power.”

We also modify the discussion section as follows:

“… relatively short SMF part of the cavity due to the strong self-phase modulation effect. Since the pulses have a nearly parabolic shape defined by the filter (see Fig. 1(b)) they propagate self-similarly [38]. As a result the bandwidth increased almost by twenty times (up to 30 nm) without optical wave breaking.”

...

“To compensate for the effect of increased gain on a low power radiation it is necessary to reduce ρ_min value. Thus, the most unexpected parameter plays the same important role as the length of SMF part and the pump power. A quite similar …

It should be noted that achieving such small value of ρ_min in an experiment is a subject for further investigations as the limiting factors are still unknown. Moreover, even the achieved level of less than 4% is quite surprising for a NPE-based SAM and provides a new perspective on the development of a fiber master oscillator. It will definitely result in a new class of high-power ultrashort fiber laser thanks to the use of modern fiber amplifiers, including those based on tapered fibers [28].”

Reviewer 2 Report

The paper is devoted to experimental and numerical study of dissipative solitons with ultra-broadband spectrum in NPE mode-locked Yb-doped fiber laser. Chirped 14-ps pulses with 60-nm bandwidth and relatively high energy of 2.2 nJ has been achieved. Almost 100-fold compression with diffractive gratings has been demonstrated leading to the pulse duration of about 150 fs. The reason of generating ultra-broadband spectrum has been convincingly presented. The results are solid and interesting. They have a sufficient novelty and potential impact on the development of mode-locked fiber lasers operating in the normal dispersion range and producing ultra-broadband pulses. The paper is well written and well illustrated. Thus, I strongly recommend the paper for publication in Photonics.

I have only a few optional suggestions and questions to the authors.

 

·                The information about the active rare-earth ions or the operating wavelength is missed in the abstract. It may be useful to say in the abstract that the paper is devoted to an Yb-doped fiber laser and/or specify the central laser wavelength.

·                The experimental spectra presented in Fig. 2(a) have fringes. What is their origin?

·                What nonlinear crystal was used for the FROG measurements?

·                To support and explain the experimental results and predict the system behavior for different parameters, the numerical model with allowance for the Raman nonlinearity is used. How strongly does the Raman term affect the nonlinear dynamics of dissipative solitons? Will there be a qualitative or quantitative difference if this term is switched off in the simulation?

Author Response

We thank the Reviewer for his valuable comments, which resulted in improving the paper quality. We have made corresponding changes to the revised manuscript, in which we believe that we have addressed all the questions and suggestions. Below are line-by-line responses to all the comments.

1. The information about the active rare-earth ions or the operating wavelength is missed in the abstract. It may be useful to say in the abstract that the paper is devoted to an Yb-doped fiber laser and/or specify the central laser wavelength.

We thanks the Reviewer for this suggestion. The first sentence of the abstract was rewritten as follows:

“A dissipative soliton mode-locked Yb-doped fiber laser ...”

 

2. The experimental spectra presented in Fig. 2(a) have fringes. What is their origin?

The fringes are the result of spurious interference in the APC output connector and OSA input and do not have physical meaning. We believe that much more important that the spectral shapes and bandwidth are in excellent agreement with a high contrast.

 

3. What nonlinear crystal was used for the FROG measurements?

BBO crystal with the thickness of 200 mkm. The info was added into the text.

 

4. To support and explain the experimental results and predict the system behavior for different parameters, the numerical model with allowance for the Raman nonlinearity is used. How strongly does the Raman term affect the nonlinear dynamics of dissipative solitons? Will there be a qualitative or quantitative difference if this term is switched off in the simulation?

We thank the Reviewer for this question. In fact Raman term plays only limiting role in our case. In particular, it limits the upper boundary of the area (Fig.5) and does not affect the shape of stable solutions. So, the further increase in Psat leads to the formation of noise in the right part of the soliton spectrum and reduces its stability. Next it results in the complete distortion of the pulse. To highlight this fact we add the next sentence in the text:

“At the same time, the maximal pulse energy limited by the value of rho_min from the right side of the area and by the Raman threshold from the top one. It should be noted that Raman term in the numerical model plays only limiting role and does not affect the shape of stable solutions.”

And rewrite the next sentence as follows:

The presented area shows that higher energy can be obtained only by reducing rho_min.

Reviewer 3 Report

Abdrakhmanov et al. shows a discussion of a dissipative soliton mode-locked fiber laser. However, there are some comments for the authors.

Major comments:

1.       What is the phase information from FROG measurement in Figure 2? Since this is a optimization process, the phase of the pulse is very important for the readers to evaluate this laser.

2.       What is the reason for the unsymmetric FROG pattern in Figure 2? Is this an alignment issue? In general, the FROG pattern should be symmetric.

Minor comments:

1.       Line 53, “required, like cell imaging and laser microscopy.” This sentence needs reference.

Author Response

We thank the Reviewer for his valuable comments, which resulted in improving the paper quality. We have made corresponding changes to the revised manuscript, in which we believe that we have addressed all the questions and suggestions. Below are line-by-line responses to all the comments.

Major comments:

1. What is the phase information from FROG measurement in Figure 2? Since this is a optimization process, the phase of the pulse is very important for the readers to evaluate this laser.

We agree that the phase information is the most important. However, as was mentioned in the text, the FROG trace got cut off due to limited range of our measurement equipment. So, we were not able to retrieve the phase from the measured traces but we can just sum up the intensity of all wavelengths at each time delay to plot the ACF. Also we can look qualitatively at the captured part of the FROG traces. E.g. Fig.2(d) shows the third order dispersion pattern. We rewrite the next sentence as follows:

The FROG trace got cut off due to limited range of our measurement equipment, so it was not able to retrieve a phase and pulse shape but the pulse duration was retrieved from the FROG traces simplified to auto-correlation function by summing up the intensity of all wavelengths at each time delay.

2. What is the reason for the unsymmetric FROG pattern in Figure 2? Is this an alignment issue? In general, the FROG pattern should be symmetric.

FROG trace got cut off by wavelength due to limited range of used spectrometer. In the time domain it looks symmetric enough to eliminate an alignment issue.

Minor comments:

1. Line 53, “required, like cell imaging and laser microscopy.” This sentence needs reference.

The sentence was rewritten on the following form:

… “required, such as multiphoton and time-resolved microscopy, harmonic generation and material nano-processing.”