High–Efficiency, Widely Tunable MgO: PPLN Optical Parametric Oscillator

Round 1

Reviewer 1 Report

The article presented by Yueyue Lian et al. entitled “High-efficiency, Widely tunable MgO: PPLN Optical Parametric Oscillator for Mid-infrared Difference Frequency Generation” reported a femtosecond MgO: PPLN OPO pumped by an all-solid-state mode-locked Yb: KGW laser, which can be used for the generation of femtosecond mid-infrared laser by DFG. With 6 W pump power, the OPO based a MgO: PPLN crystal generated 2.68 W of signal power and 1.2 W of idler power, which corresponds to the total conversion efficiency added up to 67.4%. By comparison, this is the highest conversion efficiency of a femtosecond OPO in the similar output wavelength range. Meanwhile, the signal wavelength tunable from 1.43 to 1.78 μm together with idler tunable from 2.44 to 3.68 μm were obtained. This result is surely of interest to the community, I recommend this paper for publication in Photonics after addressing the following comments.

1.     As shown in Figure 4, the signal output power drops sharply after the wavelength of 1.7 μm, what is the reason?

2.     It is better to provide the idler wavelength tunable range for the illustration in Figure 6, which could be more clearly.

3.     How to realize the wide wavelength tuning in DFG?

4.     For the conversion efficiency of 67.4%, please explain the detailed calculation process.

Author Response

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Reviewer 2 Report

The authors reported a femtosecond OPO, wavelength tunable in near- and mid-infrared. According to the authors, they believe it’s the highest total conversion efficiency reported to date. Other than that, this paper lacks innovative points.

1.     Mg:PPLN based OPO has been studied thoroughly since the 1990s. Commercial femtosecond Mg:PPLN OPOs are also available on the market, such as Coherent's Chameleon OPO. Since this paper is mainly focused on the high conversion efficiency, however, a description of how such a high conversion efficiency was achieved is missing.

2.     Regarding the experimental configuration, the authors used a 1 mm long PPLN crystal. The authors should explain the reason for using this length, or whether this length is optimal. The authors describe the spot size in the crystal as 72 µm, but do not say the transverse mode size of the OPO resonant cavity.

3.     Can the authors discuss the effect of crystal dispersion on conversion efficiency and pulse length, and what method should be used if dispersion compensation is to be performed.

 

4.     Although the authors give a comparison of the efficiency of multiple reports in Table 1, this comparison is not very meaningful in the absence of analysis.

Author Response

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Reviewer 3 Report

The authors demonstrated a high-efficiency, widely tunable femtosecond laser generation using optical parametric oscillator (OPO) based on a multi-period MgO-doped periodically poled lithium niobite (MgO: PPLN) crystal. Up to 2.68 W of signal power at 1540 nm and 1.2 W of idler power at 3110 nm was obtained, and the achieve total conversion efficiency was 67.4%. The results are interesting and could find some possible applications in many fields. I recommend to publish this work in photonics, however, there are some concerns should be addressed.

 

1.     I strongly suggest the author modify the title, since the author focused on the generation of signal and idler in an OPO, and the DFG in MIR didn’t have any experimental results.

2.     The calculation of the conversion efficiency may mislead the readers considering the different output ratio for the signal and idler.

3.     The maximum peak intensity was about 16.4 GW/cm^2, which is far beyond the damage threshold of the PPLN. The author needs to explain how they protect the crystal.

4.     In terms of phase matching condition, I suggest the author change the color of the idler spectra in Fig. 7(b) to match the color in Fig. 7(a).

Author Response

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Reviewer 4 Report

In this work, a tunable MgO:PPLN OPO is realized. The most impressive aspect of this work is that compared to other previous work, the overall conversion efficiency of OPO (conversion of pump light to signal light and idler light) has reached 67.4%, which is very interesting. However, there are still the following problems:

(1)       67.4% conversion is impressive for a femtosecond mode-locked laser pumped extra-cavity OPO. However, the author does not seem to analyze in detail the reasons for such high conversion efficiency. That is, compared to other work, what are the innovations in the structure of OPO in this work?

(2)       The resonator length of the OPO is the half of the pump laser’s, resulting in the frequency of the signal light being twice that of the pump light. This means that every two times the signal light passes through the MgO: PPLN crystal, only one of the pulses is amplified. Considering the loss of nonlinear media, this seems to be detrimental to the improvement of conversion efficiency. In addition, the output temporal pulse sequence output from OPO should be given.

(3)       The analysis of some graphs is somewhat simplified. For example, (c) and (d) in Figure 5 should be analyzed in more detail. What does the dash line in the figure represent?

(4) How is RMS calculated? When calculating it, what is the sampling rate and sampling time?

(5) Finally, the article has some problems in English expression. Please pay attention to the unification of tenses and the application format of clauses.

In conclusion, I think this manuscript has interesting work worth attention, but it needs to be reorganized and supplemented with experiments.

Author Response

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Round 2

Reviewer 4 Report

There still exist some problems in English expression in this manuscript. For example, in line 164-165, the sentence of “As shown in Figure 8(a), which displays the normalized out-put signal spectra in the tuning range” lacks a main sentence. In line 187-189, the sentence of “Such a high conversion efficiency was achieved firstly because the maximum peak intensity of pump is 16.4 GW/cm2 in this work that can greatly promote the improvement of conversion efficiency” is grammatically extremely confusing.

In addition to the examples given above, there are other grammatical issues in this manuscript. Please pay attention to the application format of clauses and revise the text carefully.

Author Response

We thank the reviewer’s comments very much. As your suggestion, the corresponding grammatical modifications have been made in the revised manuscript.