A Passively Wavelength-Stabilized Mid-Infrared Optical Parametric Oscillator
Round 1
Reviewer 1 Report
In this manuscript, the authors have reported wavelength and power stability of an output-coupled cw SRO based on MgO:PPLN by exploiting the technique of air tight chamber and water cooling. The discussion of thermal and pressure effects on the cavity stability and free spectral range (Sections 3.2 and 3.3) is the most interesting and novel results. The results presented includes study of frequency drift due to air pressure variation and temperature change in the base plate deployed. Further discussions have been done on mode hopes which has been claimed to be due to the larger variation of these parameters (air pressure, temperature). I recommend this work to be published in Photonics, with the following modifications are required:
1. In line 34, the authors should illustrate clearly what is YAG.
2. In line 68, the authors miss the space in “M3-5”.
3. In line 73, the authors do not show how to calculate the waist radius, and I also wonder the beam quality factor of idler beam.
4. In line 103, the authors say the material absorption limits the idler power, but they do not illustrate the data or references.
5. In line 150, the authors should illustrate detail about the derivation of the equation (7).
6. Please check the whole figures, there are some spelling mistakes in Figure 7-8.
7. As shown in Figure 6-7, why will the idler power change suddenly when longitudinal modes are hopping?
8. As shown in Figure 6, The deviation of idler power is ~100 mW, which is nearly 2.5 times the deviation of idler power in Figure 7, the spaces of mode hop, however, are nearly equal. Please describe the reason.
9.As shown in Figure 9, the SRO is capable of operating without mode hops over 13 hours. Figure 6-7, however, shows the longitudinal modes of the SRO hops. Why Figure 9 is in accord with Figure 6-7?
Author Response
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Author Response File: 
Author Response.docx
Reviewer 2 Report
The novelty of this work is not very clear because 1um pumped continuous-wave(CW) PPLN OPOs are know to be widely tunable from 1.45-4um in the signal and idler combined. There are commercially available 1um-pumped CW PPLN OPOs which provide output wavelengths from 1.45-4um with average power >5W in the wavelength tuning range such as the Radiantis TITAN (https://radiantis.com/products/titan-cw-opo-based-laser-system/). And it is also a well known fact that CW OPOs are very sensitive to temperature and air-pressure fluctuations. There have been few reports of passively stabilized long-term mode-hop-free CW OPOs with mode-hop-free operation of 24 hours (Ikegami et al., Opt. Comm. 184(1-4), 13-17 (2000)).
Also, the paper needs some spelling check and grammar correction. The literature review is incomplete and doesn't mention the prior-art properly. The authors need to provide a thorough comparison with other works and prove the novelty.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
In this manuscript, the authors investigated the effect of temperature and air pressure of the OPO cavity on the stability of output mid-IR light frequency. The results showed the importance of controlling both temperature and air pressure to achieve a long-term stable light frequency for a CW OPO laser, which is a big improvement for CW mid-IR OPO laser. For this reason, I commented that this manuscript can be published in the journal of Phtonics. But I just have one question:
1. What is the linewidth of the pump laser and the idler light? Maybe the authors can add this information to the manuscript.
Author Response
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Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Thank you for making the necessary changes in the manuscript. However, I still think that the manuscript is incomplete. There is not much information provided on the actual building and development of the setup which would allow another researcher to effortlessly repeat the work. Every bit of information including actual photos of the setup, the water cooling system, air-sealed cavity, etc. needs to be provided for the purpose of repeatability of this work!
There needs to be full discloure of the the cooling system (design and every components used and justification on any particular material choice, thermal simulations if done, 3D drawings of the advanced cooling setup which helps in getting such high thermal stability). The same applies to the air-pressure sealed chamber, you need to provide actual setup pictures, designs, specific materials/components used, etc. Since, the information mentioned above is the only and actual novelty of this manuscript, so without a full disclosure of the making of this setup which can easily allow any other researcher to repeat this work, this manuscript cannot be published.
Author Response
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Author Response File: Author Response.docx
