Continuous-Wave Room-Temperature External Cavity Quantum Cascade Lasers Operating at λ~8.5 μm

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the authors designed a reasonable external cavity system by adjusting the external cavity length and thoroughly investigated the effects of temperature and current on the output performance. A tuning range of 103.3 cm⁻¹ and a high performance output with a side mode suppression ratio of 30 dB were achieved at 25°C. This work provides good insights for optimizing external cavity designs and offers comprehensive experimental validation.

 I think this paper is suitable for Photonics, while there are still some points need further clarifications.

 1) In the "2. External cavity setup" section, the authors mention that in order to ensure the accuracy of the side mode suppression ratio (SMSR) measurement, the 'N-B* Strong' parameter was set as the apodization function in the FTIR testing equipment. Could the authors explain why this parameter was chosen and whether there is any experimental validation to support this choice?

2) In Figure 3, the authors verify that a 25 cm external cavity length is optimal at a diffraction angle of 39°. However, based on the simulation results, it appears that 25 cm should also be the optimal external cavity length at other diffraction angles. Was there any experimental validation conducted to support this at other diffraction angles?

3) In Figure 3, "9.5 um" is incorrect and should be corrected to "9.5 μm". The authors should carefully check the article for similar errors.

4) The authors need to further check the figure legend format. For example, "Figure. 4." is incorrect and should be corrected to "Figure 4.".

5) The authors should revise the reference format, such as using the standardized abbreviations for the journal names in the cited references.

Author Response

Please see the attachment！

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript, "Continuous wave room temperature external cavity quantum cascade lasers operating at λ ~ 8.5 μm," primarily focuses on the experimental characterization of an External Cavity Quantum Cascade Laser (EC-QCL). Following a relatively brief introduction, the authors provide details about the experimental setup, including a schematic diagram and specific information about the QCL used in their study. The manuscript then presents the results of their experimental measurements, investigating the effects of three key parameters on the performance of the EC-QCL: (1) cavity length, (2) injection current, and (3) temperature. The authors have examined and reported how variations in these parameters affect the power output, tuning bandwidth, linewidth, and threshold current of the EC-QCL.

The manuscript is well-organized and structured. While the presentation is clear, in some of the sentences, it could benefit from some polishing and minor revisions to enhance readability and improve clarity, so I suggest the author do another round of proofreading. The introduction provides an overview of the subject and concludes with a summary of the authors' approach. The experimental setup section provides insights into the experimental setup, but the last paragraph of this section should be improved. The authors have conducted measurements across various scenarios and experimental conditions. The discussion of the experimental results is generally clear and well-organized.

·       Comments:

·       Lines 85-89:

1.     Are the authors referring to “Nuttall-Blackman apodization function” when they mention N-B* strong”? Will you please clarify this abbreviation? Also, I believe it could be beneficial to the readers if the authors could add reference(s) to this in the paper.  

2.     Can the authors please add some explanation to their method for power measurement. The authors have referenced another paper, but I believe it is beneficial to the readers if the authors add some description of this method and refer the readers to reference 12 for more details.

·       Figure 2:

1.     I think the phrase” Relative Electric Field Intensity” needs to be clarified. Are the authors referring to intensity which can be written in general as I/I₀ = (E×E^* /E₀×E^*₀). In this case I believe “Relative Intensity” is a more accurate term. Or they are referring to the absolute magnitude of the electric field. The same argument can be applied to the text in line 127.

2.     There appears to be a discrepancy between the reported intensities for the respective cavity lengths on the right and left graphs in this figure. For instance, at a cavity length of 25 cm and an angle of 39°, the intensity is approximately 250 in the right-hand side graph, whereas it is about 500 in the left-hand side graph. Could the authors please clarify this inconsistency?

·       Lines 132-133:

The sentence from the paper is “The simulation results informed the adjustment of the external cavity length to 15 cm, 20 cm, 25 cm, 30 cm, and 35 cm.” I believe this sentence needs more explanation and clarification. After this sentence the text immediately moves to discussing the experimental measurements of the light-current-voltage curves. Can the authors please elaborate more on this sentence? How did the simulation results imply the mentioned cavity lengths?

·       Lines 143-145:

The sentences from the paper are “In contrast, the EC-QCL's threshold current significantly decreases to 626.5 mA at an external cavity length of 25 cm. This reduction may be attributed to the introduction of external gratings in the EC-QCL, which effectively lower mirror loss.” Can the authors please clarify their reasoning by adding more discussion? How is the effective mirror loss reduced by addition of the diffraction grating to the FP-QCL cavity?

·       Lines 162-166:

The sentences from the paper are “As illustrated in figure 4, the EC-QCLs were examined at a constant diffraction angle of 39.0° and a temperature of 25°C, with external cavity lengths set at 15 cm, 20 cm, and 25 cm respectively. The narrowest linewidth of 0.76 nm was observed at an external cavity length of 25 cm. Thus, it is evident that an increase in resonant cavity length corresponds to a decrease in linewidth [18].”

1.     It is known that in general increasing the length of cavity will result in a narrower linewidth. The authors have reported the L-I-V for cavity lengths of 30 and 35 cm also. Were linewidth measurements performed for these longer cavities as well? It can be expected that those cavity lengths will result in narrower linewidths. Assuming those measurements were performed, is the 25 cm cavity still provide the narrowest linewidth? Can the author please clarify this and modify the text accordingly?

2.     Additionally, upon reviewing reference [18], I was unable to identify its relevance to the statements made in the text with this reference. Could the authors please verify the accuracy and appropriateness of this reference?

·       Figure 6:

Based on the reported results in figure 7, I think the maximum power corresponds to the injection current of 1200 mA, Is there any specific reason the authors did not include the related results of the 1200 mA in this graph?

·       Lines 255-256:

The sentence from the paper is “We will solve the coating related problems in the future, and the performance of the EC-QCL will be further improved.”. May the authors please clarify and add discussion (in the proper section of the paper) on the nature of coating related problems, their proposed improvements, and also what are their expected improvements in the performance of EC-QCL?

Author Response

Please see the attachment!

Author Response File: Author Response.pdf