Round 1

Reviewer 1 Report

The manuscript reported the design, fabrication and characterization of multi-section, coupled cavity mid—IR quantum cascade lasers. The proposed design of 3 section coupled cavity quantum cascade lasers allows stable, single mode emission with 35 dB side mode suppression ratio. Overall, this paper is organized in a proper manner and the experimental discussion is somehow sound. Nevertheless, a few other points are suggested to be addressed.

1. It is found from Figure 6 that the temperature is used to tune the lasing peak, I wondering what the mechanism is.
2. Some definition of the abbreviation are missing.

 

Author Response

The authors would like to thank the Reviewer for the comments on our paper submitted to MDPI Photonics.

The missing definitions of abbreviations were introduced in the text. Temperature tuning is common to all types of semiconductor lasers. The change of the operating temperature (e.g. cooling in cryostat, or heating above room temperature) results in temperature induced change of material properties, like energy bandgaps and refractive indices, what results in shift of emission wavelength.

Reviewer 2 Report

The work presented by the authors is interesting both from the point of view of laser physics and from the point of view of modern technology for creating cascade lasers using the method of molecular beam epitaxy technology. But I have a few comments without correcting which, I believe, the article will be difficult for Photonics readers to understand.

The first remark appears in section 2, which describes the process of creating a quantum cascade laser. From the point of view of a reader who is not too deeply familiar with the topic but who is trying to understand the laser manufacturing process, I would like to see a more detailed presentation of the process of creating a three-cascade laser. In terms of describing this process. The description of this process on page 2 does not fully reveal the essence of Figure 1 in my opinion. In addition, in the caption to Figure 2 there are abbreviations MS and LS that are not described in the text. In the abstract, the abbreviation QCLs is used, the explanation of which is given later in the text. This is not quite accepted in the scientific literature.

The second remark concerns the sentence "only the modes that coincide in both cavities can propagate" in Section 3 line 93. What is meant by the word "coincide»? The constant propagation of modes or the distribution of their electric field in resonators? What is “FSR” and “The TRS spectrum of two—section CC— 103 QCL” in lines 96 and 103? LS and SS in line 111? It seems to me that it is necessary to attract your reader and make the text as accessible as possible for reading and understanding. I believe that it is necessary to improve the form of presentation of the material. From a scientific point of view, the content of the work is beyond doubt.

Author Response

The authors would like to thank the Reviewer for the comments on our paper submitted to MDPI Photonics.

The abbreviations in abstract and in text were corrected.

The FSR abbreviation is the free spectral range of the Fabry-Perot resonator, and is here understood as the measure of distance between the modes allowed by the resonator.

The sentence "only the modes that coincide in both cavities can propagate" was rephrased in the text, to clarify the reception of the statement.

 

Reviewer 3 Report

The paper reports on the fabrication and characterization of a QCL laser employing three coupled Fabry-Pérot cavities of a different length. Improvements in the laser performance, notably in tunability in the pulsed regime, are demonstrated by appropriate experimental evidence, and the basic mechanisms behind them briefly elucidated accounting for cavity transmission as a function of wavelength.

The manuscript suffers from a sometimes excessive conciseness (it should be considered as a follow up of previously published works by the same group of Authors, duly mentioned in the reference list, whose reading helps understanding the present paper). Nonetheless, it is sufficiently clear and presented results are overall convincing.

Therefore, my opinion is that it can be definitely considered for publication. Prior to acceptance, Authors are invited to consider the following minor points.

1. Authors state in the Introduction (lines 36-37) that the proposed architecture offers a significant reduction in fabrication complexity. This is mostly true when external cavity lasers are considered, rather than DFB devices, thanks to recent advancements in lithography techniques. However, further to my personal opinion, my feeling is that the concept should be better outlined in the text. At least, it should be mentioned again in the Conclusions.
2. A similar comment applies more in general to the performance of the device. I would like to see in the Conclusions an assessment of the achieved performance compared to those of other, more conventional, architectures.
3. I understand this point is probably out of the scope of the paper, but the reader would find useful to have some deeper insights on the physical mechanisms involved in the operation of a three coupled cavity device. The idea is first mentioned in the Introduction, lines 48-51: the relevant sentence is rather unclear, since the “stabilizing” effects of the third cavity are not truly defined. Moreover, the links between the transmission properties of the coupled cavities, presented in Section 3.1, and the “stabilizing” effects are not duly elucidated also in other parts of the manuscript. Authors are invited to better point out the basic mechanisms involved in the improved tunability of the laser.
4. Discussion on the time resolved spectral properties of the laser (lines 206-224) would take advantage of a careful revision. First of all, the statement that the non-linear wavelength shift is related with thermal properties of semiconductor lasers is unclear to me, and, at least, should be supported by appropriate references. Moreover, interpretation of the findings shown in Fig. 9, based on the change of refractive index (as a function of what?), is unclear. Authors should at least present details of the calculations they did, which are too vaguely mentioned in the text.
5. The comparison with “standard” FP laser shown in Fig. 3 is interesting, but also in this case clarity can be improved. A few more details should be given on such a standard laser (cavity length, current injection electrodes, structure of the laser, etc.). Furthermore, curves in Fig. 3(a) should be clearly attributed to either left or right vertical axis.
6. Not all acronyms mentioned in the text appear to have been duly defined: Authors are invited to check for the correct spelling of all acronyms. There are a few typos such as “guaranties” and “ration”.    

Author Response

The authors would like to thank the Reviewer for careful reading of the manuscript and accurate comments on our paper submitted to MDPI Photonics. We have corrected the manuscript following the remarks.
Some information concerning the FP laser has been added in the manuscript.

The stabilizing effect of SS on spectra was probably not defined properly in the manuscript.  The purpose of the work was to propose 3-section device, emitting single mode with longer range of continuous tuning during the pulse, in contrast to 2-section device. This effect was obtained and demonstrated in the work. Additional information was added in the text.

As for the nonlinear wavelength shift, it is the result of thermal properties of the device. Numerical modelling, as well as experiments, show, that for most of types of semiconductor lasers, the device is heating very fast at the start of the current pulse. This effect saturates with time, and for long pulses (ms), the temperature can be considered as constant. When short pulses are considered, (ns – us range) this effect is well visible e.g. when registering temperature induced wavelength shift, which is a standard technique used for estimation of the temperature of semiconductor lasers.

Both FP and CC devices were fabricated within the same processing batch, thus, the only difference is the resonator modification in CC laser.

Figure 5 was edited to clearly attribute curves to the left and right axis.

The missing abbreviations in abstract and in text were corrected

Reviewer 4 Report

1. Introduction. Authors mentioned that, single mode emission is one of the crucial requirements in spectroscopy applications, however did not cited important spectroscopy applications related to hyperfine spectroscopy in which single mode emission and narrow band are crucial parameters. There are reports about use of QCL in cavity ring down spectroscopy (Photonics 2020, 7(3), 74; https://doi.org/10.3390/photonics7030074 ) and others laser sources for hyperfine spectroscopy of muonic hydrogen.

2. Introduction. “In direct absorption laser spectroscopy, the emission line of laser source is swept (tuned) across the absorption line or several lines of gas.” This sentence needs a reference related to spectroscopy of gas using narrow band, single mode, tunable QCL for detection of gases, for molecular trace gas detection.

3. There are many acronyms, which for skilled person are known but for general reader are not; therefore, the used abbreviated terms must be defined at the first use.
4. The experimental part presented poor description of the characterization experimental procedure, the devices used are not described but only cited in other papers. It is important to report also the linewidth of the single mode emission, the pulse duration, jitter, wavelength stability, etc.
5. The authors plotted wavelength versus pulse duration but to confirm the single mode operation, the pulse shape and jitter must be also presented.

What (if any) is the difference in the pulse duration and build up time as a function of the selected wavelength?

6. The change of ambient temperature will affect the cavity length, leading to the drift of resonant cavity mode. How this effect was controlled?.
7. Information about parameters such as energy stability, wavelength stability and pulse-pulse stability has no mention in the manuscript. The authors need to perform test about stability. It’s important to report also long term stability. How long time was the laser working without mode hopping?.
8. The authors should include the frequency stability of the source and to correlate it with linewidth.
9. What about the energy per pulse?. No any data was reported.
10. The authors should mention about the precision/ accuracy and response time of the system during the measurement.
11. How is the laser source stability reported in this manuscript different from the one that was developed earlier?.
12. How does the transverse beam profile looks like?.
13. The quality of figures should be improved.
14. The authors should take care of the citations. " Coupled Cavity Mid-IR Quantum Cascade Lasers Fabricated by Dry Etching" is not citation 14.
15. Figure 4, the axis unit is missing. For the caption, the authors must give the values of section lengths of the 3 different cavities.
16. How these numerical results in Fig.9 show the thermal tuning during the pulse or change of temperature?
17. For the References the authors must respect format of the journal.

Author Response

The authors would like to thank the Reviewer for careful reading of the manuscript and accurate comments on our paper submitted to MDPI Photonics. We have corrected the manuscript following the remarks. Mentioned citations were added.

Missing abbreviations and acronyms definitions were added.

In general, we agree that more detailed experiments concerning the linewidth of single mode spectra could be presented increasing overall quality of the paper. At present, we are developing appropriate setups and methodology to perform experiments. The linewidth will be approximated by tuning the emission through the absorption line of NO2. Those results will be published in near future.

The stabilizing effect of SS on spectra was probably not defined properly in the manuscript. The purpose of the work was to propose 3-section device, emitting single mode with longer range of continuous tuning during the pulse, in contrast to 2-section device. This effect was obtained and demonstrated in the work. Additional information clarifying the aim was added in the text, to avoid unclear phrase of “stabilizing effect” on the emission spectra.

Concerning the driving pulse shape, all the measurements were performed using very fast voltage source power supply (Avtech), delivering pulses with rise times of 5ns.

The temperature of the device under test was stabilized using the Peltier element to minimize ambient temperature effect on the spectra, as well as on device parameters.

The experiments (energy stability, wavelength stability and pulse-pulse stability) which the Reviewer mentioned are undoubtfully required to fully assess the quality and performance of the proposed design. They will be performed in near future.

The energy per pulse can be calculated giving c.a. 0.05 uJ.

The far field profile of emission was measured. The device emits in fundamental single spatial mode (TEM00).

Figure 4 and its caption were corrected according to remarks of the Reviewer.

Round 2

Reviewer 4 Report

The authors improved the paper and I am also very satisfied with their answers, therefore i suggest acceptance of the manuscript