A Terahertz Fast-Sweep Optoelectronic Frequency-Domain Spectrometer: Calibration, Performance Tests, and Comparison with TDS and FDS

Round 1

Reviewer 1 Report

The paper discusses about the calibration and performance benchmarking of a terahertz fast-sweep optoelectronic frequency-domain spectrometer. In addition to that the Authors compared this novel instrumental platform with two already validated technologies: time-domain spectroscopy (TDS) and photonic-based frequency-domain spectroscopy (FDS). After calibrating and analysing the performance of the T-Sweeper, the following materials were tested with the three different instruments: resonant mesh filters, lactose monohydrate and UHMWPE. Comparing the T-Sweeper results with two other instruments TeraFlash Pro (TDS) and TeraScan (FDS) the Authors found that the results were in agreement.

Although well defined, the calibration procedure described is not a totally innovative contribution (but it is configured in the standardized procedures). Despite this, the work fit the journal scope and the results obtained give the scientific community the awareness of being able to perform THz spectroscopies even with a fast-sweep optoelectronic frequency-domain spectrometer. Validating another analysis technique and instrumentation is of great interest. The manuscript is clear, easy to read and well structured, written in good English with some minor mistakes. The References provided are recent to the state of the art, extensive and well selected. The experimental method is quite easy to reproduce, has a correct methodological approach.

The reviewer’s opinion is that this works is interesting, appropriate for the readership of this Journal and deserves to be published after some changes. The following specific suggestions are proposed for the Authors: 

(1) Title: It could be important to increase the visibility of the article to include in the title some words to highlights the comparison with TDS and FDS. That is a key aspect of the article. 

(2) Abstract, lines 1-6: The abstract appears extremely concise and is similar to a sort of index of the content, it should be reformulated focusing on the goal of the study and also adding an anticipation of the results obtained.

(3) Keywords: I suggest increasing the total number of keywords to five.

(4) Section 1, lines 32-34: A references may be necessary to prove the claim of a slow frequency scanning rate.

(5) Section 2, Figure 1a: The text reference to this image should be inserted. Furthermore, if possible, it would be useful to insert a straight photo with the optics within the frame as well.

(6) Section 2.2, line 72: Have they been calibrated and validated with the same procedure as T-Sweeper? More specifications are needed.

(7) Section 2.2, line 89: It would be extremely useful for the reading to insert a comparison table with the main characteristics of the three instruments.

(8) Section 2.3, line 95: Add acronyms in parentheses (Smax) and (NF).

(9) Section 2.3, line 108: Did you find an explanation for the sharp dip at 0.88 THz? Or do you think the causes are attributed to emitter and / or receiver geometry for that too?

(10) Section 2.3, Figure 2a: The image is not perfectly legible in black and white, it might be useful to reverse the order of the legend (starting from the bottom 1 up to 100,000 at the top).

(11) Section 3.1, line 143: “A high-resistivity silicon wafer can serve as a low-finesse etalon.” It would be useful to insert a Reference.

(12) Section 3.1, line 157: the formula reported is different from Figure 4b description. Also is missing the meaning of Np.

(13) Section 4.2, Figure 6: It would be advisable to unify the legend and not divide it into three graphs, it can cause confusion. Furthermore, the readability in Black / white is not optimal, lines with different strokes could be used. The frequency scale for the three graphs should be uniform, and the axe title frequency (THz) should be repeated for each graph (a, b, c).

(14) Section 4.3, Figure 7: Similar considerations on poor visibility in black and white. Figure 7a should be enlarged in order to correctly plot the absorption max. In the text there is no reference or comments to Figure 7b.

(15) Section 4.3, Line 233: There is no figure 8c, should be 7c instead, correct?

(16) Section 4.4, Figure 8: Similar considerations on poor visibility in black and white. In the text there is no reference or comments to Figure 8b.

(17) General final note: it is advisable to carry out a careful review of English from the point of view of spelling, grammar and typos.

In conclusion, the manuscript deserve publication after the suggested minor revisions.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

In the manuscript ApplSci-1857479-v1, entitled “A terahertz fast-sweep optoelectronic frequency-domain spectrometer: calibration and performance test”, the authors report calibration and performance tests of a terahertz (THZ) frequency-domain spectrometer (FDS) designed for industrial applications. The FDS made use of a fast-sweep optoelectronic and was calibrated using water lines and silicon wafer etalon, and its tests were performed by measuring its transmission properties and comparing with other spectrometers. More impressively, the FDS has made use of optoelectronic system that is capable of faster scanning than conventional temperature-tuned FDS, and is comparable with time-domain spectrometer (TDS). After reading the manuscript, I found it well-written. The operation and specifications of the system are described in detail. Both the calibration and performance test data seem convincing. These factors above are sufficient to merit a publication in Appl. Sci. Before recommendation, I would like the authors to address the follow concerns.

1)     Please clarify and define the abbreviations “Tx” and “Rx” in Figure 1b, although it’s easy to guess they might refer to "transmitter" and "receiver".

2)     Figure 2a is a bit confusing when combining with the statement “For random (Gaussian) noise, averaging lowers the noise floor by…” in Line 100. It seems to me from Fig. 2a that the noise floors remain the same when increasing the averages. Or this is a normalized plot? Please clarify this.

3)     Figure 5b, caption, it should be “The fitted slopes …” instead of “The calculated slopes …”.

4)   As this optoelectronic FDS is for industrial applications, and probably ultimately would become a commercially available product, I wonder if it is possible to have a table showing the specifications/parameters of current FDS and that of the other two spectrometers?

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript, the authors report calibration and performance tests of a terahertz fast-sweep optoelectronic frequency-domain spectrometer designed for industrial applications. The frequency scale is calibrated using atmospheric water vapour lines and a silicon wafer etalon; amplitude linearity is verified using a set of silicon plates. Instrument performance is tested by measuring transmission properties of a variety of representative samples and comparing with a time-domain spectrometer and a frequency- domain spectrometer. The manuscript is interesting, it is in general deserves publication in Appl. Sci.. My comments below will improve the content of the manuscript. My comments are as follows:
1. For the three figures in Figure 6, the data measured by T-Sweeper instrument is higher in the high-frequency part, please explain the reasons.
2. The novelty of the manuscript should be clearly highlighted.
3. Are these optimized results? the authors should use the optimized results.
4. The author claims that “but overall, good agreement is 235 observed among three instruments, in particular between T-Sweeper and TeraFlash.”. Which result is accurate and reliable?

5. The author should include a few recent references in the manuscript as follows:

“Amplitude and frequency tunable absorber in the terahertz range”, Results in Physics, 2022, 34:105263；“Wave-thermal effect of a temperature-tunable terahertz absorber”, Optics Express, 2021, 29(23): 38557-38566.；

Author Response

Please see the attached as a response to your review.

Author Response File: Author Response.docx