Simultaneous Detection of Gas Concentration and Light Intensity Based on Dual-Quartz-Enhanced Photoacoustic-Photothermal Spectroscopy

Round 1

Reviewer 1 Report

 

In Hao Liu et al., the authors present a photoacoustic gas sensor using two quartz tuning fork (QTF). One QTF is used to measure the gas concentration, the second to measure the laser light intensity. This approach makes it possible to correct the measurement of the concentration by taking into account the variations in power of the laser. In their paper, the authors demonstrate experimentally that it is possible to apply this correction thanks to the linearity of the power measurement. The manuscript was well organized and can be easily understood. For me, I can recommend its publication with the following addressed.

 

L40 : The authors cite Alexander Graham Bell but reference [7] is by A.A. Kosterev. Similarly, it seems that the reference [8] is not the right one either. 

 

L46 : The authors say that the QEPAS technique has the advantage of “low air consumption”. I don’t see what means “low air consumption”.

 

L47 : The authors say that the QEPAS technique has the advantage of “unmatchable level of compactness”. It is untruth, as an example you can read the following reference.  Glière, A., Barritault, P., Berthelot, A., Constancias, C., Coutard, J. G., Desloges, B., … Nicoletti, S. (2020). Downsizing and Silicon Integration of Photoacoustic Gas Cells. International Journal of Thermophysics, 41(2), 1–18. https://doi.org/10.1007/s10765-019-2580-7

 

L56 : Reference [20] is not dealing with QEPTS but it is a comparison between two optical detection techniques, one based on a standard Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) detection module; and the other one based on a Photothermal Spectroscopy (PTS) module where a Fabry-Perot interferometer acts as transducers to probe refractive index variations.

 

Fig 4 : The x-axis should not be a time but a least the drive current or/and the laser emitting wavelengths.

 

Fig 5a : The shape of the QEPTS signal should be explain (idem Fig 7b).

 

L204 : The authors should have given the exact time used for the measurement of the Allan deviation and the lock-in time constant used.

 

Section3.5 : Is the normalized signal (with power correction) used for the measurement of the Allan deviation ? Is long-term drift improving with the normalized signal ?

 

The authors have previously published similar results using a single QTF, reference [25] in the article. The difference between these two results must be described to explain the novelty of the submitted article.

 

The sensitivity of the laser intensity detection is also not discussed. The technique is only of interest if one can estimate the smallest power variation that the system can correct. That’s why an estimation should be given by the author.

Author Response

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Reviewer 2 Report

In this work, the authors present a method based on the dual-quartz-enhanced photoacoustic-photothermal spectroscopy for simultaneous measurement of gas concentration and laser power. And the influence of laser power fluctuation on photoacoustic signal is eliminated. Before considering acceptance for publication, a minor revision is recommended with my comments listed as follows:

a) All Figures in the manuscript are not clear. Please adjust the clarity of the Figures.

b) In Figure 3, it is suggested that the author change the abscissa unit to wavelength or wave number, because different lasers have different conversion coefficients (current to wavelength).

c) In lines 90 and 91, there is an extra dot, the wavelength should be 1653.72 nm, not 1.653.72 nm.

d) All mentioned instruments need to note their manufacturer and model.

e) Why set the scan frequency to 0.05 Hz ? Why not use faster scanning frequency ?

f) The integration time should be included when reporting the minimum detection limit in abstract and conclusion.

g) Some publications about the measurement of laser power by photothermal spectroscopy should be discussed. 1) IEEE Photonics Technology Letters, 2019, 31(19): 1592-1595. 2) Measurement, 2020, 156: 107601.

Author Response

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Reviewer 3 Report

The QEPAS system measurement outcomes will be negatively impacted by variations in laser light intensity. This study describes a dual tuning fork system that combines QEPAS and QEPTS to normalize the photoacoustic spectroscopy light intensity. This method is crucial for long-term measurements. The benefits are particularly clear in the terahertz and mid-infrared bands. However, I want to discuss with the authors here about a few issues.

1.      Does the laser beam strike the root of QTF2 directly after passing through QTF1 or is it focussed via the converging lens? Please include a clear explanation of this in the publication by the authors.

2.      How large is the volume inside the photoacoustic cell? Does absorption also occur in the QEPTS system? Please explain in the paper.

3.      Two commercially available 32~KHz tuning forks were selected for this study to detect the photoacoustic and photothermal signals, respectively. However, why the Q values of QTF1 and QTF2 are very different? The authors are requested to briefly explain the reasons.

4.      The electrical excitation method described in line 132 of the text to measure the response frequency of QTF, the obtained frequency response curve is usually asymmetric, which seems to be different from the frequency response curve of Fig. 2. Please explain in the paper.

 

5.      The 2f signal and 1f signal at different concentrations are shown in Figure 7. The 2f signal increases with increasing concentration while the 1f signal is almost constant. Please give a more adequate explanation.

 

Author Response

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Reviewer 4 Report

The article proposes to use tuning forks instead of photodetectors to achieve the measurement of light intensity. Especially when the light source is in the mid-infrared band, it can not only reduce the cost, but also the system is simpler. The method has shown great advantages in the practical application of QEPAS technology. It provides the solution to the problem of light intensity normalization for QEPAS technology. However, I have some questions and suggestions to present here.

1. Figure 1 shows the QEPTS signal added to the QEPAS signal by an adder, which is amplified by a preamplifier. However, the intensity of QEPAS and QEPTS signals are very large differences. Is there a problem with signal saturation when using the same preamplifier?

2. In this paper, the measurement of QEPAS and QEPTS signals is realized by using dual frequency modulation. Will there be any mutual interference between the two frequencies?

3. Temperature and atmospheric pressure can negatively affect the measurement of QEPAS and QEPTS signals. How did the authors exclude the influence of temperature and atmospheric pressure on the measurement results?

4. Different scanning frequencies will affect the amplitude of the QEPAS and QEPTS signals. The scanning frequency chosen in the paper is 0.05Hz. What is the author's rationale for choosing the scanning frequency?

Author Response

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