Main > Chinese Optics Letters >  Volume 17 >  Issue 6 >  Page 063001 > Article
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Accepted: Mar. 22, 2019

Posted: Jun. 12, 2019

Published Online: Jun. 12, 2019

The Author Email: Cunjun Ruan (ruancunjun@buaa.edu.cn)

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Cunjun Ruan, Deyin Kong, Jun Dai, Kanglong Chen, Sujie Guo, Xiaojun Wu. High-resolution frequency-domain spectroscopy for water vapor with coherent and continuous terahertz wave[J]. Chinese Optics Letters, 2019, 17(6): 063001

## Abstract

High-resolution frequency-domain spectroscopy (FDS) is set up using a coherent and continuous wave terahertz (THz) emitter and receiver. THz waves are generated and detected by two photomixers with two distributed feedback (DFB) lasers. Atmospheric water vapor with different relative humidity is systematically investigated by the FDS. A high-frequency resolution of ∼14 MHz is obtained with the help of Hilbert transformation, leading to a well resolved and distinct transmittance characterization of water vapor. Compared with conventional THz time-domain spectroscopy, the high-resolution continuous wave THz spectrometer is one of the most practical systems in gas-phase molecular sensing, identification, and monitoring.

## 2 R-FCN+ResNet的火焰检测模型介绍

Figure 1. Flame detection algorithm model diagram

## 2.2 R-FCN模型介绍

R-FCN效仿R-CNN[10],采用流行的物体检测策略,分为两步:1) 由RPN[11]根据特征提取网络提供的特征谱图求出疑似火焰区域集合(ROIS); 2) 对RPN求出的ROIS逐个进行火焰识别(ROI为ROIS中任意一个疑似火焰区域)。

In order to verify the sensitivity of THz-FDS, we measure the atmospheric water vapor absorption of $7.5%±0.6%$, $12.0%±0.9%$, $26.0%±0.8%$, $33.5%±0.7%$, and $40.2%±1.6%$. The temperatures of them are $24.8°C±0.2°C$, $25.0%±0.4°C$, $25.0°C±0.3°C$, $25.5°C±0.2°C$, and $26.5°C±0.2°C$. The calculated p-p is shown in Table 1, and the results are shown in Fig. 5. Due to crowded absorption lines and strong absorption coefficients at higher frequencies, it is very difficult to obtain obviously resolved water vapor absorption lines for the frequencies. The other reason is the output power of the THz emitter at higher frequencies is relatively low, less than 0.1 μW. The water vapor absorption signatures reduce when the RH decreases. For the three lower frequencies of $∼0.558$, $∼0.753$, and $∼0.989 THz$, they can hardly be detected when the RH is $∼7.5%$. Figure 5(b) exhibits the enlarged absorption lines at 0.558 THz measured for RHs of 40.2%, 33.5%, 26%, 12%, and 7.5%, respectively. The frequency difference between the simulated results and the measured ones may be caused by the influence of water clusters[9]. The water clusters in high humidity may have special properties.

• #### Table 1. Partial Pressure

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#### Table 1. Partial Pressure

Temperature (°C)Relative Humidity (%)Partial Pressure (Pa)
24.87.5234.9
25.012.0380.4
25.026.0824.2
25.533.51094.0
26.540.21392.8

Figure 5. (a) Water vapor transmittance spectra measured by THz-FDS at RHs of 40.2%, 33.5%, 26%, 12%, and 7.5%, respectively. (b) The enlarged view of the transmittance spectra at 0.558 THz for different RHs. Green, measured results; red, simulated results; blue, smoothed result.

When the RH decreases, the interference signals from the standing waves of the system caused by the FP effect, for example, from the interface between the air and the silicon lens, turn out to be dominant. This noise limits the system’s ability for trace substance. Up to now, there is no good way to eliminate these phenomena. In our case, we try to remove the oscillations by converting the spectrum to a calculated time-domain signal, where the oscillations and main signal can be distinguished by the intensity. This method is like Fourier self-deconvolution (FSD)[27]. The differences are that FSD is suitable for the absorption spectroscopy, but, in our work, the targeted data is the original data, which contains the transmission coefficient spectrum. The results are shown in Figs. 3, 4, and 5(b).

During our experiments, we find that the THz-FDS is very sensitive to the RH. Therefore, it may become a highly sensitive remote monitoring system for water vapor, especially in some specific conditions, such as vacuum chambers, high altitudes, and outer space water vapor detection. For the fact that the temperature difference of the five data is small, we can get the peaks’ transmittance changed with RH at 0.558, 0.753, and 0.989 THz.

In Fig. 6, for the three frequencies, the transmittance decreases with RH and frequency. The error at RH 7.5% is caused by the low SNR, which is due to FP interference. RH cannot directly represent the amount of water vapor, but, in our results, the deviation of temperature is low, so the RH is approximately in direct proportion to the p-p. In our system, 0.558 THz is near the peak intensity of the source output energy, so the signal is strong, and the absorption of water is large enough for identification under the effect of FP interference. It means that the absorption peak at 0.558 THz is more appropriate for remote sensing of atmospheric water vapor in this system.

Figure 6. Transmittance at 0.558, 0.753, and 0.989 THz for different RHs of 40.2%, 33.5%, 26%, 12%, and 7.5%, respectively.