Qualitative and quantitative analysis of atmospheric
methanol using a continuous-wave terahertz
spectrometer
Han Zhang (张 寒)
1,2
, Zhaohui Zhang (张朝晖)
2,3,
*, Xiaoyan Zhao (赵小燕)
3
,
Xiaotong Zhang (张晓彤)
1
, Tianyao Zhang (张天尧)
3
,CanCao(曹 灿)
3
,
and Yang Yu (于 洋)
3
1
School of Computer and Communication Engineering, University of Science and Technology Beijing,
Beijing 100083, China
2
Beijing Engineering Research Center of Industrial Spectrum Imaging, Beijing 100083, China
3
School of Automation and Electrical Engineering, University of Science and Technology Beijing, Beijing 100083, China
*Corresponding author: zhangzhaohui@ustb.edu.cn
Received July 9, 2018; accepted August 30, 2018; posted online September 20, 2018
We present a specific-window method to subtract the interference of water vapor on terahertz frequency-domain
spectroscopy (THz-FDS) at ambient temperature and pressure. A continuous-wave spectrometer based on photo-
mixing was utilized to obtain THz-FDS of methanol vapor in the range of 50–1200 GHz. The distinctly spaced
absorption features in the neighborhood of atmospheric windows of transparency were selected to perform linear
fitting versus the calculated absorption cross section and obtain the concentration of methanol. Furthermore, the
gradually decreased methanol vapor was quantified to demonstrate the reliability of the method.
OCIS codes: 300.6495, 300.6390, 300.1030, 010.1120.
doi: 10.3788/COL201816.103001.
The qualitative and quantitative analysis of pollution gas
in atmosphere is vital to environmental monitoring. Since
the terahe rtz (THz) wave is sensitive to many gas species,
more and more research efforts have been devoted to the
development of spectroscopic THz gas sensors
[1]
. However,
the monitoring and analysis of gas in the atmosphere is
far from practical application, since much interference is
unavoidable, for instance, the atmospheric water vapor
strongly attenuates the THz radiation.
It is worth mentioning that some studies concerning
the application in real atmosphere have been reported.
Hsieh et al.
[2]
utilize asynchronous-optical-sampling THz
time-domain spectroscopy (THz-TDS) with a resolution
of 1 GHz to achieve a detect limit of 200 ppm (1 ppm =
10
−6
) for acetonitrile (CH
3
CN) gas in the presence of
smoke under atmospheric pressure, which undoubtedly
proves that THz spectroscopy is a promising method
for environmental monitoring. The non-mechanical time-
delay scanning in this method provides more advantages
on spectral resolution, accuracy, and measurement time
than the conventional THz-TDS. The cost is the simulta-
neous water vapor that has to also be quantified.
Another major THz spectroscopy is the THz frequency-
domain spectroscopy (THz-FDS), realized by frequency
scanning narrow-band continuous-wave THz radiation.
The latest study released by Tekawade et al.
[3]
explores
the potential of THz wave electronics for measuring meth-
anol in ambient air at 500 Torr total pressure within the
220–330 GHz region using a radio-frequency multiplier
source. Although the frequency band is narrow, such that
the interference from water vapor is not included, this
might lose some spectral features and is limited by less
gas species that can be detected.
THz-FDS obtained by the photomixing technique is
another promising approach to achieving broader con-
tinuous tuning range of sub-THz to THz order while main-
taining moderate spectral resolution. Recently, optical
frequency combs (OFCs) have combined with the photo-
mixing technique to achieve higher frequency accuracy,
stability, and resolution at some expense of bandwidth.
With this technique the CH
3
CN gas is investigated but
under very low pressure
[4]
.
Since there are limited researches on gas in real atmos-
phere, this work aims to explore the feasibility of high-
sensitivity measurement of target gas under normal
pressure and temperature. In this study, a fiber-based pho-
tomixing THz spectrometer with a broad bandwidth was
utilized to detect methanol vapor with various concentra-
tions in the atmosphere. The important point is that we do
not purge the water vapor before experiments but utilize a
specific-window method, which chooses several atmos-
pheric windows of transparency to perform the quantita-
tive analysis.
Continuously tunable measurements were performed on
a TOPTICA TeraScan 1550 platform. As shown in Fig.
1,
the interference of the beams from two distributed feed-
back (DFB) lasers generated a laser beat, one splitting
beam of which irradiated on the InGaAs photomixer to
generate a THz wave. A coherent detection scheme was
applied; the THz wave was focused into the InGaAs de-
tector, and meanwhile, another splitting beam of the laser
beat irradiated on the detector. The induced photocurrent
COL 16(10), 103001(2018) CHINESE OPTICS LETTERS October 10, 2018
1671-7694/2018/103001(5) 103001-1 © 2018 Chinese Optics Letters