Ammonia Sensor based on QEPAS with HC-PBF as reference cell
Jiang Meng*, Feng Qiaoling, Wang Congying, Wei Yufeng, Liang Tongli,Wang Xuefeng
Institute of Beijing Aerospace Control Devices, No.1 Fengying East Road, Beijing, China 100094
ABSTRACT
Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors involves with many desirable features, such as being small and
portable, with fast continuous in situ measurements possible. In QEPAS systems, reference cells filled with gas mixtures for
wavelength locking and calibration are used to improve the precision and stability of the trace gas concentration measurement. For this
study, a 5 m length hollow core photonic bandgap fiber (HC-PBF) splicing with single mode fibers at two ends was manufactured as
reference cell, which has long absorption path, low transmission loss and easy connectivity. Hollow cores under high pressure (3.0×
10
5
Pa) were filled with a certified mixture of ammonia and nitrogen gas to reach equilibrium rapidly. The experiment results indicated
that absorption spectra of reference cell with a low loss <3.5dB maintained stable after 150 days at atmosphere. For trace gas
detection, 1.531 μm DFB laser with wavelength modulation technique was demonstrated based on QEPAS. A normalized noise
equivalent absorption coefficient (NNEA) of 1.18×10
-7
cm
-1
W/√Hz was obtained at room temperature and pressure of 760 Torr.
This results in a minimum detection limit of 3.6ppm for noise equivalent concentration within a 1s lock in integration time.
Keywords: quartz enhanced photoacoustic spectroscopy, hollow core photonic bandgap fiber, ammonia sensor, trace gas
detection, wavelength calibration
1. INTRODUCTION
At present, the ability to detect small amounts of trace gas plays an important role in many real world applications. Much
scientific research is focused on environmental issues including air pollution, car exhausts, and noninvasive exhalation
analysis in medicine. The medical significance of the presence of ammonia and carbon dioxide naturally occurring
component in exhalations have been studied previously, demonstrating the usefulness of sensors to diagnose and monitor
a variety of medical conditions, including chronic kidney disease (CKD), helicobacter pylori infection, and
encephalopathy. Healthy individuals typically have a few hundred parts per billion by volume (ppbv) of ammonia in
their exhalations, while patients diagnosed with CKD, for example, could have over one part per million by volume
(ppmv)
[1,4]
.
Photoacoustic spectroscopy is a well-established method used in trace gas sensing applications due to its high sensitivity.
As one kind of photoacoustic spectroscopy technology, QEPAS gas sensors have advantages of high sensitivity, high
selectivity and rapid responses. Tuning forks and micro resonance tubes are used instead of photoacoustic cells and
microphones to realize quartz-enhanced photoacoustic spectroscopy detection. In particular, micro resonance tubes are
used for signal enhancement. The total size of the system can be produced sufficiently small to be used for portable in-
situ measurements.
In 2007, A.A Kosterev
[2, 3]
demonstrated a QEPAS system employing a 2µm DFB diode laser with power equal to
6.2mW. The minimum detection limit of ammonia was 3ppm. The influence of water vapor on the relaxation rate
enhancement in human exhalation gas detection was discussed with a simple theoretical model that can be
experimentally confirmed. Previous works included the demonstration of gas detection with distributed feedback
quantum cascade laser in middle infrared to improve the sensitivity, and performance improvement with different
operating wavelength
[4]
. However the reference cell as an important component of the stable system also should be
taken into account. As a gas sensor, the time taken for gas into the hollow core photonic bandgap fiber is too long for
practice application. However as a reference cell, hollow core photonic bandgap fiber shows low transmission loss, long
absorption light path compared with traditional self- focusing system. In this letter, we report on a 5m HC-PBF used as a
reference cell for wavelength locking to improve the precision and stability.
2. MANUFACTURE OF REFERENCE CELL
The HC-PBF (NKT HC-1550-02) with cross section as shown in Fig.1 (a) is connected between single mode fibers. The
HC-PBF with a transmission window between 1490nm to 1680nm is prepared as the reference cell into the gas detection
system. The diameter of the center hollow core is 10μm. A triangular array of air holes in a silica background
*aboobjm@163.com; phone +86 010 88104445; fax 86 010 88104254;
2015 International Conference on Optical Instruments and Technology: Optical Sensors and Applications,
edited by Xuping Zhang, David Erickson, Xudong Fan, Zhongping Chen, Proc. of SPIE Vol. 9620,
96200F · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2192986
Proc. of SPIE Vol. 9620 96200F-1
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