2010 International Conference On Computer Design And Appliations (ICCDA 2010)
An Optical Fiber Sensor based on Absorption Spectroscopy for Carbon Monoxide Detection
Shanying Zhu, Youping Chen, Gang Zhang
Jiming Sa
School of Mechanical Science and Engineering
School of Information Engineering
Huazhong University of Science and Technology
Wuhan University of Technology
Wuhan, PR C hina
Wuhan, PR C hina
e-mail:
[email protected]
Abstract-An
optical
fiber
sensor
based
on
absorption
spectroscopy has been developed for the determination of carbon
monoxide
(CO)
in
the
harsh
environment.
Conventional gas sensor used to detect carbon monoxide concentration is often difficult to achieve high sensitivity, selectivity, and specificity in presence of a mixture of gases. This sensor uses DFB LD as light source, low cost near infrared
components,
and
differential
optical
absorption
spectroscopy. The experimental results indicated that the sensor has been developed to operate with high sensitivity and stability so it does not suffer from other gases interferences. The lower limit of detection for the sensor was found to be not less than 50 ppm for carbon monoxide.
Keywords- Near-infrared;
Optical fiber
and carbon dioxide of the exhaust emissions with absorption spectroscopy.
sensor;
Carbon
An
optical
sensor
presented
is
capable
of
significant feature of an optical fiber is that the information is transmitted using light signals as opposed to electrical signals. So optical fiber sensors are immune to electrical and electromagnetic interferences. In addition, the more advantage of the sensor is that it is possible to remotely monitor CO concentrations due to the optical fiber transmission characteristics. These features make optical fiber sensor highly suitable for using in harsh environments and remote real-time online measurements.
monoxide; Absorption spectroscopy
I.
fiber
monitoring the presence of carbon monoxide, based on its optical absorption in the near-infrared region. The most
II.
THEORETICAL BACKGROUND
INTRODUCTION
C arbon monoxide is one of the most common and dangerous pollutants present in the environment. It has been
-18.0
termed as a silent killer because of its colorless, odorless and non-irritating. Research has shown that it is a threat to human health and the environment. The poisoning level of
-19.0
carbon monoxide is proportional to the concentration and time. The man will be a slight headache, fatigue, dizziness
-20.0
and nausea when he is in the air in which the concentration of the CO reaches 100 ppm more than two hours. C arbon
-21.0
monoxide is produced by the incomplete combustion of
-22.0
fossil fuels-gas, oil, coal and wood used in boilers, engines, oil burners, gas fires, water heaters, solid fuel appliances and open fires. Therefore, it is needed to fit CO gas sensor in these occasions in order to prevent CO poisoning.
WAYENUMBERS
[4-5]
and
electrochemical
reaction
[6-8].
Semiconductor method is often difficult to achieve high sensitivity, selectivity, and specificity in presence of a mixture of gases [3]. Besides the sensors based on metal oxide tend to suffer from baseline drifts upon interaction with poisoning species [9]. C atalyst-combustion and electrochemical methods are prone to poisoning and aging. In contrast to these sensors, an optical fiber sensor is suitable for use in harsh environment, and overcome the shortcoming of traditional gas sensor. Such as the literature [10-11] reported, an optical fiber sensor detects carbon monoxide
978-1-4244-7164-51$26.00 © 2010 IEEE
.,
Figure L The absorption of CO in the infrared region of the spectrum
The traditional sensor used to detect carbon monoxide concentration is based on semiconductor [1-3], catalyst combustion
em
As
each
pollutant
gas
has
a
characteristic
optical
absorption spectrum it is possible to determine which gas is present and in what quantity by analyzing its unique optical spectrum.
C learly,
a
gas
sensor
based
on
absorption
spectroscopy does not suffer from being cross-sensitive to other species present in the gas cell provided its spectrum can be uniquely defined and the spectral resolution of the detector is sufficient. Optical absorption lines occur throughout the electromagnetic spectrum. To date, most optical fibers based sensing have concentrated in the near infrared due to the greater availability of components designed and optimized for use in the telecommunications
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2010 International Conference On Computer Design And Appliations (ICCDA 2010)
industry [11]. Fig. 1. shows the absorption of the co in the
absorption lines. Absorption intensity weakened in tum. The
infrared wavelength range [12]. It is clear from Fig.1 that CO
second overtone absorption lines in which the quartz fiber is
has higher absorption in the mid-infrared wavelength range
low loss of 1.1-1.7 �m is selected. From the Fig.2, absorption
than the near-infrared.
mid-infrared components and the immature technology. L ow cost near-infrared components and sophisticated in
lines in near-infrared region include P branch and R branch, the center wavelength of P branch is 1 580nm (wave number l is 6320cm- ) and the center wavelength of R branch is I 1567nm (wave number is 6377 cm- ). Spectral lines of R
technology make it is capable of detecting CO in this
branch is denser, so it is selected.
However,
it
is
not
feasible
to
construct a mid-infrared optical fiber sensor due to the costly
It should be noted that there is no significant absorption
wavelength. The absorption of CO in near-infrared is shown in Fig. 2[12].
by CO2, N2, H20 (which has significant absorption through
the infrared) or an?, other pollutants at these wavelengths. So select 6380.3 cm- spectral line as the center wavelength of
light source. Its wavelength is 1567.3nm. And this molecular 23 1 absorption spectrum peak intensity is 2.13 x 1O- cm- / 2 (molecule x cm- ). It was shown in Fig. 3.
2.5
20
2.5
1.5
2.0 1.0
1.' :x10 -23
0.5
II W AV E N U M B E R S
1.2
em
-1
0.8
Figure 2. The absorption of CO in the near-infrared region of the spectrum
-23 X10
0.4
The gas concentration can be measured by detecting the optical power absorption. The Beer-L ambert law defines the relationship between absorbance and concentration of an
1$312.0
Where I is the transmitted intensity, 10 is the incident
optical path length.
�.o
�.o
6384.0 em
G38t1.o
6388.0
-1
The light source should be choosed after the absorption lines is determined. The DFB LD (Distributed feed-back L aser diode) which is a narrow-band light source with a bandwidth less than O.2nm is selected because it is steady and the center wavelength can be adjusted for 0-5nm. And
From the (1):
1 C=-ln(lolI) aL
1$3711.0
Figure 3. The fine spectral line of CO absorption
(1)
intensity, is the molar absorption coefficient of the species in question, C is the concentration of the sample, and L is the
1$37(1.0
WAVENUMBERS
absorbing species and is shown in Equation (1).
1= 10 exp[-aCL]
1S314D
(2)
the center wavelength of DFB LD can be adjusted by changing the injection current and die temperature.
From the (2) we can see that if the gas absorption coefficient and the optical path length were certain, the gas concentration only relate to the attenuation of light intensity.
Gas
The gas absorption coefficient and the optical path length is
Input
proportional to the gas concentration, so in order to improve
cell
fiber
accuracy, the wavelength selected in which the absorption coefficient is to be as large as possible and the optical path length is to be as long as possible. However, the longer the optical path, then the greater the attenuation of light, it should choose the right light source and determine the appropriate length of optical path. III. A.
Output
EXPERIMENTAL DETAILS
Spectral analysis and Light Source Selection
fiber
Figure 4. Gas cell constructed by lens couples
The spectral absorption of carbon monoxide in the infrared region was shown in Fig. 1. Besides the base band absorption lines, there are also the first and second overtone
Gas cell is the sensitive element of the optical sensor. The light intensity will be attenuated when there is some detecting gas in the cell and the light passes through it. And
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2010 International Conference On Computer Design And Appliations (ICCDA 2010)
the requirements of the gas cell are mainly two things: First,
After half an hour, the CO in the gas cell evanesces
the light intensity should be attenuated as few as possible when there is no detecting gas in the cell; second, the path
completely, then repeat above process and purge the gas cell with 2000ppm, 100ppm, SOppm of CO in tum, and save
length can not to be longer when the structure of the gas cell is determined. The system uses the gas cell based on
processing.
C ascaded GRIN L ens which was composed of one group of
experimental data. Fig. 6. shows the result of the data
input lenses and one group of output lenses. Input lenses
In the experiment, it is needed to manually adjust the size of Die temperature and injection current in order to
collimated the emission light that emit from optical fiber, and
determine the output central wavelength of DFB LD, and
then formed a collimated beam. The beam passes through the
ensure that the center wavelength within the absorption line.
gas cell, and then it is coupled into output optical fiber by
There will be certain errors more or less because of manual
output lenses. Specific structure of the gas cell was shown in
adjustment. Although the gas used in the experiment is national
Fig. 4. B.
standard product, the gas concentration in the gas cell has
Structure of the opticalfiber sensor
deviation from the standard gas concentration due to the flow
The schematic diagram of the optical fiber sensor based
velocity and time of the gas purged into the gas cell.
on spectral absorption was shown in Fig.S. DFB LD-driven
However,
mainly includes three parts:
experiments, and this error should be very small.
current-driven,
temperature
the present
study
is
the
control and Active light modulation. The current-driven is to
result
of
repeated
f . J voltage Valuel �=::::� :;::
control the size of the injection current, and the temperature control is to determine the die temperature of the DFB LD.
5.0 "-��--.-�-r-�--.-�:;:::::
The Active light modulation is to load the DFB LD with sine
4.5
wave. The light from the DFB LD is sent to the coupler, and it is divided into two-way by the coupler. One way is sent to Reference gas cell, and the other way is sent to detection gas
4.0 '" " 3.5
cell. Then the light from the two gas cells were attached to the detector(PIN), and the weak voltage signal on the
�/
detector was to be treatment initially, such as amplification,
�
�
filtering, and difference. Finally, the DSP minimum system completes final analysis and processing, and the DC voltage was gained.
3.0
2.5
2.0
1000
2000
3000
4000
5000
CO Concentration (ppm) Figure 6. The experimental results
IV.
CONCLUSIONS
This work demonstrates the capabilities of an optical sensor to detect carbon monoxide concentrations in the near infrared region, at IS67.3nm. The sensor presented can measure concentrations of carbon monoxide up to SOppm. The sensitivity of the proposed optical fiber sensor is directly related to the path length of the gas cell and so by increasing the path length of the sensor a more sensitive sensor can be
Figure 5. Schematic diagram of the fiber optic sensor
C.
realized that is capable of measuring lower concentrations with increased resolution.
Experiment results and analyses
By
At first, it must adjust the center wavelength ofDFB LD which should be coincided with the absorption lines thought changing the size of injection current and die temperature. And Then the DFB LD was loaded by the sine wave with 1747HZ. Before purging CO into the gas cell, the two output voltages on the signal adjust circuit must be adjusted to sameness, in other words, two-way voltage differential is zero. Then the gas cell was initially purged with SOOOppm standard CO gas from a cylinder for 10 minutes. Glass Rotameter controls the gas flow rate at 200ml/min. And
using
the
simple
double-light
path
measuring
principle it can eliminate the influence of thermal zero shift, electromagnetic interferences and the attenuation of light intensity. Optical fiber used in the gas sensor make it response quickly, anti-interference, corrosion proof and suitable for using in the harsh environments. ACKNOWLEDGMENT
The authors would like to acknowledge the support of Natural Science Foundation of Hubei Province science and technology plan (Grant No. 2008CDA017) and National
recorde the output voltage value.
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2010 International Conference On Computer Design And Appliations (ICCDA 2010)
Natural Science Foundation Project(Grant No. 50974062) for
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funding this work.
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