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1
OPTICAL METHANE SENSOR BASED ON A FIBER LOOP AT
1665 NM Kuanglu YU Supervisor
Chongqing WU, Zhi WANG Institute of Optical
Information E-mail 06118331_at_bjtu.edu.cn
Beijing Jiaotong University
An experimental setup was built, and a narrow
optical pulse sequence emitted from the DFB LD
was launched into the fiber loop, then a decaying
pulse train was coupled out as the pulse rang
around, which are shown in (a) and (b) for air or
99.9 methane filled in the chamber,
respectively. In (a), five pulses could be
observed, but the first pulse of the output does
not pass through the gas chamber as it is coupled
out directly, so it gives no information about
the gas concentration. it means a light pulse
passes through the chamber for 4 times and the
gas gap is exactly four times the length of
itself. The output pulse trains are
exponentially-fitted except the first pulse in
Matlab, and the fitted curves are given in (c).
From these figures, the 1/e ring-down time with
air in the gas chamber is 82.92ns, so the total
loss is 5.23dB. Finally, the input optical
wavelength was adjusted to 1665.380nm which is
the one of the strongest peaks near 1665nm, then
99.9 methane was filled into the chamber, the
output pulse train is shown in Fig. 4(b). The
total loss is 9.77dB, so the absorption loss is
about 4.54dB, it has a good agreement with the
data given in section 3.1.
(a)
(b)
(c)

(d)
1. BACKGROUND

• Methane is known to be
• a greenhouse effect gas
• an explosive gas
• a safety and environmental problems causer
• a clean-burning, high-energy future fuel
• So methane online real-time monitoring has become
  very important.
• Whats new in our experiment?
• Use a 50 mm gas chamber, which makes the system
  more simple and reliable.
• Use a 1665 nm light source and a fiber loop,
  which makes the system more sensitive.

2. THEORY
Configuration of the fiber loop gas
detecting system We coupled a temporally narrow
pulse of light into the loop and as the pulse
went around the cavity a decaying pulse train was
coupled out. The 1/e ring-down time te, which
can be evaluated by a fitting progress of the
output pulse train, satisfies the relationship
of,
(1) where
T represents the round-trip time of the light,
and the total loss G can be written as G fiber
loss coupler loss collimators loss gas
absorption loss. (2) The gas concentration can
be obtained from Beer-Lambert Law with the value
of the gas absorption loss which can be deduced
from Equations (1) and (2).
4. CONCLUSIONS FUTURE WORK
Conclusions With the novel methane sensing
method, the methane concentration is measured
with a only 50mm long gas gap, while the system
is more simple, reliable and sensitive. The
absorption spectrum, wavelengths as well as the
depths of the absorption peaks of methane near
1665nm is accurately measured with a
super-luminescent diode, which can be used for
the further work. Future work Obviously, the
round-trip numbers of the decaying pulse trains
are only 4 or 5 due to the high insertion loss of
the components, which limits the improvements of
the sensor. If there is an amplifier in the fiber
loop, the signal pulse will be amplified, the
traveling loops and the sensitivity will be
increased.
3. EXPERIMENTS
The methane absorption spectrum from 1664.8 nm to
1667.2 nm was exactly measured with a SLD. The
strongest peaks are at 1665.740nm, 1665.370nm and
1665.538nm with absorption losses of 6.93dB,
5.43dB and 4.32dB. Methane absorption lines
5. ACKNOWLEDGEMENTS
This project is funded by the Key Project of
Beijing Jiaotong Univ. (NO. 2005SZ001).
By Kuanglu YU BJTU-IOI