An all optical remote sensing system utilizing long distance, ultralow loss optical fiber networks is studied and discussed for near infrared absorption measurements of combustible and/or explosive gases such as CH4 and C3H8 in our environment, including experimental results achieved in a diameter more than 20 km. The use of a near infrared wavelength range is emphasized

Inaba, H.

English

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N87-10354
WIDE-AREA REMOTE-SENSING SYSTEM OF POLLUTION AND GAS DISPERSAL BY
NEAR-INFRARED ABSORPTION BASED ON LOW-LOSS OPTICAL FIBER NETWORK
Humio Inaba
Research Institute of Electrical Communication
Tohoku University, Sendai 980, Japan
Abstract-An a11-optical remote-sensing system utilizing long-distance,
ultralow-loss optical fiber networks is studied and discussed for near-
infrared absorption measurements of combustible and/or explosive gases such
as CH and C_HR in our environment, including experimental results achieved
in a _iamete_ _ore than 20 km.
I. Introduction
Optical fiber sensor technology is being developed currently for a wide
variety of applications, and the technical advantages are well recognized so
that its impact is now evident. Based on the idea of the first author [I]
for a new capability of this technology, an optical network system employing
low-loss optical fibers has been analyzed and examined for remote sensing of
environmental pollution and spilled dispersals of inflammable, explosive and
toxic gases/vapors in various industrial and mining facilities as well as in
urban and residential areas by the spectroscopic absorption method [2, 3].
This regime of fiber-optic remote monitoring and detection has various
advantages since the optical energy can be concentrated and transmitted in
low-loss optical fibers even for long distances over several tens km, inst-
ead of the open atmosphere. Hence fully optical, reliable, sensitive, econo-
mical with low-power lasers or even incoherent sources, feasible, real-time,
nonhazardous, e.g., eye-safe and explosion-free technique can be realized,
along with capabilities of little optical interference and of continuous sur-
veillance with easy calibration and selectivity as well as no electrical in-
duction, for various stressing environments and severe conditions. The ex-
perimental feasibility of a fiber-optic gas sensor system has been demonst-
rated for remote absorption measurement of NO2 in the visible spectrum using
an Ar ion laser [3, 4], and CHA in the near-infrared region employing InGaAsP
light emitting diodes (LED's) [3, 5-7] by our group.
This paper reports and summarizes experimental results of a low-loss opt-
ical fiber-based remote-sensing system of CHA and C_HR gases which have been
investigated in our laboratory, with specifid empha_i_ on the use of a near-
infrared wavelength range for this purpose.
II. Near-lnfrared Absorption Spectra and Basic Arrangement
Currently, ultralow-loss silica optical fibers exhibiting transmission
loss as low as 1 dB/km or less over the range of about 1.05 - 1.7 pm as well
as high-quality optical sources and detectors have been developed primarily
for long-distance, high-bit-rate optical communications, as shown In Fig. I.
Typical absorption wavelengths of various molecular species present in the
atmosphere and environment are also depicted. Consequently, it becomes fair-
ly apparent that the fiber-optic remote gas sensor system should be substan-
tially operated in this near-infrared region to achieve a wide area coverage
such as, for instance, a few tens km in diameter [I-3].
From this point of view, we have performed spectroscopic measurements
251
https://ntrs.nasa.gov/search.jsp?R=19870000921 2020-03-23T14:46:14+00:00Z
and analysesof combination r
and overtone bandsof a num- Potber of molecules such as CHa, APO
C_H., C_H., C_H2, C_H_, C4H;',,
NI_R_,Hp6,4tho_e contained i_u
LNG, LPG and city gases, and
others in this near-infrared
range [3, 5-9].
On the basis of this sp-
ectral information, we are
pursuing the re_te absorp-
tion measurements using InGa-
AsP and InGaP LED's in conju-
nction with a few to several
tens km-long, ultralow-loss
silica optical fiber links. _
Figure 2 shows the basic arr- i-
angement of a fully optical
remote gas sensor system em- z
ploying a compact absorption
cell as a sensor head. Near-
infrared LED's used for the
experiment are laser diodes _
(LD's) operated under thres-
hold current level with out-
put powers less than O.l n_.
The optical beam from the L-
ED was transmitted through a Fig. l
transmitting multimode opti-
cal fiber to a remotely loc-
ated gas sensor head and then
returned to the transmitter/
receiver location via the re-
ceiving multimode optical fi-
ber. The transmission loss of
the silica optical fibers was
lower than 1 dB/km in the 1.3 - 1.7
pm region. Absorption measurement
was performed by using a grating sp-
ectrometer to change the detecting
spectral width or a dielectric int-
erference filter for a practical si-
mplification in the system operation
[6, 7].
Ill. Remote Monitoring of Methane
Gas at 1.33 and 1.66 pm
The experimental demonstration of
remote detection of low-level CHa gas
was carried out for the first tingeem-
ploying a 2-km-long low-loss silica
optical fiber link and a 50-cm-long
compact absorption cell incorporating
InGaAsP LED's operated around 1.34 and
1.61 pm. We observed that the detec-
InGaAs. I nGaAsP
Sl" I _. Ge
InGaAsPl lnP
!
| ....
I
I nGaA_Slbl GaSh
._ NH3CH4CO2 C2H,
H20. CH_| No2J /._/c_\ / CH,
/ _ I I / _ // //"H3\/_ _O
II lit1Ill Ill Ill1I i
3 I
I
| t t t I ! I I I I
0.8 1.0 1.2 1.& 1.6 !.8
WAVELENGTH (J/m)
Currently available high-quality semi-
conductor light sources (LD and LED) and de-
tectors (PD and APD) in conjunction with ul-
tralow-loss silica optical fibers in the ne-
ar-infrared region, and typical absorption
wavelengths of various molecular species pr-
esent in the atmosphere and environment.
TRANSMITTING
OPTICAL Irl DER
OAS CELLLEO .A A.
,,,v'1o''"OUTMONOCHROMATOR
RECEI¥1NO
OPTIdAL
L.._J RECORDERJ FI BEll
J_,JM,CRO-I
Ico.pu,c.I
Fig. 2 Block diagram of an all-opti-
cal fiber-based remote-sensing sys-
tem for absorption measurement of
low-level combustible, explosive
and toxic gases/vapors in the near-
infrared region.
252
tion sensitivity of CHA gas in air is
approximately 4 %of tBe lower explo-
sion limit (LEL) at the 1.331-_m Q-
branch of the v_+ 2v3 band using a gr-
ating monochrom_tor with the spectral
t.
100
o,o'Cm 3sec
A,A 1", I0 sec
n.II 1:.30 $ec
0
W
resolution of 0.3 nm [5], and about
50
1.3 % of the LEL at the 1.666-um Q-
branch of the 2v_ band employing a di- _
electric interference filter with the
spectral width of nearly 3 nm [6], re- u
LOWER EXPLOXION
.
g_ 1.6B
spectively.
A compact and reliable remote-sen- " A
sing system was also realized by utili- s
zing the power-balanced, two-waveleng-
th differential absorption method [10],
which e,ables one direct detection of x
differential absorption signals for the _
specific substance to be monitored [7]. _ I_
o 2 _ 6 o io _2 14 is m 20
Thus we confirmed that this technique
is capable of achieving the sensitivi-
ty of about 0.8 % of the LEL of CH4
density in air with easy calibratidn,
using two dielectric interference fil-
ters centered at 1.666 _m as the on-
absorption filter and at 1.528 _m as
the off-absorption filter, respective-
ly [7].
Based on these experiments demonst-
rating the feasibility of low-level CH4
gas remote sensing employing very low-
loss optical fiber links, we planned to
extend the length of optical fibers to
OPTICAL FIBER LENGTH (kin)
Fig. 3 Minimum detectable pressure
of CH_ gas in air at 1.331 and I.
666 _ as a function of the optic-
al fiber length and the detection
time in the all-optical remote me-
asurement employing 5 - 20 km-long
ultralow-loss silica optical fiber
links in conjunction with InGaAsP
LED's and a 50-cm-long absorption
cell as the gas sensor head.
achieve a capability of a wider area coverage of this fully optical system.
Figure 3 summarizes the measured result for the silica optical fiber links
of the length of 5, lO, 16 and 20 km. The minimum detectable pressure of CHd
gas in air was measured for each optical fiber link at 1.33 and 1.66-um band_
using a dielectric interference filter with the transmission center at 1.331
pm and about 2.5 nm spectral width, and a similar filter around 1.666 pm as
mentioned above, respectively. It was verified that the detection sensitivi-
ty of 25 % of the LEL of CH_ gas density in air, which is normally required
for any practical CHA gas s_nsor, can be realized up to a 20-km-long optical
fiber link with the detection time longer than 3 sec for 1.666-pm band, and
also up to nearly 12 km with the detection time of 30 sec for 1.331-pm band.
We should note that since the absorption of CH4 molecules in 1.66-_m region
is usually stronger than that in 1.33-um region, higher sensitivity or longer
detectable range for the fiber-optic CH4 gas remote sensing is achievable in
1.66-_m region as is seen in this figure.
IV. Remote Monitoring of Propane Gas at 1.68 um
As a further extension of our all-optical fiber-based remote gas sensing
scheme, we have performed the remote measurement of low-level C_e8 gas in I.68-_m region for the first time [Ill. This gas is known to be predomi
nant component of LPG, and an inflammable and explosive species.
From the spectroscopic measurement using InGaAsP and InGaP LED's, C3H8
253
molecules were found to have absorption
spectra in 1.3 - 1.7 _m region and the
strongest absorption at 1.6837 um [9,
11]. Then remote detection of C_HR gas
was pursued employing the experi_eMtal
setup shown in Fig. 2 incorporating an
InGaP LED and a 50-cm-long compact abs-
orption cell.
Figure 4 shows the measured depend-
ence of absorbance at the wavelength of
1.6837 _m on C3H8 partial pressure in
the cell using a 2-km-long low-loss si-
lica optical fiber link. Here, I and
I represent the intensity of opti_al
signal which passed through the evacua-
ted and C_HR-air mixture contained abs-
orption c_IT, respectively. The spect-
ral resolution was 1.0 nm and the dete-
ction time was 3 sec. In this measure-
ment, the minimum detectable pressure of
C_HR gas in air was confirmed to be Iow-
eP _han 2 Torr, which corresponds to ne-
arly 12 % of the LEL.
T o
li
' i
3 t
| f ! LIMIt (L(L)
_-i, I , , ,I, I , , , , I •
I
tO 20 30
PROPANE PARTIAl. PRESSURE (Torr)
Fig. 4 Measured dependence of the
absorbance at 1.6837 _m on C H
partial pressure in I atm. 3 8C Ho-
air mixture contained in a 5d-_m-
long gas sensor head using a 2-km-
long very low-loss silica optical
fiber link.
Moreover, we have realized that the same system utilizing a 5-km-long si-
lica optical fiber link is capable of achieving reproducibly the detection
sensitivity of about 2.4 Torr for C2H8 gas in air with the resolution of 1.2
- 1.5 nm, i.e., 14 % of the LEL [llJ. The similar remote-sensing system is
being applied to other gases by suitably selecting the measuring wavelength
in the near-infrared region as illustrated in Fig. l[12].
V. Conclusion
On the basis of the present state of the art of optical electronics tech-
nology, this kind of optical fiber network incorporating LED's or LD's and
detectors in the near-infrared range should prove a very powerful scheme and
provide extensively an all-optical safe way for remote sensing of various da-
ngerous and toxic gases/vapors at strategic points over a wide area within
the environment, e.g., industrial complexes, factories, mines, fuel storage
yards, tunnels, undergrounds, ships, offices, hospitals, hotels, apartments,
and so on.
References
[1] H. Inaba, In Conference Abstracts, 9th International Laser Radar Conference, Munich, July
1979, Invited paper 2 - 2, pp. 61 - 67.
[23] H. lnaba, T. Kobayasl, M. Hirama, and M. Hamza, Electron. Lett. 15, 749 (1979).H. Inaba, in Optlcal and Laser Remote Sensing, D. K. Kt111nger an--JA. Mooradlan, Eds.
(Springer, Berlin, 1983) p. 288.
E_ T. Kobayasi, M. Hirama, and H. lnaba, Appl. Opt. 20, 3279 (1981).K. than, H. Ito, and H. Inaba, App1. Phys. Lett. _, 634 (1983).
[6] K. Chan, H. Ito, and H. Inaba, IEEE/OSA J. Lightwave Techno1. LT-____.22,234 (1984).
E_] K. than, ,,. Ito, and H. Inaba, Appl. Opt. 23, 1 Oct. 1984 issue, to be published.K. Chan, H. lto, and H. lnaba, Appl. Opt. _"2", 3802 (1983).
[9] H. Inaba, K. Chart, and H. lto, tn Otg. Tecl_'. Papers, Conference on Lasers and Electro-
Optics (CLEO'84), Anaheim, California, June 1984, Invited paper WN3, pp. 118 - 120.
[10] M. Hamza, T. Kobayast, and H. Inaba, Opt. and Quantum Electron. 13, 187 (1981) and 1._4, 339
(]gBZ).
Jill K. Chan, 11. Ito, and H. Inaba, Appl. Phys. Lett. 45, 220 (1984).
[12J H. Inaba, in Laser/Optoelectronics in Engineering-- Laser 85 Opto-
e]ektronik, W. Waidelich, Ed. (Springer, Berlin, 1986) p. 619.
254
8-bits digitizer units. Signal averaging and system control is handled by an ITT
XT computer with a hard disk and a floppy drive for data storage. The corn=
puter has graphic capabilities to present data. A printer and a plotter are
incorporated for the final presentation of evaluated data. The computer can, on
its own, perform a complete measurement sequence once the directions have
been entered. System steering is performed with an additional microprocessor,
which takes care of stepper motors, chopper, laser etc.
The system has the function of a laboratory when no field campaign is perfor-
med. It has a stationary docking place at our institute that makes it possible to
walk directly from the fixed laboratories into the bus. So far, the system has
participated in two field campaigns) one for SO 2 and one for NO 2. A result from
the SO 2 campaign is shown in figure 2.
Future use of the system will be in development of lidar techniques) where we
try to widen the range of species that can be monitored. In specific) we will in=
vestigate range resolved measurements of NO and mercury. Other experiments
are also planned - a monochromator on the detection side allows measurements
with the DOAS (Differential Optical Absorption Spectroscopy) technique to be
performed. This method) where the absorption of light from a high pressure
Xenon lamp is measured in a way that eliminates atmospheric turbulence) gives
the average concentration of the monitored species over the measured path. The
sensitivity can be made extremely high. Remote laser induced fluorescence
activities such as mineral monitoring or monitoring of pollution effects on
plants might be considered. Other applications can be to provide a mobile lab or
laser source for combustion studies in the industry.
References:
I. K. Fredriksson, B. Galle, K. Nystrgm, and S. Svanberg, "Mobile lidar system
for environmental probing ", Applied Optics 20, qlgl (198t).
2. R. 3. Allen and W. E. Evans, Rev. Sci. Instruments _=_3, 1_22
(1972).
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Wide-area remote-sensing system of pollution and gas dispersal by near-infrared absorption based on low-loss optical fiber network

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