COL 10(5), 050501(2012) CHINESE OPTICS LETTERS May 10, 2012
Comparative study of fiber optic liquid level sensors based
on long-period fiber gratings with different doping
concentrations
Barerem-Melgueba Mao
1∗
, Yizhen Wei (
)
1
, and Bin Zhou (
±±±
RRR
)
1,2
1
Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou 310058, China
2
Institute of Optics, Zhejiang University, Hangzhou 310027, China
∗
Corresponding author: francoismao@gmail.com
Received October 20, 2011; accepted December 12, 2011; posted online March 6, 2012
The performances of two liquid level sensors based on long-period fiber gratings are studied. The long-
period gratings (LPGs) have similar characteristics (length and period), but are fabricated with two pho-
tosensitive B-Ge co-doped fibers with different dopant concentrations. We investigate the temperature
sensitivities of LPGs and exploit their refractive index sensitivity to implement liquid level measurement.
By controlling fiber parameters, such as the dopant concentrations, the measurement sensitivity of a LPG-
based fiber optic liquid level sensor can be improved.
OCIS codes: 050.2770, 060.2370, 230.1150.
doi: 10.3788/COL201210.050501.
Liquid level sensing (i.e., the measurement of liquid vol-
ume) has been intense ly studied as a gauging technique
because of its essential applications in modern industry.
For example, in a refinery or chemical plant, large num-
bers of liquid level sensors (LLSs) are usually employed
to monitor liquid storage tanks. A mass of systems, like
aircraft fuel ga uging and ink jet printing, also rely on
liquid level measurement to enable proper functioning.
The properties of a liquid, such as being flammable or
inert, must be carefully considered in designing LLSs
because they can result in diverse types of sensors for
different applications. LLSs can be generally classified
by their mechanisms: mechanical
[1]
, electrical
[2]
, and
optical
[3−5]
. Among these techniques, optical fiber-based
LLSs present s ome remarkable advantages, such as their
immunity to electromagnetic interference, high sensitiv-
ity, and resistance to rugged environments. Optical fiber-
based sensors are lightweight and small in size. They
are especially attractive for applications in explos ive en-
vironments and flammable atmospheres beca us e light
is confined inside the fiber, a dielectric material, and
does not interact with the surrounding material. Re-
cently, LLS based on long-period grating (LPG) has at-
tracted considerable attention due to its s implicity and
reliability. LPG is a type of fiber devic e that couples
light between the fundamental core mode and forward-
propagating cladding modes. This coupling results in a
transmission spectrum consisting of a series of attenu-
ation bands at distinct wavelengths
[6]
. These attenua-
tion bands are sensitive to temperature, strain
[6]
, bend-
ing curvature
[7]
, and the refrac tive index (RI) of the sur-
rounding material
[8]
. The purpose of LPG-based sens ors
is to measure the wavelength shift of a specific attenua-
tion band while one of the external physical parameters
(i.e., temperature, strain, bend, and RI) is changing.
In this letter, we co mpare the pe rformances of fiber op-
tic LLSs based on long-period fiber gratings. Two LPG-
based LL Ss were fabricated using two B-Ge co- doped
fibers with different doping concentrations. These LLSs
were car efully studied in the application of liquid level
sensing with different solutions. The temperature sen-
sitivities of the LP Gs were also measured. The experi-
mental results show that the fiber dopant concentrations
have significant effects on the shift of the resonance wave-
lengths of the LPG. Our work provides valuable insight
into the mechanism of LPG-based LLS, as well as an ap-
proach to improve measurement sensitivity.
LPGs are obtained by introducing a periodic modula-
tion of the RI in the core of photosensitive fibers, with
a s patial per iod ranging from 100 µm to 1 mm. The
light coupling between core mode and the co-propagating
cladding modes occurs at distinct wavelengths given un-
der the phase matching condition as
[9]
λ
l
= [n
eff
core
(λ
l
) − n
eff
clad,l
(λ
l
)]Λ, (1)
where λ
l
is the coupling wavelength, Λ is the period of
the LPG, n
eff
core
is the effective index of the propagat-
ing core mode, and n
eff
clad,l
is the effective index of the
lth cladding mode susceptible to the RI of surrounding
medium. When variations of external physical parame-
ters, such as RI, alter the LPG’s period and/or the core-
cladding index difference, the phase-matching condition
is changed, resulting in a wavelength shift of the trans-
mission dips. Using LPG-based LLS, one can measure
the liquid level by detecting the wavelength shift, which
depends on surrounding medium’s RI. Changes in reso-
nant wavelength according to the surr ounding medium’s
RI (n
ext
) can be expressed as
[10]
dλ
l
dn
ext
= λ
l
γΓ
ext
, (2)
where γ is the general sensitivity fa c tor and is given by
γ =
(dλ
l
/dΛ)
n
eff
core
− n
eff
clad,l
, (3)
where γ is positive for low-order cladding modes, whereas
it is negative for high-order c ladding modes. The symbol
1671-7694/2012/050501(4) 050501-1
c
2012 Chinese Optics Letters