COL 11(11), 110602(2013) CHINESE OPTICS LETTERS November 10, 2013
Miniature temperature sensor based on
optical microfiber
Zhengtong Wei (
¥¥¥
ÚÚÚ
), Zhangqi Song (
yyy
ÙÙÙ
ééé
)
∗
, Xueliang Zhang (
ÜÜÜ
ÆÆÆ
),
Yang Yu (
uuu
), and Zhou Meng (
³³³
)
College of Optoelectronic Science and Engineering, National University of Defense Technology,
Changsha 410073, China
∗
Corresponding author: songzhangqi@nudt.edu.cn
Received June 27, 2013; accepted September 4, 2013; posted online November 4, 2013
Optical microfibers (OMs) are good alternatives in the field of sensing. In this letter, a simple and effective
miniature temperature sensor based on OM is proposed and experimentally verified. Using pure water and
fiber coating as the OM clad, an additional loss will occur due to the absorption of outer medium. The
temperature of the outer environment can be estimated by monitoring the change in additional loss. In
the demonstrated experiments, a series of OM with different diameters, waist lengths, and constructions
are used as sensing elements. The correlation coefficients between the experimental results and the linear
fittings are better than 0.99, and the temperature sensitivity obtained by the linear fittings can achieve
–0.151 dB/
◦
C (in pure water) and –0.405 dB/
◦
C (covered by fiber coating). Moreover, higher sensitivity
can be obtained by decreasing the diameter, increasing the waist length of the OM, or choosing the proper
operating wavelength.
OCIS codes: 060.2370, 060.2300, 060.4005.
doi: 10.3788/COL201311.110602.
Temperature is an important physical parameter that
should be accurately determined in many contexts
[1]
,
such as in environmental measure ment, chemical indus-
try, and remote s e nsing. Temperature s e nsors based on
optical fiber have been widely developed because of their
immunity to electromagnetic interference and possibility
to work in contact with explosives
[2]
. Fiber Bragg grat-
ings (FBGs) ar e the most popular sensing elements for
temper ature measurement
[3]
; however, cross-sensitivity is
a key challenge of FBG applications
[4]
. Numerous stud-
ies have been conducted to solve such problem
[5−8]
, but
the pr ocess a nd construction are more complicated.
Optical microfiber (OM) has been extensively resear-
ched because of the enormous progress in the fabrication
of low-loss structures that allow for low-loss evanescent
wave guiding
[9,10]
. In the sensing field, OM has distinct
advantages bec ause of its enabling optical properties, in-
cluding large evanescent fields, high-nonlinearity, stro ng
confinement, large waveg uide dispers ion, and small bend-
ing radius. When light is transmitted in the OM, a
relatively large fraction of the guided power propagates
out of the physical bo unda ry as an evanescent field,
making it highly sensitive to the outer medium. OM
has been successfully used to sense humidity
[11]
, refrac-
tive index
[12,13]
, gas concentration
[14]
, current
[15]
, micro
particle
[16]
, and temperature
[17−24]
. T he temperature
sensor in Refs.[17-21] obtains the parameter by using an
optical spectrum analyzer (OSA) a nd by monitoring the
shift of interference fringe, which makes the whole sys-
tem expens ive and unpractical. Meanwhile, temperature
is measured by recording the change of peak reflected
wavelength
[22−24]
; however, the fabricatio n of FBG in
OM (MFBG) is difficult and complicated.
In this letter, a miniature temperature sensor based
on OM is proposed and demonstrated. The sensing OM
is used as a sensing element and a transmissio n chan-
nel. After OM fa brication, the profile is surrounded
by ma terials with lower re fractive index and absorption
coefficient, s uch as pure water and optical fiber coating.
The absorption coefficient of the outer-surrounding ma-
terial changes due to the change in tempe rature, thereby
inflecting the transmitted power. Temperature can be es-
timated by mo nitoring the additional loss. The proposed
sensor has the advantages of conventional optical fiber
temper ature sensor, as well as the characteristics of ultra
compactness, high res olution, good repeatability, simple
construction, and easy op e ration.
The sensing OMs used in this letter were fabr ic ated
from conventional single-mode fiber (SMF) by modified
flame-brush method
[9]
. The heat intensity can be con-
trolled to ensure a sufficiently high temperature to soften
the SMF by adjusting the micro-heater current. OMs
with different diameters can be fabricated by controlling
the taper-drawing speed. To demonstrate the facility of
our proposal, two kinds of materials were used as the
outer medium in our demonstr ation exper iments.
When the fabricated OM is surrounded by lower re-
fractive index material (such as pure water and optical
fiber coating), a two-layer waveguide consisting of the
core and the outer surrounding is composed. A relatively
large fraction power is propagated in the outer medium
due to the ultra-tiny scale of OM. Moreover, a fraction of
the power trans mitted in the clad can be calculated by
η =
R
∞
a
S
z2
dA
R
a
0
S
z1
dA +
R
∞
a
S
z2
dA
, (1)
where a is; OM radius; S
z1
and S
z2
represent the z-
components of the Poynting vectors in the OM and the
clad (surrounding medium), respectively
[25]
. If the outer
material is pure wa ter and the core is silica (in this let-
ter, OM is fabricated from silica), a fraction of the power
transmitted in the outer diameter can be obtained, as
shown in Fig. 1 .
1671-7694/2013/110602(4) 110602-1
c
2013 Chinese Optics Letters
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