556 CHINESE OPTICS LETTERS / Vol. 7, No. 7 / July 10, 2009
Experimental study on vibration frequency response
of micro-bend optic-fiber sensor
Fuxiang Qin (覃覃覃付付付祥祥祥)
1∗
, Honggang Li (李李李洪洪洪刚刚刚)
2
, Wande Fan (范范范万万万德德德)
1
,
and Qiuqin Sheng (盛盛盛秋秋秋琴琴琴)
1
1
Institute of Physics, Nankai University, Tianjin 300071, China
2
Tianjin East Harbor Science and Technology Development Limited Company, Tianjin 300384, China
∗
E-mail: qinfx16@yahoo.com.cn
Received Octob er 22, 2008
We make an experimental study on vibration frequency response of micro-bend optic-fiber sensor, and
single-mo de fib ers and multi-mode fibers are used as the sensitive optic-fibers. Contrast between the
two sensitive fibers is presented. Result shows that the micro-bend optic-fiber sensor has good frequency
resp onse characteristics and strong ability to restore the waveform. With the frequency varying in the range
of 500 − 4762 Hz, the vibration sensors using multi-mode optic-fiber as the sensitive fiber is more sensitive
than that using single-mode optic-fiber. And the former has better frequency response characteristics and
stronger capacity of waveform revivification. But with the frequency in the range of 287 − 500 Hz, the
latter is better.
OCIS codes: 060.0060, 280.0280, 120.0120.
doi: 10.3788/COL20090707.0556.
Optic-fiber sensor is widely used now
[1−3]
, and Micro-
bend optic-fiber sensor is an important optic-fiber sen-
sor. Since Fields et al.
[4]
brought out the principle of
Micro-bend optic-fiber sensors (MFS) for the first time
in 1980, MFS has gotten much attention because of its
advantages such as simple structure, low cost, easy equip-
ment, and so on
[5]
. MFS was first applied in an optic-
fiber hydrophone system by the Institute of the U.S.
Navy, but without technical report. After that, Micro-
bend optic-fiber modulator principle was expanded to
the applications of sensing displacement, acceleration,
and other physical parameters. The resolution ratio
of detecting displacement can reach up to the level of
0.1 nm, and the detection dynamic range can reach to
more than 100 dB
[6]
. In 1991, Lumholt et al.
[7]
pointed
out that MFS showed high sensitivity in temperature
in the range of 20 − 180 K. Also, sensors could be
more sensitive on temperature and suffer less changes
of its sensitivity while changing the wavelength of light
by selecting the appropriate signal wavelength. MFS
used in multi-parameters measurements has also been
reported
[8,9]
. Someone brought out an MFS with grating
fiber for double-parameters which could sense strain and
temperature at the same time. The optic-fiber micro-
bend mechanism is used to modulate the reflected light
of FBG center wavelength and the light intensity at the
same time, and only the varying of temperature can
change the FBG center wavelength. By detecting the
signal reflection of its peak center wavelength and inten-
sity at the same time, measurements of the strain and
temperature distinctively can be effectively realized. Si-
multaneously, the temperature and strain cross-sensitive
problem has been effectively solved. Moreover, its linear
response can reach to 0.995.
Since vibration phenomenon can be found in nature
easily, e.g., the high-rise buildings or the suspension
beams bridge can gain vibration in the action of wind,
under the shock of explosions wave, even caused by the
earthquake, and so on. Sensing the vibration mentioned
above can study whether the subjects are working in
normal or there are the security risks. So we can take
actions to evaluate the original design and construction.
In other words, monitoring the seismic is meaningful.
Early detection of vibration is made by using mechanical
sensors, but its detection range is very narrow. Then,
a typ e of electromagnetic sensors came up, which still
had shortcomings such as low sensitivity and poor anti-
jamming capability. Compared with electromagnetic sen-
sors, micro-bend optic-fiber sensors have a wide dynamic
range, high sensitivity, strong anti-jamming ability, and
other advantages. However, the work of using micro-
bend optic-fiber sensor to sense the frequency response
of vibration has not yet been reported. The experiment
in this letter is to study the micro-bend optic-fiber sensor
used for sensing the frequency response vibration.
MFS makes use of the intensity modulation induced
by micro-bend loss in sensitive fibers as a transduction
mechanism for detecting environmental changes, such as
displacement, strain, temperature, and so on. In the
MFS, the transmission of high-end total internal reflec-
tion mode in the optical fiber is affected by environmental
changes. Part of the energy escap es from the side of the
fiber in the curved section. By detecting the changes in
light intensity, corresponding physical parameter can be
gained
[10−13]
. The structure of MFS is shown in Fig. 1.
Usually, the deformer is formed by a pair of tooth plates,
and sensitive fiber goes through the middle tooth plates.
Under the action of the tooth plates, sensitive fiber can
get a periodic micro-b end. When the tooth plate affected
by external disturbances, the micro-bend degree of the
optic-fiber changes, leading to the change in output light
intensity. By measuring the change in the output light
intensity, the level of the external disturbances can be
measured indirectly. And the function of a micro-bend
optic-fiber sensor is achieved.
In response to an appropriate environmental distur-
1671-7694/2009/070556-04
c
° 2009 Chinese Optics Letters