The Design of Fiber Bragg Grating Temperature Measurement System Based on Labview
Open Access JCC
Figure 1. Schematic diagram of temperature demodulation
system.
lume grating. Out from the optical circulator, the light
through the optical fiber collimator parallel incident into
the volume grating and transmission of the light from
which projected on the line array of PIN after convex
lens.
After the parallel light irradiate into the volume grat-
ing, the light with the same wavelengths will be emitted
from the same direction and then the light of different
wavelengths will focus on the different positions after
convex lens. Based on the output signal of PIN line array
tested, the input spectrum can be rebuilt and the Bragg
wavelength of fiber grating can be gained so that the
temperature or strain can be finally inverted. In order to
achieve the temperature sensor data, the system adopts
the optical fiber grating temperature sensor which is on ly
sensitive to temperature.
After the direction of incident parallel light is deter-
mined, the line array pixel of the PIN and the wavelength
has a one-to-one corresponding relationship. When tem-
perature changes, the Bragg wavelength of the sensing
grating will drift, leading to the change of the position of
the maximum light power point incident to the line array
PIN. Through the subsequent circuit processing, the po-
sition of the maximum power point can be obtained and
then the changed Bragg wavelength.
By the Labview software, the VI program is written,
demodulation module of gathering data can be read and
processed to instantly display the corresponding relation
between environment temperature and center wavelength.
3. Design of the PC Software Using Labview
This project adopting the optical fiber Bragg grating tem-
perature demodulation software developed by Labview
can realize the acquisition and processing of sensor data,
real-time display the center wavelength and the corres-
ponding temperature and set the sensor parameters and
automatically save data.
The Software design includes the following parts: pa-
rameters setting, spectral fitting, temperature display,
waveform display, data storage and temperature alarm.
3.1. Spectral Fitting-LM Algorithm
The PIN structure has a relatively higher sensitivity and
internal gain, which make the demodulation module have
a high signal noise ratio (SNR). But the PIN has a small
number of pixels and the acquisition point of the light
power is limited. The bigger spacing of adjacent pixel
makes the received optical power not certainly the max-
imum intensity of the center wavelength location, which
probably cause measurement error. We adopt LM algo-
rithm of Gaussian to linear fitting by multiple sampling
point near the peak and then get the highly accurate Gaus-
sian spectrum.
The received data are taken from the sampling point to
peak fitting. Given fitting function:
2
1
()exp4ln 2
nBi
i
iBi
fR
νν
νν
=
−
=−
∆
∑
Where n is the number of Bragg grating.
is the
wavelength.
is the initial value of the ith center wa-
velength.
is the initial FWHM (full width half
maximum) of grating spectrum.
121 23
1
(;,, , )(;,,)
k
njjj j
j
yfvx xxfvxxx
=
= …=
∑
The purpose of spectrum line fitting is to obtain the
coefficient by minimizing
() ()
[ ]
∑
=
−=
m
iinin
yxxxvfxxx
1
2
2121
,,;,,
ϕ
,
where,
is the measured spectrum data, m is the
number of measured da ta.
3.2. Data Collection, Processing and Display
The typical external interface includes the call to DLL,
COM and ActiveX. The data interf ace between hardware
and software in the project needs the communication
with the modulation module through dynamic library file
WIN32Shared.DLL to realize the calls to objective func -
tion. The dynamic link library includes the library files of
multiple functions or resources. Functions and data are
stored in the DLL for export use, in which the executable
files contain only the link library files referring to func-
tions and the description information of those functions
in documents. The call can be achieved to DLL through a
call library function s (CLF) node. But before the call, w e
need to guide the dynamic link library functions, as shown
in Figure 2. The return value of a function can be empty,
integer or floating point pointer. The function’s parame-
ters, data type, transfer sequence need setting and proce-
dure and storage location need calling to complete the
calls to the objective function.
The main objective functions in WIN32Shared. DLL
is as follows:
BOOL DLL_Open_Device(int SerialType);