Optics and Photonics Journal, 2013, 3, 232-235
doi:10.4236/opj.2013.32B054 Published Online June 2013 (http://www.scirp.org/journal/opj)
Recognition of Bragg Wavelength Disturbed by Time
Delay of Fiber Length in Prepositive Tunable Filter
Chuan Li, Xiaoyong Chao, Yingna Li, Tao Xie, Zhengang Zhao, Xin Xiong
Faculty of Information Engineering and Automation, Kunming University of Science and Technology, Kunming City, China
Email: lichuankmust@163.com
Received 2013
The wavelength shift in fiber Bragg grating does not depend directly on the total light levels, losses in the connecting
fibers and couplers, or source power. However, if the tunable Fabry-Perot filter is place on the end of incident fiber, the
detected time delay of modulation light is occurred due to the unmatch between the scanning time and light transmis-
sion time in the transmission fiber. Consequently, the detected peak wavelength shifts with the length of transmission
fiber. Thus, the peak wavelength shift effect of Bragg reflective light transmitted in fiber with different fiber length can
be obvious in the demodulator with a prepositive tunable Fabr y-Perot filter. The experiment indicates the shift rates of
0.109 - 0.126 nm/km increase approximately linearly with the original peak wavelength of 1532.917 - 1560.300 nm at
the fiber length of 0 - 6 km. To certify the consistency of measurement data, the criterion correction is introduced. By
using the differential method of two fiber Bragg gratings with an optical path, the differential worth is compensated
from the disturbance modulated by the time delay of fiber length.
Keywords: Optic Fiber; Fiber Length; Bragg Wavelength; Fabry-Perot Filter; Time Delay
1. Introduction
Fiber photosensitivity was first observed in germanium-
doped silica fiber in experiments performed by K. Hill
and coworkers at the communication research center in
Canada in 1978 [1,2]. Since then, the fiber Bragg grat-
ings represent a key element in the established and
emerging fields of optical communications and optical
fiber sensing. The principal advantage is that the meas-
urand information is wavelength-encoded (an absolute
quantity, or a state value), thereby making the sensor
self-referencing, rendering it independent of fluctuating
light levels and the system immune to source power and
connector losses [3-5]. In the sensing system of fiber
Bragg grating, the precision of shift of peak wavelength
B should be superior to 0.001 nm for proving the
measurement precision of 0.1 or 1  Thus, the meas-
urement precision
B defined the measurement preci-
sion of whole system. However, the intensity variations
in the light source, the bending on lead/in-out fibers, and
a tunable filter can cause signal deviations when a tun-
able filter scans the Bragg grating sensors [6-8]. H. G.
Limberger etc. found a reflectivity of 94% and a band-
width of 1.7 nm in communication fiber with three side-
lobes [6]. V. Gaillarda etc. research the optical spectrum
feature in the fiber of low coherence interferometer and
fiber Bragg grating [7]. Y. L. Lo etc. found that the in-
tensity variations from the macrobending of the lead-in/
out fibres during high frequency disturbances can cause
deviations of the signal when a tunable filter scans the
Bragg wavelength [ 8] .
2. Peak Wavelength Shift with the Fiber
Length in a Prepositive Tunable Filter
The fiber Bragg grating is a permanent perlodic modula-
tion of the refractive index along a given length of opti-
cal fiber. Due to the coupling between the forward and
backward propagating modes, the specific wavelength
light depending on the modulation period of the refrac-
tive index is reflected at the location of the fiber Bragg
grating. The other wavelength lights are transmitted
through the fiber Bragg grating. The incident light is re-
flected when its peak wavelength is equal to the Bragg
wavelength [1-5]:
max 2
Bragg eff
 (1)
where, neff is the effective refractive index of reverse
couple mode,
is the period of grating. The most im-
portant property of FBG is that it will reflect the incident
light with particularly predetermined wavelengths, while
passing all the other wavelengths of light at the same
time. As the wavelength of the reflected light varies with
the strain, temperature and the other environmental fac-
Copyright © 2013 SciRes. OPJ
C. LI ET AL. 233
tors, detection of the wavelength will yield information
about these quantities. Figure 1 is the schematic diagram
of fiber Bragg grating sensing modulation system, where-
into, (a) is the reflection-type measurement, and (b) is the
transmission-type measurement. In the modulation sys-
tem, the light sources can be broadband light, tuning light,
pulse light, and laser etc., the connectors are coupler and
circulator, the sensing gratings are fiber Bragg grating
and long period fiber grating, and the photoelectric de-
tector is used [5].
In the modulation process, the incident light directly
accessed the sensing grating through the transmission
optical path or the optical coupler. Under the static, quasi-
static, and time varying action of external filed, such as:
strain filed, temperature filed ect., the incident light is
modulated; subsequently, the reflected (or transmitted)
modulated light is detected by the photoelectric detector.
A very important character of fiber Bragg grating is that
the peak wavelength
B of reflected light will change
when there is a change in strain, temperature and other
environment factor. The reason is that change of the
strain of the grating or its environmental temperature can
result in change in the efficient refractive index of the
core neff as well as the period of index modulation
Therefore, the the shift of Bragg wavelength
B can be
expressed as:
Bragg effeff
 (2)
where,  is the elastic deformation of fiber; neff is the
elasto-optical effect of fiber. Eq. (2) indicate that the ef-
fective measurement of
B determine the measurement
precision of the whole system.
The peak wavelength shift effect of Bragg reflective
light transmitted in fiber is observed, as shown in Figure
2. In this experiment, the fiber is the G.652 standard
communication monomode fiber, and the fiber Bragg
grating is inscribed in the Hydrogen-loaded G.652 fiber.
Figure 1. The schematic diagram of fiber Bragg grating
sensing demodulation system.
In Figure 2, the incident broadband light source is
modulated to the narrowband light by the tunable Fabry-
Perot (TFP) filter. The filter is characterized by bandpass
resonances of Lorentzian lineshape and bandwidth of
typically 0.3 nm, with a wide operating range of 40 nm,
depending on the spacing between the mirrors of Fabry-
Perot etalon. Electrical control of this mirror spacing via
piezoelectric stacks allows for tuning the passband
wavelength. In operation, the passband light is injected to
each fiber Bragg grating through two couplers, a circula-
tor. The return passband light modulated by the grating is
detected by the detector through the circulator. As the
filter is tuned, the passband scans over the return signal
from the grating, and the wavelength can be determined
and recorded from the matching peak wavelength of re-
flective light.
In this scanning optical filter, the wavelength range is
1525 - 1565 nm, the wavelength resolution is 0.001 nm,
and the wavelength repetition is 0.01 nm. In this experi-
ment, the tunable Fabry-Perot filter discovers that the
peak wavelength is shifted with the variation of fiber
length, as shown in Figure 3.
In Figure 3, the original peak wavelengths of grating
max(l = 0 ) = 1532.917 nm, 153 7.0 91 nm, 139. 899 nm,
1540.568 nm, 1545.063 nm, 1550.444 nm, 1555.573 nm,
and 1560.300nm, and the widths of gratings are
= 0.2
nm. As the fiber length is l = 6 km, the peak wavelength
shifts are 0.654 - 0.759 nm. By using fitting, the peak
wavelength shift of Bragg reflective light
max(l) -
is related to the fiber length l:
Figure 2. The schematic diagram of the fiber Bragg grating
measurement system demodulated via a prepositive tunable
Fabry-Perot passba nd filte r.
Figure 3. The peak wavelength shift of Bragg reflective light
related to the fiber length.
Copyright © 2013 SciRes. OPJ
 
max max
0.10901532.9176.823 10
 
where, the standard error between the fitting results and
the experimental results is 0.009 nm, the shift rates of the
Bragg wavelength to the fiber length are 0.109 - 0.126
3. Transmission Component of Bragg
Reflective L igh t
This wavelength shift effect of Bragg reflective light
caused by the fiber length is important to the fib er Bragg
grating sensor network. Figure 4 shows the strain ex-
periment based on the cantilever beam of constant bend-
ing rigidity [9,10]. The bottom of the beam is fixed on
the bracket, and the top is hung through the blade, the
pothook, and the weight. The cantilever beam is made
from the stainless steel material at the size of l = 300.0
mm, h = 3.0 mm, and B = 45.9 mm.
In Figure 4, the fiber Bragg gratings mounted on the
up and down surfaces of cantilever are separately suf-
fered the strains of
and -
, but is located in the same
temperature field T. Thus, the strain
can b e expressed
as the function of the shifts of Bragg wavelength in the
gratings [ 7- 1 0]:
 
 
where, S = 1.22 × 10-3 nm/ is the strain sensitivity co-
efficient of grating,
, T) and
, T) are the
wavelength shifts of gratings mounted on the up and
down surfaces of the cantilever. The strain of the beam
caused by the loading can be measured directly by the
tunable Fabry-Perot detector (TFP detector), as shown in
Figure 5.
In Figure 5, the Bragg wavelengths of gratings mounted
on the up and down surface of cantilever are separately
subject to wavelength levels of 1550.2 - 1552.2 nm and
1538.8 - 1540.8 nm. In the loading, the solid and virtual
line groups denote the wavelength sensitivities of the
gratings mounted on the up and down surface with the
Figure 4. The schematic diagram of loading test.
Figure 5. The loading-wavelength variaiton diagram of
fiber Bragg gratings mounted on the cantilever with the
fiber length of l = 0 km, 2 km, 4 km, and 6 km.
fiber length of l = 0 km, 2 km, 4 km, and 6 km. Accord-
ing to Eq. (4), through the least-square algorithm fitting,
the strain can be formatted by the weight g:
where, the strain precision of the gratings is 0.5% in the
strain range of 0 - 815.6 . Eq. (5) indicates the Bragg
wavelength shift of sensing grating can be obtained by
processing these Bragg wavelengths of gratings in the
same fiber length, i.e., the Bragg wavelength shift of
sensing grating is independent to the Bragg wavelength
shift of transmission fiber with different lengths. By us-
ing the differential method of two fiber Bragg gratings
with an optical path, the differential worth is compen-
sated from the disturbance modulated by the time delay
of fiber length.
4. Conclusions
In this paper, the shift effect of peak wavelength of
Bragg reflective light is produced by changing the length
of transmission fiber. The experiment indicates that the
peak wavelength shifts of Br agg ref lec tive lig ht ar e 0.109
- 0.126 nm/km, which increased with the original peak
wavelength of 1532.917 - 1560.300 nm at the fiber
length of l = 0 km. The further sensing experiment indi-
cates that the shift of Bragg wavelength in the prepositive
tunable filter includes the sense component of grating
and the transmission component of Bragg reflective lig ht.
The measured peak wavelength should be corrected
when the transmission shift distributes the measurement
accuracy. It is noteworthy that the differential worth is
compensated from the disturbance modulated by the time
delay of fiber length by using the differential method of
two fiber Bragg gratings with an optical path.
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