Environmental Engineering Reference
In-Depth Information
Fig. 9.12 Schematic of DPP-BOTDA sensing system
BGS (Brillouin frequency), while its position along the fiber is calculated from the
light round-trip time. The differential pulse-width pair (DPP) technique in BOTDA
is implemented as follows [
49
,
50
]: first, two time traces of the Brillouin signal
(I
0
s
ð ;
I
0
s
ð
) are obtained by using two pulses (s
1
;
s
2
) with different pulse
widths; second, the differential signal (I
0
D
ð
) is obtained by making a subtraction
between the two Brillouin signals, and then the differential Brillouin spectra can be
obtained by sweeping the frequency offset in the vicinity of the Brillouin fre-
quency shift (BFS). In the differential Brillouin spectra, the spatial resolution is
determined by the differential pulse, i.e., the pulse-width difference of the pulse
pair, rather than the original pulses. So a high spatial resolution can be achieved by
using a small difference of the pulse pair.
The peak frequency of the BGS, called the Brillouin frequency shift is given by
[
51
]:
m
B
¼
2nV
a
=
k
ð
9
:
14
Þ
where n is the refractive index of the fiber core, V
a
is the velocity of the acoustic
wave, and k is the vacuum wavelength of the pump light. Brillouin frequency shift
increases linearly with strain and temperature, so the Brillouin frequency shift at
position z is expressed as [
52
]:
m
B
ðÞ¼
m
B
ðÞþ
C
s
e
ðÞþ
C
t
t
ðÞ
t
r
ð
Þ
ð
9
:
15
Þ
