Post on 14-Dec-2015
OFS Perth 2008
A multiplexed CW Brillouin system, for precise interrogation of a sensor array made from short discrete sections of optical fibre
John Dakin1, Sanghoon Chin2, and Luc
Thévenaz2
1 Optoelectronics Research Centre, University of
Southamptonjpd@orc.soton.ac.uk
2 Ecole Polytechnique Fédérale de Lausanne, Switzerlandsanghoon.chin@epfl.ch; luc.thevenaz@epfl.ch
Summary• We present, for the first time, a novel multiplexed sensing architecture
for real-time monitoring of a small array of optical fibres.
• Signal separation, is in the frequency domain, rather than the usual time domain, and relies on each fibre having a different Brillouin shift.
• The ability to monitor with a 100% duty cycle gives enhanced signal to noise ratio, allowing precision measurement.
• We will show first the basic feasibility of the method, then show how the it may be used for precise temperature measurement.
Schematic diagram of basic sensing system
The DFB laser pump into the fiber, via a 1W EDFA a circulator, and 3 Brillouin signals return to be mixed with part of the pump wave on a ~12 GHz photo-
receiver .
(The delay line is used to break coherence of the pump beam, to reduce interference between the pump wave and undesirable pump-wave residues returning from port 3 of
circulator)
DFB-LD
PC
ESADet
F3 F2 F1
Chamber
Delay
1W
Beating notes
EDFA VOA
B3B2
B1c
c
c
1
Schematic diagram of basic sensing system
DFB-LD
PC
ESADet
F3 F2 F1
Chamber
Delay
1W
Beating notes
EDFA VOA
B3B2
B1c
c
c
• Configuration of the cascaded fibers
50 m of DSF 15 m of DCF 50 m of special fiber with small size of core
diameter: 7 cm diameter: 7 cm
2
OFS Perth 2008
Theory for Brillouin Shift in Optical Fibres•The Brillouin shift is proportional to the longitudinal acoustic velocity in the glass material, mainly of the fibre core region in which the majority of energy propagates
•Fibres of different composition can have significantly different acoustic velocities, as the latter is a function of the density and the Young’s modulus of the glass.
•Doping with heavy elements will generally increase the density markedly with respect to pure silica, and many dopants also reduce the Young’s modulus, both parameters therefore tending to reduce the acoustic velocity.
It is therefore relatively easy to select fibres having markedly different Brillouin shifts to suit our multiplexing method!
2 aB
nV
aV , being acoustic velocity
Measured Brillouin Stokes Spectrum from the 3-fibre array
This figure shows three different Brillouin Stokes signals, one from each fibre, as displayed on the electrical spectrum analyzer
(ESA).
9.6 9.8 10.0 10.2 10.4 10.6 10.8 11.0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
fibre-3
fibre-1
Am
plit
ud
e, a
.u.
Frequency, GHz
fibre-2
3
Temperature dependence of Stokes Frequency.
Frequency change of the Stokes signal, as the temperature of the chamber changes, in steps, from 25 oC to 85 oC. It is clearly seen
that only fiber-3 shows a variation, whilst the others scatter light at constant frequency.
4
9.6 9.8 10.0 10.2 10.4 10.6 10.8 11.0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
increasing temp.
fibre-3
fibre-1
Am
plit
ud
e, a
.u.
Frequency, GHz
fibre-2
(a)
10.70 10.72 10.74 10.76 10.78 10.80
0.0
0.3
0.6
0.9
1.2
1.5
Am
plit
ud
e, a
.u.
Frequency, GHz
(b) 25 oC
35 oC
45 oC
55 oC
65 oC
75 oC
85 oC
Linear variation of the Stokes on Temp.
This display shows the frequency change of the Stokes as the temperature of the chamber changes from 25 oC to 85 oC. It is
clearly seen that only fiber-3 varies, whilst the others stay at the same frequency.
Temp oCBrillouin Stokes
shift, GHz
Change in Brillouin shift MHz, relative to
the 250C value
25.1 10.724 0.00
35.2 10.734 9.3
45.2 10.745 21.0
55.3 10.755 30.3
65.3 10.764 49.7
75.2 10.776 51.3
85.2 10.785 60.7
20 30 40 50 60 70 80 90
10.72
10.73
10.74
10.75
10.76
10.77
10.78
10.79
Bri
llou
in s
hif
t, G
Hz
Temperature, oC
5
Precision temperature sensor
VOA
DFB-LDEOM
FBG 1 FBG 2
Probe Pump
1WEDFA
F1 F2 F3
Chamber
Frequency CounterDet
Delay1-km SMF
BPF
To create a precise temperature sensor, the Stokes scattered light was down-converted, using a modulation sideband of the pump laser as local oscillator,
to give a beat signal of order 145 MHz.
NOTE: The intermediate-frequency beat signals were then mixed with a 150 MHz local oscillator in a second electrical (i.e. post-detector) , down-conversion stage, to give ~ 5 MHz signal for frequency measurement. This 2nd mixing stage will be shown in next
slide.6
Detection system, now showing the second (electrical) down-conversion mixing stage
RF L.O signal
Photoreciever: Bandwidth, 125 MHz
High pass filter: Cut-off, 150 MHz
Low-pass filter: Cut-off, 15 MHz
Electrical down-conversion
Frequency Counter
+
-
7
Results of heating and cooling cycle
0 1 2 3 4 5 6
6
7
8
9
10
F
req
ue
nc
y, M
Hz
Delay time, hour
0 1 2 3 4 5 6
6
7
8
9
10
F
req
ue
nc
y, M
Hz
Delay time, hour
The counter frequency was monitored during slow heating and cooling, with a total temperature excursion of ~ 3.5 C.
The magnified insert shows the short-term frequency fluctuation was only of ~10 kHz RMS, equivalent to ~ 0.01 C, despite fast
(1s) update time. 8
Conclusions
•We have conceived and demonstrated a new frequency-division- multiplexed Brillouin sensor system
•We have shown that it is possible to get good separation of Brillouin signals by simple selection of commercial fibres
•We have shown that the sensor operates with low crosstalk between sensors (NOTE : WE NEED TO DEMONSTRATE THIS NEXT)
•We have achieved a noise-limited temperature precision of ~ ± 0.01 C0 RMS, with a fast update time of only 1 second