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OFS Perth 2008. A multiplexed CW Brillouin system, for precise interrogation of a sensor array made from short discrete sections of optical fibre. John Dakin 1 , Sanghoon Chin 2 , and Luc Thévenaz 2 1 Optoelectronics Research Centre, University of Southampton

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slide1

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 Southampton

jpd@orc.soton.ac.uk

2 Ecole Polytechnique Fédérale de Lausanne, Switzerland

sanghoon.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.

Summary
slide3

F3

F2

F1

DFB-LD

1W

EDFA

VOA

Chamber

PC

nB3

nB2

nB1

nc

nc

nc

Delay

Beating notes

ESA

Det

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)

1

slide4

F3

F2

F1

DFB-LD

1W

EDFA

VOA

Chamber

PC

nB3

nB2

nB1

nc

nc

nc

  • Configuration of the cascaded fibers

Delay

Beating notes

ESA

Det

diameter: 7 cm

diameter: 7 cm

50 m of DSF

15 m of DCF

50 m of special fiber with small size of core

Schematic diagram of basic sensing system

2

slide5

OFS Perth 2008

, being acoustic velocity

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!
slide6

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).

3

slide7

(b)

(a)

25 oC

45 oC

65 oC

85 oC

35 oC

55 oC

75 oC

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

slide8

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.

5

slide9

n

FBG 1

FBG 2

DFB-LD

EOM

n

n

Probe

Pump

1W

EDFA

Chamber

Delay

1-km SMF

VOA

F1

F2

F3

Frequency Counter

BPF

Det

n

Precision temperature sensor

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

slide10

Electrical down-conversion

Frequency Counter

-

+

RF L.O signal

Photoreciever: Bandwidth, 125 MHz

High pass filter: Cut-off, 150 MHz

Low-pass filter: Cut-off, 15 MHz

Detection system, now showing the second (electrical) down-conversion mixing stage

7

slide11

Results of heating and cooling cycle

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

slide12

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
slide13

Acknowledgements

Prof John Dakin wishes to thank EPFL for granting him a short visiting professorship

All the authors wish to thank Andrew Sansom, of Golledge Electronics, UK, for providing a number of complimentary filter samples at short notice.