Presentation of EPFL-Group for FibreOptics: Ultra-High ...Rayleigh-based sensing principle •An optical pulse is use to interrogate the fiber •Backscattered light originated from

Presentation of EPFL-Group for Fibre Optics:

Ultra-High Spatial Resolution in Distributed Fibre Sensing

Prof. Luc THEVENAZ& co-workers

2 Postdoc, 4 PhD students, 3 visiting students

1/5 Adm. Assistant - 1/10 Technician

Presentation EPFL - Group for Fibre Optics Luc Thévenaz

GFO Activities

• Optical Fibre Sensors

Electrical current sensors (closed)

Distributed fibre sensing using stimulated Brillouin scattering (core research)

• Optical signal processing using Fibres

Slow & fast light, optical storage

Dynamic fibre gratings, microwave photonics

• Sensing using laser spectroscopy

Photoacoustic gas trace sensing (closed)

Photonic crystal fibres and waveguides


Fibre sensing, a response to a societal concern

Anticipatingcatastrophic failures

Saferinfrastructures

More efficient energy use

Environmental& human threats


«Optical fibre nerves»a realistic answer to this concern

The optical fibre can play the role of a sensitive nerve that is seamlesslyand densely integrated in a structure or the environment

The optical fibre can inform about the amplitude and the position of the «sensation»


A standard optical fibre is the sensing element and gives a value of measurand for each point along the fibre.

The optical fibre may replace manythousands of point sensors.

Range: 50 km, ext. to 150 kmTemperature resolution: 0.1 degStrain resolution: 0.001 %Spatial resolution: 0.01-3 m (range dependent)

Optical fibre cable

February 6May 12

November 14

Position along the fibre, m

Te

mp

era

ture

, d

eg

C

Examples of distributed fibre sensing


Spontaneous scattering-based sensors

6

Fibre under testDetection

Pulsed laser

Directionalcoupler

Lightpulse

P(z) = 12vgr Po tt S(z) ad (z) e

- 2az

S(z) = 38

lon pwo(z)

2

: Recapture factor of the backscattered light (wo: mode radius)

ad(z) : Scattering coefficient

a : Attenuation

t : Pulse temporal width

vgr : Group velocity


Rayleigh-based sensing principle

• An optical pulse is use to interrogate the fiber

• Backscattered light originated from the different scattering points interfere, resulting in a zigzag-shaped trace.

fiber

optical frequency, refractive index and pitch

Coherent optical time-domain reflectometry

7


0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

Distance [m]

Sig

nal am

plitu

de [

a.u

.]

frequency f0, temperature T

0

Characteristic of Rayleigh traces

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

Distance [m]

Sig

nal am

plitu

de [

a.u

.]


0


0 - 26 mK

3

Temperature dependence

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

Distance [m]

Sig

nal am

plitu

de [

a.u

.]


0

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

Distance [m]

Sig

nal am

plitu

de [

a.u

.]


0

frequency f0 + 20 MHz, temperature T

0

Frequency dependence


Rayleigh-based sensing principle

��(��, ��)freq change

�′(�� + ∆�, �� + ∆�)

Trace comparison, ∆� ∝ ∆�

The similarity is determined by cross-correlation

�� , �� ∗ �′(�� + ∆�, �� + ∆�) 4

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

Distance [m]

Sig

nal a

mplit

ud

e [a

.u.]


0

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

Distance [m]

Sig

nal a

mplit

ud

e [a

.u.]


0


0 - 26 mK

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

Distance [m]

Sig

nal a

mplit

ude [a.u

.]


0


0 - 26 mK

frequency f0+ 30 MHz, temperature T

0 - 26 mK

�(��, �� + ∆�)temp change

The shape change induced bytemperature can be fullycompensated by the effect ofchanging optical frequency.

relative temperature change


C-OTDR – Spectral measurementsSpectral cross-correlation

-200 -150 -100 -50 0 50 100 150 200

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Frequency shift (MHz)

Co

rre

late

d v

alu

e (

a.u

.)

Experimental data

Quadratic fitting

Frequency accuracy:

▫ 3 MHz (2mK @ 300K)

▫ 1k averages

▫ Scanning step: 10 MHz

▫ Time: 40s

Distance [m]

Fre

qu

en

cy s

hif

t [M

Hz]

0 4 8 12 16 20 24

-150

-100

-50

0

50

100

150

200

(b)

10


Km-range C-OTDR measurements

• Temperature change: -0.018 K (around 2-km distance)

▫ Frequency shift in standard fibre: 18.96 MHz

Distance [m]

Fre

qu

en

cy c

han

ge [

MH

z]

1995 2000 2005 2010 2015

150

100

50

0

-50

-100

-150

connector

11


Nonlinear coupling-based sensors

12

The nonlinear interaction creates a dynamic Bragg grating and 100% of the diffracted light is back-coupled: ideal recapture!

But the probe signal is needed to generatethe dynamic grating and the response ison top of the CW probe signal.

Pp

Ps

Bs P s

eff

gP P P z

A

1.00

0 10 20 30 40

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

Re

sp

on

se

Distance (km)


Brillouin sensing principle

t

f

13

Pp

Ps

Bs P s

eff

gP P P z

A


Generation of local stationary gratings

0 0 0 0 0 0 0 0 0 0 0 0 0 0p p p p p p p p p p p p

Signal phase-modulated by a PRBS or chaotic signal

+ + + + + + + + + + + + + +- - - - - - - - - - - -

The driving force for the acoustic wave ~ the product of the wave amplitudes

X (time averaged)

010

++++++++++++++ ------------



0 0 0 0 0 0 0 0 0 0 0 0 0 0p p p p p p p p p p p p

Signal phase-modulated by a PRBS

+ + + + + + + + + + + + + +- - - - - - - - - - - -

The driving force for the acoustic wave ~ the product of the wave amplitudes

++++++++++++++ ------------

-0.2 -0.1 0 0.1 0.2z [m]




Results• Fibre length: 40 m

• PRBS Clock rate: 8.14 GHz Spatial resolution: 1.2 cm

• PRBS length 215-1=32,767 symbols


Results• Fibre length: 40 m

• PRBS Clock rate: 8.14 GHz Spatial resolution: 1.2 cm

• PRBS length 215-1=32,767 symbols

A. Zadok, Y. Antman, N. Primerov, A. Denisov, J. Sancho, and L. Thevenaz,"Random-access distributed fiber sensing,“ Laser & Photonics Reviews, online preview, 2012.


Examples of fibre components inspection

Distance (mm)

Brillo

uin

fre

qu

en

cy (

GH

z)

0 50 100 150

10.8

10.82

10.84

10.86

10.88

10.9

0 50 100 15010.82

10.83

10.84

10.85

10.86

10.87

10.88

10.89

Distance (mm)

Brillo

uin

fre

qu

en

cy (

GH

z)


Examples of fibre components inspection

Distance (mm)

Brillo

uin

fre

qu

en

cy (

GH

z)

0 20 40 60 80

10.8

10.81

10.82

10.83

10.84

10.85

10.86

10.87

10.88

10.89

0 20 40 60 80 10010.81

10.82

10.83

10.84

10.85

10.86

10.87

10.88

Distance (mm)

Brillo

uin

fre

qu

en

cy (

GH

z)


Perspectives

State-of-the-art: distributed temperature and strain sensing over50km with 1m spatial resolution

Research art: >100km with 1m spatial resolution, 10km with 1 cmspatial resolution (1’000’000 resolved points).

Future: Contact detection with instantaneous response

Future: Distributed pressure sensing

Future: Distributed sensing of magnetic field, illumination,radiation, etc…

Future: Shape sensing

Documents

Presentation of EPFL-Group for FibreOptics: Ultra-High ...Rayleigh-based sensing principle •An optical pulse is use to interrogate the fiber •Backscattered light originated from