Optical fiber sensors: overview and recent advances...Coherent Rayleigh scattering L P If L coh >> L...

Optical fiber sensors: overview and

recent advances

Claudio Oton

Scuola Superiore Sant’Anna, Pisa, Italy

18th Annual Workshop of the IEEE Photonics Benelux Chapter

Mons, Belgium 22 May 2015

Claudio Oton - Optical fiber sensors 2

Outline

Introduction to optical fiber sensors

• Rayleigh

• Raman

• Brillouin

• FBG

Applications and market

Recent advances

Conclusions


Optical fiber sensors

Distributed

Reading

unit

50 km

fiber

Continuous profile information

Input light

Distance (km)

Discrete


Optical fiber sensors

Reading

unit

Discrete

Discrete parameter information

Input light

Distance (km)(if many, quasi-distributed)

Distributed


Backscattering in an optical fiber

Laser

Detector

3 spectral bands:

• Rayleigh (elastic scattering)

• Brilluoin (acoustic phonons)

• Raman (optical phonons)

In absence of elements along the fiber


Rayleigh scattering

Elastic scattering produced by the atoms

𝛼𝑅 =𝐶

𝜆4C constant (0.7-0.9 dB mm4/km)

Some of the scattered ligth is guided backwards

𝛾 = 𝑆𝛼𝑅 S: capture factor

g ~ 10-4 km-1

Reflected photon is in phase with the incident


Distributed Rayleigh backscattering

Laser

Detector

𝐸 =

𝑖=0

𝑁

𝐸𝑖𝑒𝑗𝜙𝑖

LP

If Lcoh << LP incoherent

𝐼 =

𝑖=0

𝑁

𝐼𝑖

Typical OTDR (Optical time domain reflectometry) Typically P 10ns, resolution 1m


Coherent Rayleigh scattering

LP

If Lcoh >> LP Coherent

𝐼 =

𝑖=0

𝑁

𝐼𝑖

Speckles harm the OTDR trace for loss estimation

Speckles

100 200 300 400 500 600 700 800 900 1000

0

0.05

0.1

0.15

0.2

0.25

Coherent (Phase) OTDR Trace

Distance(m)

Am

plit

ude (

V)

But speckles are phase sensitive

They change with variations of strain, temperature, etc

Phase OTDR or -OTDR

𝐸 =

𝑖=0

𝑁

𝐸𝑖𝑒𝑗𝜙𝑖

Distributed acoustic sensor (DAS)


Distributed acoustic sensor

A. Masoudi, M. Belal, T. Newson, Meas. Sci. Tech. 24 (2013)

Thousands of microphones along the fiber!

Geological surveys


Raman scattering

Raman

Stokes

Raman

Anti-Stokes

Interaction with optical phonons

𝑃𝐴𝑆𝑃𝑆= 𝑒−

ℎΔ𝜐𝑅𝑘𝑇

Δ𝜐𝑅~13THz for silica glass

Sensitivity: 0.035 dB/K


Raman distributed temperature sensors

0 5 10 15-20

0

20

40

60

Distance [km]

Tem

pera

ture

[°C

]

TCC at 50°C

TCC at 26°C

TCC at 10°C

TCC at -10°C

Raman traces are smooth

Spontaneous Raman is incoherent Experimental result

Sensing fiber

RDTS

Sensing fiber

RDTS

RDTS

Sensing fiber

RDTS

Sensing fiber

Single-ended

configuration

Double-ended

configuration

(immune to wavelength

dep. loss variations)

PAS is very weak

Pump powers typically high (>1W)

Multimode fiber

Broadband laser


Brillouin scattering

Interaction with acoustic phonons (long-range vibrations)

Speed of sound 5200 km/s

Δ𝑣 =2𝑉𝑎𝑛

𝜆010GHz (80pm)

Doppler effect:

B~ 0.05 MHz / m

νB dependent on temperature and strain

Can be a strain/temperature distributed sensor


Stimulated Brillouin scattering

A counter-propagating probe beam in the Stokes band can be amplified


Brillouin Optical Time Domain Analyisis (BOTDA)

A pump pulse and a cw probe can extract the gain profile

CW

Laser

Waveform

generator

EDFA

t

MZM

DCRF

Oscilloscope

FBG

MZM

Fiber

20 km

EDFA

Typical BOTDA setup

PumpProbe


Fiber Bragg grating sensors

Typical strain response: 1 pm/m

Typical temperature response: 10 pm/K

Monitoring the peak position,

we can sense vibration and

temperature

Typical bandwidth 100-200 pm

Advantage: fast measurements


Multiplexed FBG sensors

WDM (limited total grating number)

WDM & TDM (many more gratings, using pulsed source)

WDM & SDM (FBG sets read in sequence)


Fiber Bragg grating sensors

Typical sensing parameters:

Strain/Vibration

Temperature

Special FBGs:

Pressure

Acceleration

Chemical substances

Electrical current

Magnetic field


Application sectors

FBG-based(strain, vibration, pressure...)

Fire detection

Industrial plants

GasoductsGeothermal

Solar power plants

Wind farmsAeronautic

Structural health

Railtrack monitoring

Oleoducts

Oil rigs

Fracking


Distributed fiber sensor market

Over 1.5 Billion$ distributed fiber optic sensors market forecast in 2013-2017 in

strategic industrial sectors

Photonic Sensor Consortium Market Survey Report,

http://www.igigroup.com/st/pages/photonic_sensor_report.html


Distributed fiber sensor market

Photonic Sensor Consortium Market Survey Report,

http://www.igigroup.com/st/pages/photonic_sensor_report.html


The hype cycle


Challenges

Cost

Sensing distance

Speed (strain/vibration)

Cost

Spatial resolution (cracks are small)

Cross sensitivity (temperature & strain)

Cost

...did I mention cost?


How to improve SNR?

Increase peak power (nonlinear effects!)

Increase measurement time (I lose speed!)

Spatial averaging (I lose spatial resolution!)

Any other idea?


How to improve SNR?

Weighing

scale

2

1

3

4

5

6

7

8

9

10

x y z

3 unknown weights

3 weighing tests

A B C

SNR improves!x + y = WA + s

y + z = WB + s x , y , z

x + z = WC + s

Simple but inaccurate


SNR improvement: Coding

][][1

0

kHixpjHiyM

kMkj

Single pulse response samples TR

Example of backscattered trace with the 7-bit binary

Pattern P = { 0,1,1,1,0,1,0 }

M-bit moving window

0 1 2 1

1 2 1 0

2 3 0 1

1 0 3 2

...

...

... *

: : : : :

:

M M

M

M M M

p p p p

p p p p

Y p p p p X

p p p p

M

MC

x

y

gain2

1

s

sTheoretical Coding Gain

P = { p0 , p1 , p2 , p3 , … … , pM-1 , p0 , p1 , p2 , … pM-1}

Acquired Samples

Reshaping

*Y S X

Cyclic Coefficients MatrixDecoding:

MxM linear system

1 *X S Y


Raman DTS with cyclic coding

0 5 10 15 20 250.1

0.2

0.3

0.4

0.5

0.6

0.7

Distance [km]

Vo

lta

ge

[V

]

Stokes

Anti-Stokes

(a)

0 5 10 15 20 25-30

-20

-10

0

10

Distance [km]

No

rma

lize

in

ten

sit

y [

dB

]

Conventional RDTS

Simplex-coded RDTS

ExperimentalCoding Gain: ~6dB

M. Soto, T. Nannipieri, A. Signorini, et al. Opt. Lett. 36 (13) 2557 (2011)

63-bit code

SNR can be improved without

increasing peak power

Less noise

Longer distances

Faster measurements

Lower peak powers

Simple decoding: one matrix multiplication


Fast BOTDA with coding

M. Taki, Y. Muanenda, C. J. Oton, et al, Opt. Lett. 38 (15) 2877 (2013)

Subsecond measurements achieved


Dynamic BOTDA sensing

R. Bernini, A. Minardo, and L. Zeni. Opt. Lett. 34, (17) 2613 (2009)

200 Hz sampling rate, 12Hz vibration detected Natural vibration modes can be detected

Probe fixed at max. slope


Dynamic BOTDA through phase modulation

J. Urricelqui, A. Zornoza, M. Sagues, A. Loayssa, Opt. Express 20, (24) 26942 (2012)

Immune to gain variations

fRF = 850 MHz

1.6 kHz sampling rate

1m resolution

160 m length


BOTDA with better SNR

A. Lopez-Gil, A. Dominguez-Lopez, S. Martin-Lopez, M. Gonzalez-Herraez, J. Lightwave Tech. (in print, 2015)

45km sensing length

No pol. scrambler needed

SNR improvement


High-spatial resolution BOTDA

Can we make resolution < 1m?

Use shorter pulses?

Phonon lifetime: 10ns

Intrinsic limitation of BOTDA spatial resolution


Sub-meter BOTDA

Differential pulse pair (DPP) technique

W. Li, X. Bao, Yun Li, L. Chen Opt. Express 16, (26) 21616 (2008)

Substracting slightly different pulses

15 cm resolution achieved!

L = 1km

SNR penalty


Brillouin with 1cm resolution

1.2 cm spatial resolution!

20m fiber length

PM fiber

K.Y. Song, S. Chin, N. Primerov, L. Thévenaz, J. Lightwave Tech. 28, (14) 2062 (2010).

Brillouin dynamic grating

Pump pulses: 30ns

Probe pulse: 116 ps


Hybrid fiber sensors

Raman/BOTDA sensor

Challenge: SMF limits pump peak power Coding!

1m spatial resolution

80m resolution, 3ºC temp resolution

10 km sensing distance

Same laser, same fiber, same coding

M. Taki, A. Signorini, C. J. Oton, et al, Opt. Lett. 38, (20) 4162 (2013)


Hybrid fiber sensors

Raman/FBG sensor

I. Toccafondo, M. Taki, A. Signorini, et al, Opt. Lett. 37, (21) 4434 (2012)

10 km sensing range

8kHz sampling rate

Same laser, fiber and

detection system


Conclusions

Optical fiber distributed sensors:

Unique technology

Growing market and application range

Interesting physics & engineering


Acknowledgements

Monitoring Gas Compressors and Turbines using FBG sensors

(GE Oil&Gas)

Fiber Optic Sensors for High Energy Physics Experiments

(CERN)

Hybrid Raman/FBG sensors for railways infrastructure

monitoring (RFI)


Acknowledgements

People at Scuola Superiore Sant’Anna involved in fiber sensing

Farhan ZaidiStefano FaralliFabrizio Di Pasquale Yonas Muaenda

Alessandro Signorini Tiziano Nannipieri Claudio OtonIacopo Toccafondo

Area leader


email: [email protected]

thank you!

Optical fiber sensors: overview and recent advances...Coherent Rayleigh scattering L P If L coh >> L...

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Transcript of Optical fiber sensors: overview and recent advances...Coherent Rayleigh scattering L P If L coh >> L...