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Optical fiber sensors: overview and recent advances Claudio Oton Scuola Superiore SantAnna, Pisa, Italy 18 th Annual Workshop of the IEEE Photonics Benelux Chapter Mons, Belgium 22 May 2015 Outline Introduction to optical fiber

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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

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Outline

  • Introduction to optical fiber sensors
  • Rayleigh
  • Raman
  • Brillouin
  • FBG
  • Applications and market
  • Recent advances
  • Conclusions
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Optical fiber sensors

  • Distributed

Reading unit

50 km fiber

Continuous profile information Input light

Distance (km)

  • Discrete
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Optical fiber sensors

Reading unit

  • Discrete

Discrete parameter information Input light

Distance (km)

(if many, quasi-distributed)

  • Distributed
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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

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Rayleigh scattering

Elastic scattering produced by the atoms 𝛽𝑆 = 𝐷 𝜇4

C constant (0.7-0.9 dB mm4/km)

Some of the scattered ligth is guided backwards

𝛿 = 𝑇𝛽𝑆

S: capture factor

g ~ 10-4 km-1 Reflected photon is in phase with the incident

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Distributed Rayleigh backscattering

Laser Detector

𝐹 =

𝑗=0 𝑂

𝐹𝑗𝑓𝑘𝜚𝑗

LP

If Lcoh << LP  incoherent 𝐽 =

𝑗=0 𝑂

𝐽𝑗

Typical OTDR (Optical time domain reflectometry)

Typically P  10ns, resolution 1m

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Coherent Rayleigh scattering LP

If Lcoh >> LP  Coherent 𝐽 =

𝑗=0 𝑂

𝐽𝑗 Speckles harm the OTDR trace for loss estimation

Speckles

100 200 300 400 500 600 700 800 900 1000 0.05 0.1 0.15 0.2 0.25 Coherent (Phase) OTDR Trace Distance(m) Amplitude (V)

But speckles are phase sensitive They change with variations of strain, temperature, etc

Phase OTDR or -OTDR

𝐹 =

𝑗=0 𝑂

𝐹𝑗𝑓𝑘𝜚𝑗

Distributed acoustic sensor (DAS)

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Distributed acoustic sensor

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

Thousands of microphones along the fiber! Geological surveys

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Raman scattering

Raman Stokes Raman Anti-Stokes

Interaction with optical phonons

𝑄

𝐵𝑇

𝑄

𝑇

= 𝑓−ℎΔ𝜑𝑆

𝑙𝑈

Δ𝜑𝑆~13THz for silica glass Sensitivity: 0.035 dB/K

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Raman distributed temperature sensors

5 10 15

  • 20

20 40 60

Distance [km] Temperature [°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
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Brillouin scattering

Interaction with acoustic phonons (long-range vibrations)

Speed of sound 5200 km/s

Δ𝑤 =

2𝑊

𝑏𝑜

𝜇0 10GHz (80pm)

Doppler effect:    

B

~ 0.05 MHz / m

νB dependent on temperature and strain Can be a strain/temperature distributed sensor

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Stimulated Brillouin scattering

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

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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

DC RF

Oscilloscope

FBG MZM Fiber 20 km EDFA

Typical BOTDA setup

Pump Probe

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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

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Multiplexed FBG sensors

WDM (limited total grating number) WDM & TDM (many more gratings, using pulsed source) WDM & SDM (FBG sets read in sequence)

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Fiber Bragg grating sensors

Typical sensing parameters:

  • Strain/Vibration
  • Temperature

Special FBGs:

  • Pressure
  • Acceleration
  • Chemical substances
  • Electrical current
  • Magnetic field
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Application sectors

FBG-based

(strain, vibration, pressure...)

Fire detection Industrial plants Gasoducts Geothermal Solar power plants Wind farms Aeronautic Structural health Railtrack monitoring Oleoducts Oil rigs Fracking

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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

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Distributed fiber sensor market

Photonic Sensor Consortium Market Survey Report, http://www.igigroup.com/st/pages/photonic_sensor_report.html

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The hype cycle

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Challenges

  • Cost
  • Sensing distance
  • Speed (strain/vibration)
  • Cost
  • Spatial resolution (cracks are small)
  • Cross sensitivity (temperature & strain)
  • Cost

...did I mention cost?

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How to improve SNR?

  • Increase peak power (nonlinear effects!)
  • Increase measurement time (I lose speed!)
  • Spatial averaging (I lose spatial resolution!)
  • Any other idea?
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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 = W

A + s

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

Simple but inaccurate

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SNR improvement: Coding

] [ ] [

1

kH i x p jH i y

M k M k j

  

   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

1 2 1 1 2 1 2 3 1 1 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 M C

x y gain

2 1    s s

Theoretical Coding Gain

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

Acquired Samples

Reshaping

* Y S X 

Cyclic Coefficients Matrix

Decoding: MxM linear system

1 *

X S Y

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Raman DTS with cyclic coding

5 10 15 20 25 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Distance [km] Voltage [V]

Stokes Anti-Stokes

(a) 5 10 15 20 25

  • 30
  • 20
  • 10

10

Distance [km] Normalize intensity [dB]

Conventional RDTS Simplex-coded RDTS Experimental Coding 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

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Fast BOTDA with coding

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

Subsecond measurements achieved

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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

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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
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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
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High-spatial resolution BOTDA

Can we make resolution < 1m?

Use shorter pulses?

Phonon lifetime: 10ns

Intrinsic limitation of BOTDA spatial resolution

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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
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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

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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)
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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

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Conclusions Optical fiber distributed sensors:

  • Unique technology
  • Growing market and application range
  • Interesting physics & engineering
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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)

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Acknowledgements

People at Scuola Superiore Sant’Anna involved in fiber sensing

Farhan Zaidi Stefano Faralli Fabrizio Di Pasquale Yonas Muaenda Alessandro Signorini Tiziano Nannipieri Claudio Oton Iacopo Toccafondo

Area leader

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email: c.oton@sssup.it

thank you!