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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
Claudio Oton - Optical fiber sensors 2
Outline
Introduction to optical fiber sensors
• Rayleigh
• Raman
• Brillouin
• FBG
Applications and market
Recent advances
Conclusions
Claudio Oton - Optical fiber sensors 3
Optical fiber sensors
Distributed
Reading
unit
50 km
fiber
Continuous profile information
Input light
Distance (km)
Discrete
Claudio Oton - Optical fiber sensors 4
Optical fiber sensors
Reading
unit
Discrete
Discrete parameter information
Input light
Distance (km)(if many, quasi-distributed)
Distributed
Claudio Oton - Optical fiber sensors 5
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
Claudio Oton - Optical fiber sensors 6
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
Claudio Oton - Optical fiber sensors 7
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
Claudio Oton - Optical fiber sensors 8
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)
Claudio Oton - Optical fiber sensors 9
Distributed acoustic sensor
A. Masoudi, M. Belal, T. Newson, Meas. Sci. Tech. 24 (2013)
Thousands of microphones along the fiber!
Geological surveys
Claudio Oton - Optical fiber sensors 10
Raman scattering
Raman
Stokes
Raman
Anti-Stokes
Interaction with optical phonons
𝑃𝐴𝑆𝑃𝑆= 𝑒−
ℎΔ𝜐𝑅𝑘𝑇
Δ𝜐𝑅~13THz for silica glass
Sensitivity: 0.035 dB/K
Claudio Oton - Optical fiber sensors 11
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
Claudio Oton - Optical fiber sensors 12
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
Claudio Oton - Optical fiber sensors 13
Stimulated Brillouin scattering
A counter-propagating probe beam in the Stokes band can be amplified
Claudio Oton - Optical fiber sensors 14
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
Claudio Oton - Optical fiber sensors 15
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
Claudio Oton - Optical fiber sensors 16
Multiplexed FBG sensors
WDM (limited total grating number)
WDM & TDM (many more gratings, using pulsed source)
WDM & SDM (FBG sets read in sequence)
Claudio Oton - Optical fiber sensors 17
Fiber Bragg grating sensors
Typical sensing parameters:
Strain/Vibration
Temperature
Special FBGs:
Pressure
Acceleration
Chemical substances
Electrical current
Magnetic field
Claudio Oton - Optical fiber sensors 18
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
Claudio Oton - Optical fiber sensors 19
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
Claudio Oton - Optical fiber sensors 20
Distributed fiber sensor market
Photonic Sensor Consortium Market Survey Report,
http://www.igigroup.com/st/pages/photonic_sensor_report.html
Claudio Oton - Optical fiber sensors 22
Challenges
Cost
Sensing distance
Speed (strain/vibration)
Cost
Spatial resolution (cracks are small)
Cross sensitivity (temperature & strain)
Cost
...did I mention cost?
Claudio Oton - Optical fiber sensors 23
How to improve SNR?
Increase peak power (nonlinear effects!)
Increase measurement time (I lose speed!)
Spatial averaging (I lose spatial resolution!)
Any other idea?
Claudio Oton - Optical fiber sensors 24
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
Claudio Oton - Optical fiber sensors 25
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
Claudio Oton - Optical fiber sensors 26
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
Claudio Oton - Optical fiber sensors 27
Fast BOTDA with coding
M. Taki, Y. Muanenda, C. J. Oton, et al, Opt. Lett. 38 (15) 2877 (2013)
Subsecond measurements achieved
Claudio Oton - Optical fiber sensors 28
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
Claudio Oton - Optical fiber sensors 29
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
Claudio Oton - Optical fiber sensors 30
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
Claudio Oton - Optical fiber sensors 31
High-spatial resolution BOTDA
Can we make resolution < 1m?
Use shorter pulses?
Phonon lifetime: 10ns
Intrinsic limitation of BOTDA spatial resolution
Claudio Oton - Optical fiber sensors 32
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
Claudio Oton - Optical fiber sensors 33
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
Claudio Oton - Optical fiber sensors 34
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)
Claudio Oton - Optical fiber sensors 35
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
Claudio Oton - Optical fiber sensors 36
Conclusions
Optical fiber distributed sensors:
Unique technology
Growing market and application range
Interesting physics & engineering
Claudio Oton - Optical fiber sensors 37
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)
Claudio Oton - Optical fiber sensors 38
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