FIBER OPTIC SENSORS
FOR HTS MAGNETS
28.09.11
Melanie Turenne
Acknowledgements
2
This work was partially supported by the US
DOE SBIR Program
 Collaborators include:

Muons, Inc.
Muons, Inc.
Dr. Rolland Johnson
Dr. Gene Flanagan
CERN special topic seminar: Fiber optic sensors for HTS magnets
North Carolina State University
Department of Materials
Science and Engineering
Dr. Frank Hunte
Dr. Justin Schwartz
Liyang Ye
mturenne@gmail.com
Overview
3
Background on optical fibers
 Optical fiber sensors

 Application
in HTS magnets
 Sensor technologies: Brillouin, FBGs, Rayleigh

Research
 Bi2212
process monitoring
 Cryogenic calibration
 YBCO quench detection

Conclusions and Future work
CERN special topic seminar: Fiber optic sensors for HTS magnets
mturenne@gmail.com
Overview
4
Background on optical fibers
 Optical fiber sensors

 Application
in HTS magnets
 Sensor technologies: Brillouin, FBGs, Rayleigh

Research
 Bi2212
process monitoring
 Cryogenic calibration
 YBCO quench detection

Conclusions and Future work
CERN special topic seminar: Fiber optic sensors for HTS magnets
mturenne@gmail.com
Basics of optical fibers
5

Optical fibers act as waveguides to transmit
light down their core
 Core
and cladding have slightly different indexes
of refraction which causes total internal reflection

Made of high purity glass – silicon dioxide
(SiO2), aka silica
 Can
be doped with Ge, F, H, RE depending on
application
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How optical fibers are made
Glass pre-form is made, heated
(to 2000 °C) in a vertical
orientation and pulled through
a draw tower
 Coatings are applied for
individual applications

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www.fiberinstrumentsales.com/blog/2010/04/26/how-are-fiber-optics-made/
6
mturenne@gmail.com
Types of optical fibers
7
en.wikipedia.org/wiki/Optical_fiber
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Properties of optical fibers (SMF)
8
Gold coating = 155 μm
Cladding = 125 μm
Core = 9 μm
Tensile strength > 700 MPa
 Minimum bend radius is typically 100 times the
fiber’s radius

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Various sensing applications
9
Sensors for temperature, strain, radiation,
pressure, position
 Extrinsic sensors measure a change outside the
fiber body, e.g. Fabry-Perot interferometers
 Intrinsic sensing takes place within the fiber,
where physical changes to the fiber reflect
changes in the light passing through it

CERN special topic seminar: Fiber optic sensors for HTS magnets
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Overview
10
Background on optical fibers
 Optical fiber sensors

 Application
in HTS magnets
 Sensor technologies: Brillouin, FBGs, Rayleigh

Research
 Bi2212
process monitoring
 Cryogenic calibration
 YBCO quench detection

Conclusions and Future work
CERN special topic seminar: Fiber optic sensors for HTS magnets
mturenne@gmail.com
Motivation for HTS
11

Quench detection in HTS magnets is difficult due to
the slow normal zone propagation velocity
 Traditional
voltage tap methods do not have the spatial
resolution necessary for practical application in HTS
systems, and are more cumbersome
 Thus more advanced sensors capable of distributed
sensing are required

HT processing of Bi2212
 Peak

temperature held for only a short time period
Temperature reaches ~900 °C in a pure oxygen environment
 Precise
local temperature information is needed to
optimize performance
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Advantages of optical fiber sensors
12
Immune to RF and electromagnetic waves
 Small and lightweight
 Low loss
 Available in long lengths (50 km)
 Distributed (or quasi-distributed) sensors

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Major challenges
13





Low cryogenic sensitivity – coatings required
Bi2212 heat treatment survivability and
compatibility of fiber, coating, and conductor
Separation of temperature and strain effects
Balance of temporal and spatial resolution
Integration into large-scale magnet systems
 Placement
 Protection
during winding, processing and operation
 Calibration
CERN special topic seminar: Fiber optic sensors for HTS magnets
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Overview
14
Background on optical fibers
 Optical fiber sensors

 Application
in HTS magnets
 Sensor technologies: Brillouin, FBGs, Rayleigh

Research
 Bi2212
process monitoring
 Cryogenic calibration
 YBCO quench detection

Conclusions and Future work
CERN special topic seminar: Fiber optic sensors for HTS magnets
mturenne@gmail.com
15
Brillouin Scattering
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Brillouin scattering theory
16

Light is scattered off acoustic phonons and is
shifted by some Brillouin frequency
where n is the index of refraction, Va is the
acoustic velocity, and λ is the wavelength of
incoming light
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Brillouin sensing
17

Three properties are measured in order to
simultaneously determine temperature and
strain:
 Brillouin
power (intensity)
 Frequency shift
 Line-width (FWHM)

At room temperature, these values vary
linearly; however at cryogenic temperatures
they are nonlinear
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Previous Brillouin research
18

Mahar et al., explored Brillouin scattering for
LTS magnets (thesis 2008, MIT)
 Mapped
the Brillouin parameters under various
strain and temperature conditions for
superconducting magnet application

Achieved moderate temporal resolution of 0.5 s,
but only a 5 m spatial resolution
 Might
be acceptable for LTS, but probably not for
HTS systems
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19
Fiber Bragg gratings (FBGs)
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FBG fabrication
20
Gratings are a local, periodic change in the
refractive index of a fiber’s core
 Written into a fiber by
exposure to UV light
through a phase mask

www.ibsen.dk/phasemasks/fbgphasemasks
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Bragg wavelength
21

Each grating reflects a single Bragg wavelength, lB:
lB = 2nL where n is the refractive index and L is the
grating pitch
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en.wikipedia.org/wiki/Fiber_Bragg_grating
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Basic principles
22

Changes in the temperature or strain state of the
fiber change the Bragg wavelength:
ΔλB = λB[(1-ρα)ε + (α+ξ)ΔT]
where ρα is the photoelastic constant, α is the
coefficient of thermal expansion and ξ is the
thermo-optic coefficient
lB = 2nL
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Sample FBG signal
Reflected power (dB)
23
λB
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λB+Δλ
Wavelength (nm)
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Application of FBGs
24

Multiplexing possible – write many gratings on
one fiber
 Time
division multiplexing (TDM)
 Wavelength division multiplexing (WDM)
Systems can monitor up to 64 fibers at once
 Interrogation system costs ~$20K-40K with
speeds of 1Hz - 2 MHz

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25
Rayleigh scattering
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26
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27
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28
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Sample signal from Luna OBR
29
Distance from Luna OBR (m)
5.8
-99.4
-40
-60
-70
-99.6
-99.8
Amplitude (dB/mm)
Amplitude (dB/mm)
-50
5.9
6.0
6.1
6.2
6.5
6.6
Reference
scan
(RT)
-6.3raw 6.4amplitude
data
-80
-100.0
End of short
patchcord
(splice location)
End of 5 m
patchcord
-100.2
-100.4
-100.6
-100.8
FC/APC
connector from
-101.0
Luna OBR to patchcord
End of fiber
-101.2
-101.4
-90
-100
-110
0
1
2
3
4
5
6
7
Distance from Luna OBR (m)
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Application of Rayleigh scattering
30
Entire fiber serves as distributed sensor
 Calibration over temperature range and for
specific coating conditions may be necessary
 Balance must be found between temporal,
spatial resolution and monitoring length

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Comparison of relevant sensing
techniques for HTS applications
31
Fiber Bragg Gratings
+
−
Pros
+ Conceptually simple
+ Grating multiplexing possible
(TDM, WDM)
+ Multiple fiber sampling
+ Fast (2 MHz sampling rate)
Cons
− Cost of gratings ($10-$100
each)
− Only quasi-distributed sensing
− Erasure above 650 °C
− No separation of
strain/temperature
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Rayleigh Scattering
+
−
Pros
+ No post-processing of fiber
needed
+ Survive high temperatures
+ Truly distributed sensing
+ High spatial resolution
Cons
− High processing time (>10s)
− Calibration dependent
− Hardware more expensive
− Sensitive to vibrations
− No separation of
strain/temperature
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Overview
32
Background on optical fibers
 Optical fiber sensors

 Application
in HTS magnets
 Sensor technologies: Brillouin, FBGs, Rayleigh

Research
 Bi2212
process monitoring
 Cryogenic calibration
 YBCO quench detection

Conclusions and Future work
CERN special topic seminar: Fiber optic sensors for HTS magnets
mturenne@gmail.com
NOT TO SCALE
33
Bi2212 Heat Treatment Process Monitoring
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Bi2212 process monitoring
Compatibility of gold fibers and Bi2212
34
SEM/EDS images
66.3% Ag
33.7% Au
Fiber
78.4% Ag
21.6% Au
Ic testing
Ic varied only 2% from
control, well within
statistical variation
Sample
D = 1000 μm
1.0 mV/cm 0.1 mV/cm
IC (A)
IC (A)
With fiber
279
231
W/o fiber
284
235
4.2 K, 5 T background field
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Bi2212 process monitoring
FBGs
35
FBG Heat Treatment Test
1578
1577
Bragg Wavelength (nm)
1576
1575
1574
1573
Erasure at ~650 °C
1572
1571
1570
1569
1568
1567
0
100
200
300
400
500
600
700
800
900
Temperature (C)
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Bi2212 process monitoring
Rayleigh scattering
36
to TC monitoring computer
Insulation
FO Loc 1
FO Loc 2
Gold furnace
Quartz tube
Thermocouples
Gold coated fiber
Patchcord
Splice
LUNA OBR
Experimental set-up
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to LUNA
monitoring
computer
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Bi2212 process monitoring
Rayleigh scattering
37
Temperature profile over time
900
Monitoring TC
800
Furnace TC
Temperature (deg C)
700
FO Loc 1
600
FO Loc 2
500
400
300
200
100
0
0
50
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100
Time (min)
150
200
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Bi2212 process monitoring
Rayleigh scattering
38
Temperature Profile vs Distance
800
700
600
500
Temp (C)
FO Loc 1
700
662
586
503
556
487
452
325
256
200
129
70
23
400
FO Loc 2
300
200
100
0
6.8
6.9
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7.0
7.1
Distance from Luna OBR (m)
7.2
7.3
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39
Cryogenic Calibration
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Cryogenic Calibration
FBGs (1)
40
Two “strain-free” FBGs with different Bragg
wavelengths were attached to a copper plate
fitted with a heater
 The plate was submerged in liquid nitrogen
and a heater was used to heat the sample to
room temperature
 Thermocouples and a Cernox thermometer
monitored the temperature of the plate

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Cryogenic Calibration
FBGs (2)
41
FBG Cryogenic Calibration Curve
Normalized Bragg Wavelength (nm/nm)
1.0002
1.0000
0.9998
0.9996
0.9994
1516 nm
0.9992
1568 nm
0.9990
50
100
150
200
250
300
Temperature (K)
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Cryogenic calibration
Rayleigh scattering (1)
42

Four coated fibers were calibrated down to 50 K
using a cryocooler
 Acrylate
(telecommunications grade) fiber
 Gold as-received fiber
 Gold annealed (120 °C for 20 hours) fiber
 Gold + nGimat doped titanate fiber

The average spectral shift (from a 281 K
reference scan) along the length of the fiber was
found for various temperatures between 50 K and
295 K, in a “strain-free” state
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Cryogenic calibration
Rayleigh scattering (2)
43
450
y = 2E-05x3 - 0.0126x2 + 0.3702x + 404.32
40 nGimat

Spectral Shift from 281 K ref (GHz)
400
41 Acrylate
350
42 Gold AR
300
y=
8E-06x3
-
0.0058x2
+ 0.0223x + 273.07
43 Gold
Annealed
250
200

y = 7E-06x3 - 0.0055x2 + 0.1112x + 255.68
150
y = 6E-06x3 - 0.0046x2 - 0.0398x + 249.5
100
50
0
-50
0
50
100
150
200
250
Acrylate fiber
most sensitive
(largest
spectral shift)
Gold fibers
approximately
the same; need
better bonding
of gold to
silica fibers
300
Temperature (K)
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44
Quench Detection in YBCO CC
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Quench Detection
FBGs (1)
45

Operating on coldhead of cryocooler at 65 K

Applied constant current at 60% IC (150A/240A)

Quench initiated by a heater pulse
TC37
TC36
TC35
TC34
TC33
TC32
TC31
V20
FO Temp
Probe
FO Strain
Gauge
HTR
V25
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V24
V = voltage tap
TC = thermocouple
V22
V21
V23
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Quench Detection
FBGs (2)
46
160
1514.730
(a) Thermocouple response
TC34
12
Wavelength Peak (nm)
Pulse Height (V)
10
120
8
6
TC35
4
2
0
0.0
0.5
100
1.0
TC33
1.5
Time (s)
TC36
80
TC32
60
TC38
TC37
40
0
1
2
3
4
TC30
5
1514.725
1514.720
1514.715
1514.710
1514.705
1514.700
TC31
0
6
1
2
3
4
(b) Voltage Vij
Wavelength Peak (nm)
0.10
0.08
V23
0.06
0.04
V24
0.02
V22
V21
0.00
0
1
2
3
Time (s)
6
7
8
9 10 11 12
8
9 10 11 12
1515.3 (d) Strain FBG
V20
0.12
5
Time (s)
Time (s)
0.14
Vij (mV)
(c) Temperature FBG
4
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1515.1
1515.0
1514.9
1514.8
1514.7
V25
5
1515.2
F. Hunte et al, PAC09
Temperature (K)
140
Heater pulse
6
0
1
2
3
4
5
6
7
Time (s)
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Quench Detection
Rayleigh scattering (1)
47
A 10 cm long straight sample of AMSC YBCO
was fitted to the cold head of a cryocooler
 Instrumented for quench measurement

 One
side with thermocouples, voltage taps
 Gold + nGimat fiber attached to opposite side
Sample temperature varied from 45 to 55 K
 Cryocooler was switched off for measurements;
transport current of 200 A was applied and
quench triggered with pulse from heater wire;
fiber was scanned every 4 seconds

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Quench Detection
Rayleigh scattering (2)
48
Voltage taps
TC1
TC2
TC3
TC5
TC6
Coldhead
Heater
Optical fibers attached
with GE varnish
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Quench Detection
Rayleigh scattering (3)
49
Quench response from thermocouples and fiber as a function of time
70
45
Temperature change from initial (K)
40
TC5
60
TC6
35
Coldhead
50
30
FO3
FO5
40
25
FO6
20
30
15
20
10
10
5
0
Inverse spectral shift from initial scan (GHz)
TC3
0
0
2
4
6
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8
Time (s)
10
12
14
16
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Quench Detection
Rayleigh scattering (4)
50
Quench response from optical fiber and thermocouples along length of sample
70
Fiber - 4 seconds
Fiber - 8 seconds
Fiber - 12 seconds
Fiber - 16 seconds
TC - 4 seconds
TC - 8 seconds
TC - 12 seconds
TC - 16 seconds
50
40
30
60
50
40
20
30
10
20
0
10
-10
11.05
YBCO
TC6
11.07
TC5
11.09
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TC3
11.11
TC2
TC1
11.13
11.15
Distance from OBR (m)
Temperature change from initial (K)
Inverse spectral shift from initial scan (GHz)
60
0
11.17
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Overview
51
Background on optical fibers
 Optical fiber sensors

 Application
in HTS magnets
 Sensor technologies: Brillouin, FBGs, Rayleigh

Research
 Bi2212
process monitoring
 Cryogenic calibration
 YBCO quench detection

Conclusions and Future work
CERN special topic seminar: Fiber optic sensors for HTS magnets
mturenne@gmail.com
Conclusions (1)
52
Investigating FBGs and Rayleigh
backscattering for application in HTS magnets
 Bi2212 process monitoring

 Bi2212
and gold fibers compatible during HT
 Type I FBGs not suitable; chemical composition
gratings (CCGs) and Type II fsIR FBGs may be
 Rayleigh scattering looks promising!

YBCO quench detection
 Successful
proof-of-concept with FBGs
 Preliminary success using Rayleigh scattering
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Conclusions (2)
53

Optical fiber sensors have the capability to
provide distributed temperature and strain
measurements for superconducting magnet
systems, but a large-scale effort needs to be
undertaken in order to make them a viable,
reliable, and trusted sensing technology
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Future Work
Short Term Goals
54
Investigate additional coatings for enhanced
cryogenic sensitivity and protection of the fibers
 Introduce strain gauges for quantitative
measurements of strain/temperature on Rayleigh
quench experiment
 Optimize low-temperature Rayleigh DAQ
 Instrument small YBCO coil with coated optical
fibers and measure quench using Rayleigh
 Heat treat small Bi2212 coil using Rayleigh
backscattering; possibly test at cryogenic
temperatures

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Future Work
Long Term Goals
55

Further quench studies using FBGs
 Faster
scanning rates (1 kHz)
 Multiple FBGs in small YBCO coil



Investigate CCGs and fsIR FBGs for Bi2212
process monitoring
Implement a faster processing platform for
analyzing Rayleigh data, with the aim of
significantly improving temporal resolution (new
SBIR phase I grant)
Develop fiber sensing package, including fiber
coatings and sheathing/sleeve for magnet
integration
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One more thing…
56

Need a collaboration between people/groups
with experience in:
 Optical
fiber technology
 Superconducting magnet design
 Cryogenic and vacuum systems
 Signal processing

Together we can make this a reality!
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END
Thank you for your attention!
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Relevant publications
58





High temperature compatibility of Bi2212 and goldcoated optical fibers (M. Turenne, PAC09)
High temperature testing of traditional FBGs (M.
Turenne, SOFE09) and fs-IR FBGs (unpublished)
High temperature testing of gold coated fibers using
Rayleigh backscattering (M. Turenne, ASC 2010)
Cryogenic quench measurements of YBCO using FBGs
(F. Hunte, PAC09)
Brillouin scattering for superconducting magnets
(S. Mahar, thesis, MIT 2008)
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High temperature FBGs
Chemical composition gratings (CCGs)
59
Similar to FBGs but with much higher thermal
capacity
 Formed by annealing fibers with existing FBGs
near 1000 °C

 FBG
is erased
 CCG grows in its place

May be solution to
Bi2212 monitoring
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spie.org/x39542.xml?pf=true&ArticleID=x39542
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High temperature FBGs
Ultrafast laser writing
60

Temperature resistant FBGs can also be
formed by writing the grating using ultrafast
lasers (fsIR)
 This
creates color centers and/or local damage at
the grating location, which requires more energy to
diffuse
 Reduces the need for hydrogen pre-loading

fsIR gratings can withstand temperatures
upwards of 1000 °C for hundreds of hours
CERN special topic seminar: Fiber optic sensors for HTS magnets
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