principle and application of fiber optic...
TRANSCRIPT
Il-Bum KwonCenter for Safety Measurements
Korea Research Institute of Standards and Science
Principle and Application of Fiber Optic Scattering Sensors
2018
1Contents
I. IntroductionII. Phi-OTDRIII. OFDRIV. Raman OTDRV. BOTDAVI. BOCDAVII. Conclusion
2Introduction of FOS
§ Embeddable§ Long Gage Lengths (If Needed)§ Chemically Inert§ Serial Multiplexibility (WDM)§ No Ground Loops
§ Compatibility With Telecom§ Very Small Gage Lengths (If Needed)§ High temperature measurement§ Can Have Very Long Stand-off
Distances§ EMI no EMP
Light sourceLight
sourcePhoto
detectorPhoto
detector
measurand field M(t)
optical fiber
output M(t)
Signal Processor
Signal Processor
l : WavelengthF: Phasen : frequencyI : intensity
3Intrinsic Distributed Sensing
2 2 , ( ) ( )l nlt f t f lv c
= = Þ
OTDR (Optical Time domain Reflectometry)We can know the spatial distribution (or variation) of optical medium with analysis of backscattered light in the time domain-First invented to test the optical links (disconnection, splicing point, etc)
Back Scattering
We can know the physical quantity in spatial domain from the time domain
Pump Laser
§ Scattering Sensors
4Light scattering in fiber
Name Origin Sensing parameters
Rayleigh Scattering Non-uniformity of transmission medium
- (light loss)
Brillouin Scattering Coupling with acoustic phonon Strain or temperature
Raman Scattering Coupling with optical photon Temperature
Phi-OTDR
uUnlike the OTDR, the signal of the Φ-OTDR is a result of intra-pulsecoherent interference from the Rayleigh backscattering reflections in anoptical fiber.
• Phase-sensitive technique with localized intensity-change.• Highly sensitive for dynamic perturbation e.g. vibration• High Signal-to-Noise ratio (SNR)
Φ-OTDROTDR
Ampl
itude
Distance
(A) Direct detection scheme (B) Heterodyne detection scheme
Pros:-Simple setup like a conventional OTDR
Cons:-Ultra-narrow line-width laser source (< 3 KHz)
-Stable laser source (<5 KHz per sec)-Monitoring fiber length ~ 15-20 Km
Applications:-Intrusion detection
-Defense, perimeter security, SHM of civil structures (Bridges, Dams etc.)
Pros:-High SNR , High sensitivity, Highly narrow line-width
laser source (< 50 KHz)-Long monitoring length (>100 Km)
-Hybridization with other distributed sensing schemesCons:
-Complicated & Bulky setup-Intensive signal processing required.
Applications:-Large variety of sensing applications
-Simultaneous sensing of two parameters
Phai-OTDR
Experimental setup of Φ-OTDR
SOA: Semiconductor Optical AmplifierEDFA: Erbium-doped Fiber AmplifierPG: Pulse GeneratorPC: Polarization ControllerBPF: Band Pass FilterFBG: Fiber Bragg GratingVOA: Variable Optical AttenuatorFUT: Fiber under TestPD: Photo detectorDAQ: Data Acquisition (card)
PG
FUT
Circulator
PC
DAQ
EDFA
VOA 1.2 Km 1 Km
9m
Laser
Vibration
SOA
PD
Isolator
FBG
Circulator
BPF
SOALaser
PCPD
EDFABPF
Isolator
VOAOptical
Circulator
Experimental setup of Φ-OTDR
Results: Raw data analysis§ Rio Laser: Line width= 5 -15 KHz§ Pulse width = 90 ns (Spatial Resolution = 9m)§ Pulse Rep. rate = 10 KHz§ Sampling rate= 50.2 MS/s
Raw data contains:• Low frequency oscillation• Spurious peaks• High frequency noise
Event: 170 Hz is applied..
@ 1226 m
(a) f = 102 Hz (b) f = 170 Hz (c) f = 306 Hz (d) f = 510 Hz
Length
Time
Length axis: 1 data point =1.9902 mTime axis: 1 data point =0.0001 sec
Spat
ial d
omai
nFr
eq.
dom
ainResults: Raw data analysis
Serial
Frequency (Hz) of vibration
Applied Measured1 102 102.552 136 136.573 170 169.334 204 203.545 255 254.266 306 305.577 357 356.828 408 408.059 510 510.45
Measurement error < 1 Hz !!
Various vibration events with different frequencies are measured:
Measurement Range: 16 Hz < f < 5 KHzMeasurement Range: 16 Hz < f < 5 KHz
Table
Results: Raw data analysis
Acoustic detection Acoustic vibrations: Amplitude = 10 V
Fig. 2 Frequency response of acoustic vibrations
Fig. 1 Acoustic detection setup
Signal processing
• Raw traces data:
• Moving Average traces:
• Differential Trace:
N= Average No. n= Step size
(1)
(2)
M = Total No. of raw traces
Moving Averaging Method: Intrusion detection, an Impact detection!!
Zoom region
ResultsVibration is applied at 170 Hz.Event location can be detected.
Results
Zoom region
SNR depends on N and n!!
Vibration is applied at 170 Hz.Event location can be detected.
16OFDR§ Optical Frequency Domain Reflectometry
5%
APD
PC Sensing fiber
C1
5:95
C3 C4
Auxiliary Interferometer80 m delay fiber
TLS
C2
P
S
APD
APD
PBS
DAQ
LO Cleaved
Strain inducing translator
Main Interferometer
TLS: Tunable Laser SourcePBS: Polarization Beam SplitterPC: Polarization ControllerAPD: Avalanche Photo detectorDAQ: Data AcquisitionSB: Sampling BoardC1 : 5:95 Fiber CouplerC2, C3, C4: 3 dB Fiber Couplers
Polarization diversity scheme with Michelson Interferometer
SB
17OFDR§ Principle of OFDR (1)
u Main interferometry
§ C2 에서의 간섭 신호의 세기 (PBS 전단에서의 신호 세기) :
§ 기준 광 :
§ 감지 광 :
{ }20 0( ) exp 2 2 ( )rE t E j f t t e tp pg pé ù= + +ë û
{ }[ ]
20 0( ) ( ) exp 2 ( ) ( ) 2 ( )
( ) ( ) exp /sE t R E j f t t e t
where R r c n
t p t pg t p t
t t at
é ù= - + - + -ë û= -
( ) ( ) ( )2 20 0
12 ( ) Cos 2 ( ) ( ) ; ( )2bI f R E f f t e t e t I f I f tt p t gt té ùì ü= + + + - - =í ýê úî þë û
(1)
(2)
(3)
§ PBS 후단에서 출력되는 센서 신호 :
Ip(f)I(f)Is(f)
18OFDR
§ 샘플링된 S와 P 편광 신호 데이터를 FFT 변환 처리 ; ü FFT 변환은 주파수 영역의 데이터를 시간 영역 데이터로 변환 :
ü OTDR 신호 특성을 갖는 유효 시간 영역 데이터 :
ü 게이지 길이와 일치하게 유효 시간 영역 데이터를 여러 개의 작은 데이터 그룹으로 분리함.
§ 각각의 게이지 길이 데이터를 IFFT 변환하고 교차-상관관계를 구하여 주파수 이동량을 구함.ü 동일한 게이지 길이의 IFFT 변환한 센싱 데이터와 기준 데이터 사이의 교차-상관관계를 구하면 해당 게
이지 길이에서의 주파수 이동량을 구할 수 있음. 이 주파수 이동량은 변형률 변화에 해당함.
§ 보조 간섭계의 상승 에지 트리거 시점에서 S 편광과 P 편광 신호 데이터의 샘플링 ;
ü TLS의 비선형성을 제거하기 위함.
ü 보조 간섭계에 의한 샘플링에 의하여 같은 주파수 간격의 데이터만 얻게 됨.
s gf gt=
Ip(t), Is(t)Ip(f), Is(f) FFT
22( ) ( ) ( )OTDR s pR I It t t= +
(4)
(5)
u OFDR 원리 : Auxiliary Interferometer
u OFDR 원리 : Signal Processing
19OFDRu OFDR 구성 : 주요 간섭계와 부가 간섭계 및 신호 취득 및 처리 시스템
§ 감지 광섬유에 약 5 cm 길이에 변형률을 인가하기 위한 이송 장치를 설치
§ 보조 간섭계 샘플링 보드 제작
Tunable laser source
APD
PC with DAQ
Monitor
Optical configuration
Strain inducing translator
OFDR setup
Polarization controler
Fiber Isolator
Fiber coupler
To sensing
fiber
From TLS
Main interferometer
Fiber delay line
Fiber coupler (C3)Fiber coupler (C4)
Aux. interferometer
20OFDRu OFDR의 신호처리 화면 및 LabVIEW 프로그램, 샘플링 보드
§ 교차 상관관계 처리에 의하여 광섬유 거리에 따
른 주파수 이동량 (변형률)을 결정한 후 화면에
표시
§ P와 S 편광 신호 데이터를 취득한 이후에 디지털
신호처리를 위한 LabVIEW 프로그램
§ 주파수 변조 선형화를
위한 보조간섭계 신호
샘플링 보드
21OFDRu OFDR의 공간 분해능과 변형률 분해능
22OFDRu 직경 10 cm, 길이 4 m pvc 파이프에 광섬유 부착(x,y 2 축)
23OFDR
파이프 중간부분이 눌릴 경우 파이프 한쪽 끝 부분이 눌릴 경우
u 모사 케이블 (플라스틱 파이프) 분포 변형률 측정 결과
24Raman OTDR
R(t) : Ratio of Stokes light intensity due to anti-Stoke light intensityA : Constantls : Wavelength of Stokes lightla : Wavelength of Stokes lighth : Voltzman’s constant k : Plank’s constantn : Velocity of light
Stokes light intensity
Anti-Stokes light intensity
Pulsed pumping light
Anti-Sfilter
Test Fiber
heating
Light source
Stokesfilter
Photo detector
Photo detector
§ Basic principle
4
25
00 ( ) 1( )( ) ( log( 1) 1))
( )B
hcf T z kB
f
I zkT zhc I z
n
n
D-= - +
D(?
Optics Express (2010)
An auto-correction method for temperature measurement was developed by a unique signal processing method with a mirror at the end of the fiber.
( ) ( ( ) )( ( ) )f n rI z I z C I z C= - -
( )nI z
( )rI z
: Intensity of normal back scattering
: Intensity of reflected back scattering
By applying a mirror at the end of the fiber, we can get normal back scatterings as well as reflected back scatterings. After gathering these signals, a unique equation, which is drew by us, is used to determine the distributed temperature.
Raman OTDR§ Auto correction for temperature measurement
26
Optics Express (2010)
Raman OTDR sensor was constructed with a pulsed laser, an EDFA, a Raman filter, and APD. Two tests are designed to show the feasibility of temperature measurement and the bending loss immunity.
Raman OTDR§ Raman OTDR for temperature measurement
27
2150 2200 2250 230020
40
60
80
100
Distance m Tem
pera
ture
o C
100 oC90 oC80 oC70 oC60 oC50 oC40 oC30 oC23 oC
0 1000 2000 3000 400020
40
60
80
100
Distance m Tem
pera
ture
o C
100 oC90 oC80 oC70 oC60 oC50 oC40 oC30 oC23 oC
Anti-Stokes Raman scattering signals were obtained sequentially, at first normal back scattering, In(z), and secondly the reflected back scattering, Ir(z), shown in the left figure. By processing our method, the temperature data could be obtained shown in the two right figures.
Anti-Stokes Raman scattering signals
Processed temperature through fiber
Optics Express (2010)
Raman OTDR§ Temperature sensing results
28
According to apply the bending on the fiber at 2170 m, and 2225 m, the light was leaked at that point shown in the left figure and the right upper figure. After processing the signals, we can see there are no temperature changes on the processed temperature signal in the left lower figure.
Anti-Stokes Raman scattering signals
Processed temperature through fiber
Optics Express (2010)
Raman OTDR§ Immunity on bending loss
29Brillouin OTDA
Temperature or Strain Bn
Index or acoustic velocity change
Pulsed pumping light CW probe light
PD
Test Fiber
Indexchange
heating
0
10
20
30
40 10.8
10.85
10.9
10.95
0
1
2
Frequency (GHz)Distance (km)
Inte
nsity
(mW
)
Temperature
§ BOTDA (Brillouin Optical Time Domain Analysis) Sensor
2 aB
P
nVn l=
νB : Brillouin frequencyVa : Acoustic wave velocityn : Refractive indexλP : the wavelength of the incident pump lightwave
If the temperature is changed on the fiber, then the Brillouinfrequency of the fiber will be changed linearly.In the case of strain, Brillouin frequency also linearly changed.
30Brillouin OTDA§ Spatial resolution enhancement by two pulse technique
Location, z
x x+Dz1 x+Dz2
Brillouin scattering 1
Brillouin scattering 2Dz2 - D z1
Location, z
x x+Dz1 x+Dz2
Brillouin scattering 1
Brillouin scattering 2Dz2 - D z1
Pulse 1 has a pulse width of DZ1, and Pulse 2 has a pulse of DZ2.
After gathering the back scattering light, the ratio of two scattering lights can give us the enhanced spatial resolution.
US, China, EU patents (2004)
31Brillouin OTDA
( )( )
( ) ( ) úûù
êëé ¢¢D¢== ò
D+
D+
2
1
,,exp,,),( )1(
)2(zx
zx pucw
cw zdzIzgxIxIxNBGS nn
nnn
( ) ( ) ( ) ( ) ( ) úûù
êëé ¢¢D¢-= ò
D+ 1 ,,expexp,,)1( zx
x pucwcw zdzIzgLLIxI nnann
( ) ( ) ( ) ( ) ( ) úûù
êëé ¢¢D¢-= ò
D+ 2 ,,expexp,,,0)2( zx
x pucwcw zdzIzgLLIxI nnann
§ Signal processing of two back scattering lights
- First back scattering light from pulse 1
- Second back scattering light from pulse 2
- Normalized Brillouin gain spectrum
US, China, EU patents (2004)
32Brillouin OTDA
-2 0 2 4 6 8 10-4.0x10-5
0.0
4.0x10-5
8.0x10-5
1.2x10-4
1.6x10-4
2.0x10-4
2.4x10-4
Stra
in
Balanced Location (m)
Spatial resolution: 1 m without signal-processing with signal-processing
(Spatial resolution enhancement) Theoretical strain
0 2 4 6 8 10 12 14 1620
22
24
26
28
30
32
34
36
Line
widt
h (M
Hz)
Spatial Resolution (m)
Before Signal Processing After Signal Processing
SR 1 m : 200 nsec / 190 nsec SR 2 m : 170 nsec / 150 nsec SR 3 m : 200 nsec / 170 nsec SR 5 m : 200 nsec / 150 nsec SR 7 m : 170 nsec / 100 nsec
§ Effect of signal processing
- Line width of BGS
- Strain measurement of a beam
US, China, EU patents (2004)
33Brillouin OTDA§ Spatial resolution enhancement by double pulse technique
section 1, 3 = 2m, section 2 = 2m
D_40-20 nsS_80 ns
S: single pulseD: double-pulseD_A-B:
A: pulse widthB: separation width
Optics Express (2004)
34
Brillouinfrequency
Temperature
Coefficient of Temperature(1MHz/°C)
( ) (0)(1 )B B TT C T Cen n e= + +
Brillouinfrequency
Strain
Coefficient of Strain
(500MHz/%)
Two fibers (strain fiber and temp. fiber) were applied on a steel beam, the length of 8 m. 6 heaters raised the temperature of the beam.
( ) (0)(1 )B B TT C Tn n= +
• Temperature fiber • Strain fiber
4000 4000
8000
100
50
FOBOTDASensor
unit (mm)
ESG1 ESG2 ESG3 ESG4
Optical Fiber
2.5
2000 1000 1000 1000
Load Heater
Strain fiber
Temperature fiber
§ Strain measurement with temperature compensation
Brillouin OTDA
Key Engineering Materials (2004)
35
The strain distribution of the heating beam can be obtained those values accurately after compensation.
2015 2020 2025 2030 2035-500-450-400-350-300-250-200-150-100-50
050
100150200250300350
Stra
in (m
e)
Distance (m)
FOS at self weight FOS at self weight + load ESG at self weight ESG at self weight + load
2015 2020 2025 2030 2035-500-450-400-350-300-250-200-150-100-50
050
100150200250300350
ESG at self weight FOS at self weight ESG at self weight + load FOS at self weight + load
Stra
in (m
e)
Distance (m)
- Before compensation - After compensation
§ Strain measurement with temperature compensation
Brillouin OTDA
Key Engineering Materials (2004)
36
12.7
35.0
Light in
Light out Right View 1.0
7.2
16.3A
17.4
7.2
Light in
Light out
Front View
14.4
13.5
5.1
B
13.5
36
Light in
Light out Left View
12.3
7.2
17.4
C
Fiberlength= 468 m
Fiberlength= 255 m
Fiberlength= 542 m
Total Fiber length = 1265 m
An optical fiber line was installed on the walls, south wall, west wall, north wall of the research building for Center for Safety Measurement in KRISS.
SPIE Smart Structures Conf (2002)
§ Building of an optical fiber
Brillouin OTDA
37
Distributed temp. display
Mode setup
Display direction setup
Period setup
Max/Min/Avg temp.
Max/Min/Avg data
§ Software of BOTDA for temperature monitoring
Brillouin OTDA
SPIE Smart Structures Conf (2002)
38
(a) 11:00, Avr. T.: 16.8 ℃ (b) 15:00, Avr. T.: 19.3 ℃ (c) 19:00, Avr. T.: 11.0 ℃
(d) 23:00, Avr. T.: 5.2 ℃ (e) 03:00, Avr. T.: 4.7 ℃ (f) 07:00, Avr. T.: 4.0 ℃
2003. 3. 25. ~ 3. 26., South wall
A daily change of temperature measurement was shown on the south wall. The averaged temperature maximum is 19.3 oC at 3 PM. The minimum is 4.0 oC at 7 AM.
§ Temperature distribution
Brillouin OTDA
SPIE Smart Structures Conf (2002)
39
In order to show the strain measurement by BOTDA, an optical fiber was attached on the surface of a 8 m long beam. A 20 kg weight was loaded on the center of the beam. Then, according to the bending of the beam, the upper part of the beam will be compressed, also, the lower part of the beam will be extended. The maximum strain is to be acquired at the center of the beam.
L = 8000
Load, P
Optical FiberSensors
W = 45
H = 45
L/2
30
Smart Mater. Struct. (2002)
§ Strain measurement of a beam
Brillouin OTDA
40
The distributed measurement can make an intrinsic error on their measurment result because this measurement average through every spatially resolved distance. The compensation method was contrived in the end-parts of the measurement range.
-2 -1 0 1 2 3 4 5 6 7 8
0.0
2.0x10-5
4.0x10-5
6.0x10-5
8.0x10-5
1.0x10-4
Stra
in (e
)
Location (m)
Actual Strain Measured Strain Calibrated Strain
x=0 but eB¹0 2/B
xx D=¢
x
DxB
0
DxB
x
LMeasurement range
§ Error correction on the measured strain
Brillouin OTDA
Smart Mater. Struct. (2002)
41Brillouin OCDA
• DFB 레이저 다이오드의 출력광을 위상변조하여 펌프와 프로브 광으로 사용
• 위상 변조 광신호가 임의의 감지 광섬유 위치에서 연속적인 브릴루앙 이득을 발생하여
신호로 출력
• 위상 변조 주파수를 변경하면 연속적인 브릴루앙 이득 발생 위치가 변경됨
위상 변조형 브릴루앙 상관 영역 해석 (BOCDA) 센서 시스템 구축
Composites Sci. and Tech. (2017)
42Spatial Resolution of BOCDA
Smart Mater. Struct. (2002)
• 단일 모드 광섬유 사이에 1 cm 길이의
DSF 광섬유를 연결
• 1 cm의 DSF 광섬유가 잘 검출됨.
• 주파수는 약 3 MHz의 잡음이 있음.
위상 변조형 BOCDA) 센서 성능 시험
1cm type II
15cm type II
43
Composite cylinder lay-up was [90°1/OF/90°1/+-20°1/90°3/+-20°1/90°3 /+-20°2/EPDM].Optical fiber was wound on the cylinder with 12 mm interval.Impact energy levels were 10, 20, and 40 J.The impactor has hemispherical shape of the diameter of 12.5 mm.
§ Embedded optical fiber in composite cylinder
Damage Detection
Composites Sci. and Tech. (2017) in preparing
44
Damage location and severity were clearly detected.
§ Detection result
Damage Detection
Composites Sci. and Tech. (2017) in preparing
45FOS Market Snapshot
• >$2.0B 2016; >$3.2B 2021– 9.9% CAGR 2016-2021
• Major segments– Military/Aerospace– Oil & Gas– Industrial– Security
• Diverse supply base– Large Cap (10)- Defense, Energy– Small Cap and Private (50+)
• Near term incremental growth segments– Geophysical and Downhole Oil and Gas– Infrastructure Monitoring
The global fiber optic sensors market should reach $3.2 billion by 2021 from $2.0 billion in 2016 at a compound annual growth rate of 9.9 %, from 2016 to 2021. The FOS market for defense should reach $999 million by 2021 from $687 million in 2016 at a CAGR of 7.8%, from 2016 to 2021. The FOS market for medical should reach $395 million by 2021 from $205 million in 2016 at a CAGR of 14.0%, from 2016 to 2021.
46ConclusionFiber optic sensors are now well preparing to be applied in real world. In this talk, Four sensors, Phi-OTDR, OFDR, BOTDA, and BOCDA sensors, are introduced and explained for their applications.
Phi-OTDR is suitable for event detection. OFDR can be used to do quantitative measurement.BOTDA is suitable for long range measurement with meter spatial range.BOCDA can measure sub meter spatial resolution with several kilometer sensing fiber.
We can apply these distributed fiber optic sensors on the fields of aerospace, civil, railway, marine and ocean structures etc.
“FOS market will grow 1.5 times until 2021. Therefore, this is the right time to start business.”
Thanks for your attention.