Principle and Application of Fiber Optic...

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


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


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


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


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


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


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


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


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


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


• 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.

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