Principle and Application of Fiber Optic Scattering Sensors 2018 Il-Bum Kwon Center for Safety Measurements Korea Research Institute of Standards and Science
Contents 1 I. Introduction II. Phi-OTDR III. OFDR IV. Raman OTDR V. BOTDA VI. BOCDA VII. Conclusion
Introduction of FOS 2 Light source measurand field M(t) optical fiber l : Wavelength F: Phase n : frequency I : intensity Photo detector Signal Processor output M(t) 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
Intrinsic Distributed Sensing 3 Scattering Sensors Pump Laser Back Scattering 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) 2l 2nl t = =, f ( t) Þ f ( l) v c We can know the physical quantity in spatial domain from the time domain
Light scattering in fiber 4 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 u Unlike the OTDR, the signal of the Φ-OTDR is a result of intra-pulse coherent interference from the Rayleigh backscattering reflections in an optical fiber. Φ-OTDR OTDR Amplitude Distance Phase-sensitive technique with localized intensity-change. Highly sensitive for dynamic perturbation e.g. vibration High Signal-to-Noise ratio (SNR)
Phai-OTDR (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 schemes Cons: -Complicated & Bulky setup -Intensive signal processing required. Applications: -Large variety of sensing applications -Simultaneous sensing of two parameters
Experimental setup of Φ-OTDR BPF Laser Isolator PC PG SOA EDFA FBG Circulator 9m Circulator VOA FUT 1.2 Km Vibration 1 Km DAQ PD SOA: Semiconductor Optical Amplifier EDFA: Erbium-doped Fiber Amplifier PG: Pulse Generator PC: Polarization Controller BPF: Band Pass Filter FBG: Fiber Bragg Grating VOA: Variable Optical Attenuator FUT: Fiber under Test PD: Photo detector DAQ: Data Acquisition (card)
Experimental setup of Φ-OTDR Optical Circulator VOA Laser SOA BPF EDFA Isolator PC PD
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 @ 1226 m Event: 170 Hz is applied.. Raw data contains: Low frequency oscillation Spurious peaks High frequency noise
Results: Raw data analysis Time (a) f = 102 Hz (b) f = 170 Hz (c) f = 306 Hz (d) f = 510 Hz Spatial domain Length Freq. domain Length axis: 1 data point =1.9902 m Time axis: 1 data point =0.0001 sec
Results: Raw data analysis Various vibration events with different frequencies are measured: Measurement Range: 16 Hz < f < 5 KHz Serial Frequency (Hz) of vibration Applied Table Measured 1 102 102.55 2 136 136.57 3 170 169.33 4 204 203.54 5 255 254.26 6 306 305.57 7 357 356.82 8 408 408.05 9 510 510.45 Measurement error < 1 Hz!!
Acoustic detection Acoustic vibrations: Amplitude = 10 V Fig. 1 Acoustic detection setup Fig. 2 Frequency response of acoustic vibrations
Signal processing Moving Averaging Method: Intrusion detection, an Impact detection!! Raw traces data: M = Total No. of raw traces Moving Average traces: (1) N= Average No. n= Step size Differential Trace: (2)
Results Vibration is applied at 170 Hz. Event location can be detected. Zoom region
Results Vibration is applied at 170 Hz. Event location can be detected. Zoom region SNR depends on N and n!!
OFDR 16 Optical Frequency Domain Reflectometry Polarization diversity scheme with Michelson Interferometer Main Interferometer LO Cleaved C2 PC Sensing fiber Strain inducing translator TLS C1 5:95 5% PBS P S APD APD DAQ C3 C4 APD SB 80 m delay fiber Auxiliary Interferometer TLS: Tunable Laser Source PBS: Polarization Beam Splitter PC: Polarization Controller APD: Avalanche Photo detector DAQ: Data Acquisition SB: Sampling Board C1 : 5:95 Fiber Coupler C2, C3, C4: 3 db Fiber Couplers
OFDR 17 Principle of OFDR (1) u Main interferometry 기준광 : 감지광 : 2 { p pg p } Er ( t) = E0 exp é ë j 2 f0t + t + 2 e( t) C2 에서의간섭신호의세기 (PBS 전단에서의신호세기 ) : ù û 2 { } Es ( t) = R( t ) E0 exp é ë j 2 p f0( t - t ) + pg ( t - t ) + 2 pe( t -t ) where R( t ) = r( t )exp -at c / n [ ] é ì 1 üù I f R E0 ê í f0 fbt e t e t ý I f I f t 2 ú ë î þû 2 2 ( ) = 2 ( t ) Cos 2 p t + + gt + ( ) - ( - t ) ; ( ) = ( ( )) ù û (1) (2) (3) PBS 후단에서출력되는센서신호 : I(f) I p (f) I s (f)
OFDR 18 u OFDR 원리 : Auxiliary Interferometer 보조간섭계의상승에지트리거시점에서 S 편광과 P 편광신호데이터의샘플링 ; ü TLS의비선형성을제거하기위함. ü 보조간섭계에의한샘플링에의하여같은주파수간격의데이터만얻게됨. f s = gt g (4) u OFDR 원리 : Signal Processing 샘플링된 S 와 P 편광신호데이터를 FFT 변환처리 ; ü FFT 변환은주파수영역의데이터를시간영역데이터로변환 : I p (f), I s (f) FFT I p (t), I s (t) ü OTDR 신호특성을갖는유효시간영역데이터 : ü 게이지길이와일치하게유효시간영역데이터를여러개의작은데이터그룹으로분리함. 각각의게이지길이데이터를 IFFT 변환하고교차 - 상관관계를구하여주파수이동량을구함. ü R ( t ) = I ( t ) + I ( t ) OTDR s p 2 2 동일한게이지길이의 IFFT 변환한센싱데이터와기준데이터사이의교차 - 상관관계를구하면해당게이지길이에서의주파수이동량을구할수있음. 이주파수이동량은변형률변화에해당함. (5)
OFDR 19 u OFDR 구성 : 주요간섭계와부가간섭계및신호취득및처리시스템 감지광섬유에약 5 cm 길이에변형률을인가하기위한이송장치를설치 보조간섭계샘플링보드제작 OFDR setup Tunable laser source To sensing fiber Fiber coupler Fiber Isolator From TLS Monitor Polarization controler Main interferometer APD Optical configuration Aux. interferometer Fiber delay line PC with DAQ Strain inducing translator Fiber coupler (C4) Fiber coupler (C3)
OFDR 20 u OFDR 의신호처리화면및 LabVIEW 프로그램, 샘플링보드 교차상관관계처리에의하여광섬유거리에따 른주파수이동량 ( 변형률 ) 을결정한후화면에 표시 주파수변조선형화를 위한보조간섭계신호 샘플링보드 P 와 S 편광신호데이터를취득한이후에디지털 신호처리를위한 LabVIEW 프로그램
OFDR 21 u OFDR 의공간분해능과변형률분해능
OFDR 22 u 직경 10 cm, 길이 4 m pvc 파이프에광섬유부착 (x,y 2 축 )
OFDR 23 u 모사케이블 ( 플라스틱파이프 ) 분포변형률측정결과 파이프중간부분이눌릴경우 파이프한쪽끝부분이눌릴경우
Raman OTDR 24 Basic principle Light source Pulsed pumping light heating Stokes light intensity Stokes filter Anti-S filter Anti-Stokes light intensity Photo detector Photo detector Test Fiber 4 R(t) : Ratio of Stokes light intensity due to anti-stoke light intensity A : Constant ls : Wavelength of Stokes light la : Wavelength of Stokes light h : Voltzman s constant k : Plank s constant n : Velocity of light
Raman OTDR 25 Auto correction for temperature measurement An auto-correction method for temperature measurement was developed by a unique signal processing method with a mirror at the end of the fiber. I ( ) n z Ir ( z) : Intensity of normal back scattering : Intensity of reflected back scattering I ( z) = ( I ( z) - C)( I ( z) - C) f n r T z k I ( z ) hcdn I ( z) hcdn 0 ( ) -1 B f T z0 kb ( ) = ( log( (? - 1) + 1)) f 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. Optics Express (2010)
Raman OTDR 26 Raman OTDR for temperature measurement 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. Optics Express (2010)
Raman OTDR 27 Temperature sensing results 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. Processed temperature through fiber Anti-Stokes Raman scattering signals Temperature o C 100 80 60 40 23 o 30 o C 40 o C 50 o C 60 o C 70 o C 80 o C 90 o C C 100 o C 20 0 1000 2000 3000 4000 Distance m Temperature o C 100 80 60 40 20 23 o 30 o C 40 o C 50 o C 60 o C 70 o C 80 o C 90 o C C 100 o C 2150 2200 2250 2300 Distance m Optics Express (2010)
Raman OTDR 28 Immunity on bending loss 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)
Brillouin OTDA 29 BOTDA (Brillouin Optical Time Domain Analysis) Sensor If the temperature is changed on the fiber, then the Brillouin frequency of the fiber will be changed linearly. In the case of strain, Brillouin frequency also linearly changed. Pulsed pumping light PD n B = 2nVa l P heating Index change Test Fiber CW probe light Intensity (mw) 2 1 0 0 10 Distance (km) 20 30 40 10.8 Temperature 10.85 10.9 10.95 Frequency (GHz) νb : Brillouin frequency Va : Acoustic wave velocity n : Refractive index λp : the wavelength of the incident pump lightwave Temperature or Strain Index or acoustic velocity change n B
Brillouin OTDA 30 Spatial resolution enhancement by two pulse technique Brillouin scattering 2 Dz 2 - D z 1 Pulse 1 has a pulse width of DZ1, and Pulse 2 has a pulse of DZ2. x x+dz 1 x+dz 2 Location, z After gathering the back scattering light, the ratio of two scattering lights can give us the enhanced spatial resolution. Brillouin scattering 1 US, China, EU patents (2004)
Brillouin OTDA 31 Signal processing of two back scattering lights - First back scattering light from pulse 1 I (1) cw (2) cw NBGS x ( ) = ( ) (- ) ò + D z1 x, n I L, n exp al exp g( z, n ) DI ( z, n ) cw - Second back scattering light from pulse 2 I (2) ( x, n ) ( x, n ) I ( ) ( ) ù êë é x = = ò + D z2 cw x, n ) exp g z, n DI, (1 pu z n dz x+dz I 1 úû ( ) cw ù êë é pu dz x úû x ( ) = ( ) (- ) ò + D z2 0, x, n I L, n exp al exp g( z, n ) DI ( z, n ) cw - Normalized Brillouin gain spectrum ù êë é pu dz x úû US, China, EU patents (2004)
Brillouin OTDA 32 Effect of signal processing - Line width of BGS Linewidth (MHz) 36 34 32 30 28 26 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 2.4x10-4 2.0x10-4 1.6x10-4 - Strain measurement of a beam Spatial resolution: 1 m without signal-processing with signal-processing (Spatial resolution enhancement) Theoretical strain 24 1.2x10-4 22 Strain 8.0x10-5 20 0 2 4 6 8 10 12 14 16 Spatial Resolution (m) 4.0x10-5 0.0-4.0x10-5 -2 0 2 4 6 8 10 Balanced Location (m) US, China, EU patents (2004)
Brillouin OTDA Spatial resolution enhancement by double pulse technique 33 S: single pulse D: double-pulse D_A-B: A: pulse width B: separation width section 1, 3 = 2m, section 2 = 2m S_80 ns D_40-20 ns Optics Express (2004)
Brillouin OTDA 34 Strain measurement with temperature compensation 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. Load Optical Fiber ESG1 ESG2 ESG3 ESG4 Heater 50 Strain fiber 2000 1000 1000 1000 2.5 4000 4000 8000 100 Temperature fiber unit (mm) FOBOTDA Sensor Temperature fiber Brillouin frequency Temperature n ( T ) = n (0)(1 + C T ) B B T Coefficient of Temperature (1MHz/ C) Strain fiber n B ( T ) = n B (0)(1 + CTT + C e e ) Brillouin frequency Strain Coefficient of Strain (500MHz/%) Key Engineering Materials (2004)
Brillouin OTDA 35 Strain measurement with temperature compensation The strain distribution of the heating beam can be obtained those values accurately after compensation. - Before compensation - After compensation Strain (me) 350 300 250 200 150 100 50 0-50 -100-150 -200-250 -300 ESG at self weight FOS at self weight ESG at self weight + load FOS at self weight + load -350-400 -450-500 2015 2020 2025 2030 2035 Distance (m) Strain (me) 350 300 250 200 FOS at self weight FOS at self weight + load ESG at self weight ESG at self weight + load 150 100 50 0-50 -100-150 -200-250 -300-350 -400-450 -500 2015 2020 2025 2030 2035 Distance (m) Key Engineering Materials (2004)
Brillouin OTDA 36 Building of an optical fiber 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. Light in Light out Right View 1.0 Light in Light out Left View 13.5 C 17.4 12.7 A 16.3 12.3 36 7.2 35.0 7.2 Fiber length = 542 m Light out 17.4 Front View B Light in 13.5 Fiber length = 468 m Fiber length = 255 m 7.2 5.1 14.4 Total Fiber length = 1265 m SPIE Smart Structures Conf (2002)
Brillouin OTDA 37 Software of BOTDA for temperature monitoring Mode setup Distributed temp. display Display direction setup Period setup Max/Min/Avg temp. Max/Min/Avg data SPIE Smart Structures Conf (2002)
Brillouin OTDA 38 Temperature distribution 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. 2003. 3. 25. ~ 3. 26., South wall (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 SPIE Smart Structures Conf (2002)
Brillouin OTDA 39 Strain measurement of a beam 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/2 L = 8000 Load, P W = 45 Optical Fiber Sensors 30 H = 45 Smart Mater. Struct. (2002)
Brillouin OTDA 40 Error correction on the measured strain 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. x x 1.0x10-4 Actual Strain Measured Strain Calibrated Strain Dx B 0 Measurement range L Dx B Strain (e) 6.0x10-5 4.0x10-5 8.0x10-5 x DxB x=0 but e B ¹0 = / 2 2.0x10-5 0.0-2 -1 0 1 2 3 4 5 6 7 8 Location (m) Smart Mater. Struct. (2002)
Brillouin OCDA 41 위상변조형브릴루앙상관영역해석 (BOCDA) 센서시스템구축 DFB 레이저다이오드의출력광을위상변조하여펌프와프로브광으로사용 위상변조광신호가임의의감지광섬유위치에서연속적인브릴루앙이득을발생하여신호로출력 위상변조주파수를변경하면연속적인브릴루앙이득발생위치가변경됨 Composites Sci. and Tech. (2017)
Spatial Resolution of BOCDA 42 위상변조형 BOCDA) 센서성능시험 단일모드광섬유사이에 1 cm 길이의 DSF 광섬유를연결 1 cm의 DSF 광섬유가잘검출됨. 주파수는약 3 MHz의잡음이있음. 1cm type II 15cm type II Smart Mater. Struct. (2002)
Damage Detection 43 Embedded optical fiber in composite cylinder 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. Composites Sci. and Tech. (2017) in preparing
Damage Detection 44 Detection result Damage location and severity were clearly detected. Composites Sci. and Tech. (2017) in preparing
FOS Market Snapshot 45 >$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.
Conclusion 46 Fiber 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.