Fibre Optic Materials. Stephen Kukureka

Transcription

1 Fibre Optic Materials Stephen Kukureka

2 Outline Fibre optics history, principles and materials Optical fibres and cables for telecommunications Sensors and smart structures Reliability and mechanical testing Applications and the future

3 Early telecommunications systems

4 In 1870 John Tyndall discovered..

5 but Bell, Morse and Strowager s inventions were more popular!

6 1966 Kao and Hockham proposed optical fibres at STC Laboratories (STL), Harlow however. losses were then 1000 db km -1 for glass and 5 to 10 db km -1 for coaxial cable!

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8 Transatlantic telecommunications cables Cable and Wireless, 1995

9 twisted pair 64 kb per second single fibre 10 Mb per second typical cable 1024 twisted pairs typical cable 16 or 32 fibres 64 Mb per second per cable (8 TV signals) 280 Mb per second (35 TV channels, phone calls) prone to interference not prone to interference possible to tap high security relatively easy to recycle more difficult to recycle

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11 Structure of an optical fibre Outer coating 250µm Inner coating buffer Core 8-9µm Cladding 125µm

12 Fibre optics principle

13 Total internal reflection hyperphysics.phy-astr.gsu.edu

14 Propagation in an optical fibre

15 Materials Silica SiO 2 with dopants to modify refractive index and improve processability. Most common dopant is germania GeO 2 which raises the refractive index. Dopants which raise refractive index include: ZrO 2, TiO 2, Al 2 O 3, P 2 O 5. Dopants which lower refractive index include: B 2 O 3 and F.

16 Modified chemical vapour deposition J M Senior, Optical Fiber Communications, 2 nd ed, Prentice Hall, 1992

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18 Measurement of attenuation Attenuation = 10 log 10 (Power in /Power out ) db 0.2 db = 97 % power transmitted 3 db = 50 % power transmitted 5 db = 68 % power transmitted 10 db = 10 % power transmitted 100 db = 1 % power transmitted 1000 db approx zero power transmitted!

19 Intrinsic attenuation UV band (Urbach) edge Electronic transitions from valence to conduction band. IR band edge Lattice vibrations for doped glass. Rayleigh scattering Small variations in refractive index from density fluctuations.

20 Extrinsic attenuation Hydroxyl (OH) ions From absorbed water. Fundamental stretching frequency at 2.7 µm. Overtones at 1.38 µm and 1.25 µm. 1ppm Si-OH gives losses of 48 db/km at 1.38 µm 2.5 db/km at 1.25 µm Transition metal impurities Small variations in refractive index from density fluctuations. Mie scattering Defects between λ/10 and λ from glass imperfections.

21 First window Losses below 5 db/km at µm by % of power transmitted. Second window Losses below 1 db/km at 1.3 µm. 79% of power transmitted. Third window Losses of 0.2 db/km at 1.55 µm. 95 % of power transmitted.

22 Bending losses Macrobending: In a fibre, all macroscopic deviations of the fibre s axis from a straight line, that will cause light to leak out of the fibre, causing signal attenuation. Microbending: Mechanical stress on a fibre that introduces local discontinuities, which results in light leaking from the core to the cladding by a process called mode coupling.

23 Dispersion and fibre types Multimode step-index fibre Multimode graded-index fibre Single-mode fibre

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25 Aerial cable Hybrid cable Armoured cable Low smoke cable Breakout cable Tight buffer cable

26 Modern telecommunications

27 Optical fibre sensors and smart structures Sensors for smart structures Fibre Bragg Gratings (FBG) Interfaces and coatings Mechanical reliability and testing

28 Smart people? Nervous system + Muscular system Sensors + Actuators

29 Smart structures

30 Optical fibres for sensors Optical fibres can be more than mere signal carriers. Any disturbance of the fibre alters the characteristics of the guided light. Such alterations can be monitored. Characteristics of light monitored in sensing applications include: amplitude polarisation phase modal distribution wavelength time-of-flight

31 Parameters which may be detected using fibre optic sensors strain displacement damage residual strain acceleration cracking vibration deformation wear frequency impact corrosion acoustic emission liquid levels ph levels pressure index of refraction temperature load angular velocity linear velocity chemical composition chemical reactions electric fields Future Fibre Technologies Pty, Australia:

32 The yacht Jacquelina, instrumented by

33 Advantages of fibre optic sensors small size very wide frequency bandwidth response low weight simultaneous sensing of more than one parameter robust very wide operating temperature range low unit cost high tensile strength high sensitivity high fatigue life high spatial resolution fast response times corrosion resistance immunity to electromagnetic interference (EMI) non-conductive plus numerous systems-related advantages

34 What is a Fibre Bragg Grating and how does it work?

35 Manufacturing Fibre Bragg Gratings

36 Fibre Bragg grating principle Sensitivity can be 8µε or % ie 8 mm in 1 km! (but greater response to temperature than strain)

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38 Safety-critical applications for sensors..

39 Strength and Reliability of Optical Fibres and Sensors Flaws Fatigue and Stress Corrosion Ageing Fatigue knee Interfaces in sensors

40 Distribution of fibre strengths Probability mass function Strength depends on fibre diameter PDF strain-rate environment Strength; MPa gauge length

41 Weibull Probability P = 1 exp σ σ 0 m P Probability of failure ln ln 1 1 P = m ln ( σ ) m ln ( ) σ 0 σ Strength 3 Chart Title σ 0 Scale Parameter 2 1 y = x R 2 = m Weibull modulus 0-1 ln(ln(1/(1-p))) ln(stress)

42 Time-to-failure models Time-to-failure models are generally based on two assumptions The applied stress is concentrated by cracks on the surface of the fibre These surface flaws grow under a load which is less than the ultimate tensile strength of the fibre

43 Standard time-to-failure model where t f = AY 2 2 ( n 2 ) S K i IC σ n = B σ n t f = time to failure K IC = critical stress-intensity factor n = stress-corrosion susceptibility parameter S i = intrinsic strength Y = crack-shape parameter A = materials constant B = strength preservation parameter n 2

44 Degradation of fibre strength Optical fibre strength is degraded by water in two ways stress corrosion which is responsible for dynamic and static fatigue zero-stress ageing Stress-corrosion susceptibility parameter (n) is different for short and long-term tests in wet environments at elevated temperatures

45 Static Fatigue and Ageing applied stress stress time-to-failure ageing time static fatigue knee (left) ageing knee (right)

46 Summary of fibre properties Strength about 6 GPa Modulus about 72 GPa (Hence failure strain about 7-8 % (high!) Weibull modulus high varies with fibre: may be 60 Stress-corrosion susceptibility parameter (n) is 20 before the knee and 7 afterwards

47 Reliability of sensors the interfaces and interphases What might affect the sensitivity of the strain measurement? Coating Adhesive

48 Effect of coupling agent on the fibre Strength Bonding Hydrolytic and chemical stability of the surface of the fibre; presence of the fatigue knee ; and effect on the surface flaws Effect on the sensitivity of the sensor embedded in GFRP/CFRP subjected to ageing cycle.

49 E f = 72 GPa G coating =95 MPa G adhesive =1.2 GPa R f =0.06 mm R adhesive =0.3 mm R coating =(.06-.2)mm

50 Optical Fibre and Sensor Mechanical Testing and Reliability Laboratory

51 Two-point Bend Apparatus Loading type Constant velocity Constant strain rate Constant stress rate

52 2-POINT BEND TESTER FOR FIBER STRENGTH MEASUREMENT

53 Two-Point Bend - Constant Strain Rate Cumulative Failure Probability F(%) [ln(ln(1/(1-f))) scale] Constant strain rate: 50%/min Temperature: 23 o C Cumulative Failure Probability F (%) [ln(ln(1/(1-f))) scale]] Constant strain rate: 50%/min Temperature: 23 o C Breaking Stress (MPa) [lnσ scale] Breaking Strain (%) [lnσ scale] Average breaking stress: 7024MPa Average breaking strain: 7.54%

54 4-POINT BEND TESTER FOR FIBER AND TAPE STRENGTH MEASUREMENT

55 Tensile Test Apparatus Loading type Constant velocity Constant strain rate Constant stress rate

56 Tensile Testing

57 Mechanical Reliability of Fibre Bragg Gratings Cumulative Failure Probability (%) [lnln(1/(1-f)) scale] As-received fibre Stripped and recoated fibre Grating (T: 25 o C and RH: 70%) Breaking Stress (MPa) [ln(stress) scale] Median Breaking Stress (MPa) as-received 5565 stripped and recoated 5183 Grating Median breaking stress difference between as- received and stripped-and and-recoated fibres: ~10% Median breaking stress difference between as- received fibres and gratings: ~16% Grating contribution to median breaking stress change: 16% - 10% = 6%. This is in good agreement with the result obtained in the laboratory environment

58 Fractography laboratory environment Stripped and recoated fibre Large slow propagation zone No damage at fracture initiation site

59 Optical Fibre Sensors for Remote Tunnel Displacement Monitoring (OFSTUNN)

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63 Estimation of Stress and Strain in Smart Rod ε max = r/(r+r) R σ max = Eε max Where r = radius of rod R = radius of tunnel Parameters Old tunnel (R=2095mm) Rod diameter 6mm 10mm New tunnel (R=4500mm) Rod diameter 6mm 10mm Stress (MPa) Strain (%) Assuming constant modulus E = 43GPa

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68 Damage Detection and Structural Health in Carbon-Fibre Reinforced Composites Carrie L Spence

69 TL emission Triboluminescent mechanism TL crystal Sucrose Mechanical 2 new surfaces Gas excitation Fracture

70 Triboluminescent material Structure of TL material TL emission spectrum P O Mn Br2 TL intensity, normalised Wavelength, nm

71 Proof of principle experiments Glass coverslips diameter = 9mm Glass capillary Results Doped resin Storage Oscilloscope Photomultiplier Triboluminescent signal, mv Impact energy, mj

72 Other applications of optical fibre sensors

73 and instrumentation of wind turbines (.but not really by me!)

74 The future Erbium-Doped Fibre Amplifiers (EDFA) Amplification of power with fewer repeaters. Wavelength-Division Multiplexing (WDM) Greater capacity through using wavelength modulation as well as pulse amplitude modulation. Plastic Optical Fibres (POF) Developments in materials to improve attenuation for telecommunications and FBG sensors.

75 Acknowledgements Current group Yehia El Shazly Dr Pifeng Miao Carrie Spence Former students Dr Darran Cairns Dr Chunyang Wei Current collaborators Dr Nicole Metje, Prof Chris Rogers, Dr David Chapman (Civil Engineering) Project students Felicity Croft Ian Wands