Optical fibers. Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações)

Transcription

1 Optical fibers

2 Outline Fundamental Components and Concepts. Intensity Based and Fabry-Perot Interferometric Sensors. Applications of the Fiber Optic Sagnac Interferometer. The Mach-Zehnder and Michelson Interferometers and Multiplexing. Fiber Optic Smart Structures for Natural, Civil, and Aerospace Applications. Fiber Optic Grating Sensors and Applications

3 Fundamental Components and Concepts Sensor capabilities Advantages Historical trends Sensor types Fundamental components Fiber fabrication / coupling Light sources Modulators Detectors

4 Fiber Optic Sensor Capabilities Rotation, acceleration Electric and magnetic fields Temperature and pressure Acoustics and vibration Strain, humidity, and viscosity

5 Fiber Optic Sensor Advantages Lightweight / nonobtrusive Passive / low power EMI resistant High sensitivity and bandwidth Environmental ruggedness Complementary to telecom / optoelectronics

6 Fiber Optic Sensors Historical Trends Few, high-priced Low-cost basic Low-cost complex components, laser diodes, microoptics components, laser diodes pigtailed, fiber beamsplitters components, mass produced integrated optics

7 Fiber Optic Sensors Historical Trends Niche markets -RF Mass markets emerge - Fiber optic systems -fiber temperature fiber gyros, medical, lab instruments, manufacturing optic smart structures, industrial systems

8 Fiber Optic Sensors Historical Trends Niche markets -RF Mass markets emerge - Fiber optic systems -fiber temperature fiber gyros, medical, lab instruments, manufacturing optic smart structures, industrial systems

9 Extrinsic Fiber Optic Sensors Light modulator Input fiber Output fiber Environmental signal

10 Intrinsic Fiber Optic Sensors Optical fiber Environmental signal

11 Intrinsic Fiber Optic Sensors Sensing Light Source Optical Fiber Region Detection unit

12 Fundamental Components Optical fiber Light sources Beam conditioning optics Modulators Detectors

13 Opticalfiber John Tyndal ( ) 13

14 Total Internal Reflection Air Water Bottom of pool

15 Total Internal Reflection α < α m ax A B α > α m ax n 2 n 0 n 1 Lost B θ < θ c θ > θ c Fiber axis Cladding Propagates A Core Maximum acceptance angle α max is that which just gives total internal reflection at the core-cladding interface, i.e. when α = α max then θ = θ c. Rays with α > α max (e.g. ray B) become refracted and penetrate the cladding and ar eventually lost S.O. Kasap, Optoelectronics (Prentice Hall) 15

16 Refraction and Reflection When Φ 2 = 90, Φ 1 = Φc is the Critical Angle Snell s Law: n 1 Sin Φ 1 = n 2 Sin Φ 2 Φc=Sin -1 (n 2 /n 1 ) 16

17 StepIndexFiber n 1 n 2 n 1 >n 2 Core and Cladding are glass with appropriate optical properties Buffer is plastic for mechanical protection 17

18 Optical Fiber Structure Numerical aperture n (clad) n (core) Waveguide axis

19 Types of Optical Fiber Multimode step index Graded index multimode Single mode Polarization preserving Polarizing

20 StepIndexFiber r Buffer tube: d = 1mm n n 1 n 2 Protective polymerinc coating Cladding: d = µm Core: d = 8-10 µm The cross section of a typical single-mode fiber with a tight buffer tube. (d = diameter) 1999 S.O. Kasap, Optoelectronics (Prentice Hall) 20

21 Fiber Types 2006, VDV Works LLC

22 Ray description of different fibers 22

23 Ray description of different fibers Fibra monomodo Fibra multimodo Fibra multimodo de índice gradual Capítulo 2 23

24 Step and Graded Index Fibers n 2 O n 1 n (a) Multimode step index fiber. Ray paths are different so that rays arrive at different times. O O' O'' n 2 n 1 n (b) Graded index fiber. Ray paths are different but so are the velocities along the paths so that all the rays arrive at the same time. n S.O. Kasap, Optoelectronics (Prentice Hall) 24

25 Graded Index Fiber O n c c We can visualize a graded index B' fiber by imagining a stratified medium with the layers of refracti θ B' Ray 2 n θ b B B' c/n b indices n a > n b > n c... Consider two b A B'' close rays 1 and 2 launched from O θ n a at the same time but with slightly 2 θ A B 1 c/n a Ray 1 a different just suffers launching total internal angles. reflection Ray 1 O' M Ray 2 becomes refracted at B and reflected at B' S.O. Kasap, Optoelectronics (Prentice Hall) 25

26 Step Index Multimode Fiber = n n n 2 n n

27 Graded Index Fiber O n c c We can visualize a graded index B' fiber by imagining a stratified medium with the layers of refracti θ B' Ray 2 n θ b B B' c/n b indices n a > n b > n c... Consider two b A B'' close rays 1 and 2 launched from O θ n a at the same time but with slightly 2 θ A B 1 c/n a Ray 1 a different just suffers launching total internal angles. reflection Ray 1 O' M Ray 2 becomes refracted at B and reflected at B' S.O. Kasap, Optoelectronics (Prentice Hall) 27

28 Types of Polarization Preserving Fiber Elliptical clad Soft glass side pit Stress rod

29 Fiber Depolarizer Broadband input light beam Polarization preserving fiber sections at 45 degrees

30 Fiber Fabrication Types Modified chemical vapor deposition Outside chemical vapor deposition Vapor axial deposition Rods in glass tube