1. Optical Fiber

2. A thin (2-125  m) flexible strand of glass or plastic  Light entering at one end travels confined within the fiber until it leaves it at the other end  As fiber bends around corners, the light remains within the fiber through multiple internal reflections  Lowest losses (attenuation) with ultra pure fused silica glass… but expensive and more difficult to manufacture  Reasonable losses with multi- component glass and with plastic Pure Glass Multi- component Glass Plastic Quality, Cost, Difficulty of Handling Attenuation (Loss)

3. Optical Fiber: Construction An optical fiber consists of three main parts  Core A narrow cylindrical strand of glass/plastic, with refractive index n 1  Cladding A tube surrounding each core, with refractive index n 2 The core must have a higher refractive index than the cladding to keep the light beam trapped inside: n 1 > n 2  Protective outer jacket Protects against moisture, crushing Individual Fibers: (Each having its core & Cladding) Multiple Fiber Cable Single Fiber Cable Important: Each core surrounded by a cladding

5. Reflection and Refraction At a boundary between a denser (n 1 ) and a rarer (n 2 ) medium, n 1 > n 2 (e.g. water-air, optical fiber core- cladding) a ray of light will be refracted or reflected depending on the incidence angle Total internal reflection Critical angle Refraction denser rarer 11 22 n1n1 n2n2  critical 11 22 n 1 > n 2 Increasing Incidence angle, 11 v 1 = c/n 1 v 2 = c/n 2 Angles With the Normal

6. Optical Fiber n1n1 n 1 > n 2 Denser Rarer n1n1 n2n2 ii Total Internal Reflection at core-cladding boundary for  i >  critical Refraction at boundary for. Escaping light is absorbed in jacket  i <  critical

7. Total Internal Reflection As  1 increases (or  1 decreases) then there is no reflection cc n2n2 n1n1 n 1 > n 2 11 The incident angle  1 =  c = Critical Angle Beyond the critical angle, light ray becomes totally internally reflected 1>c1>c n2n2 n1n1 n 1 > n 2 1<c1<c 11 When  1 = 90 o (or  c = 0 o ) n 1 sin  1 = n 2 Thus the critical angle

8. Ray Propagation in Fibre - Bound Rays From Snell’s Law: n 0 sin  = n 1 sin (90 -  )  =  max when  =  c Thus, n 0 sin  max = n 1 cos  c 1 2 3 4 5 Cladding n 2 Core n 1 Air (n o =1)  cc  >  c,  >  max a

9. Ray Propagation in Fibre - contd. Since Then NA determines the light gathering capabilities of the fibre NA determines the light gathering capabilities of the fibre n 0 sin  max = n 1 (1 - sin 2  c ) 0.5 Or

10. Ray Propagation in Fibre - contd. Therefore n 0 sin  max = NA Fibre acceptance angle Thus 0.14< NA < 1

11. Attenuation in Guided Media Larger Frequency

12. Optical Fiber - Benefits Greater capacity  Fiber: 100’s of Gbps over 10’s of Kms  Cable: 100’s of Mbps over 1’s of Kms  Twisted pair: 100’s of Mbps over 10’s of meters Lower/more uniform* attenuation (Figure)  An order of magnitude lower  Relatively constant over a larger range of frequencies* Electromagnetic isolation  Not affected by external EM fields: No interference, impulse noise, crosstalk  Does not radiate: Not a source of interference Difficult to tap (data security) * With careful selection of operating band

13. Optical Fiber – Benefits, Contd. Greater repeater spacing: Lower cost, Fewer Units  Fiber: 10-100’s of Kms  Cable, Twisted pair: 1’s Kms Smaller size and weight:  An order of magnitude thinner for same capacity Useful in cramped places Reduced cost of digging in populated areas Reduced cost of support structures

14. Optical Fiber - Applications Long-haul trunks  Telephone traffic over long routes between cities, and undersea: Fiber & Microwave now replacing coaxial cable  1500 km, Up to 60,000 voice channels Metropolitan trunks  Joining exchanges inside large cities:  12 km, Up to 100,000 voice channels Rural exchange trunks  Joining exchanges of towns and villages:  40-160 km, Up to 5,000 voice channels Subscriber loops  Joining subscribers to exchange: Fiber replacing TP, allowing all types of data LANs, City Exchange Main Exchange

15. Optical Fiber - Transmission Characteristics Acts as a wave guide for light (10 14 to 10 15 Hz)  Covers portions of infrared and visible spectrum Transmission Modes: Single Mode Multimode Step Index Graded Index

16. Modes in Fibre A fiber can support:  many modes (multi-mode fibre).  a single mode (single mode fiber). The number of modes supported in a fiber is determined by the indices, operating wavelength and the diameter of the core, given as. V<2.405 corresponds to a single mode fiber. By reducing the radius of the fiber, V goes down, and it becomes impossible to reach a point when only a single mode can be supported. or

17. Types of Fibre There are two main fibre types: (1) Step index: Multi-mode Single mode (2) Graded index multi-mode Multi-mode SIMulti-mode GI Total number of guided modes M for multi-mode fibres:

18. Step-index Multi-mode Fibre Advantages: Allows the use of non-coherent optical light source, e.g. LED's F acilitates connecting together similar fibres Imposes lower tolerance requirements on fibre connectors. Cost effective Disadvantages: Suffer from dispersion (i.e. low bandwidth (a few MHz) High power loss Output pulse Input pulse n 2 = 1.46 50-200  m 120-400  m n 1 =1.48-1.5 100 ns/km

19. Graded-index Multi-mode Fibre Output pulse Input pulse n2n2 n1n1 50-100  m 120-140  m Advantages: Allows the use of non-coherent optical light source, e.g. LED's F acilitates connecting together similar fibres Imposes lower tolerance requirements on fibre connectors. Reduced dispersion compared with STMMF Disadvantages: Lower bandwidth compared with STSMF High power loss compared with the STSMF 1ns/km

20. Step-index Single-mode Fibre Output pulse Input pulse Advantages: Only one mode is allowed due to diffraction/interference effects. Allows the use high power laser source F acilitates fusion splicing similar fibres Low dispersion, therefore high bandwidth (a few GHz). Low loss (0.1 dB/km) Disadvantages: Cost 8-12  m 100-120  m n 2 = 1.46 n 1 =1.48-1.5 5ps/km

21. Optical Fiber – Transmission modes Spread of received light pulse in time (dispersion) is bad:  Causes inter-symbol interference  bit errors (similar to delay distortion)  Limits usable data rate and usable transmission distance Caused by propagation through multiple reflections at different angles of incidence Dispersion increases with:  Larger distance traveled  Thicker fibers with step index  Less focused sources Can be reduced by:  Limiting the distance  Thinner fibers and a highly focused light source  Single mode (in the limit): High data rates, very long distances  Or Graded-index multimode thicker fibers: The half-way (lower cost) solution

22. Optical Fiber Transmission System– Light Source + Fiber + Light Detector Light Emitting Diode (LED)  Incoherent light- More dispersion  Lower data rates  Low cost  Wider operating temp range  Longer life Injection Laser Diode (ILD)  Coherent light- Less dispersion  Higher data rate  More efficient  Faster switching  Higher data rate Light Sources

23. Optical Fiber – Wavelength Division Multiplexing (WDM) A form of FDM (Channels sharing the medium by occupying different frequency bands) Multiple light beams at different light frequencies (wavelengths) transmitted on the same fiber Each beam forms a separate communication channel Separated at destination by filters Example: 256 channels @ 40 Gbps each  10 Tbps total data rate WDM

24. Optical Fiber – Four Transmission bands (windows) in the Infrared (IR) region Band selection is a system decision based on:  Attenuation of the fiber  Properties of the light sources  Properties of the light receivers L S C Note: in fiber = v/f = (c/n)/f = (c/f)/n = in vacuum/n i.e. in fiber < in vacuum Bandwidth, THz 33 12 4 7