Fiber Optic Transmission

1. Fiber Optic Transmission SL/HL – Option F.3

2. Reflection/Refraction • Reflection • A wave encounters a boundary between two mediums and cannot pass through • The angle of incidence is always equal to the angle of reflection • Refraction • When a wave passes through boundary into a new medium its speed changes • The wave will change directions based on the change in its speed

3. Refraction • If the wave speeds up it will bend away from the normal line • If the wave slows down it will bend towards the normal line • Snell’s Law

4. Critical Angle • When traveling into air from some medium, light will always speed up, thus increasing the angle • If it speeds up enough, the angle of refraction will be 90 degrees • This means that the refracted ray will travel along the edge of the boundary

5. Critical Angle • When light strikes the boundary at the critical angle or greater, the wave is totally reflected back into the first medium • Here n1 is the index of refraction of the medium the light starts in

6. Total Internal Reflection • Usually when a wave reaches a boundary between mediums it is partially reflected and partially refracted • When the critical angle is exceeded the entire wave is reflected back within the medium • The wave doesn’t lose any energy

7. Optical Fiber • Fiber optic cable is made of thin, clear glass or plastic • Once light enters the cable it is totally internally reflected until it reaches the far end • Actual optical fiber is step indexed • There is another layer between the core and the outside • This is so the fibers can be bundled together

8. Dispersion • Modal • Not all the waves that enter make it to the other end, only certain ones • The possible paths are called modes • Material • Because different frequencies have different refractive indices, they have different paths • These can both cause problems if the bits of data arrive out of order • More direct modes are faster • Laser light and single mode cable reduce these effects

9. Material Dispersion

10. Attenuation • Attenuation is the opposite of amplification • As a signal travels through a cable it will slowly lose intensity as energy is lost • Attenuation is measured in decibels (dB) • The 10 at the beginning is to convert to decibels • Generally measured in dBkm-1

11. Wavelength/Attenuation

12. Noise • One advantage to using fiber optics is that it is not particularly susceptible to noise • Any noise that does occur is generally due to random light entering the end of the cable • The power ratio of noise to signal in fiber optics is generally in the range of 10-17 or 10-18

13. Reshapers • Monomode fibers can eliminate modal dispersion and lasers cut down on material dispersion, but it is not completely eliminated • Over a long distance individual pulses can start to overlap each other • Every 40-60km is a reshaper which will detect and reshape the signal • Has its own laser which sends a ‘new’ signal

14. Amplifiers • Even with reshaping, signals still attenuate over the length of the cable • Amplifiers along the cable increase the signal strength to keep it going