Introduction to Fiber Optics What are optical fibers

Introduction to Fiber Optics

What are optical fibers Thin strands of pure glass n Carry data over long distances n At very high speeds n Fiber can be bent or twisted n

Fiber optic technology Sources n Transmission medium n Detectors n Fig: The fiber optic communication system

Sources of light Light emitting diodes n Lasers n

Sources Modulate electrical signals into optical signals n Mostly modulate at 850 nm, 1300 nm and 1550 nm n Lasers give high intensity, high frequency light n LEDs are economical n

Transmission medium Optical fiber is replacing copper n Light is used as the carrier of information n Much higher data rate n The optical fiber

Physics of optical fibers n n Index of refraction of material : ratio of speed of light in vacuum to speed of light in medium Refraction of light : bending of light as it travels from one media to another

Refraction of light Speed of light changes as it across the boundary of two media n Angles w. r. t normal n

Refraction Indices n n n n n Vacuum……. 1. 00000 (exactly) Air …… 1. 00029 Alcohol. . . 1. 329 Diamond. . . 2. 417 Glass. . . . 1. 5 Ice. . . . 1. 309 Sodium Chloride (Salt). . 1. 544 Sugar Solution (80%). . . . 1. 49 Water (20 C). . . . 1. 333

Snell’s Law n Critical angle: Angle of incidence at which angle of refraction = 900

Total internal reflection n Trapping light in the fiber

Fibers can be bent!! Fig: Illustration of total internal reflection

Types of optical fibers n Single mode n n n only one signal can be transmitted use of single frequency Multi mode Several signals can be transmitted n Several frequencies used to modulate the signal n

Losses in optical fibers Attenuation loss n Dispersion loss n Waveguide loss n

Splices and Connectors To connect to fibers mechanically or by fusion n Lot of signal loss possible n Very accurate alignment necessary n Most important cost factor n Now being replaced by optical amplifiers n

Optical Receivers Must be very sensitive n Capable of picking up and amplifying signals of nanowatts n Photodiodes and phototransistors n These devices get ‘turned ON’ by light n Produce photocurrent n

Advantages of optical fibers Can carry much more information n Much higher data rates n Much longer distances than co-axial cables n Immune to electromagnetic noise n Light in weight n Unaffected by atmospheric agents n