1. Snell’s Law    n2n2 n1n1 n1n1 Light rays bend when traversing boundaries between media with different refractive index: in out See http://micro.magnet.fsu.edu/primer/java/scienceopticsu/refraction/index.html

2. Optical fiber Cladding Core n n’ n n Optical fibers are cylindrical waveguides, providing light confinement by total internal reflection along all directions which are perpendicular to the propagation direction. These are essentially bendable “light pipes”. Cross-section 1 – 10 μm ~ 100 μm size n > n’ always

3. Fibers are made of ultrapure SiO 2 glass (silica). Different dopants are added both to the core and cladding, such that the refractive index of the core is slightly larger than that of the cladding. Optical loss in fiber-quality fused silica. (circa 1995) Optical loss in fiber-quality fused silica. (circa 2001) To optimize fibers for telecommunications applications it was necessary to purify them to a very high degree and remove all traces of water. This eliminated the high absorption losses in the “communications window”. Communications window

4. Fiber-Optic Communications Systems Example of fiber-optical communication link. Electrical current pulses representing digital data drive a semiconductor laser. The emitted light pulses pass through a fiber and are detected by a photo-detector at the far end. Laser Input electric pulses ~10Gb/sec Light pulses travel in fiber (short or long) Output electric pulses

5. Amplifying optical signals How far can an optical signal (light) travel in fiber before absorption causes significant losses and signal deterioration? Communications window Fibers can typically transmit information over a distance of 80km, after which signals require amplification and/or regeneration. Fibers also have a very large bandwidth – the communications window where absorption losses in the fiber are small is broad. This allows transmitting many wavelengths (frequencies) simultaneously.

6. Connecting fibers – optical communications systems MUX = Multiplexing DEMUX = Demultiplexing SCL = semiconductor laser Mod = modulator Det = detector Different frequency for each channel