2017年2月10日星期五

Fiber Optical Transceiver Basic Information ( 1 )


Why fiber optics?
 * Bandwidth is a compelling reason
 * 1970’s – Copper cable, 672 simultaneous data streams, with 2 km spacing
 * between amplification points
 * Today – With a single fiber, in excess of 130,000 simultaneous data streams,
 * with 60 km spacing between amplification points

Two primary types of fiber
 * Multi mode fiber
 * Single mode fiber

Multi mode fiber
 * Core is 50 μm to 62.5 μm, cladding is 125 μm (point of reference: human hair is about 100 μm)
 * Multiple light modes propagate thru the fiber. Different modes travel
 * different paths, some longer than others, resulting in a spreading of the light pulse - modal dispersion
 * Modal dispersion is a performance limiting factor in multi mode fiber

Single mode fiber
 * Core is approx. 9(8) μm, cladding is 125 μm
 * Core is sized proportionately (9 μm) to the wavelength (1100 nm and above) as to only propagate one mode efficiently
 * Modal dispersion does not exist in single mode fiber

Dispersion in single mode fiber
 * Chromatic (material) dispersion
 *  Chromatic dispersion is the result of different wavelengths traveling at different speeds

Attenuation is the loss of optical power as light travels thru the fiber
 * Varies with wavelength
 * Constant across frequency (unlike copper cable)
 * To minimize attenuation, use a source (laser) that emits in the low-loss region of the fiber

Multi mode fiber
 * Modal dispersion is the performance limiting factor
 * Chromatic (material) dispersion exists, but is not significant
 * Multi mode fiber is used for short distance links (up to 500 meters at
 * 850 nm and Gigabit frequencies) due to bandwidth limitation caused by modal dispersion

 Single mode fiber
 * Modal dispersion does not exist
 * Chromatic (material) dispersion is a performance limiting factor
 * Single mode fiber is used for long distance links (up to 80+ km), at 1310 nm and 1550 nm

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