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Chromatic dispersion

The effect where light at different wavelengths spread out (disperse in the material). Chromatic dispersion determines both the data-carrying capacity of a single-mode optical fiber and the optimum distance between repeaters in a communications-link. Different wavelengths of light will travel at different (phase) velocities within the core of an optical fiber.

The optical power produced by laser, LED or SLED sources used in communications and sensors is usually distributed throughout a range of wavelengths and not within a single, discreet wavelength.  This means that as the pulse of light travels within the core of the optical fiber, it spreads out because the different wavelengths of the pulse travel at different velocities.  As data rates increase these pulses will begin to overlap and affect the quality of the data.

Dispersion increases both with the length of fiber involved and the spectral width of the optical source used and is measured in ‘picoseconds per nanometer kilometer’ or (ps/nm.km).  That is, if a communications link has a dispersion of 2ps/nm.km, is 2km long, and is used with a source that has a linewidth of 5nm, then the duration of the pulse will have increased by 20 picoseconds by the time it reached the detector.

Dispersion also increases with wavelength and in most telecoms fibers it changes from negative to positive between the 1300nm and 1550nm transmission windows.  The wavelength at which the value of dispersion passes through zero is known as the ‘lambda zero’ (λ0).  ‘Dispersion shifted’ fibers are specially designed so their lambda zero occurs within the 1550nm transmission window.

Fibercore HiBi Fiber is birefringent, which means light travelling in the core ‘sees’ one of two values of refractive index, depending on its polarization axis.  This produces two distinct values of dispersion.

 

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