Glossary
Attenuation
Reduction in transmitted optical power. Attenuation as a function of distance in optical fiber is logarithmic. Attenuation as a function of optical wavelength is dominated by the degree to which light is scattered by the molecular structure of the optical fiber (�Rayleigh scattering�).
Chromatic Dispersion
Chromatic dispersion is a measure of the time based broadening which occurs in pulses of light as they propagate along a length of fiber. The spectrum of the optical light pulsed from a transmitter into a fiber includes multiple wavelengths; not just a single wavelength. Chromatic dispersion is caused when different wavelengths of light within the pulse propagate at different velocities. The delay difference between the wavelengths transmitted and those received results in a broadening of the optical pulse. Chromatic dispersion impairs the recovery of the data signal. The wavelength at which the dispersion is minimized (approximately zero) is referred to as the zero-dispersion wavelength, characterized by the symbol lo. Chromatic Dispersion is the most distinguishing difference between the applicable ITU single-mode fiber types. Chromatic dispersion is typically expressed in ps/nm/km (picoseconds of pulse spreading, nanometers of optical wavelength, kilometers of fiber traveled).
At the same center wavelength a broad-spectrum source, like a light emitting diode (LED), will produce much more chromatic dispersion than a narrow spectrum source, like a laser. Chromatic dispersion is the principal dispersion component of singlemode fiber systems, while modal dispersion dominates in laser-based multimode fiber systems.
Cutoff Wavelength
Above this wavelength, optical signals are single-mode. Signals are multi-mode below the cutoff wavelength. The cutoff wavelength for cabled fiber is lower than that for bare fiber due to mechanical stresses exerted on the fiber by the cabling process. For standard single-mode fiber, the standardized (IEC and ITU) cutoff wavelength for cabled fiber is below 1260 nm. With fiber designed for single-mode applications, transmission at wavelengths below the cutoff is rarely if ever attempted, as bandwidth and distance are significantly reduced and more optimal multimode performance at shorter wavelengths is achieved with fibers designed specifically for that purpose.
Differential Mode Delay (DMD)
The difference in the time delay between modes is called differential mode delay. The tuning of these modes is quantified by thedifferential mode delay measurement.
Erbium Doped Fiber Amplifier (EDFA)
An amplifier which boosts the optical power of a signal without the need for electrical regeneration. An EDFA, in simple terms, is a length of optical fiber doped with Erbium and �pumped� by a shorter wavelength laser. Information bearing signals transmitted through the doped fiber will have additional energy imparted to them by the excited Erbium, thus increasing their optical power. EDFAs are only effective in the higher wavelength regions (typically 1525-1625 nm).
Four Wave Mixing
The generation of light at a new wavelength due to the interaction of transmitted signals at two or more wavelengths. Efficient four wave mixing requires proper phase matching, where signals at adjacent wavelengths are essentially coincident in time.
Intermodal Dispersion
The time it takes for light to travel through a fiber is different for each mode resulting in a spreading of the pulse at the output of the fiber referred to as intermodal dispersion or intermodal distortion. This mainly applies to multimode fiber.
Modal Bandwidth
Measure of the highest frequency signal that can be supported over a given distance of multimode fiber, as limited by modal dispersion. Modal bandwidth is typically expressed in MHz*km.
Mode Field Diameter (MFD)
The MFD is used to describe the distribution of the optical power in a fiber by providing an �equivalent� diameter, sometimes referred to as the spot size
Optical Nonlinearity
Variations in optical properties of an optical fiber as a function of optical power. For example, a high-powered optical pulse can induce changes in the chromatic dispersion of an optical fiber.
Polarization Mode Dispersion (PMD)
Difference in propagation velocity between different optical polarization states. An optical signal can be represented by two orthogonally polarized components, each of which will travel at different velocities due to inherent geometric flaws in a length of optical fiber. Since receivers used in optical communications do not discriminate between different polarization states, the two delayed polarization components will be mixed at the receiving end. This mainly applies to single-mode fiber.
References
1. �Bellcore�s fiber measurement audit of existing cable plant for use with high bandwidth systems�, J. Peters, A. Dori, and F. Kapron, Proceedings of NFOEC 1997.
2. �PMD assessment of installed fiber plant for 40 gbit/s transmission�, P. Noutsios and S. Poirier, Proceedings of NFOEC 2001.