Equilibrium mode distribution
Encyclopedia
The equilibrium mode [power] distribution of light travelling in an optical waveguide
or fiber
, is the distribution of light that is no longer changing with fibre length or with input modal excitation. This phenomenon requires both mode filtering and mode mixing to occur in the fibre to produce a state that is independent of the mode power distribution launched by the light source. At propagation distances exceeding the equilibrium length, intramodal pulse distortion increases (bandwidth decreases) as the square root of length.
The term equilibrium length is sometimes used to describe a stationary mode distribution, which is the length of multi-mode optical fiber
necessary to attain a static mode
distribution from a specific excitation condition. Equilibrium length is, strictly, the longest such length, as would result from a widely variable range of input excitation. Other terms for equilibrium length are equilibrium coupling length and equilibrium mode distribution length.
Equilibrium mode [power] distributions were reported in early multimode transmission systems at propagation distances as short as a few hundred metres. However, as fibre manufacturing improved, the minute waveguide dimensional and structural changes that produce mode-mixing have been greatly reduced. The length of fibre required to attain true equilibrium is now much greater than the length of practical multimode transmission systems, which makes the term effectively obsolete.
In the absence of strong mode-mixing, high order mode filtering is the primary remaining mechanism for potential change of an input mode power distribution. If a well-aligned laser is the optical source, the mode power distribution is highly concentrated in the lowest order modes, and remains essentially unchanged with distance due to the lack of mode-mixing. If an optical source that overfills the fibre is used, only the highest order guided mode group experiences excess attenuation, and the mode power distribution becomes slightly filtered as a result. (Mandrel wrapping
is a viable method to artificially create this state.) Such mode power distributions are stationary; neither changes with fibre length, but equilibrium does not exist in either case because the distributions remain dependent on the input power distribution.
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
or fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
, is the distribution of light that is no longer changing with fibre length or with input modal excitation. This phenomenon requires both mode filtering and mode mixing to occur in the fibre to produce a state that is independent of the mode power distribution launched by the light source. At propagation distances exceeding the equilibrium length, intramodal pulse distortion increases (bandwidth decreases) as the square root of length.
The term equilibrium length is sometimes used to describe a stationary mode distribution, which is the length of multi-mode optical fiber
Multi-mode optical fiber
Multi-mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus...
necessary to attain a static mode
Transverse mode
A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...
distribution from a specific excitation condition. Equilibrium length is, strictly, the longest such length, as would result from a widely variable range of input excitation. Other terms for equilibrium length are equilibrium coupling length and equilibrium mode distribution length.
Equilibrium mode [power] distributions were reported in early multimode transmission systems at propagation distances as short as a few hundred metres. However, as fibre manufacturing improved, the minute waveguide dimensional and structural changes that produce mode-mixing have been greatly reduced. The length of fibre required to attain true equilibrium is now much greater than the length of practical multimode transmission systems, which makes the term effectively obsolete.
In the absence of strong mode-mixing, high order mode filtering is the primary remaining mechanism for potential change of an input mode power distribution. If a well-aligned laser is the optical source, the mode power distribution is highly concentrated in the lowest order modes, and remains essentially unchanged with distance due to the lack of mode-mixing. If an optical source that overfills the fibre is used, only the highest order guided mode group experiences excess attenuation, and the mode power distribution becomes slightly filtered as a result. (Mandrel wrapping
Mandrel wrapping
In multimode fiber optics, mandrel wrapping is a technique used to preferentially attenuate high-order mode power of a propagating optical signal...
is a viable method to artificially create this state.) Such mode power distributions are stationary; neither changes with fibre length, but equilibrium does not exist in either case because the distributions remain dependent on the input power distribution.