Bonjour
Quand on aborde des considérations d'ingéniérie, en optique guidée
(ie fibres) il est forcément pratique d'utiliser la notion
BP=(bande de fréquence transmissible théorique) multipliée par (la distance)
sous forme d'une constante. C'est bien sûr un expédient, mais
est représentatif d'une ressource. Donc dès lors que la
distance s'accroit, sans régénération ou dispositif quelconque
la ressource est dégradée par la dispersion. C'est la question
posée initialement qui désignait cette réponse. ;-)
Ce que vous écrivez est juste mais ne répond pas à la question posée. ;-)
"Bandwidth
Apart from attenuation the bandwidth is the second parameter
characterizing the properties of an optical fiber. The bandwidth
is a measure of the dispersion of an optical fiber."
In "Corning Cable Systems - Fiber Optics Glossary"
<http://snipurl.com/7ptt>
Allez, un petit supplément de chez les hamburgers
<http://www.its.bldrdoc.gov/fs-1037/dir-004/_0531.htm>
"bandwidth (of an optical fiber)
bandwidth (of an optical fiber):
1. The lowest modulation frequency at which the RMS peak-to-valley
amplitude (optical power) difference of an intensity-modulated
monochromatic signal decreases, at the output of the fiber, to a specified
fraction (usually one-half) of the RMS peak-to-valley amplitude (optical
power) difference of a nearly-zero (arbitrarily low) modulation frequency,
both modulation frequencies having the same RMS peak-to-valley amplitude
(optical power) difference at the fiber input.
Note 1:
In multimode fibers, multimode distortion is usually the most significant
parameter limiting fiber bandwidth, although material dispersion may also
play a significant role, especially in the first (850-nm) window.
Note 2:
In multimode fibers, the bandwidthdistance product (colloquially, "fiber
bandwidth" ) is customarily specified by vendors for the bandwidth as
limited by multimode distortion only.
The spectral width of the optical source is assumed to be extremely narrow.
In practice, the effective fiber bandwidth will also be limited by
dispersion, especially in the first (850- nm) window, where material
dispersion is relatively high, because optical sources have a finite
spectral width. Laser diodes typically have a spectral width of several
nanometers, FWHM. LEDs typically have a spectral width of 35 to 100 nm,
FWHM.
Note 3:
The effective risetime of multimode fibers may be estimated fairly
accurately as the square root of the sum of the squares of the material-
dispersion-limited risetime and the multimode-distortion-limited risetime.
Note 4:
In single-mode fibers, the most important parameters affecting fiber
bandwidth are material dispersion and waveguide dispersion. Practical
fibers are designed so that material dispersion and waveguide dispersion
cancel one another at the wavelength of interest.
Note 5:
Regarding effective fiber bandwidth as it affects overall system
performance, it should be recognized that optical detectors such as
PIN diodes are square-law devices. Their photocurrent is proportional to
the optical power of the detected signal. Because electrical power is a
function of the square of the current, when the optical power decreases by
one-half (a 3-dB decrease), the electrical power decreases by three-fourths
(a 6-dB decrease).
2. Loosely, synonym bandwidthdistance product ."
"Mais c'est pas grave" [(c) Kiscool]
Bien cordialement.
--
Post by Fred et surtout Fredje ne souviens plus trop où j'ai planté quoi.....Merci... Dodo
Tu te souviens des noms ?
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