1. The difference between the
limiting frequencies within which performance of a device, in respect to some characteristic, falls within specified limits. (
188 )
2. The difference between the limiting frequencies of a continuous
frequency band. (
188 ) href="http://www.its.bldrdoc.gov/fs-1037/dir-001/_0063.htm#188">188 )
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 bandwidth•distance 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 bandwidth•distance product .
Of an
optical fiber, under specified launching and cabling conditions, at a specified
wavelength, a figure of merit equal to the product of the fiber's length and the 3-
dB bandwidth of the optical
signal.
Note 1: The bandwidthdistance product is usually stated in
megahertz
kilometer (MHzkm) or
gigahertzkilometer (GHzkm).
Note 2: The bandwidthdistance product, which is normalized to 1 km, is a useful figure of merit for predicting the effective fiber bandwidth for other lengths, and for concatenated fibers.
Synonym bandwidthlength product.